diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
index 895339ca18..f18cc0ef10 100644
--- a/src/mcp/client/session.py
+++ b/src/mcp/client/session.py
@@ -4,10 +4,10 @@
 import logging
 from collections.abc import Callable, Mapping, Sequence
 from dataclasses import dataclass
-from functools import reduce
+from functools import cache, reduce
 from operator import or_
-from types import TracebackType
-from typing import TYPE_CHECKING, Annotated, Any, Final, Literal, Protocol, TypeAlias, cast, overload
+from types import TracebackType, UnionType
+from typing import TYPE_CHECKING, Annotated, Any, Final, Literal, Protocol, TypeAlias, cast, get_args, overload
 
 import anyio
 import anyio.abc
@@ -30,6 +30,7 @@
 from mcp_types import methods as _methods
 from mcp_types.version import (
     HANDSHAKE_PROTOCOL_VERSIONS,
+    KNOWN_PROTOCOL_VERSIONS,
     LATEST_HANDSHAKE_VERSION,
     LATEST_MODERN_VERSION,
     MODERN_PROTOCOL_VERSIONS,
@@ -77,6 +78,45 @@ def _clamp_inbound_ttl(raw: dict[str, Any]) -> None:
         raw["ttlMs"] = 0
 
 
+@cache
+def _wire_fields(target: type[BaseModel] | UnionType) -> frozenset[str]:
+    """Top-level wire keys `target` declares (its members', for a union).
+
+    A `RootModel` row (e.g. an empty result carried as `RootModel[Result]`)
+    reports its wrapped type's keys, not the pydantic-internal `root`.
+    """
+    members: tuple[Any, ...] = get_args(target) if isinstance(target, UnionType) else (target,)
+    models = [m for m in members if isinstance(m, type) and issubclass(m, BaseModel)]
+    fields: set[str] = set()
+    for model in models:
+        if getattr(model, "__pydantic_root_model__", False):  # a RootModel wrapper row
+            fields |= _wire_fields(model.model_fields["root"].annotation)
+        else:
+            fields.update(field.alias or name for name, field in model.model_fields.items())
+    return frozenset(fields)
+
+
+@cache
+def _later_revision_fields(method: str, version: str) -> frozenset[str]:
+    """Result keys a revision newer than `version` declares for `method` but `version` doesn't.
+
+    The version-free result types carry every revision's fields, so such a key
+    (e.g. 2026-07-28 `ttlMs`/`cacheScope` on a pre-2026 session) is outside the
+    negotiated contract yet would still parse into the model and trip that later
+    revision's constraints. Empty at the newest known revision.
+    """
+    current = _methods.SERVER_RESULTS.get((method, version))
+    if current is None or version not in KNOWN_PROTOCOL_VERSIONS:
+        return frozenset()
+    newer = KNOWN_PROTOCOL_VERSIONS[KNOWN_PROTOCOL_VERSIONS.index(version) + 1 :]
+    later: set[str] = set()
+    for revision in newer:
+        row = _methods.SERVER_RESULTS.get((method, revision))
+        if row is not None:
+            later |= _wire_fields(row)
+    return frozenset(later) - _wire_fields(current)
+
+
 def _same_schema(a: dict[str, Any] | None, b: dict[str, Any] | None) -> bool:
     """JSON equality for two output schemas.
 
@@ -558,6 +598,11 @@ async def send_request(
             _methods.validate_server_result(method, version, raw)
         except KeyError:
             pass
+        # Drop a later revision's fields (e.g. 2026-07-28 cache hints on a pre-2026
+        # session): they are outside the negotiated contract, and the version-free
+        # result type would otherwise apply that revision's constraints to them.
+        if not (foreign := _later_revision_fields(method, version)).isdisjoint(raw):
+            raw = {key: value for key, value in raw.items() if key not in foreign}
         if isinstance(result_type, TypeAdapter):
             return result_type.validate_python(raw, by_name=False)
         return result_type.model_validate(raw, by_name=False)
diff --git a/tests/client/test_session.py b/tests/client/test_session.py
index 6e5e0a14d8..6663fb47a2 100644
--- a/tests/client/test_session.py
+++ b/tests/client/test_session.py
@@ -1759,6 +1759,75 @@ async def test_a_boolean_inbound_ttl_is_not_clamped_only_coerced_by_validation(w
     assert result.ttl_ms == int(wire_ttl)
 
 
+_LEGACY_HINTED_RESULTS: list[tuple[str, dict[str, Any]]] = [
+    ("list_tools", {"tools": []}),
+    ("list_prompts", {"prompts": []}),
+    ("list_resources", {"resources": []}),
+    ("list_resource_templates", {"resourceTemplates": []}),
+    ("read_resource", {"contents": []}),
+]
+
+_LEGACY_TAGGED_RESULTS: list[tuple[str, dict[str, Any]]] = [
+    ("call_tool", {"content": []}),
+    ("get_prompt", {"messages": []}),
+]
+
+
+def _legacy_init(version: str) -> dict[str, Any]:
+    return InitializeResult(
+        protocol_version=version,
+        capabilities=ServerCapabilities(),
+        server_info=Implementation(name="mock-server", version="0.1.0"),
+    ).model_dump(by_alias=True, mode="json", exclude_none=True)
+
+
+async def _call_legacy(session: ClientSession, verb: str) -> Any:
+    if verb == "read_resource":
+        return await session.read_resource("mem://x")
+    if verb == "call_tool":
+        return await session.call_tool("t", {})
+    if verb == "get_prompt":
+        return await session.get_prompt("p")
+    return await getattr(session, verb)()
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("version", HANDSHAKE_PROTOCOL_VERSIONS)
+@pytest.mark.parametrize(("verb", "body"), _LEGACY_HINTED_RESULTS)
+async def test_cache_hints_from_a_legacy_server_never_reach_the_result(
+    version: str, verb: str, body: dict[str, Any]
+) -> None:
+    """SDK-defined: on any pre-2026 session the caching fields are outside the negotiated
+    schema, so whatever a server puts in them - even values the 2026-07-28 enum
+    rejects - is dropped and the model shows its conservative defaults."""
+    dispatcher = _ScriptedDispatcher(_legacy_init(version), {**body, "ttlMs": -1, "cacheScope": "session"})
+    with anyio.fail_after(5):
+        async with ClientSession(dispatcher=dispatcher) as session:
+            await session.initialize()
+            result = await _call_legacy(session, verb)
+    assert (result.ttl_ms, result.cache_scope) == (0, "private")
+    assert not {"ttl_ms", "cache_scope"} & result.model_fields_set
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(("verb", "body"), _LEGACY_TAGGED_RESULTS)
+async def test_a_2026_result_type_tag_from_a_legacy_server_never_reaches_the_result(
+    verb: str, body: dict[str, Any]
+) -> None:
+    """SDK-defined: `resultType` is 2026-07-28 vocabulary that also feeds result-union
+    routing, so a tag on a pre-2026 wire (even one no union arm claims) is dropped and
+    the plain result is returned rather than mis-routing or failing."""
+    # `call_tool` re-lists tools to validate structured content; that answer trails harmlessly for `get_prompt`.
+    dispatcher = _ScriptedDispatcher(
+        _legacy_init(LATEST_HANDSHAKE_VERSION), {**body, "resultType": "task"}, {"tools": []}
+    )
+    with anyio.fail_after(5):
+        async with ClientSession(dispatcher=dispatcher) as session:
+            await session.initialize()
+            result = await _call_legacy(session, verb)
+    assert result.result_type == "complete"
+
+
 @pytest.mark.anyio
 async def test_session_call_tool_returns_input_required_result_when_opted_in() -> None:
     """`ClientSession.call_tool(..., allow_input_required=True)` surfaces the