_requirements.py

"""Requirements manifest for the interaction-model test suite.

Every user-facing behaviour the SDK must satisfy, keyed by a stable `<area>:<feature>[:<variant>]`
ID. Each entry owns the tests that exercise it: tests declare `@requirement("<id>")` (a test that
proves several behaviours stacks several decorators) and `test_coverage.py` enforces the contract
in both directions: every non-deferred requirement has at least one test, and every test carries
at least one requirement.

Sources:
    spec URL    -- externally mandated by the MCP specification (deep link to the section)
    `sdk`       -- a behavioural guarantee the SDK chose; not spec-mandated
    `issue:#n`  -- regression lock-in for a previously fixed bug

The `behavior` sentence describes the REQUIRED behaviour -- what the specification (or the SDK's
own contract) says should happen. Tests always pin the SDK's current behaviour. Where current
behaviour falls short of `behavior`, the gap is recorded as data: `divergence` on entries whose
tests pin the divergent behaviour, or `deferred` on entries that are tracked but not yet covered
by a test in this suite. An entry may carry both: `divergence` records the spec-compliance gap
(issue-able) and `deferred` records why no test exists; `divergence` alone implies a test pins
the divergent behaviour. `issue` carries the tracking link for a recorded gap once one is filed.

`deferred` reasons take one of three shapes: where the behaviour is exercised elsewhere in this
repo the reason names the covering test path; where the SDK does not implement the behaviour at
all the reason starts with "Not implemented in the SDK"; and where an interaction-level test is
planned but not yet written the reason starts with "Not yet covered here".

`transports` records which transports a behaviour applies to (or is observable on); None means
the behaviour is transport-independent.

The ID vocabulary and entry granularity are aligned with the TypeScript SDK's end-to-end
requirements suite, so coverage and recorded divergences can be compared across the two SDKs
entry by entry; IDs that exist in only one SDK reflect genuinely different API surface.
"""

import re
from collections.abc import Callable, Sequence
from dataclasses import dataclass
from typing import Any, Literal, TypeVar

import pytest
from mcp_types.version import KNOWN_PROTOCOL_VERSIONS

SpecVersion = Literal["2025-11-25", "2026-07-28"]
"""A protocol version the suite parametrizes over. Both values are typed even though only one is
on the active axis (SPEC_VERSIONS) until the 2026-07-28 implementation lands."""

SPEC_VERSIONS: tuple[SpecVersion, ...] = ("2025-11-25", "2026-07-28")
"""The active spec-version matrix axis, ordered oldest to newest. Every entry must be in KNOWN_PROTOCOL_VERSIONS."""

SPEC_BASE_URL = "https://modelcontextprotocol.io/specification/2025-11-25"
"""Deep-link base for entries citing the 2025-11-25 revision (the bulk of the manifest). Pinned --
not derived from SPEC_VERSIONS -- so adding a newer revision to the active axis does not silently
repoint existing source URLs."""

SPEC_2026_BASE_URL = "https://modelcontextprotocol.io/specification/2026-07-28"
"""Deep-link base for entries citing the 2026-07-28 revision."""

Transport = Literal["in-memory", "stdio", "streamable-http", "streamable-http-stateless", "sse"]

CONNECTABLE_TRANSPORTS: tuple[Transport, ...] = ("in-memory", "sse", "streamable-http", "streamable-http-stateless")
"""Transports the connect fixture fans out over (the subset with a factory in conftest._FACTORIES)."""

TRANSPORT_SPEC_VERSIONS: dict[Transport, tuple[SpecVersion, ...]] = {
    "sse": ("2025-11-25",),
    "in-memory": ("2025-11-25", "2026-07-28"),
    # At the newer revision the protocol-version header check runs before the stateless branch is
    # taken, so a stateless connection at that revision behaves identically to the stateful one.
    # Locked to avoid a redundant matrix column; revisit if the header/stateless ordering changes.
    "streamable-http-stateless": ("2025-11-25",),
}
"""Transports that only serve a subset of SPEC_VERSIONS. Absent => serves all. Consulted by compute_cells()."""

ArmExclusionReason = Literal[
    "asserts-legacy-handshake",
    "method-not-in-modern-registry",
    "legacy-only-vocabulary",
    "modern-error-surface",
    "requires-session",
    "drives-transport-directly",
    "server-initiated-request",
]
"""Machine-readable reasons a requirement is excluded from a (transport, spec_version) matrix cell.
The set doubles as a re-admission checklist: when a feature lands, grep for its reason to find the
cells to re-admit. Values are kept byte-identical to the typescript-sdk's EntryExclusionReason."""

_TestFn = TypeVar("_TestFn", bound=Callable[..., object])

_SOURCE_PATTERN = re.compile(r"https://modelcontextprotocol\.io/specification/.+|sdk|issue:#\d+")

_TASKS_DEFERRAL = (
    "Tasks have been removed from the draft spec and from this SDK; they are expected to return "
    "as a separate MCP extension. These 2025-11-25 requirements are tracked but intentionally "
    "unimplemented."
)


@dataclass(frozen=True, kw_only=True)
class Divergence:
    """A documented gap between the SDK behaviour this suite pins and what `source` mandates."""

    note: str
    issue: str | None = None


@dataclass(frozen=True, kw_only=True)
class ArmExclusion:
    """Excludes a requirement from a (transport, spec_version) matrix cell, with a typed reason."""

    reason: ArmExclusionReason
    transport: Transport | None = None
    spec_version: SpecVersion | None = None
    note: str | None = None

    def __post_init__(self) -> None:
        if self.spec_version is not None and self.spec_version not in KNOWN_PROTOCOL_VERSIONS:
            raise ValueError(f"spec_version {self.spec_version!r} is not in KNOWN_PROTOCOL_VERSIONS")


@dataclass(frozen=True, kw_only=True)
class KnownFailure:
    """A (transport, spec_version) cell where the requirement's test is expected to fail (strict xfail)."""

    note: str
    transport: Transport | None = None
    spec_version: SpecVersion | None = None
    issue: str | None = None

    def __post_init__(self) -> None:
        if not self.note.strip():
            raise ValueError("note must be non-empty")
        if self.spec_version is not None and self.spec_version not in KNOWN_PROTOCOL_VERSIONS:
            raise ValueError(f"spec_version {self.spec_version!r} is not in KNOWN_PROTOCOL_VERSIONS")
        if self.issue is not None and not re.fullmatch(r"#\d+|https://github\.com/\S+", self.issue):
            raise ValueError(f"issue must be '#<n>' or a GitHub URL, got {self.issue!r}")


@dataclass(frozen=True, kw_only=True)
class Requirement:
    """A single testable behaviour and the provenance of why it must hold."""

    source: str
    behavior: str
    transports: tuple[Transport, ...] | None = None
    divergence: Divergence | None = None
    deferred: str | None = None
    issue: str | None = None
    note: str | None = None
    added_in: SpecVersion | None = None
    removed_in: SpecVersion | None = None
    supersedes: tuple[str, ...] = ()
    superseded_by: str | None = None
    arm_exclusions: tuple[ArmExclusion, ...] = ()
    known_failures: tuple[KnownFailure, ...] = ()

    def __post_init__(self) -> None:
        if not _SOURCE_PATTERN.fullmatch(self.source):
            raise ValueError(f"source must be a specification URL, 'sdk', or 'issue:#n', got {self.source!r}")
        if self.added_in is not None and self.added_in not in KNOWN_PROTOCOL_VERSIONS:
            raise ValueError(f"added_in {self.added_in!r} is not in KNOWN_PROTOCOL_VERSIONS")
        if self.removed_in is not None and self.removed_in not in KNOWN_PROTOCOL_VERSIONS:
            raise ValueError(f"removed_in {self.removed_in!r} is not in KNOWN_PROTOCOL_VERSIONS")
        if (
            self.added_in is not None
            and self.removed_in is not None
            and KNOWN_PROTOCOL_VERSIONS.index(self.added_in) >= KNOWN_PROTOCOL_VERSIONS.index(self.removed_in)
        ):
            raise ValueError(f"added_in {self.added_in!r} must be earlier than removed_in {self.removed_in!r}")


REQUIREMENTS: dict[str, Requirement] = {
    # ═══════════════════════════════════════════════════════════════════════════
    # Lifecycle & version negotiation
    # ═══════════════════════════════════════════════════════════════════════════
    "lifecycle:capability:client-not-declared": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#operation",
        behavior=(
            "The client rejects sending notifications or registering handlers for capabilities it did not declare."
        ),
        divergence=Divergence(
            note=(
                "The client does not check its own declared capabilities before sending notifications or "
                "serving callbacks; nothing prevents a caller from violating the spec's MUST."
            ),
        ),
        deferred=(
            "Not implemented in the SDK: the client does not check its own declared capabilities before "
            "sending notifications or serving callbacks."
        ),
    ),
    "lifecycle:capability:server-not-advertised": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#operation",
        behavior=(
            "The client rejects calls to methods (e.g. resources/list) for capabilities the server did not advertise."
        ),
        divergence=Divergence(
            note=(
                "The client sends any request regardless of the server's advertised capabilities and "
                "surfaces whatever the server answers; the spec's MUST is not enforced."
            ),
        ),
        deferred=(
            "Not implemented in the SDK: the client sends any request regardless of the server's "
            "advertised capabilities and surfaces whatever the server answers."
        ),
    ),
    "lifecycle:initialize:basic": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#initialization",
        behavior=(
            "Connecting sends initialize with the protocol version, client capabilities, and client "
            "info; the server responds with its own and the connection is established."
        ),
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:initialize:server-info": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#initialization",
        behavior="The initialize result identifies the server: name and version, plus title when declared.",
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:initialize:instructions": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#initialization",
        behavior="A server may include an instructions string in the initialize result; the client exposes it.",
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:initialize:capabilities:from-handlers": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#capability-negotiation",
        behavior=(
            "The server advertises a capability for each feature area it has a registered handler for, "
            "and omits the capability for areas it does not."
        ),
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:initialize:capabilities:minimal": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#capability-negotiation",
        behavior="A server with no feature handlers advertises no feature capabilities.",
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:initialize:client-info": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#initialization",
        behavior="The client's name, version, and title are visible to server handlers after initialization.",
        removed_in="2026-07-28",
        superseded_by="lifecycle:envelope:stamped-on-every-request",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
        arm_exclusions=(ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),),
    ),
    "lifecycle:initialize:client-capabilities": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#capability-negotiation",
        behavior=(
            "The client capabilities visible to the server reflect which client callbacks are configured "
            "(sampling, elicitation, roots)."
        ),
        removed_in="2026-07-28",
        superseded_by="lifecycle:envelope:stamped-on-every-request",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
        arm_exclusions=(ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),),
    ),
    "lifecycle:initialized-notification": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#initialization",
        behavior=(
            "After successful initialization, the client sends exactly one initialized notification, "
            "before any non-ping request."
        ),
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:ping": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/ping#behavior-requirements",
        behavior="ping in either direction returns an empty result.",
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); ping deleted from the schema, no replacement.",
    ),
    "ping:client-to-server": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/ping#behavior-requirements",
        behavior="A client-initiated ping receives an empty result from the server.",
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); ping deleted from the schema, no replacement.",
    ),
    "ping:server-to-client": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/ping#behavior-requirements",
        behavior="A server-initiated ping receives an empty result from the client.",
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); ping deleted from the schema, no replacement.",
        arm_exclusions=(ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),),
    ),
    "lifecycle:requests-before-initialized": Requirement(
        source="sdk",
        behavior=(
            "A request other than ping sent before the initialization handshake completes is rejected with an error."
        ),
        removed_in="2026-07-28",
        note="initialize handshake removed at 2026-07-28; per-request _meta envelope replaces it.",
    ),
    "lifecycle:pre-initialization-ordering": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#initialization",
        behavior=(
            "Before initialization completes, the client sends no requests other than pings, and the "
            "server sends no requests other than pings and logging."
        ),
        divergence=Divergence(
            note=(
                "The server's send methods (create_message / elicit_form / list_roots) do not check "
                "initialization state before sending; on the client side, Client always completes the "
                "handshake before any caller code runs."
            ),
        ),
        deferred=(
            "Not implemented in the SDK: neither side enforces sender-side restraint. The server's send "
            "methods (create_message / elicit_form / list_roots) do not check initialization state before "
            "sending, and there is no natural hook to issue a server-to-client request between the "
            "initialize response and the initialized notification through the public API; on the client "
            "side, Client always completes the handshake before any caller code runs."
        ),
    ),
    "lifecycle:version:downgrade": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#version-negotiation",
        behavior=(
            "When the server returns an older supported protocol version, the client downgrades to it "
            "and the connection succeeds at that version."
        ),
        removed_in="2026-07-28",
        note="initialize-time version negotiation removed at 2026-07-28; version carried per-request in _meta.",
    ),
    "lifecycle:version:match": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#version-negotiation",
        behavior=(
            "When the server supports the requested protocol version it echoes that version in the "
            "initialize result, and the connection proceeds at that version."
        ),
        removed_in="2026-07-28",
        note="initialize-time version negotiation removed at 2026-07-28; version carried per-request in _meta.",
    ),
    "lifecycle:version:server-fallback-latest": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#version-negotiation",
        behavior=(
            "An initialize request carrying a protocol version the server does not support is answered "
            "with another version the server supports — the latest one — rather than an error."
        ),
        removed_in="2026-07-28",
        note="initialize-time version negotiation removed at 2026-07-28; version carried per-request in _meta.",
    ),
    "lifecycle:version:reject-unsupported": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#version-negotiation",
        behavior=(
            "A client that receives an initialize response carrying a protocol version it does not "
            "support fails initialization with an error rather than proceeding with the session."
        ),
        removed_in="2026-07-28",
        note="initialize-time version negotiation removed at 2026-07-28; version carried per-request in _meta.",
    ),
    "lifecycle:stateless:request-envelope": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/lifecycle#stateless-operation",
        behavior=(
            "At protocol_version 2026-07-28, every request carries io.modelcontextprotocol/protocolVersion "
            "and /clientCapabilities in params._meta (/clientInfo is optional); no initialize handshake occurs."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:stateless:no-initialize": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/lifecycle#stateless-operation",
        behavior=(
            "A ClientSession pinned to 2026-07-28 is born initialized: initialize() is idempotent "
            "and returns the synthesized result without any frame sent."
        ),
        added_in="2026-07-28",
        deferred="covered by a tests/client/ unit test; not observable as an interaction",
    ),
    "lifecycle:stateless:caller-meta-preserved": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/lifecycle#stateless-operation",
        behavior=(
            "Caller-supplied _meta keys on a request survive the per-request envelope merge: the "
            "three io.modelcontextprotocol/* envelope keys overwrite any caller-supplied values for "
            "those keys; non-colliding caller keys are preserved."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:stateless:unpinned-legacy-wire": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/versioning",
        behavior=(
            "An unpinned session that negotiates an earlier protocol version emits no 2026-07-28 "
            "vocabulary on any JSON-RPC frame in either direction."
        ),
        deferred=(
            "bare-ClientSession seam; the high-level Client + HTTP-seam scan in "
            "hosting:http:legacy-no-modern-vocabulary covers the same vocabulary set"
        ),
    ),
    "lifecycle:envelope:stamped-on-every-request": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic#_meta",
        behavior=(
            "Every client→server request on a modern-negotiated session carries "
            "_meta.{protocolVersion,clientInfo,clientCapabilities}; notifications do not."
        ),
        added_in="2026-07-28",
        supersedes=("lifecycle:initialize:client-info", "lifecycle:initialize:client-capabilities"),
    ),
    "lifecycle:envelope:header-matches-meta": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http#headers",
        behavior="On HTTP, the MCP-Protocol-Version header on every POST matches _meta.protocolVersion in the body.",
        transports=("streamable-http", "streamable-http-stateless"),
        added_in="2026-07-28",
        note="HTTP-only: the header is a streamable-http transport concern; stdio and in-memory carry no headers.",
    ),
    "lifecycle:discover:basic": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/lifecycle#discover",
        behavior=(
            "Calling discover() sends server/discover with no params and returns a typed DiscoverResult "
            "carrying supportedVersions, capabilities and the cache hint fields; the server's identity "
            "travels as the io.modelcontextprotocol/serverInfo stamp in the result _meta."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:discover:retry-on-32022": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/lifecycle#version-errors",
        behavior=(
            "When server/discover returns -32022 UnsupportedProtocolVersion, the client retries once with "
            "the intersection of error.data.supported and its own modern versions; an empty intersection raises."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:discover:fallback-method-not-found": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/stdio#backward-compatibility",
        behavior=(
            "When server/discover returns any JSON-RPC error or a bare HTTP 4xx, an auto-negotiating "
            "client falls back to the legacy initialize handshake and the connection succeeds at a "
            "handshake-era version (legacy servers reject the probe with various codes)."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:discover:network-error-raises": Requirement(
        source="sdk",
        behavior=(
            "A network/connection error during server/discover propagates to the caller without "
            "falling back to initialize; any rpc-error or 4xx falls back (legacy servers reject the "
            "probe with various codes). An outage is never an era verdict."
        ),
        transports=("streamable-http", "streamable-http-stateless"),
        added_in="2026-07-28",
        note="HTTP-only: distinguishes transport-level failures from server-side rejection.",
    ),
    "lifecycle:mode:legacy-never-probes": Requirement(
        source="sdk",
        behavior=(
            "A Client constructed with mode='legacy' sends initialize as its first request "
            "and never sends server/discover."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:mode:pin-never-handshakes": Requirement(
        source="sdk",
        behavior=(
            "A Client constructed with mode='2026-07-28' sends no initialize and no server/discover; its "
            "first wire request is the caller's first call, carrying the full _meta envelope."
        ),
        added_in="2026-07-28",
    ),
    "lifecycle:mode:prior-discover-zero-rtt": Requirement(
        source="sdk",
        behavior=(
            "A Client constructed with prior_discover=<DiscoverResult> sends no negotiation traffic; "
            "server_info and capabilities are populated from the prior result."
        ),
        added_in="2026-07-28",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Protocol primitives: cancellation, timeout, progress, errors, _meta
    # ═══════════════════════════════════════════════════════════════════════════
    "protocol:request-id:unique": Requirement(
        source=f"{SPEC_BASE_URL}/basic#requests",
        behavior=(
            "Every request sent on a session carries a unique, non-null string or integer id; ids are "
            "never reused within the session."
        ),
    ),
    "protocol:request-id:caller-supplied": Requirement(
        source="sdk",
        behavior=(
            "A caller can supply the id of a request it sends, so the id is known before any response "
            "arrives; subscriptions/listen streams are demultiplexed by exactly that id."
        ),
        note=(
            f"The demux-by-listen-request-id obligation is the spec's "
            f"({SPEC_2026_BASE_URL}/basic/patterns/subscriptions#receiving-notifications); supplying the "
            "id up front is the SDK surface that makes it satisfiable."
        ),
        added_in="2026-07-28",
    ),
    "protocol:notifications:no-response": Requirement(
        source=f"{SPEC_BASE_URL}/basic#notifications",
        behavior=(
            "Notifications are never answered: every message the server delivers is either the response "
            "to a request the client sent or a notification carrying no id."
        ),
    ),
    "protocol:cancel:abort-signal": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#cancellation-flow",
        behavior=(
            "Abandoning an in-flight request client-side (cancelling the task awaiting it) cancels the "
            "request itself: the server-side handler stops and the session serves later requests "
            "normally."
        ),
        note=(
            "The per-transport wire spelling (frame vs response-stream close) is pinned separately by "
            "protocol:cancel:stream-frame and the client-transport:http:cancel-* pair."
        ),
        arm_exclusions=(
            ArmExclusion(
                reason="requires-session",
                transport="streamable-http-stateless",
                note=(
                    "The 2025-era cancel frame POSTs on a fresh per-request transport that shares no "
                    "in-flight state with the blocked request, so the handler is never interrupted."
                ),
            ),
        ),
    ),
    "protocol:cancel:abort-scoped": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior=(
            "Abandoning one in-flight request cancels only that request: a concurrent request on the "
            "same connection keeps running and returns its result."
        ),
        arm_exclusions=(ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),),
    ),
    "protocol:cancel:handler-abort-propagates": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior="On the receiving side, a cancellation notification stops the running request handler.",
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "protocol:cancel:in-flight": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior=(
            "A cancellation notification for an in-flight request stops the server-side handler, and the "
            "receiver does not send a response for the cancelled request - no result and no error."
        ),
        divergence=Divergence(
            note=(
                "The 2025-era streamable HTTP transport still answers a cancelled request, with a "
                "REQUEST_CANCELLED (-32800) error - deliberate and era-scoped: that wire ends a request's "
                "stream only with a response for its id, so silence would leave the POST (and any "
                "resuming client's replay) open. Every other transport sends nothing, and the "
                "2026-07-28 MUST NOT applies only there. Retires with the legacy transport; see "
                "transport:streamable-http:cancelled-request-terminated."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "protocol:cancel:initialize-not-cancellable": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior="The client never sends notifications/cancelled for the initialize request.",
    ),
    "protocol:cancel:late-response-ignored": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior=(
            "A response that arrives after the sender issued notifications/cancelled is ignored; the "
            "request stays failed and no error is raised."
        ),
    ),
    "protocol:cancel:server-survives": Requirement(
        source="sdk",
        behavior="The session continues to serve new requests after an earlier request was cancelled.",
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "protocol:cancel:server-to-client": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior=(
            "A server that abandons an in-flight server-initiated request (sampling, elicitation, roots) "
            "cancels it, and the client stops processing the cancelled request."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "protocol:cancel:stream-frame": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/patterns/cancellation#transport-specific-cancellation",
        behavior=(
            "On stream (stdio-shaped) wires at 2026-07-28, abandoning an in-flight request sends exactly "
            "one notifications/cancelled naming its request id - streams keep the frame spelling of "
            "cancellation that streamable HTTP dropped."
        ),
        added_in="2026-07-28",
        note="Exercised over the in-memory stream pair, the same dual-era wire stdio serves.",
    ),
    "protocol:cancel:unknown-id-ignored": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#error-handling",
        behavior=(
            "The receiver silently ignores a cancellation notification referencing an unknown or "
            "already-completed request id; no error response is sent and no exception is raised."
        ),
    ),
    "protocol:cancel:sender-targeting": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#behavior-requirements",
        behavior=(
            "Cancellation notifications reference only requests that were previously issued in the same "
            "direction and are believed to still be in flight."
        ),
        deferred=(
            "Not implemented in the SDK: there is no public client-side cancel API to drive (see "
            "protocol:cancel:abort-signal), so the sender-side targeting rule has nothing to pin."
        ),
    ),
    "protocol:error:connection-closed": Requirement(
        source="sdk",
        behavior="Closing the transport fails all in-flight requests with a connection-closed error.",
    ),
    "protocol:error:internal-error": Requirement(
        source=f"{SPEC_BASE_URL}/basic#responses",
        behavior=(
            "An unhandled exception in a request handler is returned to the caller as JSON-RPC error "
            "-32603 Internal error."
        ),
        divergence=Divergence(
            note=(
                "The low-level Server returns code 0 (not a defined JSON-RPC code) instead of -32603 and "
                "leaks str(exc) as the error message."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(
                reason="modern-error-surface",
                spec_version="2026-07-28",
                note=(
                    "The modern entry maps Exception->INTERNAL_ERROR (-32603) with an opaque message, so the "
                    "2026 arm SATISFIES this requirement; the test pins the legacy code-0 divergence and "
                    "needs an era-aware assertion before re-admission."
                ),
            ),
        ),
    ),
    "protocol:error:invalid-params": Requirement(
        source=f"{SPEC_BASE_URL}/basic#responses",
        behavior="A request with malformed params is answered with JSON-RPC error -32602 Invalid params.",
    ),
    "protocol:error:method-not-found": Requirement(
        source=f"{SPEC_BASE_URL}/basic#responses",
        behavior="A request whose method has no registered handler is answered with a METHOD_NOT_FOUND error.",
    ),
    "protocol:error:null-id": Requirement(
        source="sdk",
        behavior=(
            "An error response carrying a null id — the JSON-RPC shape for a peer reporting a failure it "
            "could not attribute to a request, such as a parse error — is surfaced to the application "
            "rather than silently discarded."
        ),
        divergence=Divergence(
            note=(
                "The dispatcher drops null-id error responses with a debug log; in v1, JSONRPCError.id was "
                "non-nullable, so a null-id error response failed transport validation and the resulting "
                "ValidationError was surfaced to message_handler as an exception. A typed fault channel "
                "restoring visibility is planned."
            ),
        ),
        deferred=(
            "Not yet covered here: the current drop is pinned at the dispatcher level by "
            "tests/shared/test_jsonrpc_dispatcher.py; an interaction-level test waits on the planned "
            "fault channel."
        ),
    ),
    "protocol:meta:related-task": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#related-task-metadata",
        behavior="Messages may carry related-task _meta associating them with a task.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "meta:request-to-handler": Requirement(
        source=f"{SPEC_BASE_URL}/basic#_meta",
        behavior="The _meta object the client attaches to a request is visible to the server handler.",
        arm_exclusions=(ArmExclusion(reason="asserts-legacy-handshake", spec_version="2026-07-28"),),
    ),
    "meta:result-to-client": Requirement(
        source=f"{SPEC_BASE_URL}/basic#_meta",
        behavior="The _meta object a handler attaches to its result is delivered to the client.",
    ),
    "protocol:progress:callback": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior=(
            "Progress notifications emitted by a handler during a request are delivered to the caller's "
            "progress callback, in order, with their progress, total, and message."
        ),
    ),
    "protocol:progress:token-injected": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior=(
            "Supplying a progress callback attaches a progress token to the outgoing request, which the "
            "server-side handler can observe in its request metadata."
        ),
        arm_exclusions=(ArmExclusion(reason="asserts-legacy-handshake", spec_version="2026-07-28"),),
    ),
    "protocol:progress:token-unique": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior=("Concurrent in-flight requests that each supply a progress callback carry distinct progress tokens."),
        note=(
            "Tested as the consequence: each callback receives only its own request's progress under "
            "interleaved emission. Token distinctness is the JSON-RPC mechanism for that; the in-process "
            "direct dispatcher carries the callback per-request without a wire-level token."
        ),
    ),
    "protocol:progress:monotonic": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior=(
            "The progress value increases with each notification for a given token, even when the total is unknown."
        ),
        divergence=Divergence(
            note=(
                "The spec MUST is not enforced: progress values are not validated on either side, so a "
                "handler that emits non-increasing values has them forwarded to the callback unchanged."
            ),
        ),
    ),
    "protocol:progress:stops-after-completion": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#behavior-requirements",
        behavior="Progress notifications for a token stop once the associated request completes.",
        divergence=Divergence(
            note=(
                "send_progress_notification does not check whether the token's request has already "
                "completed; the late notification is sent and reaches the client."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "protocol:progress:late-dropped-by-client": Requirement(
        source="sdk",
        behavior=(
            "A progress notification that arrives after its request has completed is not delivered to the "
            "original progress callback."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "protocol:progress:no-token": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior="Without a progress callback the request carries no progress token.",
    ),
    "protocol:progress:client-to-server": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior="A progress notification sent by the client is delivered to the server's progress handler.",
        arm_exclusions=(ArmExclusion(reason="requires-session", spec_version="2026-07-28"),),
    ),
    "protocol:timeout:basic": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#timeouts",
        behavior=(
            "A request that exceeds its read timeout fails with a request-timeout error instead of "
            "waiting forever for the response."
        ),
    ),
    "protocol:timeout:max-total": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#timeouts",
        behavior="A maximum total timeout is enforced even when progress notifications keep arriving.",
        divergence=Divergence(
            note=(
                "There is no maximum-total-timeout option; only the per-request read timeout exists, so the "
                "spec's SHOULD that an overall maximum is always enforced cannot be satisfied."
            ),
        ),
        deferred=(
            "Not implemented in the SDK: there is no maximum-total-timeout option; only the per-request "
            "read timeout exists."
        ),
    ),
    "protocol:timeout:reset-on-progress": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#timeouts",
        behavior="When configured to do so, each progress notification resets the request's read timeout.",
        deferred=(
            "Not implemented in the SDK: progress notifications do not reset the request read timeout and "
            "no option exists to enable that."
        ),
    ),
    "protocol:timeout:sends-cancellation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#timeouts",
        behavior=(
            "When a request times out, the sender issues notifications/cancelled for that request before "
            "failing the local call."
        ),
    ),
    "protocol:timeout:session-survives": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#timeouts",
        behavior="The session continues to serve new requests after an earlier request timed out.",
    ),
    "protocol:timeout:session-default": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#timeouts",
        behavior="A session-level read timeout applies to every request that does not override it.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Tools
    # ═══════════════════════════════════════════════════════════════════════════
    "tools:call:content:audio": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#audio-content",
        behavior="A tool result can carry audio content: base64 data with a mimeType.",
    ),
    "tools:call:content:embedded-resource": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#embedded-resources",
        behavior="A tool result can carry an embedded resource with full text or blob contents.",
    ),
    "tools:call:content:image": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#image-content",
        behavior="A tool result can carry image content: base64 data with a mimeType.",
    ),
    "tools:call:content:mixed": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool-result",
        behavior="A tool result can carry multiple content blocks of different types; order is preserved.",
    ),
    "tools:call:content:resource-link": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#resource-links",
        behavior="A tool result can carry a resource_link content block referencing a resource by URI.",
    ),
    "tools:call:content:text": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#text-content",
        behavior="tools/call delivers arguments to the tool handler and returns its text content to the caller.",
    ),
    "tools:call:concurrent": Requirement(
        source="sdk",
        behavior=(
            "Multiple tool calls in flight on one session are dispatched concurrently, and each caller "
            "receives the response to its own request."
        ),
    ),
    "tools:call:elicitation-roundtrip": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#user-interaction-model",
        behavior=(
            "A tool handler that issues an elicitation receives the client's result and can embed it in "
            "the tool call result."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "tools:call:is-error": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#error-handling",
        behavior=(
            "A tool execution failure is returned as a result with isError true and the failure described "
            "in content, not as a JSON-RPC error."
        ),
    ),
    "tools:call:logging-mid-execution": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#log-message-notifications",
        behavior=(
            "Log notifications emitted by a tool handler during execution reach the client's logging "
            "callback before the tool result returns."
        ),
    ),
    "tools:call:progress": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/progress#progress-flow",
        behavior=(
            "Progress notifications emitted by a tool handler reach the caller's progress callback before "
            "the tool result returns."
        ),
    ),
    "tools:call:sampling-roundtrip": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#creating-messages",
        behavior=(
            "A tool handler that issues a sampling request receives the client's completion and can embed "
            "it in the tool call result."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "tools:call:structured-content": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#structured-content",
        behavior="A tool result can carry structuredContent alongside content; the client receives both.",
    ),
    "tools:call:structured-content:text-mirror": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#structured-content",
        behavior="A tool returning structured content also returns the serialized JSON as a text content block.",
    ),
    "tools:call:unknown-name": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#error-handling",
        behavior="tools/call for a name the server does not recognise returns a JSON-RPC error.",
    ),
    "tools:capability:declared": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#capabilities",
        behavior="A server with a list_tools handler advertises the tools capability in its initialize result.",
        arm_exclusions=(ArmExclusion(reason="legacy-only-vocabulary", spec_version="2026-07-28"),),
    ),
    "tools:input-schema:json-schema-2020-12": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool",
        behavior=(
            "A tool registered with a JSON Schema 2020-12 inputSchema (nested objects, $defs references) "
            "is discoverable and callable."
        ),
    ),
    "tools:input-schema:preserve-additional-properties": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool",
        behavior="tools/list preserves inputSchema additionalProperties as registered.",
    ),
    "tools:input-schema:preserve-defs": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool",
        behavior="tools/list preserves inputSchema $defs as registered.",
    ),
    "tools:input-schema:preserve-schema-dialect": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool",
        behavior="tools/list preserves the inputSchema $schema dialect URI as registered.",
    ),
    "tools:list-changed": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#list-changed-notification",
        behavior=(
            "When the tool set changes, the server sends notifications/tools/list_changed and it reaches "
            "the client's handler."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "tools:list:basic": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#listing-tools",
        behavior="tools/list returns the registered tools with name, description, and inputSchema.",
    ),
    "tools:list:metadata": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool",
        behavior=(
            "Optional Tool fields supplied by the server (title, annotations, outputSchema, icons, _meta) "
            "are delivered to the client unchanged."
        ),
    ),
    "tools:list:pagination": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#response-format",
        behavior=(
            "tools/list supports cursor pagination: the nextCursor returned by a list handler round-trips "
            "back to the handler as an opaque cursor until the listing is exhausted."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Tools: SDK guarantees
    # ═══════════════════════════════════════════════════════════════════════════
    "client:output-schema:skip-on-error": Requirement(
        source="sdk",
        behavior="The client skips structured-content validation when the tool result has isError true.",
    ),
    "client:output-schema:validate": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#output-schema",
        behavior=(
            "A tool result whose structuredContent does not conform to the tool's declared outputSchema "
            "is rejected by the client: the call raises instead of returning the invalid result."
        ),
    ),
    "client:output-schema:missing-structured": Requirement(
        source="sdk",
        behavior="A tool that declares an output schema but returns no structuredContent fails client-side validation.",
    ),
    "client:output-schema:auto-list": Requirement(
        source="sdk",
        behavior=(
            "Calling a tool whose output schema is not yet cached issues an implicit tools/list to "
            "populate the cache; subsequent calls of the same tool do not."
        ),
        divergence=Divergence(
            note=(
                "Design concern rather than spec violation: the implicit request is invisible to the "
                "caller, and against a server that registers only on_call_tool a successful call surfaces "
                "as METHOD_NOT_FOUND from a tools/list the caller never asked for."
            ),
        ),
    ),
    "mcpserver:output-schema:missing-structured": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#output-schema",
        behavior="A tool with an output schema whose function returns no structured content produces a server error.",
    ),
    "mcpserver:output-schema:server-validate": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#output-schema",
        behavior=(
            "MCPServer validates structured content against the tool's output schema before returning; a "
            "mismatch produces a server error."
        ),
    ),
    "mcpserver:output-schema:skip-on-error": Requirement(
        source="sdk",
        behavior="Server-side output schema validation is skipped when the tool returns an isError result.",
    ),
    "mcpserver:tool:duplicate-name": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#tool-names",
        behavior="Registering a tool with a name already in use is rejected at registration time.",
        divergence=Divergence(
            note=(
                "MCPServer logs a warning and keeps the first registration instead of rejecting; "
                "warn_on_duplicate_tools defaults to True and warning is the only effect -- there is "
                "no rejection mode."
            ),
        ),
    ),
    "mcpserver:tool:extra": Requirement(
        source="sdk",
        behavior=(
            "Tool functions can access request metadata (request id, client params, session) through the "
            "Context parameter."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="asserts-legacy-handshake", spec_version="2026-07-28"),
        ),
    ),
    "mcpserver:tool:handler-throws": Requirement(
        source="sdk",
        behavior=(
            "An exception raised by a tool function (ToolError or otherwise) is caught and returned as a "
            "tool result with isError true and the failure text in content; it does not become a JSON-RPC error."
        ),
    ),
    "mcpserver:tool:input-validation": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#error-handling",
        behavior=(
            "Arguments that fail the tool's input validation produce a tool execution error (isError true "
            "with the validation failure described in content) without invoking the function."
        ),
    ),
    "mcpserver:tool:naming-validation": Requirement(
        source="sdk",
        behavior=(
            "Registering a tool whose name violates the spec's tool-naming conventions emits a warning; "
            "registration still succeeds."
        ),
    ),
    "mcpserver:tool:output-schema:model": Requirement(
        source="sdk",
        behavior=(
            "A tool returning a typed model advertises a matching generated outputSchema and returns the "
            "model's fields as structuredContent alongside a serialised text block."
        ),
    ),
    "mcpserver:tool:output-schema:wrapped": Requirement(
        source="sdk",
        behavior=(
            "A tool returning a non-object type (primitive or list) wraps the value as {'result': ...} in "
            "structuredContent, with a matching generated outputSchema."
        ),
    ),
    "mcpserver:tool:schema-variants": Requirement(
        source="sdk",
        behavior=(
            "Tool input schemas generated from complex parameter types (unions, nested models, "
            "constrained types) validate and coerce arguments before the function runs."
        ),
    ),
    "mcpserver:tool:unknown-name": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#error-handling",
        behavior="tools/call for a name that was never registered returns a JSON-RPC error.",
        divergence=Divergence(
            note=(
                "The spec classifies unknown tools as a protocol error (its example uses -32602 Invalid "
                "params); MCPServer reports a tool execution error (isError true) instead. The low-level "
                "path follows the spec example (see tools:call:unknown-name)."
            ),
        ),
    ),
    "mcpserver:tool:url-elicitation-error": Requirement(
        source="sdk",
        behavior=(
            "A tool function that raises the URL-elicitation-required error surfaces to the caller as "
            "error -32042 with the elicitation parameters intact."
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2322); error -32042 retired, replaced by an MRTR input_required result "
            "carrying inputRequests."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # MCPServer: Context helpers (SDK)
    # ═══════════════════════════════════════════════════════════════════════════
    "mcpserver:context:logging": Requirement(
        source="sdk",
        behavior=(
            "The Context logging helpers (debug/info/warning/error) send log message notifications at the "
            "corresponding severity."
        ),
    ),
    "mcpserver:context:progress": Requirement(
        source="sdk",
        behavior=(
            "Context.report_progress sends a progress notification against the requesting client's progress token."
        ),
    ),
    "mcpserver:context:elicit": Requirement(
        source="sdk",
        behavior=(
            "Context.elicit sends a form elicitation built from a typed schema and returns a typed "
            "accepted/declined/cancelled result."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "mcpserver:context:read-resource": Requirement(
        source="sdk",
        behavior="Context.read_resource reads a resource registered on the same server from inside a tool.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Resources
    # ═══════════════════════════════════════════════════════════════════════════
    "resources:annotations": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#annotations",
        behavior="Resource annotations supplied by the server round-trip to the client in the list result.",
        divergence=Divergence(
            note=(
                "The SDK Annotations model is missing the schema's lastModified field; MCPModel uses the "
                "pydantic default extra='ignore', so the value is silently dropped on parse."
            ),
        ),
    ),
    "resources:capability:declared": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#capabilities",
        behavior=(
            "A server with resource handlers advertises the resources capability, including the subscribe "
            "sub-flag when a subscribe handler is registered."
        ),
        arm_exclusions=(ArmExclusion(reason="legacy-only-vocabulary", spec_version="2026-07-28"),),
    ),
    "resources:list-changed": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#list-changed-notification",
        behavior=(
            "When the resource set changes, the server sends notifications/resources/list_changed and it "
            "reaches the client's handler."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "resources:list:basic": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#listing-resources",
        behavior=(
            "resources/list returns the registered resources with uri, name, and the optional descriptive "
            "fields supplied by the server."
        ),
    ),
    "resources:list:pagination": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#operations-supporting-pagination",
        behavior="resources/list supports cursor pagination.",
    ),
    "resources:read:blob": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#reading-resources",
        behavior="resources/read returns binary contents base64-encoded in blob.",
    ),
    "resources:read:template-vars": Requirement(
        source="sdk",
        behavior="Variables extracted from a templated resource URI reach the resource function as typed arguments.",
    ),
    "resources:read:text": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#reading-resources",
        behavior="resources/read returns text contents carrying uri, mimeType, and the text.",
    ),
    "resources:read:unknown-uri": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#error-handling",
        behavior="resources/read for an unknown URI returns JSON-RPC error -32002 (resource not found).",
    ),
    "resources:subscribe": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#subscriptions",
        behavior="resources/subscribe delivers the URI to the server's subscribe handler and returns an empty result.",
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); resources/subscribe replaced by subscriptions/listen.",
    ),
    "resources:subscribe:capability-required": Requirement(
        source="sdk",
        behavior=(
            "resources/subscribe to a server that did not advertise the subscribe capability is rejected with an error."
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); the resources/subscribe RPC is gone. The resources.subscribe "
            "capability flag is retained but reinterpreted as opt-in for the resourceSubscriptions filter on "
            "subscriptions/listen -- there is no separate subscriptions capability."
        ),
    ),
    "resources:subscribe:updated": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#subscriptions",
        behavior="After resources/subscribe, changes to that resource send notifications/resources/updated.",
        deferred=(
            "Not implemented in the SDK: the server keeps no subscription state linking subscribe to "
            "updated notifications; emitting updates is entirely handler code. The two halves are pinned "
            "separately by resources:subscribe and resources:updated-notification."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); resources/subscribe replaced by subscriptions/listen.",
    ),
    "resources:templates:list": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#resource-templates",
        behavior=(
            "resources/templates/list returns the registered templates with their uriTemplate and descriptive fields."
        ),
    ),
    "resources:templates:pagination": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#operations-supporting-pagination",
        behavior="resources/templates/list supports cursor pagination.",
    ),
    "resources:unsubscribe": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#subscriptions",
        behavior=(
            "resources/unsubscribe delivers the URI to the server's unsubscribe handler and returns an empty result."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); resources/unsubscribe replaced by subscriptions/listen.",
    ),
    "resources:unsubscribe:stops-updates": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#subscriptions",
        behavior="After resources/unsubscribe the server stops sending updated notifications for that URI.",
        deferred=(
            "Not implemented in the SDK: the server keeps no subscription state, so whether updated "
            "notifications stop after unsubscribe is entirely handler code; there is no SDK behaviour to "
            "pin beyond the unsubscribe request reaching the handler (covered by resources:unsubscribe)."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); resources/unsubscribe replaced by subscriptions/listen.",
    ),
    "subscriptions:listen:client:honored-surfacing": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/patterns/subscriptions#acknowledgment",
        behavior=(
            "Entering Client.listen() waits for the server's acknowledgment and surfaces the honored "
            "filter subset on the handle, so the client can check it against what it requested (spec SHOULD)."
        ),
        added_in="2026-07-28",
    ),
    "subscriptions:listen:client:concurrent-demux": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/patterns/subscriptions#multiple-concurrent-subscriptions",
        behavior=(
            "Concurrently open subscriptions each surface their own acknowledgment: with both listen "
            "requests in flight before either ack arrives, each handle's honored filter is the subset "
            "for its own request, routed by subscription id rather than broadcast to every open route."
        ),
        added_in="2026-07-28",
    ),
    "subscriptions:listen:client:iteration": Requirement(
        source="sdk",
        behavior=(
            "An open subscription is an async iterator of typed change events; delivered notifications "
            "still tee to message_handler so caching and observers keep working."
        ),
        added_in="2026-07-28",
    ),
    "subscriptions:listen:client:graceful-close": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/patterns/subscriptions#cancellation",
        behavior=(
            "The server's empty subscriptions/listen result (its deliberate close) ends iteration cleanly "
            "after buffered events drain; no exception is raised."
        ),
        added_in="2026-07-28",
    ),
    "subscriptions:listen:client:lost": Requirement(
        source="sdk",
        behavior=(
            "A listen stream that ends without the graceful result raises SubscriptionLost from iteration; "
            "there is no automatic re-listen."
        ),
        added_in="2026-07-28",
    ),
    "subscriptions:listen:client:era-guard": Requirement(
        source="sdk",
        behavior=(
            "Client.listen() on a pre-2026 connection raises ListenNotSupportedError steering to "
            "subscribe_resource/message_handler instead of leaking a wire -32601."
        ),
        removed_in="2026-07-28",
        note=(
            "removed_in scopes the matrix to the 2025 cells deliberately: the behavior under test is the "
            "guard on connections where the method does not exist."
        ),
    ),
    "resources:updated-notification": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#subscriptions",
        behavior=(
            "A resources/updated notification sent by the server reaches the client carrying the URI of "
            "the changed resource."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Resources: SDK guarantees
    # ═══════════════════════════════════════════════════════════════════════════
    "mcpserver:resource:duplicate-name": Requirement(
        source="sdk",
        behavior="Registering a resource or template with a duplicate identifier is rejected at registration time.",
        divergence=Divergence(
            note=(
                "MCPServer logs a warning and keeps the first registration instead of rejecting; same "
                "warn-and-ignore behaviour as duplicate tool names (mcpserver:tool:duplicate-name). "
                "Templates differ: a duplicate uri_template silently replaces the first with no warning."
            ),
        ),
    ),
    "mcpserver:resource:read-throws-surfaced": Requirement(
        source="sdk",
        behavior=(
            "A resource function that raises is surfaced to the caller as a JSON-RPC error response "
            "(-32603 Internal error), with the original exception text withheld."
        ),
    ),
    "mcpserver:resource:static": Requirement(
        source="sdk",
        behavior=(
            "A function registered with @mcp.resource() for a fixed URI is listed by resources/list and "
            "served by resources/read at that URI."
        ),
    ),
    "mcpserver:resource:template": Requirement(
        source="sdk",
        behavior=(
            "A function registered with a URI template is listed by resources/templates/list and matched "
            "by resources/read, receiving the parameters extracted from the requested URI."
        ),
    ),
    "mcpserver:resource:unknown-uri": Requirement(
        source=f"{SPEC_BASE_URL}/server/resources#error-handling",
        behavior=(
            "resources/read for a URI matching no registered resource returns JSON-RPC error -32602 "
            "(invalid params) with the requested URI in error.data, per SEP-2164."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Prompts
    # ═══════════════════════════════════════════════════════════════════════════
    "prompts:capability:declared": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#capabilities",
        behavior="A server with a list_prompts handler advertises the prompts capability in its initialize result.",
        arm_exclusions=(ArmExclusion(reason="legacy-only-vocabulary", spec_version="2026-07-28"),),
    ),
    "prompts:get:content:audio": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#audio-content",
        behavior="Prompt messages may contain audio content with base64 data and a mimeType.",
    ),
    "prompts:get:content:embedded-resource": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#embedded-resources",
        behavior="Prompt messages may contain embedded resource content.",
    ),
    "prompts:get:content:image": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#image-content",
        behavior="Prompt messages may contain image content.",
    ),
    "prompts:get:missing-required-args": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#error-handling",
        behavior="prompts/get omitting a required argument returns JSON-RPC error -32602 (Invalid params).",
        divergence=Divergence(
            note=(
                "MCPServer's prompt renderer raises a plain ValueError before the prompt function runs, "
                "which the low-level server converts to error code 0 with the exception text as the message."
            ),
        ),
        arm_exclusions=(ArmExclusion(reason="modern-error-surface", spec_version="2026-07-28"),),
    ),
    "prompts:get:multi-message": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#getting-a-prompt",
        behavior="A prompt can return multiple messages mixing user and assistant roles; order is preserved.",
    ),
    "prompts:get:no-args": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#getting-a-prompt",
        behavior="prompts/get with no arguments returns the prompt's messages.",
    ),
    "prompts:get:unknown-name": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#error-handling",
        behavior="prompts/get for an unknown prompt name returns JSON-RPC error -32602 (Invalid params).",
    ),
    "prompts:get:with-args": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#getting-a-prompt",
        behavior="prompts/get delivers the supplied arguments to the prompt handler and returns its messages.",
    ),
    "prompts:list-changed": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#list-changed-notification",
        behavior=(
            "When the prompt set changes, the server sends notifications/prompts/list_changed and it "
            "reaches the client's handler."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="requires-session", spec_version="2026-07-28"),
        ),
    ),
    "prompts:list:basic": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#listing-prompts",
        behavior="prompts/list returns the registered prompts with name, description, and argument declarations.",
    ),
    "prompts:list:pagination": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#operations-supporting-pagination",
        behavior="prompts/list supports cursor pagination.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Prompts: SDK guarantees
    # ═══════════════════════════════════════════════════════════════════════════
    "mcpserver:prompt:args-validation": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#implementation-considerations",
        behavior="prompts/get arguments that fail the prompt's argument schema are rejected before the function runs.",
        arm_exclusions=(ArmExclusion(reason="modern-error-surface", spec_version="2026-07-28"),),
    ),
    "mcpserver:prompt:decorated": Requirement(
        source="sdk",
        behavior=(
            "A function registered with @mcp.prompt() is listed with arguments derived from its signature "
            "and rendered into prompt messages by prompts/get."
        ),
    ),
    "mcpserver:prompt:duplicate-name": Requirement(
        source="sdk",
        behavior="Registering a duplicate prompt name is rejected at registration time.",
        divergence=Divergence(
            note=(
                "MCPServer logs a warning and keeps the first registration instead of rejecting; same "
                "warn-and-ignore behaviour as duplicate tool names (mcpserver:tool:duplicate-name)."
            ),
        ),
    ),
    "mcpserver:prompt:optional-args": Requirement(
        source="sdk",
        behavior="A prompt with optional arguments can be fetched without supplying them.",
    ),
    "mcpserver:prompt:unknown-name": Requirement(
        source=f"{SPEC_BASE_URL}/server/prompts#error-handling",
        behavior="prompts/get for a name that was never registered returns JSON-RPC error -32602 (Invalid params).",
        divergence=Divergence(
            note=(
                "The spec's example uses -32602 Invalid params for unknown prompts; MCPServer raises "
                "ValueError, which the low-level server converts to error code 0."
            ),
        ),
        arm_exclusions=(ArmExclusion(reason="modern-error-surface", spec_version="2026-07-28"),),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Completion
    # ═══════════════════════════════════════════════════════════════════════════
    "completion:capability:declared": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#capabilities",
        behavior="A server with a completion handler advertises the completions capability in its initialize result.",
        arm_exclusions=(ArmExclusion(reason="legacy-only-vocabulary", spec_version="2026-07-28"),),
    ),
    "completion:complete:not-supported": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#capabilities",
        behavior=(
            "A server with no completion handler does not advertise the completions capability and rejects "
            "completion/complete with METHOD_NOT_FOUND."
        ),
    ),
    "completion:context-arguments": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#requesting-completions",
        behavior="Previously-resolved argument values supplied in context.arguments reach the completion handler.",
    ),
    "completion:error:invalid-ref": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#error-handling",
        behavior=(
            "completion/complete with a ref naming an unknown prompt or non-matching resource URI returns "
            "JSON-RPC error -32602 (Invalid params)."
        ),
    ),
    "completion:prompt-arg": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#reference-types",
        behavior="completion/complete with a ref/prompt returns suggested values for the named prompt argument.",
    ),
    "completion:resource-template-arg": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#reference-types",
        behavior="completion/complete with a ref/resource returns suggested values for a URI template variable.",
    ),
    "completion:result-shape": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/completion#completion-results",
        behavior="The completion result carries values (at most 100), an optional total, and an optional hasMore flag.",
    ),
    "mcpserver:completion:capability-auto": Requirement(
        source="sdk",
        behavior=(
            "MCPServer advertises the completions capability when at least one completion source is "
            "registered, and omits it otherwise."
        ),
        arm_exclusions=(ArmExclusion(reason="asserts-legacy-handshake", spec_version="2026-07-28"),),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Logging
    # ═══════════════════════════════════════════════════════════════════════════
    "logging:capability:declared": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#capabilities",
        behavior=(
            "A server that emits log message notifications declares the logging capability in its initialize result."
        ),
        divergence=Divergence(
            note=(
                "MCPServer registers no setLevel handler, so capability derivation leaves logging unset "
                "even though the Context helpers send log message notifications."
            ),
        ),
        arm_exclusions=(ArmExclusion(reason="legacy-only-vocabulary", spec_version="2026-07-28"),),
    ),
    "logging:message:all-levels": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#log-levels",
        behavior="All eight RFC 5424 severity levels are deliverable as log message notifications.",
    ),
    "logging:message:fields": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#log-message-notifications",
        behavior=(
            "A log message sent by a server handler is delivered to the client's logging callback with its "
            "severity level, logger name, and data."
        ),
    ),
    "logging:message:filtered": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#setting-log-level",
        behavior="After logging/setLevel, log messages below the configured level are not sent.",
        divergence=Divergence(
            note=(
                "Neither MCPServer (which rejects logging/setLevel with method-not-found) nor the "
                "low-level Server (which leaves the handler entirely to the author) implements any "
                "filtering; messages are delivered at every severity regardless of the requested level."
            ),
        ),
        removed_in="2026-07-28",
        superseded_by="logging:per-request:threshold",
        note=(
            "removed in 2026-07-28 (SEP-2575); logging/setLevel removed, replaced by per-request "
            "io.modelcontextprotocol/logLevel in _meta."
        ),
    ),
    "logging:set-level": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#setting-log-level",
        behavior="logging/setLevel delivers the requested level to the server's handler and returns an empty result.",
        removed_in="2026-07-28",
        superseded_by="logging:per-request:opt-in",
        note=(
            "removed in 2026-07-28 (SEP-2575); logging/setLevel removed, replaced by per-request "
            "io.modelcontextprotocol/logLevel in _meta."
        ),
    ),
    "logging:set-level:invalid-level": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/logging#error-handling",
        behavior="logging/setLevel with an invalid level value returns JSON-RPC error -32602 (Invalid params).",
        removed_in="2026-07-28",
        superseded_by="logging:per-request:invalid-level",
        note=(
            "removed in 2026-07-28 (SEP-2575); logging/setLevel removed, replaced by per-request "
            "io.modelcontextprotocol/logLevel in _meta."
        ),
    ),
    "logging:per-request:opt-in": Requirement(
        source=f"{SPEC_2026_BASE_URL}/server/utilities/logging#per-request-log-level",
        behavior=(
            "The server does not send log message notifications for a request unless the request opts in by "
            "carrying io.modelcontextprotocol/logLevel in _meta; a handler's log calls on an un-opted "
            "request are dropped, not delivered on another stream."
        ),
        added_in="2026-07-28",
        supersedes=("logging:set-level",),
    ),
    "logging:per-request:threshold": Requirement(
        source=f"{SPEC_2026_BASE_URL}/server/utilities/logging#per-request-log-level",
        behavior=(
            "A request that opts in receives log message notifications only at or above the level named in "
            "its io.modelcontextprotocol/logLevel; entries below the level are dropped."
        ),
        added_in="2026-07-28",
        supersedes=("logging:message:filtered",),
    ),
    "logging:per-request:invalid-level": Requirement(
        source=f"{SPEC_2026_BASE_URL}/server/utilities/logging#error-handling",
        behavior=(
            "A request whose io.modelcontextprotocol/logLevel is not a recognized log level is rejected with "
            "JSON-RPC error -32602 (Invalid params)."
        ),
        added_in="2026-07-28",
        supersedes=("logging:set-level:invalid-level",),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Sampling (server → client)
    # ═══════════════════════════════════════════════════════════════════════════
    "sampling:capability:declare": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#capabilities",
        behavior=(
            "A client that handles sampling requests advertises the sampling capability in its initialize request."
        ),
        arm_exclusions=(
            ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),
            ArmExclusion(reason="asserts-legacy-handshake", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create:basic": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#creating-messages",
        behavior=(
            "A sampling/createMessage request from a server handler is answered by the client's sampling "
            "callback, and the callback's result (role, content, model, stopReason) is returned to the handler."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create:include-context": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#capabilities",
        behavior="The includeContext value supplied by the server reaches the client callback intact.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:context:server-gated-by-capability": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#capabilities",
        behavior=(
            "The server does not use includeContext values thisServer or allServers unless the client "
            "declared the sampling.context capability."
        ),
        divergence=Divergence(
            note=(
                "include_context is forwarded regardless of the client's declared sampling.context "
                "capability; the server-side validator only checks tools/tool_choice."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create:model-preferences": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#model-preferences",
        behavior=(
            "The model preferences supplied by the server (hints and the cost, speed, and intelligence "
            "priorities) reach the client callback intact."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create:system-prompt": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#creating-messages",
        behavior="The system prompt supplied by the server reaches the client callback intact.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create:tools": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#tools-in-sampling",
        behavior=(
            "A sampling request carrying tools and toolChoice reaches the client, and a tool_use response "
            "with a toolUse stop reason returns to the requesting handler."
        ),
        deferred=(
            "Not implemented in the SDK: Client does not expose ClientSession's sampling_capabilities "
            "parameter, so a client can never declare sampling.tools and the server-side validator "
            "rejects every tool-enabled request before it is sent."
        ),
    ),
    "sampling:create-message:audio-content": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#audio-content",
        behavior="Sampling messages can carry audio content: base64 data with a mimeType.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create-message:image-content": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#image-content",
        behavior="Sampling messages can carry image content: base64 data with a mimeType.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:create-message:not-supported": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#capabilities",
        behavior=(
            "A sampling request to a client that did not declare the sampling capability fails with an "
            "error rather than hanging or being silently dropped; the spec names no error code for this case."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:error:user-rejected": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#error-handling",
        behavior=(
            "A sampling request the user rejects is answered with a JSON-RPC error (the spec's code for "
            "this case is -1, 'User rejected sampling request'), surfaced to the requesting handler as an MCPError."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:message:content-cardinality": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling",
        behavior="A sampling message's content may be a single block or an array of blocks.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:result:no-tools-single-content": Requirement(
        source="sdk",
        behavior=(
            "When the request carries no tools, a sampling callback result whose content is an array is "
            "rejected by the client."
        ),
        divergence=Divergence(
            note=(
                "The client does not validate the callback result against the request shape; an array-content "
                "result for a tool-free request is accepted client-side and surfaces as a raw "
                "pydantic.ValidationError from the server's response parsing (send_request) instead."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:result:with-tools-array-content": Requirement(
        source="sdk",
        behavior=(
            "When the request includes tools, the client accepts a callback result whose content is an "
            "array including tool_use blocks."
        ),
        deferred=(
            "Not implemented in the SDK: requires declaring sampling.tools, which the high-level client "
            "cannot do (see sampling:create:tools)."
        ),
    ),
    "sampling:tool-result:no-mixed-content": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#tool-result-messages",
        behavior=(
            "A user sampling message that carries tool_result content contains only tool_result blocks; "
            "mixing tool_result with text, image, or audio content is rejected as invalid."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:tool-use:result-balance": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#tool-use-and-result-balance",
        behavior=(
            "In a sampling/createMessage request, every assistant tool_use block in messages MUST be "
            "matched by a tool_result with the same toolUseId in the immediately-following user message; "
            "an unmatched tool_use is rejected with -32602 Invalid params."
        ),
        divergence=Divergence(
            note=(
                "The client does not validate inbound tool_use/tool_result balance; the SDK enforces "
                "the rule server-side instead, before the request leaves the server (see "
                "sampling:tool-use:server-preflight)."
            ),
        ),
        deferred=(
            "Not implemented on the client receive path: validation runs only on the server send path "
            "(pinned by sampling:tool-use:server-preflight)."
        ),
    ),
    "sampling:tool-use:server-preflight": Requirement(
        source="sdk",
        behavior=(
            "The server validates tool_use/tool_result balance before sending a sampling/createMessage "
            "request; an unmatched tool_use raises ValueError and the request never reaches the wire."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "sampling:tools:server-gated-by-capability": Requirement(
        source=f"{SPEC_BASE_URL}/client/sampling#tools-in-sampling",
        behavior=(
            "A tool-enabled sampling request to a client that did not declare sampling.tools is rejected "
            "by the server before anything reaches the wire (the SDK surfaces this as an Invalid params error)."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Elicitation (server → client)
    # ═══════════════════════════════════════════════════════════════════════════
    "elicitation:capability:empty-is-form": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#capabilities",
        behavior="A client advertising an empty elicitation capability accepts form-mode elicitation requests.",
        deferred=(
            "Not implemented in the SDK: a Client with an elicitation callback always declares explicit "
            "form and url sub-capabilities, so an empty elicitation capability cannot be produced through "
            "the public API."
        ),
    ),
    "elicitation:capability:mode-mismatch": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#error-handling",
        behavior=(
            "The client answers elicitation requests for a mode it did not advertise with JSON-RPC error "
            "-32602 (Invalid params)."
        ),
        deferred=(
            "Not implemented in the SDK: a client cannot be configured form-only or url-only, so the "
            "per-mode mismatch error cannot arise (see elicitation:url:not-supported)."
        ),
    ),
    "elicitation:capability:server-respects-mode": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#capabilities",
        behavior=(
            "The server refuses to send an elicitation request with a mode the connected client did not "
            "declare in its capabilities."
        ),
        divergence=Divergence(
            note=(
                "The server does not check the client's declared elicitation modes before sending "
                "elicitation/create; the spec's MUST NOT is not enforced."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:action:accept": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#response-actions",
        behavior=(
            "A form-mode elicitation answered with action 'accept' returns the user's content to the "
            "requesting handler."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:action:cancel": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#response-actions",
        behavior="A form-mode elicitation answered with action 'cancel' returns no content to the handler.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:action:decline": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#response-actions",
        behavior="A form-mode elicitation answered with action 'decline' returns no content to the handler.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:basic": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#form-mode-elicitation-requests",
        behavior=(
            "A form-mode elicitation delivers the message and requested schema to the client callback "
            "exactly as the server sent them."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:defaults": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#requested-schema",
        behavior=(
            "Optional default values declared in a form-mode requested schema are pre-populated into the "
            "form presented to the user."
        ),
        deferred=(
            "Not implemented in the SDK: there is no form-rendering layer that could pre-populate "
            "defaults; client callbacks receive the requested schema as-is."
        ),
    ),
    "elicitation:form:mode-omitted-default": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#elicitation-requests",
        behavior="An elicitation request with no mode field is treated as form mode by the client.",
    ),
    "elicitation:form:not-supported": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#error-handling",
        behavior=(
            "An elicitation request to a client that did not declare the elicitation capability is "
            "answered with -32602 Invalid params."
        ),
        divergence=Divergence(
            note="The client's default callback answers with -32600 Invalid request instead of -32602.",
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:schema:enum-variants": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#requested-schema",
        behavior=(
            "Requested-schema enum fields (including titled and multi-select variants) reach the client "
            "callback as sent."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:schema:primitives": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#requested-schema",
        behavior="Requested-schema fields may be string (with format), number or integer, or boolean.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:schema:restricted-subset": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#requested-schema",
        behavior=(
            "Form-mode requested schemas are flat objects with primitive-typed properties only; nested "
            "structures and arrays of objects are not used."
        ),
        divergence=Divergence(
            note=(
                "ServerSession.elicit_form forwards an arbitrary dict[str, Any] schema unchanged; no shape "
                "validation at the low-level session layer (the high-level Context.elicit / "
                "elicit_with_validation helper enforces primitive-only fields before generating the schema). "
                "ClientSession likewise does not enforce it: the inbound surface gate is relaxed for "
                "requestedSchema.properties so older servers that emit anyOf for Optional fields still reach "
                "the elicitation callback."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:form:response-validation": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#form-mode-security",
        behavior=(
            "Accepted form-mode content is validated against the requested schema: the client validates "
            "the response before sending and the server validates the content it receives."
        ),
        divergence=Divergence(
            note=(
                "The client never validates outbound content; ServerSession.elicit_form returns received "
                "content unvalidated (the high-level Context.elicit / elicit_with_validation helper "
                "validates server-side, but the low-level session API does not)."
            ),
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:url:action:accept-no-content": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#response-actions",
        behavior=(
            "A URL-mode elicitation delivers the message, URL, and elicitationId to the client; an accept "
            "response carries no content (accept means the user agreed to visit the URL, not that the "
            "interaction completed)."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:url:basic": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#url-mode-elicitation-requests",
        behavior=(
            "A url-mode elicitation delivers the elicitation id and URL to the client callback exactly as "
            "the server sent them."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:url:cancel": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#response-actions",
        behavior="A URL-mode elicitation answered with cancel returns the action with no content.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:url:complete-notification": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#completion-notifications-for-url-mode-elicitation",
        behavior=(
            "An elicitation/complete notification sent by the server after an out-of-band elicitation "
            "finishes reaches the client carrying the elicitationId."
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (spec PR #2891); notifications/elicitation/complete and elicitationId removed, no "
            "replacement (under MRTR the client learns completion by retrying)."
        ),
        arm_exclusions=(ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),),
    ),
    "elicitation:url:complete-unknown-ignored": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#completion-notifications-for-url-mode-elicitation",
        behavior=(
            "The client ignores an elicitation/complete notification referencing an unknown or "
            "already-completed elicitationId without error."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (spec PR #2891); notifications/elicitation/complete removed, no replacement.",
    ),
    "elicitation:url:decline": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#response-actions",
        behavior="A URL-mode elicitation answered with decline returns the action with no content.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "elicitation:url:not-supported": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#error-handling",
        behavior=(
            "A URL-mode elicitation to a client that declared only form-mode support is rejected with an "
            "Invalid params error."
        ),
        deferred=(
            "Not implemented in the SDK: a Client with an elicitation callback always declares both the "
            "form and url sub-capabilities, so a form-only client cannot be constructed."
        ),
    ),
    "elicitation:url:required-error": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#url-elicitation-required-error",
        behavior=(
            "A handler that cannot proceed without a URL elicitation rejects the request with error "
            "-32042, carrying the pending elicitations in the error data."
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2322); error -32042 retired, replaced by an MRTR input_required result "
            "carrying inputRequests."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Roots (server → client)
    # ═══════════════════════════════════════════════════════════════════════════
    "roots:list-changed": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#root-list-changes",
        behavior="A roots/list_changed notification sent by the client is delivered to the server's handler.",
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); notifications/roots/list_changed removed, no replacement (the stateless "
            "model carries no client→server change notifications)."
        ),
    ),
    "roots:list-changed:client-emits": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#root-list-changes",
        behavior=(
            "A client that declared roots.listChanged sends notifications/roots/list_changed when its set "
            "of roots changes."
        ),
        deferred=(
            "Not implemented in the SDK: the client does not own the root set (it calls back to the host "
            "via list_roots_callback), so there is no mutation it could observe to auto-emit on; the SDK "
            "provides send_roots_list_changed() for the host to call when its roots change, and that "
            "emission path is covered by roots:list-changed."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); notifications/roots/list_changed removed, no replacement.",
    ),
    "roots:list:basic": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#listing-roots",
        behavior=(
            "A roots/list request from a server handler is answered by the client's roots callback, and "
            "the returned roots (uri, name) reach the handler."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "roots:list:client-error": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#error-handling",
        behavior="A roots callback that answers with an error surfaces to the requesting handler as an MCPError.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "roots:list:empty": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#listing-roots",
        behavior="An empty roots list is a valid response and reaches the handler as such.",
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "roots:list:not-supported": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#error-handling",
        behavior=(
            "A roots/list request to a client that did not declare the roots capability is answered with "
            "-32601 Method not found."
        ),
        divergence=Divergence(
            note="The client's default callback answers with -32600 Invalid request instead of -32601.",
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "roots:uri:file-scheme": Requirement(
        source=f"{SPEC_BASE_URL}/client/roots#root",
        behavior="Every root returned by the client identifies itself with a file:// URI.",
        deferred=(
            "Schema-level validation: the FileUrl type on Root.uri rejects any non-file:// scheme at "
            "construction and at parse, so a non-conforming root cannot reach the wire from either side; "
            "type-level coverage belongs in tests/test_types.py rather than this interaction suite."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # list_changed & dynamic registration
    # ═══════════════════════════════════════════════════════════════════════════
    "client:list-changed:auto-refresh": Requirement(
        source="sdk",
        behavior=(
            "A client configured to react to list_changed notifications automatically re-fetches the "
            "corresponding list and delivers the fresh result to its callback."
        ),
        deferred=(
            "Not implemented in the SDK: the client has no list-changed auto-refresh mechanism; "
            "notifications are only delivered to the message handler."
        ),
    ),
    "client:list-changed:capability-gated": Requirement(
        source="sdk",
        behavior=(
            "The client does not activate list-changed handling for a kind the server did not advertise "
            "with listChanged true."
        ),
        deferred="Not implemented in the SDK: no client-side list-changed handling exists to gate.",
    ),
    "client:list-changed:signal-only": Requirement(
        source="sdk",
        behavior="A client configured for signal-only list-changed handling is notified without auto-refreshing.",
        deferred="Not implemented in the SDK: no client-side list-changed handling exists.",
    ),
    "mcpserver:list-changed:debounce": Requirement(
        source="sdk",
        behavior=(
            "Bursts of registration changes on MCPServer are debounced into one list_changed notification per kind."
        ),
        deferred=(
            "Not implemented in the SDK: MCPServer does not send list_changed notifications on "
            "registration changes at all (see mcpserver:register:post-connect), so there is nothing to "
            "debounce."
        ),
    ),
    "mcpserver:register:post-connect": Requirement(
        source="sdk",
        behavior=(
            "A tool, resource, or prompt registered or removed after the client connected appears in (or "
            "disappears from) the corresponding list results, and the change is announced with a "
            "list_changed notification."
        ),
        divergence=Divergence(
            note=(
                "MCPServer never sends list_changed notifications on registration changes, so a connected "
                "client cannot learn that the set changed without polling."
            ),
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Pagination
    # ═══════════════════════════════════════════════════════════════════════════
    "pagination:exhaustion": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#response-format",
        behavior=(
            "Following nextCursor until it is absent yields every page exactly once; a result without "
            "nextCursor ends the sequence."
        ),
    ),
    "pagination:invalid-cursor": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#error-handling",
        behavior="A list request with an invalid cursor returns JSON-RPC error -32602 (Invalid params).",
    ),
    "pagination:client:cursor-handling": Requirement(
        source=f"{SPEC_BASE_URL}/server/utilities/pagination#implementation-guidelines",
        behavior=(
            "The client treats cursors as opaque tokens — it does not parse, modify, or persist them — "
            "and does not assume a fixed page size."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Tasks (experimental)
    # ═══════════════════════════════════════════════════════════════════════════
    "tasks:auth:context-isolation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-isolation-and-access-control",
        behavior=(
            "When an authorization context is available, task operations are scoped to the context that "
            "created the task: other contexts cannot get it, retrieve its result, cancel it, or see it in "
            "tasks/list."
        ),
        transports=("streamable-http",),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:bidirectional": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#definitions",
        behavior="Task APIs are bidirectional: the server may create, get, list, and cancel tasks on the client.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:cancel:no-handler-abort": Requirement(
        source="sdk",
        behavior=(
            "tasks/cancel marks the task cancelled without aborting the originating request handler "
            "(the spec says receivers SHOULD attempt to stop execution)."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:cancel:remains-cancelled": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-cancellation",
        behavior=(
            "After tasks/cancel, the task remains cancelled even if the underlying handler subsequently "
            "completes or fails."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:cancel:terminal-rejected": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-cancellation",
        behavior="tasks/cancel on a task already in a terminal state returns Invalid params (-32602).",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:cancel:working": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-cancellation",
        behavior="tasks/cancel on a working task transitions it to cancelled and returns the updated task.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:create:ttl-honored": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#ttl-and-resource-management",
        behavior=(
            "tasks/get responses include the actual ttl applied by the receiver (or null for unlimited); "
            "the create-task result carries the same value."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:create:via-tool-call": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#creating-tasks",
        behavior="A task-augmented tools/call returns a create-task result instead of the tool result.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:get": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#getting-tasks",
        behavior="tasks/get returns the task's current status, ttl, timestamps, and status message.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:lifecycle:initial-working": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-status-lifecycle",
        behavior="A newly created task has status 'working'.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:lifecycle:input-required": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#input-required-status",
        behavior=(
            "While a task awaits a side-channel client response its status is input_required; once the "
            "response arrives the task leaves input_required (typically returning to working)."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:list:invalid-cursor": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#protocol-errors",
        behavior="tasks/list with an invalid cursor returns Invalid params (-32602).",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/list dropped in the redesign)."
        ),
    ),
    "tasks:list:pagination": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#listing-tasks",
        behavior="tasks/list returns created tasks and supports cursor pagination.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/list dropped in the redesign)."
        ),
    ),
    "tasks:no-capability:ignore-task-param": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-support-and-handling",
        behavior=(
            "A receiver that did not declare task capability for a request type processes the request "
            "normally and returns the ordinary result, ignoring the task augmentation."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:progress:after-create": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-progress-notifications",
        behavior=(
            "After the create-task result, progress notifications keyed to the original progress token "
            "continue to reach the caller until the task is terminal."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:request-cancel:no-task-cancel": Requirement(
        source="sdk",
        behavior="A cancellation notification for the originating request does not auto-cancel the created task.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:result:failed": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-execution-errors",
        behavior="tasks/result for a failed task returns the failure result (isError true).",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result dropped in the redesign)."
        ),
    ),
    "tasks:result:related-task-meta": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#related-task-metadata",
        behavior="The tasks/result response carries related-task _meta naming the requested task.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result dropped in the redesign)."
        ),
    ),
    "tasks:result:terminal": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#result-retrieval",
        behavior="tasks/result for a completed task returns the stored result of the original request type.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result dropped in the redesign)."
        ),
    ),
    "tasks:side-channel:drain-fifo": Requirement(
        source="sdk",
        behavior="tasks/result drains queued related-task messages in FIFO order before returning the final result.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result side-channel dropped in the redesign)."
        ),
    ),
    "tasks:side-channel:drop-on-cancel": Requirement(
        source="sdk",
        behavior="When a task is cancelled before tasks/result, queued related-task messages are dropped.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result side-channel dropped in the redesign)."
        ),
    ),
    "tasks:side-channel:elicitation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#input-required-status",
        behavior=(
            "An elicitation issued mid-task is delivered through the tasks/result side-channel, and the "
            "client's response routes back to the handler."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result side-channel dropped in the redesign)."
        ),
    ),
    "tasks:side-channel:queue": Requirement(
        source="sdk",
        behavior=(
            "Server-to-client requests with related-task metadata sent while no tasks/result is open are queued."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result side-channel dropped in the redesign)."
        ),
    ),
    "tasks:side-channel:sampling": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#input-required-status",
        behavior=(
            "A sampling request issued mid-task is delivered through the tasks/result side-channel, and "
            "the client's response routes back to the task."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result side-channel dropped in the redesign)."
        ),
    ),
    "tasks:side-channel:stream": Requirement(
        source="sdk",
        behavior=(
            "Calling tasks/result while the task is working streams related-task messages as they are "
            "produced, then returns the result."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension (tasks/result side-channel dropped in the redesign)."
        ),
    ),
    "tasks:status-notification": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#task-status-notification",
        behavior="Task status notifications deliver status updates carrying the full task fields.",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:tool-level:forbidden-with-task-32601": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#tool-level-negotiation",
        behavior=(
            "A task-augmented tools/call on a tool that does not support tasks returns Method not found (-32601)."
        ),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:tool-level:required-no-task-32601": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#tool-level-negotiation",
        behavior=("A plain tools/call on a tool that requires task augmentation returns Method not found (-32601)."),
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    "tasks:unknown-id": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/tasks#protocol-errors",
        behavior="tasks/get, tasks/result, and tasks/cancel for an unknown task id return Invalid params (-32602).",
        deferred=_TASKS_DEFERRAL,
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2663); tasks moved out of core into the io.modelcontextprotocol/tasks "
            "extension."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Extensions (SEP-2133): client-side result claims and the capability ad
    # ═══════════════════════════════════════════════════════════════════════════
    "extensions:client:claimed-result-resolved": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic#resulttype",
        behavior=(
            "A tools/call answered with an extension-claimed resultType is finished by the owning "
            "ClientExtension's claim resolver, and Client.call_tool returns the resolver's ordinary "
            "CallToolResult. The resolver may send follow-up requests through the session it is handed."
        ),
        added_in="2026-07-28",
    ),
    "extensions:client:claimed-result-undeclared-invalid": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic#resulttype",
        behavior=(
            "A resultType unrecognized by the client is invalid: a claimed shape delivered to a client that "
            "did not construct the owning extension fails result validation (the supported set is core plus "
            "declared claims, never more)."
        ),
        added_in="2026-07-28",
        note=(
            "Known leniency: the monolith result surface still accepts an unknown tag when the payload "
            "also parses as a complete core result (open result_type, extras ignored). Rejecting tags "
            "outside core plus active claims is a tracked follow-up ruling."
        ),
    ),
    "extensions:client:capability-ad:gates-server-behaviour": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic#resulttype",
        behavior=(
            "The per-request _meta capability ad carries each declared extension's identifier and settings, "
            "and is what entitles the server to substitute that extension's claimed shapes: a server "
            "extension gating on the ad sees the declared settings, and refuses a non-declaring client with "
            "-32021 (missing required client capability)."
        ),
        added_in="2026-07-28",
    ),
    "extensions:client:capability-ad:legacy-omits-claimed": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic#resulttype",
        behavior=(
            "On a legacy connection no claim can activate, and the initialize capability ad omits "
            "claim-bearing identifiers in the same breath (claim-less identifiers still advertise), so the "
            "client never advertises an extension whose claimed shapes it would reject."
        ),
        removed_in="2026-07-28",
        note=(
            "The legacy-era half of the ad/claims coupling: only a handshake connection can exhibit it, so "
            "the version window ends where the modern era begins."
        ),
        arm_exclusions=(ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),),
    ),
    "extensions:client:notification-binding-delivery": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic#resulttype",
        behavior=(
            "A vendor server notification bound by a ClientExtension's NotificationBinding is validated "
            "against the binding's params type and delivered to its handler serially, in dispatch order."
        ),
        added_in="2026-07-28",
        deferred=(
            "Covered at session tier by tests/client/test_session_notification_bindings.py: no public "
            "server-side surface emits vendor-method notifications (ServerNotification is a closed union), "
            "and HTTP-modern arrival additionally needs the subscriptions/listen client runtime."
        ),
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Transports (in-suite coverage)
    # ═══════════════════════════════════════════════════════════════════════════
    "transport:streamable-http:stateful": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior=(
            "The interaction round trip (initialize, tool calls, tool errors) works through the "
            "streamable HTTP framing in its default stateful SSE-response mode."
        ),
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: exercises the stateful HTTP framing end to end.",
    ),
    "transport:streamable-http:json-response": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior="The interaction round trip works when the server answers with plain JSON instead of SSE.",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: JSON-response mode is an HTTP framing option.",
    ),
    "transport:streamable-http:cancelled-request-terminated": Requirement(
        source="sdk",
        behavior=(
            "A request cancelled through notifications/cancelled is terminated with a REQUEST_CANCELLED "
            "(-32800) error response, completing its POST - the JSON body in JSON-response mode, the "
            "final event of its stream in SSE mode."
        ),
        transports=("streamable-http",),
        note=(
            "An SDK choice, not spec-mandated (the spec-side gap is the Divergence on "
            "protocol:cancel:in-flight): this era's wire ends a request's stream only with a response "
            "for its id, and stores it so a resuming client's replay terminates too. The terminator is "
            "written through the same ordered channel as the request's other messages, so it cannot "
            "overtake anything already queued for the request."
        ),
    ),
    "transport:streamable-http:json-response-restrictions": Requirement(
        source="sdk",
        behavior=(
            "In JSON-response mode a handler's request-scoped server-initiated request fails fast with an "
            "INVALID_REQUEST protocol error and request-scoped notifications are not delivered, because the "
            "single JSON body carries only the response; the connection's standalone stream is unaffected."
        ),
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: JSON-response mode is an HTTP framing option.",
    ),
    "transport:streamable-http:stateless": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior=(
            "The interaction round trip works in stateless mode, where every request is served by a "
            "fresh transport with no session id."
        ),
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: stateless mode is an HTTP hosting option.",
    ),
    "transport:streamable-http:notifications": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior=(
            "Notifications emitted during a request are delivered on that request's SSE stream and reach "
            "the client's callbacks, in order, before the response."
        ),
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: per-request SSE streams are HTTP-specific.",
    ),
    "transport:streamable-http:stateless-restrictions": Requirement(
        source="sdk",
        behavior=(
            "A handler that attempts a server-initiated request in stateless mode fails with an error "
            "result, because there is no session to call back through."
        ),
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: stateless mode is an HTTP hosting option.",
    ),
    "transport:streamable-http:unrelated-messages": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior=(
            "A server-to-client message that is not related to an in-flight request is routed to the "
            "standalone GET stream and delivered to the client listening on it, not to any request's "
            "own stream."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); the standalone GET stream is replaced by subscriptions/listen.",
    ),
    "transport:streamable-http:server-to-client": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior=(
            "A server-initiated request nested inside an in-flight call round-trips over stateful streamable HTTP."
        ),
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: exercises stateful HTTP session callbacks.",
    ),
    "transport:streamable-http:resumability": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
        behavior="A client that reconnects with Last-Event-ID receives the events it missed.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); Last-Event-ID resumability/redelivery dropped, no replacement.",
    ),
    "transport:streamable-http:origin-validation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#security-warning",
        behavior="Requests with an invalid Origin header are rejected with 403 before reaching the session.",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: Origin is an HTTP header.",
    ),
    "transport:sse": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#backwards-compatibility",
        behavior=(
            "A client connected over the legacy HTTP+SSE transport completes the handshake and round-trips "
            "requests, with server messages delivered on the SSE stream."
        ),
        transports=("sse",),
        note="Only observable over the legacy SSE transport.",
    ),
    "transport:sse:endpoint-event": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#backwards-compatibility",
        behavior="Opening the SSE stream delivers an `endpoint` event naming the message-POST URL as the first event.",
        transports=("sse",),
        note="Only observable over the legacy SSE transport.",
    ),
    "transport:sse:post:session-routing": Requirement(
        source="sdk",
        behavior=(
            "The endpoint URL carries a fresh session identifier; the server registers the session before "
            "the endpoint event is sent and releases it when the stream disconnects, and a POST that names "
            "no session id, a malformed session id, or an unknown session id is rejected (400/400/404)."
        ),
        transports=("sse",),
        note="Only observable over the legacy SSE transport.",
    ),
    "transport:stdio": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#stdio",
        behavior=(
            "A Client connected to a real SDK Server over stdio initializes, calls a tool with arguments, "
            "and receives notifications and results over the child process's stdin/stdout."
        ),
        transports=("stdio",),
        note="Only observable over stdio: exercises the child-process framing end to end.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Hosting: session lifecycle
    # ═══════════════════════════════════════════════════════════════════════════
    "hosting:session:cors-expose": Requirement(
        source="sdk",
        behavior="CORS configuration exposes the Mcp-Session-Id header so browser clients can read it.",
        transports=("streamable-http",),
        deferred="Not implemented in the SDK: CORS configuration is left to the hosting ASGI application.",
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id removed, no replacement.",
    ),
    "hosting:session:create": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior=(
            "An initialize POST without a session id creates a session and returns Mcp-Session-Id in the "
            "response headers."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2567); Mcp-Session-Id and protocol-level sessions removed, no replacement "
            "(cross-call state moves to explicit server-minted handles)."
        ),
    ),
    "hosting:session:delete": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior="DELETE with a valid Mcp-Session-Id terminates the session.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); session DELETE removed with Mcp-Session-Id, no replacement.",
    ),
    "hosting:session:id-charset": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior="Generated Mcp-Session-Id values contain only visible ASCII characters.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id removed, no replacement.",
    ),
    "hosting:session:isolation": Requirement(
        source="sdk",
        behavior="Each session gets its own server instance; closing one session does not affect others.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2567); per-session server instances retired with Mcp-Session-Id, no "
            "replacement."
        ),
    ),
    "hosting:session:missing-id": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior="A non-initialize POST without Mcp-Session-Id in stateful mode returns 400.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id validation removed, no replacement.",
    ),
    "hosting:session:post-termination-404": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior=(
            "After a session is terminated, any further request carrying that session ID is answered with "
            "404 Not Found."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id removed, no replacement.",
    ),
    "hosting:session:reinitialize": Requirement(
        source="sdk",
        behavior="A second initialize on an already-initialized session transport is rejected.",
        transports=("streamable-http",),
        divergence=Divergence(
            note=(
                "The transport forwards a second initialize carrying the existing session ID to the running "
                "server, which answers it as a fresh handshake; nothing rejects re-initialization."
            ),
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2567); per-session initialize guard retired with Mcp-Session-Id, no "
            "replacement."
        ),
    ),
    "hosting:session:reuse": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior="A POST carrying a valid Mcp-Session-Id routes to that session's transport with state preserved.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id routing removed, no replacement.",
    ),
    "hosting:session:unknown-id": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior="A POST, GET, or DELETE with an unknown Mcp-Session-Id returns 404.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id removed, no replacement.",
    ),
    "hosting:stateless:concurrent-clients": Requirement(
        source="sdk",
        behavior="Multiple independent clients can connect to a stateless server concurrently.",
        transports=("streamable-http",),
        note="Stateless mode is a streamable-HTTP hosting option.",
    ),
    "hosting:stateless:no-reuse": Requirement(
        source="sdk",
        behavior="A stateless per-request transport cannot be reused for a second request.",
        transports=("streamable-http",),
        note="Stateless mode is a streamable-HTTP hosting option.",
    ),
    "hosting:stateless:no-session-id": Requirement(
        source="sdk",
        behavior="In stateless mode no Mcp-Session-Id is emitted and no session validation is performed.",
        transports=("streamable-http",),
        note="Stateless mode is a streamable-HTTP hosting option; Mcp-Session-Id is an HTTP header.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Hosting: auth
    # ═══════════════════════════════════════════════════════════════════════════
    "hosting:auth:as-router": Requirement(
        source="sdk",
        behavior=(
            "The authorization-server routes expose the authorize, token, and registration endpoints "
            "(and revocation when supported)."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the AS router is an ASGI app.",
    ),
    "hosting:auth:aud-validation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#access-token-usage",
        behavior="The resource server validates that the token audience matches its resource identifier.",
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer.",
        divergence=Divergence(
            note=(
                "BearerAuthBackend never inspects AccessToken.resource; a token issued for a different "
                "resource is accepted. Spec MUST."
            ),
        ),
    ),
    "hosting:auth:authinfo-propagates": Requirement(
        source="sdk",
        behavior="A valid token's auth info is exposed to request handlers.",
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer.",
    ),
    "hosting:auth:expired-401": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#token-handling",
        behavior="An expired token returns 401 invalid_token.",
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; 401 is an HTTP status code.",
        divergence=Divergence(
            note="The challenge carries no `scope` parameter; see the note on hosting:auth:missing-401.",
        ),
    ),
    "hosting:auth:invalid-401": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#token-handling",
        behavior="A malformed bearer token or token-verification failure returns 401 with WWW-Authenticate.",
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; 401 is an HTTP status code.",
        divergence=Divergence(
            note="The challenge carries no `scope` parameter; see the note on hosting:auth:missing-401.",
        ),
    ),
    "hosting:auth:metadata-endpoints": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-location",
        behavior=(
            "The MCP server publishes protected-resource metadata at its well-known endpoint, and the "
            "authorization server (which the SDK can also host) publishes authorization-server metadata "
            "at its own."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; well-known endpoints are HTTP routes.",
    ),
    "hosting:auth:missing-401": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#protected-resource-metadata-discovery-requirements",
        behavior=(
            "A request without an Authorization header is rejected with 401; the WWW-Authenticate header "
            "carries resource_metadata (one of the spec's two permitted discovery mechanisms)."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; 401 is an HTTP status code.",
        divergence=Divergence(
            note=(
                "The SDK never emits a `scope` parameter in any WWW-Authenticate challenge — neither the "
                "discovery-time 401 (#protected-resource-metadata-discovery-requirements SHOULD) nor the "
                "runtime 403 (#runtime-insufficient-scope-errors SHOULD); and for the no-credentials case "
                'it emits error="invalid_token", which RFC 6750 Section 3.1 says SHOULD NOT appear when no '
                "authentication information was presented."
            ),
        ),
    ),
    "hosting:auth:prm:authorization-servers-field": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-location",
        behavior=(
            "The protected-resource metadata document includes an authorization_servers array with at least one entry."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; PRM is served over HTTP.",
    ),
    "hosting:auth:query-token-ignored": Requirement(
        source="sdk",
        behavior=(
            "An access token presented in the URI query string is not accepted; the request is treated as "
            "unauthenticated."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; query strings are URL-specific.",
    ),
    "hosting:auth:scope-403": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#runtime-insufficient-scope-errors",
        behavior=(
            "A token lacking a required scope returns 403 with WWW-Authenticate carrying "
            "insufficient_scope, the required scope, and resource_metadata."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; 403 is an HTTP status code.",
        divergence=Divergence(
            note=(
                'The SDK emits error="insufficient_scope" and error_description but never the `scope` '
                "parameter the spec SHOULD include; the SDK client reads `scope` from this header to drive "
                "step-up (utils.py extract_scope_from_www_auth) — a resource-server/client asymmetry."
            ),
        ),
    ),
    "hosting:auth:as:authorize-requires-pkce": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-code-protection",
        behavior=(
            "The bundled authorization endpoint rejects an authorize request that omits "
            "`code_challenge` with `invalid_request`."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
    ),
    "hosting:auth:as:authorize-scope": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#step-up-authorization-flow",
        behavior=(
            "The bundled authorize endpoint accepts a requested scope beyond the client's registered "
            "`scope` when it is within the server's `valid_scopes`, so a client can step up without "
            "re-registering, and rejects a scope outside `valid_scopes` with an `invalid_scope` "
            "error redirect. Per-client scope policy is delegated to the provider's `authorize()`."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
    ),
    "hosting:auth:as:verifier-mismatch": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-code-protection",
        behavior=(
            "The bundled token endpoint rejects an authorization-code exchange whose `code_verifier` "
            "does not hash to the stored `code_challenge` with `invalid_grant`."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
    ),
    "hosting:auth:as:code-single-use": Requirement(
        source="sdk",
        behavior=(
            "An authorization code can be exchanged exactly once; a second exchange of the same code "
            "is rejected with `invalid_grant`. Enforced by the provider deleting the code on first use; "
            "the handler relies on `load_authorization_code` returning None."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
    ),
    "hosting:auth:as:redirect-uri-binding": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#open-redirection",
        behavior=(
            "The bundled token endpoint rejects an authorization-code exchange whose `redirect_uri` "
            "differs from the one used at authorize; the bundled authorize endpoint rejects a "
            "`redirect_uri` not in the client's registered list without redirecting to it."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
        divergence=Divergence(
            note=(
                "RFC 6749 §5.2 assigns redirect_uri mismatch at the token endpoint to invalid_grant; "
                "the SDK's TokenHandler returns invalid_request (src/mcp/server/auth/handlers/token.py:157). "
                "The rejection itself is the security-relevant property and is correct."
            ),
        ),
    ),
    "hosting:auth:as:redirect-uri-scheme": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#communication-security",
        behavior=(
            "The bundled registration endpoint accepts only redirect URIs that use HTTPS or target a loopback host."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
        divergence=Divergence(
            note=(
                "Not enforced: the registration handler models redirect_uris as AnyUrl with no scheme or "
                "host check, so http://evil.example/callback is accepted and registered. The spec's "
                "localhost-or-HTTPS rule is left to the provider implementation."
            ),
        ),
    ),
    "hosting:auth:as:token-cache-headers": Requirement(
        source="sdk",
        behavior=("Every token-endpoint response carries `Cache-Control: no-store` and `Pragma: no-cache`."),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; Cache-Control is an HTTP header.",
    ),
    "hosting:auth:as:register-echo": Requirement(
        source="sdk",
        behavior=(
            "The bundled registration endpoint returns all registered metadata about the client "
            "in its 201 response (RFC 7591 §3.2.1) - the client's `application_type` rather than a "
            "substituted default, and `client_secret_expires_at` (0 when the secret never expires) "
            "whenever a `client_secret` is issued."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
    ),
    "hosting:auth:as:register-error-response": Requirement(
        source="sdk",
        behavior=(
            "The bundled registration endpoint answers invalid client metadata with HTTP 400 and an "
            "RFC 7591 error body."
        ),
        transports=("streamable-http",),
        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Hosting: resumability
    # ═══════════════════════════════════════════════════════════════════════════
    "hosting:resume:bad-event-id": Requirement(
        source="sdk",
        behavior="A Last-Event-ID that cannot be mapped to a stream is rejected.",
        transports=("streamable-http",),
        divergence=Divergence(
            note=(
                "The replay path returns an empty SSE stream rather than rejecting an unknown "
                "Last-Event-ID; the client cannot tell an unknown ID apart from a stream with no missed "
                "events."
            ),
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); Last-Event-ID resumability dropped, no replacement.",
    ),
    "hosting:resume:buffered-replay": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#resumability-and-redelivery",
        behavior="Notifications emitted while no client is connected are replayed in order on reconnect.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE stream resumability/redelivery dropped, no replacement.",
    ),
    "hosting:resume:close-stream": Requirement(
        source="sdk",
        behavior="Handlers can close an SSE stream cleanly when an event store is configured.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); the event-store / resumability path is dropped, no replacement.",
    ),
    "hosting:resume:event-ids": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#resumability-and-redelivery",
        behavior="With an event store configured, every SSE event carries an id field.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE event-id assignment for resumability dropped, no replacement.",
    ),
    "hosting:resume:priming": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "A server-initiated SSE stream begins with a priming event carrying an event ID and an empty "
            "data field; a server that closes the connection before terminating the stream sends an SSE "
            "retry field first."
        ),
        transports=("streamable-http",),
        divergence=Divergence(
            note=(
                "The retry hint is attached to the priming event itself rather than sent as a separate "
                "event before the connection closes, and a priming event is only sent when an event store "
                "is configured and the negotiated protocol version is at least 2025-11-25."
            ),
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); the priming-event / retry-hint requirement is dropped with "
            "resumability, no replacement."
        ),
    ),
    "hosting:resume:replay": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#resumability-and-redelivery",
        behavior="GET with Last-Event-ID replays stored events for that stream after the given id.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); Last-Event-ID replay dropped, no replacement.",
    ),
    "hosting:resume:stream-scoped": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#resumability-and-redelivery",
        behavior="Replay via Last-Event-ID returns only messages from the stream that event id belongs to.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); Last-Event-ID replay dropped, no replacement.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Hosting: HTTP semantics
    # ═══════════════════════════════════════════════════════════════════════════
    "hosting:http:accept-406": Requirement(
        source="sdk",
        behavior="A request whose Accept header does not allow the response representation returns 406.",
        transports=("streamable-http",),
        note="Only observable over HTTP: 406 is an HTTP status code.",
    ),
    "hosting:http:batch": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "A POST body is a single JSON-RPC message; batched arrays are rejected for protocol revisions "
            "that forbid them."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: POST-body framing is HTTP-specific.",
    ),
    "hosting:http:content-type-415": Requirement(
        source="sdk",
        behavior="A POST with a Content-Type other than application/json returns 415.",
        transports=("streamable-http",),
        note="Only observable over HTTP: 415 is an HTTP status code.",
        divergence=Divergence(
            note=(
                "The transport-security middleware rejects a non-JSON Content-Type with 400 'Invalid "
                "Content-Type header' before the request reaches the transport, so the transport's own 415 "
                "path is unreachable through any public entry point."
            ),
        ),
    ),
    "hosting:http:disconnect-not-cancel": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "A client connection drop during an in-flight request does not cancel the server-side "
            "handler; the request continues and its result remains retrievable."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); resumability dropped and the rule is inverted (closing the response "
            "stream is now the HTTP cancellation signal), no replacement."
        ),
    ),
    "hosting:http:dns-rebinding": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#security-warning",
        behavior=(
            "The Origin header is validated on every incoming connection; a request with an invalid "
            "Origin is rejected with 403 Forbidden."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: Origin is an HTTP header.",
        divergence=Divergence(
            note=(
                "The spec's Origin validation is an unconditional MUST; the SDK enables it only when the "
                "host is a localhost address or explicit TransportSecuritySettings are passed (with no "
                "settings, no Origin validation runs), and additionally validates the Host header "
                "(returning 421 on mismatch), which the spec does not require."
            ),
        ),
    ),
    "hosting:http:json-response-mode": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior="With JSON response mode enabled, POST returns application/json instead of SSE.",
        transports=("streamable-http",),
        note="Only observable over HTTP: response Content-Type is HTTP-specific.",
    ),
    "hosting:http:method-405": Requirement(
        source="sdk",
        behavior="An unsupported HTTP method on the MCP endpoint returns 405.",
        transports=("streamable-http",),
        note="Only observable over HTTP: 405 is an HTTP status code.",
    ),
    "hosting:http:no-broadcast": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#multiple-connections",
        behavior=(
            "When multiple SSE streams are open for a session, each server-originated message is sent on "
            "exactly one stream, never duplicated."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2567); the per-session multiple-connections section is removed with "
            "Mcp-Session-Id, no replacement."
        ),
    ),
    "hosting:http:notifications-202": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior="A POST containing only notifications or responses returns 202 with no body.",
        transports=("streamable-http",),
        note="Only observable over HTTP: 202 is an HTTP status code.",
    ),
    "hosting:http:onerror": Requirement(
        source="sdk",
        behavior="Transport-level rejections are reported through an error callback on the server transport.",
        transports=("streamable-http",),
        note="Only observable over HTTP: these rejections happen at the HTTP framing layer.",
        deferred="Not implemented in the SDK: the server transport has no error callback; rejections are logged.",
    ),
    "hosting:http:parse-error-400": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "A POST body that is not valid JSON or not a valid JSON-RPC message is rejected with HTTP 400; "
            "the body may carry a JSON-RPC error response (the SDK sends a Parse error body)."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: 400 is an HTTP status code.",
    ),
    "hosting:http:protocol-version-400": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#protocol-version-header",
        behavior="An invalid or unsupported MCP-Protocol-Version header returns 400 Bad Request.",
        transports=("streamable-http",),
        note="Only observable over HTTP: MCP-Protocol-Version is an HTTP header.",
    ),
    "hosting:http:protocol-version-default": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#protocol-version-header",
        behavior=(
            "When no MCP-Protocol-Version header is received and the version cannot be determined another "
            "way, the server assumes protocol version 2025-03-26."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: MCP-Protocol-Version is an HTTP header.",
    ),
    "hosting:http:response-same-connection": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "A response is delivered on the SSE stream opened by the POST that carried its request (or "
            "that stream's resumed continuation), not on an unrelated stream."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: SSE stream affinity is HTTP-specific.",
    ),
    "hosting:http:second-sse-rejected": Requirement(
        source="sdk",
        behavior="A second concurrent standalone GET SSE stream on the same session is rejected.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); the standalone GET stream is replaced by subscriptions/listen.",
    ),
    "hosting:http:sse-close-after-response": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior="The server terminates a POST-initiated SSE stream after writing the JSON-RPC response.",
        transports=("streamable-http",),
        note="Only observable over HTTP: SSE stream lifecycle is HTTP-specific.",
    ),
    "hosting:http:standalone-sse": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#listening-for-messages-from-the-server",
        behavior="GET opens a standalone SSE stream that receives server-initiated messages.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); the standalone GET endpoint is replaced by subscriptions/listen.",
    ),
    "hosting:http:standalone-sse-no-response": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#listening-for-messages-from-the-server",
        behavior=(
            "The standalone GET SSE stream carries server requests and notifications but never a JSON-RPC "
            "response, except when resuming a prior request stream."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); the standalone GET endpoint is replaced by subscriptions/listen.",
    ),
    "hosting:http:protocol-version-rejection-literal": Requirement(
        source="sdk",
        behavior=(
            "The legacy streamable-HTTP transport's version-rejection body contains the literal substring "
            "'Unsupported protocol version', which other-SDK clients substring-match during negotiation."
        ),
        transports=("streamable-http",),
        note=(
            "Only observable over streamable HTTP: cross-SDK clients sniff this exact substring in the rejection body."
        ),
    ),
    "hosting:http:legacy-no-modern-vocabulary": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/versioning",
        behavior=(
            "A 2025-era streamable-HTTP exchange carries none of the 2026-07-28 wire vocabulary "
            "(resultType, ttlMs, cacheScope, io.modelcontextprotocol/* _meta keys, the 2026-07-28 "
            "version string, or Mcp-Method/Mcp-Name/Mcp-Param-* headers)."
        ),
        transports=("streamable-http",),
        note=(
            "Only observable over streamable HTTP: the assertion records HTTP headers and SSE frames "
            "at the transport seam."
        ),
    ),
    "hosting:http:modern:tools-call-stateless": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http",
        behavior=(
            "A 2026-07-28 tools/call POST is served without an initialize handshake and returns a "
            "result body carrying resultType: complete."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note=(
            "Only observable over streamable HTTP: the modern entry handles a 2026-07-28 POST without "
            "an initialize handshake."
        ),
    ),
    "hosting:http:modern:no-session-id": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http",
        behavior="A 2026-07-28 response never carries an Mcp-Session-Id header.",
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: Mcp-Session-Id is a streamable-HTTP response header.",
    ),
    "hosting:http:modern:initialize-removed": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/index",
        behavior="A 2026-07-28 initialize request is answered with METHOD_NOT_FOUND.",
        added_in="2026-07-28",
        transports=("streamable-http",),
        note=("Only observable over streamable HTTP: the modern entry's method registry omits initialize."),
    ),
    "hosting:http:modern:legacy-fallthrough": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/versioning",
        behavior=(
            "Non-2026-07-28 traffic on the same /mcp endpoint reaches the legacy transport "
            "byte-unchanged: a 2025-era initialize handshake still completes, and an unrecognised "
            "MCP-Protocol-Version header still produces the legacy 400 'Unsupported protocol version' literal."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note=(
            "Only observable over streamable HTTP: routing branches on the MCP-Protocol-Version "
            "header at the same /mcp endpoint."
        ),
    ),
    "hosting:http:modern:handler-exception-internal-error": Requirement(
        source="sdk",
        behavior=(
            "An unhandled handler exception on the 2026-07-28 entry is returned as JSON-RPC error "
            "-32603 with a generic message that does not echo str(exc)."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: the modern entry's exception-to-JSONRPCError boundary.",
    ),
    "hosting:http:modern:discover-response-shape": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/index",
        behavior=(
            "A 2026-07-28 server/discover response carries supportedVersions and capabilities in the "
            "result body, with supportedVersions naming the modern protocol revisions the server "
            "accepts; serverInfo is not a body field and travels as the io.modelcontextprotocol/serverInfo "
            "result _meta stamp."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: the raw result body is asserted at the wire.",
    ),
    "hosting:http:modern:removed-method-status-404": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/index",
        behavior=(
            "A method that exists at earlier protocol revisions but is removed at 2026-07-28 is "
            "answered METHOD_NOT_FOUND, and the modern entry maps that error code to HTTP 404."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note=(
            "Only observable over streamable HTTP: the HTTP status is the assertion. Kernel-origin "
            "METHOD_NOT_FOUND travels through the same status table as classifier-origin errors."
        ),
    ),
    "hosting:http:modern:envelope-missing-key-status-400": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http",
        behavior=(
            "A 2026-07-28 request whose params._meta envelope omits a required reserved key is "
            "rejected as INVALID_PARAMS at HTTP 400 before kernel dispatch."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: the HTTP status is the assertion.",
    ),
    "hosting:http:modern:handler-error-status-via-table": Requirement(
        source="sdk",
        behavior=(
            "A handler-raised MCPError on the 2026-07-28 entry reaches the wire as a top-level "
            "JSON-RPC error with its data preserved, and the HTTP status is the error-code table "
            "entry for that code (handler-origin and classifier-origin errors share one table)."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: the modern entry's JSONRPCError-to-HTTP-status mapping.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Client transport: streamable HTTP
    # ═══════════════════════════════════════════════════════════════════════════
    "client-transport:http:404-surfaces": Requirement(
        source="sdk",
        behavior="A 404 (session expired) on a request surfaces as an error to the caller.",
        transports=("streamable-http",),
        note="Only observable over HTTP: 404 is an HTTP status code.",
    ),
    "client-transport:http:session-404-reinitialize": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior=(
            "A 404 in response to a request carrying a session ID makes the client start a new session "
            "with a fresh InitializeRequest and no session ID attached."
        ),
        transports=("streamable-http",),
        divergence=Divergence(
            note=(
                "The client surfaces the 404 as an error to the caller instead of re-initializing a new "
                "session; the spec's MUST is not satisfied."
            ),
        ),
        deferred=(
            "Not implemented in the SDK: the client surfaces a Session terminated error instead of "
            "re-initializing (the surfaced error is pinned by client-transport:http:404-surfaces)."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id and protocol-level sessions removed, no replacement.",
    ),
    "client-transport:http:accept-header-get": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#listening-for-messages-from-the-server",
        behavior="The client GET to the MCP endpoint includes an Accept header listing text/event-stream.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); the standalone GET endpoint is replaced by the subscriptions/listen "
            "POST."
        ),
    ),
    "client-transport:http:accept-header-post": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "Every client POST to the MCP endpoint includes an Accept header listing both application/json "
            "and text/event-stream."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: Accept is an HTTP request header.",
    ),
    "client-transport:http:cancel-closes-stream": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http#cancellation",
        behavior=(
            "At 2026-07-28, abandoning an in-flight request closes that request's own POST stream and "
            "posts nothing further: no notifications/cancelled reaches the server (the revision defines "
            "no client-to-server notifications), and the server treats the disconnect as cancellation "
            "of exactly that request."
        ),
        transports=("streamable-http",),
        added_in="2026-07-28",
        supersedes=("client-transport:http:cancel-posts-frame",),
        note="HTTP-only by nature: the response stream that closing constitutes the signal is an HTTP exchange.",
    ),
    "client-transport:http:cancel-posts-frame": Requirement(
        source=f"{SPEC_BASE_URL}/basic/utilities/cancellation#cancellation-flow",
        behavior=(
            "At 2025-era revisions, abandoning an in-flight request POSTs exactly one "
            "notifications/cancelled naming its request id."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        superseded_by="client-transport:http:cancel-closes-stream",
        note="HTTP-only by nature: pins that the frame travels as its own POST on the legacy HTTP wire.",
    ),
    "client-transport:http:concurrent-streams": Requirement(
        source="sdk",
        behavior="Multiple concurrent POST-initiated SSE streams each deliver their response to the right caller.",
        transports=("streamable-http",),
        note="Only observable over HTTP: per-request SSE streams are HTTP-specific.",
    ),
    "client-transport:http:custom-client": Requirement(
        source="sdk",
        behavior=(
            "A caller-supplied HTTP client (and its event hooks and headers) is used for all MCP traffic, "
            "including auth flows."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: the httpx2 client is HTTP-specific.",
    ),
    "client-transport:http:custom-headers": Requirement(
        source="sdk",
        behavior="Caller-supplied headers are sent on every POST, GET, and DELETE to the MCP endpoint.",
        transports=("streamable-http",),
        note="Only observable over HTTP: headers are an HTTP concept.",
    ),
    "client-transport:http:json-response-parsed": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior="A Content-Type application/json response is parsed as a single JSON-RPC message.",
        transports=("streamable-http",),
        note="Only observable over HTTP: Content-Type is an HTTP response header.",
    ),
    "client-transport:http:no-reconnect-after-close": Requirement(
        source="sdk",
        behavior="After the transport is closed, no further reconnection attempts are scheduled.",
        transports=("streamable-http",),
        note="Only observable over HTTP: stream reconnection is HTTP-specific.",
    ),
    "client-transport:http:no-reconnect-after-response": Requirement(
        source="sdk",
        behavior="A POST-initiated stream that already delivered its response is not reconnected when it closes.",
        transports=("streamable-http",),
        note="Only observable over HTTP: stream reconnection is HTTP-specific.",
    ),
    "client-transport:http:protocol-version-header": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#protocol-version-header",
        behavior=(
            "After initialization, the client sends the negotiated MCP-Protocol-Version header on every "
            "subsequent HTTP request."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: MCP-Protocol-Version is an HTTP header.",
    ),
    "client-transport:http:protocol-version-stored": Requirement(
        source="sdk",
        behavior=(
            "The client transport stores the negotiated protocol version and sends it on every subsequent request."
        ),
        transports=("streamable-http",),
        note="Only observable over HTTP: MCP-Protocol-Version is an HTTP header.",
    ),
    "client-transport:http:reconnect-get": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#resumability-and-redelivery",
        behavior=(
            "A standalone GET SSE stream that errors is reconnected with the Last-Event-ID of the last received event."
        ),
        transports=("streamable-http",),
        deferred=(
            "The server's standalone GET stream emits no priming event or retry hint, so the client's "
            "reconnection path always sleeps the hard-coded 1 s default; a deterministic in-process test "
            "would require accepting that real-time wait. The POST-stream reconnection path is covered "
            "by client-transport:http:reconnect-post-priming."
        ),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE stream resumability/redelivery dropped, no replacement.",
    ),
    "client-transport:http:reconnect-post-priming": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior=(
            "A POST-initiated SSE stream that errors before delivering its response is reconnected only "
            "if a priming event (an event carrying an ID) was received on it."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE stream resumability/redelivery dropped, no replacement.",
    ),
    "client-transport:http:reconnect-retry-value": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#sending-messages-to-the-server",
        behavior="Reconnection delay honours the server-provided SSE retry value when one was sent.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE stream resumability/redelivery dropped, no replacement.",
    ),
    "client-transport:http:resume-stream-api": Requirement(
        source="sdk",
        behavior=(
            "The client can capture a resumption token, reconnect with the same session id, and receive "
            "the notifications it missed."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE stream resumability/redelivery dropped, no replacement.",
    ),
    "client-transport:http:session-stored": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior=(
            "The Mcp-Session-Id returned by initialize is stored by the client transport and sent on "
            "every subsequent request."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); Mcp-Session-Id and protocol-level sessions removed, no replacement.",
    ),
    "client-transport:http:sse-405-tolerated": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#listening-for-messages-from-the-server",
        behavior="Opening the standalone GET SSE stream tolerates a 405 response without failing the connection.",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); the standalone GET endpoint is replaced by the subscriptions/listen "
            "POST."
        ),
    ),
    "client-transport:http:terminate-405-ok": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior="Session termination succeeds without error if the server answers 405 (termination unsupported).",
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); session DELETE removed with Mcp-Session-Id, no replacement.",
    ),
    "client-transport:http:body-derived-headers": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports#stateless-request-headers",
        behavior=(
            "An envelope-bearing request body yields MCP-Protocol-Version, Mcp-Method, and (for tools/call) "
            "Mcp-Name headers on the outgoing HTTP request; a body without the envelope yields none."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: headers are derived from the body envelope at the transport seam.",
    ),
    "client-transport:http:custom-param-headers": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http#custom-headers-from-tool-parameters",
        behavior=(
            "On a tools/call, a client mirrors each argument annotated with x-mcp-header in the tool's "
            "inputSchema into an Mcp-Param-<name> header -- string as-is, integer as decimal, boolean as "
            "true/false, base64-sentinel-wrapped when not header-safe -- omitting null or absent arguments and "
            "never mirroring unannotated parameters. The schema is taken from the tool's last list_tools entry; "
            "a tool the client never listed emits no Mcp-Param-* headers."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: headers are derived from the cached tool schema at the seam.",
    ),
    "client-transport:http:vendor-name-param-header": Requirement(
        source="sdk",
        behavior=(
            "A vendor request type declaring name_param mirrors that wire-params key into the Mcp-Name "
            "header of its outgoing HTTP request, with no client-side registration of the method."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note=(
            "SDK mechanism honouring the per-extension Mcp-Name requirements (e.g. SEP-2663 mandates the "
            "header for tasks/*); only observable over streamable HTTP, where headers exist."
        ),
    ),
    "client-transport:http:stateless-ignores-session-id": Requirement(
        source=f"{SPEC_2026_BASE_URL}/basic/transports#stateless-request-headers",
        behavior=(
            "A pinned client never echoes a server-issued Mcp-Session-Id and never opens the standalone "
            "GET stream or the closing DELETE: the recorded wire is POST-only."
        ),
        added_in="2026-07-28",
        transports=("streamable-http",),
        note="Only observable over streamable HTTP: session-id, GET stream and DELETE are streamable-HTTP mechanics.",
        deferred="defensive against a misbehaving peer; covered by a tests/client/ unit test",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Client auth
    # ═══════════════════════════════════════════════════════════════════════════
    "client-auth:401-after-auth-throws": Requirement(
        source="sdk",
        behavior=(
            "If the server still returns 401 after a successful authorization, the client fails instead of looping."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:401-triggers-flow": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#protected-resource-metadata-discovery-requirements",
        behavior="A 401 on a request triggers the OAuth authorization flow once.",
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:403-scope-upgrade": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#step-up-authorization-flow",
        behavior=(
            "A 403 with WWW-Authenticate triggers a scope-upgrade authorization attempt; repeated 403s do not loop."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:403-scope-union": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#step-up-authorization-flow",
        behavior=(
            "On a 403 insufficient_scope step-up, the re-authorization request carries the union of the "
            "previously requested scopes and the newly challenged scopes (SEP-2350)."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:as-metadata-discovery:priority-order": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-metadata-discovery",
        behavior=(
            "The client discovers authorization-server metadata by trying, in order, the OAuth "
            "path-inserted, OIDC path-inserted, and OIDC path-appended well-known URLs (with the "
            "root-path forms when the issuer URL has no path)."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:as-metadata-discovery:issuer-validation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-metadata-discovery",
        behavior=(
            "The client rejects authorization-server metadata whose issuer does not match the URL the "
            "metadata was retrieved from (RFC 8414 section 3.3 / SEP-2468)."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:authorize:error-surfaces": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-flow-steps",
        behavior=(
            "An OAuth error redirect from the authorize endpoint aborts the flow before any token "
            "request is issued, surfacing as an error to the caller."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
        divergence=Divergence(
            note=(
                "The callback contract has no error form, so the client surfaces 'No authorization code "
                "received' rather than the redirect's `error`/`error_description` values."
            ),
        ),
    ),
    "client-auth:authorize:offline-access-consent": Requirement(
        source="sdk",
        behavior=(
            "When the authorization server's metadata advertises offline_access in scopes_supported and "
            "the client uses the refresh_token grant, offline_access is appended to the requested scope "
            "and prompt=consent is added to the authorize request."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:bearer-header:every-request": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#token-requirements",
        behavior=(
            "Once authorized, the client sends the bearer token in the Authorization header on every HTTP "
            "request to the MCP server, never in the query string."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:cimd": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#client-id-metadata-documents",
        behavior="The client can use a client-ID metadata document URL as its OAuth client_id instead of registration.",
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:client-credentials": Requirement(
        source="sdk",
        behavior=(
            "A client-credentials provider obtains a token without user interaction and the resulting "
            "bearer token authorizes subsequent requests."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:dcr:registration-error-surfaces": Requirement(
        source="sdk",
        behavior=(
            "A 400 from the registration endpoint surfaces to the caller as an OAuthRegistrationError "
            "carrying the status and the server's RFC 7591 error body."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:dcr:substituted-metadata": Requirement(
        source="sdk",
        behavior=(
            "A 201 registration response whose echoed metadata the server substituted (RFC 7591 §3.2.1) - "
            "an unregistered application_type, null redirect_uris, extra grant types - completes the flow; "
            "substituted credentials the authorization-code flow cannot apply (an unimplemented "
            "token_endpoint_auth_method; private_key_jwt, whose assertion it has no key to sign; or a "
            "secret-based method with no client_secret issued) are instead reported as an "
            "OAuthRegistrationError before the record is persisted or authorization begins."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:dcr": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#dynamic-client-registration",
        behavior=(
            "The client performs dynamic client registration against the authorization server when no "
            "client_id is preconfigured."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:as-binding": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-binding",
        behavior=(
            "Stored client credentials are bound to the issuer that registered them; when the authorization "
            "server changes, the client discards them and re-registers rather than reusing them (SEP-2352)."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:invalid-client-clears-all": Requirement(
        source="sdk",
        behavior=(
            "An invalid-client or unauthorized-client error during authorization invalidates all stored credentials."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
        divergence=Divergence(
            note=(
                "The token-response handlers do not parse the error body; an invalid_client or "
                "unauthorized_client response leaves stored client_info untouched. The TypeScript SDK "
                "clears it."
            ),
        ),
        deferred=(
            "Not implemented in the SDK: no token-response path inspects the error code to decide "
            "whether to clear client_info."
        ),
    ),
    "client-auth:invalid-grant-clears-tokens": Requirement(
        source="sdk",
        behavior="An invalid-grant error during authorization invalidates only the stored tokens.",
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:pkce:refuse-if-unsupported": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-code-protection",
        behavior=(
            "The client refuses to proceed when the authorization server's metadata does not include "
            "code_challenge_methods_supported, since PKCE support cannot be verified."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
        divergence=Divergence(
            note=(
                "The client never inspects code_challenge_methods_supported and proceeds with PKCE S256 "
                "regardless; the spec MUST is not enforced."
            ),
        ),
    ),
    "client-auth:pkce:s256": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-code-protection",
        behavior=(
            "The authorization request includes a PKCE S256 code challenge and the token request includes "
            "the matching verifier."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:pre-registration": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#preregistration",
        behavior=(
            "A client with statically preconfigured credentials skips dynamic registration and uses them directly."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:private-key-jwt": Requirement(
        source="sdk",
        behavior="The client can authenticate the client-credentials grant with a signed JWT assertion.",
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:prm-discovery:fallback-order": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#protected-resource-metadata-discovery-requirements",
        behavior=(
            "The client uses resource_metadata from WWW-Authenticate when present, then falls back to the "
            "well-known protected-resource locations in the documented order."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:prm-discovery:no-prm-fallback": Requirement(
        source="sdk",
        behavior=(
            "When every protected-resource metadata probe fails, the client falls back to discovering "
            "authorization-server metadata directly at the MCP server's origin (the legacy 2025-03-26 path) "
            "rather than aborting."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:prm-resource-mismatch": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-location",
        behavior=(
            "The client refuses to proceed when the protected-resource metadata's resource field does not "
            "match the server URL it is connecting to."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:refresh:transparent": Requirement(
        source="sdk",
        behavior=(
            "An access token the client considers expired is transparently refreshed before the next "
            "request, using the stored refresh token; the refresh request includes the resource indicator "
            "and the new token is persisted."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:resource-parameter": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#resource-parameter-implementation",
        behavior=(
            "The client includes the canonical server URI as the resource parameter in both the "
            "authorization request and the token request."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:scope-selection:priority": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#scope-selection-strategy",
        behavior=(
            "Client selects requested scope from the WWW-Authenticate scope param if present; otherwise "
            "uses scopes_supported from the PRM document; otherwise omits scope."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
        divergence=Divergence(
            note=(
                "The SDK inserts an extra fallback step between PRM and omit: if the authorization "
                "server metadata advertises scopes_supported, that list is used (client/auth/utils.py). "
                "This is beyond the spec's two-step chain."
            ),
        ),
    ),
    "client-auth:state:verify": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#open-redirection",
        behavior=(
            "A state parameter is included in the authorization URL, and authorization results with a "
            "missing or mismatched state are discarded."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:authorization-response:iss-verify": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-metadata-discovery",
        behavior=(
            "The client validates the RFC 9207 iss authorization-response parameter against the "
            "authorization server issuer (simple string comparison) and rejects a mismatch, or a "
            "missing iss when the server advertises support (SEP-2468)."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:token-endpoint-auth-method": Requirement(
        source="sdk",
        behavior="The client authenticates to the token endpoint using the auth method established at registration.",
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:token-provenance": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#token-handling",
        behavior=(
            "The client sends the MCP server only tokens issued by that server's authorization server, "
            "never tokens obtained elsewhere."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
        deferred=(
            "Untestable negative through the public API: there is no path to inject a token obtained "
            "elsewhere into the auth provider's state, so the absence cannot be observed end to end."
        ),
    ),
    "client-auth:identity-assertion": Requirement(
        source="sdk",
        behavior=(
            "The identity-assertion provider (SEP-990) presents an enterprise IdP-issued ID-JAG to the MCP "
            "authorization server via the RFC 7523 jwt-bearer grant, with no authorize or registration step, "
            "and the issued bearer token authorizes subsequent requests."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:identity-assertion:assertion-callback": Requirement(
        source="sdk",
        behavior=(
            "The identity-assertion provider sources the ID-JAG from its async assertion_provider callback, "
            "invoked with the authorization server's issuer as audience and the MCP server's resource "
            "identifier, and sends it as `assertion` on the RFC 7523 jwt-bearer request."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:identity-assertion:issuer-pinning": Requirement(
        source="sdk",
        behavior=(
            "The identity-assertion provider's authorization server is configuration: metadata is "
            "fetched only from the configured issuer's RFC 8414 well-known, the resource server is "
            "never consulted for AS selection, and the ID-JAG and client secret are not sent unless "
            "that metadata validates."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:identity-assertion:disabled-rejected": Requirement(
        source="sdk",
        behavior=(
            "When the authorization server has the identity-assertion grant disabled, the token endpoint "
            "rejects it with unsupported_grant_type and the connection fails rather than issuing a token."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:identity-assertion:invalid-assertion": Requirement(
        source="sdk",
        behavior=(
            "A jwt-bearer request whose ID-JAG the authorization server rejects surfaces as an OAuth error "
            "and the connection fails rather than proceeding with a bearer token."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "client-auth:identity-assertion:metadata-advertised": Requirement(
        source="sdk",
        behavior=(
            "When the identity-assertion grant is enabled, the authorization-server metadata advertises the "
            "jwt-bearer grant type and the id-jag grant profile in authorization_grant_profiles_supported."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # stdio transport
    # ═══════════════════════════════════════════════════════════════════════════
    "transport:stdio:clean-shutdown": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#shutdown",
        behavior="Closing the client transport closes the child process's stdin and the server exits cleanly.",
        transports=("stdio",),
        note="Only observable over stdio: child-process lifecycle is stdio-specific.",
    ),
    "transport:stdio:stream-purity": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#stdio",
        behavior=(
            "Nothing that is not a valid MCP message is written to the server's stdout, and nothing that "
            "is not a valid MCP message is written to its stdin."
        ),
        transports=("stdio",),
        note="Only observable over stdio: stdin/stdout purity is stdio-specific.",
        divergence=Divergence(
            note=(
                "While serving, stdio_server moves the wire to private descriptors and diverts fd 0/1, so "
                "handler code and its child processes can neither read protocol bytes nor write into the "
                "stream (pinned by tests/server/test_stdio.py). Remaining gaps: output flushed to stdout "
                "before the transport enters can still precede the first frame, and the claim is "
                "best-effort - skipped for explicitly injected streams and for processes without "
                "normal standard descriptors."
            ),
        ),
    ),
    "transport:stdio:no-embedded-newlines": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#stdio",
        behavior="Serialized JSON-RPC messages on stdio contain no embedded newlines; one message per line.",
        transports=("stdio",),
        note="Only observable over stdio: newline-delimited framing is stdio-specific.",
    ),
    "transport:stdio:shutdown-escalation": Requirement(
        source=f"{SPEC_BASE_URL}/basic/lifecycle#stdio",
        behavior=(
            "If the server process does not exit after stdin is closed, the client transport terminates "
            "it (and kills it if still alive) after a grace period."
        ),
        transports=("stdio",),
        note="Only observable over stdio: child-process lifecycle is stdio-specific.",
        deferred=(
            "A server that ignores stdin close takes the full PROCESS_TERMINATION_TIMEOUT (2.0 s) grace "
            "period plus up to a further 2.0 s for SIGTERM/SIGKILL escalation; testing that path is "
            "real-time-bound (the constant is module-level with no public override) and so is deliberately "
            "excluded from this suite. Covered by tests/client/test_stdio.py."
        ),
    ),
    "transport:stdio:stderr-passthrough": Requirement(
        source="sdk",
        behavior="Server stderr is available to the client and is not consumed by the transport.",
        transports=("stdio",),
        note="Only observable over stdio: stderr is a child-process stream.",
    ),
    # ═══════════════════════════════════════════════════════════════════════════
    # Composite end-to-end flows
    # ═══════════════════════════════════════════════════════════════════════════
    "flow:compat:dual-transport-server": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#backwards-compatibility",
        behavior=(
            "A single server instance can serve streamable HTTP and the legacy SSE transport "
            "concurrently; clients on either transport can call the same tools."
        ),
        transports=("streamable-http", "sse"),
        note="Exercises both HTTP transports side by side; not applicable to stdio.",
    ),
    "flow:compat:streamable-then-sse-fallback": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#backwards-compatibility",
        behavior=(
            "When a streamable HTTP initialize fails with 400, 404, or 405, falling back to the legacy "
            "SSE client transport against the same server connects successfully."
        ),
        transports=("streamable-http", "sse"),
        note="Exercises the HTTP-to-SSE fallback path; not applicable to stdio.",
        divergence=Divergence(
            note=(
                "The SDK provides no automatic streamable-HTTP-to-SSE client fallback; the spec's "
                "client-side SHOULD is left to the application to compose from streamable_http_client "
                "and sse_client. Both halves are independently proven by the matrix."
            ),
        ),
        deferred=(
            "A demonstration test would only re-prove what the matrix already covers (an SSE-only "
            "server is reachable via sse_client; an unmounted route returns 404), with the application "
            "doing the fallback in between rather than the SDK."
        ),
    ),
    "flow:elicitation:multi-step-form": Requirement(
        source="sdk",
        behavior=(
            "A single tool handler issues sequential elicitations; an accept on one step feeds the next, "
            "and a decline or cancel at any step short-circuits to a final result."
        ),
        arm_exclusions=(
            ArmExclusion(reason="server-initiated-request", transport="streamable-http-stateless"),
            ArmExclusion(reason="server-initiated-request", spec_version="2026-07-28"),
        ),
    ),
    "flow:elicitation:url-at-session-init": Requirement(
        source="sdk",
        behavior=(
            "The server can issue a URL-mode elicitation over the standalone GET stream immediately after "
            "session initialization, before any client request."
        ),
        transports=("streamable-http",),
        deferred=(
            "Not implemented in the SDK: no public per-session post-initialization hook exists on either "
            "server flavour (Server.lifespan runs at server startup, not per session; ServerSession "
            "handles the initialized notification internally with no callback). Driving 'before any "
            "client request' deterministically would also require knowing the standalone GET stream is "
            "established, which has no synchronization signal."
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2575); the standalone GET stream and session initialization are both gone, no "
            "replacement."
        ),
    ),
    "flow:elicitation:url-required-then-retry": Requirement(
        source=f"{SPEC_BASE_URL}/client/elicitation#url-elicitation-required-error",
        behavior=(
            "A tool call rejected with the URL-elicitation-required error can be retried successfully "
            "after the client completes the URL flow and the server announces completion."
        ),
        removed_in="2026-07-28",
        note=(
            "removed in 2026-07-28 (SEP-2322); the -32042 + elicitation/complete flow is replaced by the MRTR "
            "input_required/retry loop."
        ),
        arm_exclusions=(ArmExclusion(reason="requires-session", transport="streamable-http-stateless"),),
    ),
    "flow:multi-client:stateful-isolation": Requirement(
        source="sdk",
        behavior=(
            "Independent clients connected to one stateful server each receive a distinct session and "
            "only the notifications produced by their own requests."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); per-client Mcp-Session-Id sessions removed, no replacement.",
    ),
    "flow:oauth:authorization-code-roundtrip": Requirement(
        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-flow-steps",
        behavior=(
            "Connecting to a protected server walks the authorization-code flow end to end: the first "
            "attempt requires authorization, the code is exchanged, and a subsequent connection succeeds."
        ),
        transports=("streamable-http",),
        note="OAuth is HTTP-only.",
    ),
    "flow:resume:tool-call-resumption-token": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#resumability-and-redelivery",
        behavior=(
            "A tool call interrupted mid-stream is transparently resumed by the client transport using "
            "the last-seen event id, delivering only the remaining notifications and the final result."
        ),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2575); SSE stream resumability/redelivery dropped, no replacement.",
    ),
    "flow:session:terminate-then-reconnect": Requirement(
        source=f"{SPEC_BASE_URL}/basic/transports#session-management",
        behavior=("After terminating a session, a fresh connection obtains a new session id and operations succeed."),
        transports=("streamable-http",),
        removed_in="2026-07-28",
        note="removed in 2026-07-28 (SEP-2567); session DELETE removed with Mcp-Session-Id, no replacement.",
    ),
    "flow:tool-result:resource-link-follow": Requirement(
        source=f"{SPEC_BASE_URL}/server/tools#resource-links",
        behavior=(
            "A resource_link returned by a tool call can be followed with resources/read on the linked "
            "URI to retrieve the referenced contents."
        ),
    ),
}


def requirement(requirement_id: str) -> Callable[[_TestFn], _TestFn]:
    """Mark a test as exercising a requirement from :data:`REQUIREMENTS`.

    Applies the `requirement` pytest marker and records the coverage link checked by
    `test_coverage.py`. Unknown IDs fail at import time so a typo surfaces as a collection
    error on the offending test, not as a missing-coverage report later.
    """
    if requirement_id not in REQUIREMENTS:
        raise KeyError(f"Unknown requirement id {requirement_id!r}: add it to REQUIREMENTS in {__name__}")

    def apply(test_fn: _TestFn) -> _TestFn:
        covered_by(requirement_id).append(f"{test_fn.__module__}.{test_fn.__qualname__}")
        return pytest.mark.requirement(requirement_id)(test_fn)

    return apply


_COVERAGE: dict[str, list[str]] = {}


def covered_by(requirement_id: str) -> list[str]:
    """Return the (mutable) list of test names recorded as exercising `requirement_id`."""
    return _COVERAGE.setdefault(requirement_id, [])


def cell_id(transport: Transport, version: SpecVersion, *, spec_versions: Sequence[SpecVersion] = SPEC_VERSIONS) -> str:
    """Return the pytest node-id suffix for a (transport, spec_version) cell.

    While the active matrix has a single spec version, the suffix is just the transport name so
    existing node ids stay byte-identical; once a second version is on the axis the suffix becomes
    ``transport-version``.
    """
    return transport if len(spec_versions) == 1 else f"{transport}-{version}"


def compute_cells(
    requirements: Sequence[Requirement],
    *,
    spec_versions: Sequence[SpecVersion] = SPEC_VERSIONS,
    transports: Sequence[Transport] = CONNECTABLE_TRANSPORTS,
) -> list[Any]:
    """Compute the (transport, spec_version) parametrization cells for a test.

    Stacked ``@requirement`` decorators contribute multiple entries; the cells emitted are the
    INTERSECTION across all of them: a cell is dropped if it falls outside any requirement's
    ``[added_in, removed_in)`` window or matches any requirement's ``arm_exclusions``. An empty
    ``requirements`` sequence yields the full transport x spec-version grid.

    ``Requirement.transports`` is intentionally NOT consulted -- it is descriptive metadata about
    where a behaviour is observable, not a cell filter (only ``arm_exclusions`` / ``added_in`` /
    ``removed_in`` drive cell generation).

    Returns a list of ``pytest.param((transport, version), id=..., marks=...)`` values for use as
    ``metafunc.parametrize`` argvalues.
    """
    cells: list[Any] = []
    for version in spec_versions:
        version_ordinal = KNOWN_PROTOCOL_VERSIONS.index(version)
        for transport in sorted(transports):
            if transport in TRANSPORT_SPEC_VERSIONS and version not in TRANSPORT_SPEC_VERSIONS[transport]:
                continue
            # Requirement.transports is descriptive metadata only and does not filter cells.
            if any(
                (req.added_in is not None and version_ordinal < KNOWN_PROTOCOL_VERSIONS.index(req.added_in))
                or (req.removed_in is not None and version_ordinal >= KNOWN_PROTOCOL_VERSIONS.index(req.removed_in))
                for req in requirements
            ):
                continue
            if any(
                (ex.transport is None or ex.transport == transport)
                and (ex.spec_version is None or ex.spec_version == version)
                for req in requirements
                for ex in req.arm_exclusions
            ):
                continue
            matched_failure = next(
                (
                    kf
                    for req in requirements
                    for kf in req.known_failures
                    if (kf.transport is None or kf.transport == transport)
                    and (kf.spec_version is None or kf.spec_version == version)
                ),
                None,
            )
            marks = [pytest.mark.xfail(reason=matched_failure.note, strict=True)] if matched_failure else ()
            cells.append(
                pytest.param(
                    (transport, version),
                    id=cell_id(transport, version, spec_versions=spec_versions),
                    marks=marks,
                )
            )
    return cells