Document the Tasks extension

maxisbey · maxisbey · commit 53b6c653d00b · 2026-06-30T13:33:34.000Z
Adds docs/advanced/tasks.md (sibling of the MCP Apps page) covering the
server opt-in with the augment/default_ttl_ms/clock knobs, the pluggable
TaskStore protocol and its per-process caveat, the inline execution
model (tasks born terminal; acks without effect), transparent client
polling with the typed task errors, manual driving over the
mcp.shared.tasks wrappers, and the per-wire error-code table. Three
compiled docs_src tutorials back the page, each behaviorally tested; the
page is registered in the mkdocs nav (which also feeds llms.txt). The
migration note and module docstring no longer list routing headers as
deferred -- Mcp-Name stamping and validation landed with the shared
header table.
diff --git a/docs/advanced/extensions.md b/docs/advanced/extensions.md
@@ -18,7 +18,8 @@ Pass instances at construction:
 Done. The server now advertises `io.modelcontextprotocol/ui` under
 `capabilities.extensions` and serves everything the extension contributes.
 
-`Apps` is the built-in reference extension, and it gets its own page: **[MCP Apps](apps.md)**.
+Two built-in reference extensions ship with the SDK, and each gets its own page:
+**[MCP Apps](apps.md)** and **[Tasks](tasks.md)**.
 
 !!! note
     Extensions are fixed at construction. There is no `add_extension` to call later:
diff --git a/docs/advanced/tasks.md b/docs/advanced/tasks.md
@@ -0,0 +1,143 @@
+# Tasks
+
+A **task** is a `tools/call` answered by reference: instead of the `CallToolResult`,
+the server returns a `CreateTaskResult` carrying a task id, and the client fetches
+the outcome with `tasks/get`. That is
+[SEP-2663](https://modelcontextprotocol.io/seps/2663-tasks-extension.md), and the SDK
+ships it as the built-in `Tasks` extension (`io.modelcontextprotocol/tasks`).
+If [Extensions](extensions.md) are new to you, skim that page first. One minute,
+then come back.
+
+## Opting in, both sides
+
+```python title="server.py" hl_lines="5 15 16"
+--8<-- "docs_src/tasks/tutorial001.py"
+```
+
+* `extensions=[Tasks()]`: the server advertises `io.modelcontextprotocol/tasks`
+  under `capabilities.extensions` and serves `tasks/get`, `tasks/update`, and
+  `tasks/cancel`.
+* `Client(mcp, extensions={EXTENSION_ID: {}})`: the client declares the extension
+  back. Only a declaring client's `tools/call` may be answered with a task.
+* `client.call_tool(...)` does not change. When the answer comes back as a
+  `CreateTaskResult`, the client polls `tasks/get` — honoring the server's
+  `pollIntervalMs` hint, one second between polls in its absence — and surfaces
+  only the final `CallToolResult`. A `failed` task raises the typed
+  `TaskFailedError` carrying the inlined JSON-RPC error; a `cancelled` one raises
+  `TaskCancelledError`.
+
+Degradation is built in. A modern client that does not declare the extension is
+never augmented: it keeps getting plain `CallToolResult`s. And a legacy
+(2025-11-25) connection cannot negotiate the extension at all — the capability
+rides `server/discover`, which a legacy handshake doesn't have — so for that
+client the feature does not exist. Your tools don't change either way.
+
+## The server decides
+
+Augmentation is the server's call, per request: the client's declaration is
+permission, not a trigger. `Tasks()` augments every call from a declaring client;
+pass `augment=` to be choosier:
+
+```python title="server.py" hl_lines="9-10 13"
+--8<-- "docs_src/tasks/tutorial002.py"
+```
+
+* `augment` sees the validated `CallToolRequestParams` for each call. Return
+  `False` and the call passes through untouched, exactly as for a non-declaring
+  client — errors included. Here `transcode` becomes a task; `ping` never does.
+* `default_ttl_ms` bounds retention. It is stamped on the wire as `ttlMs`, and the
+  record is dropped once the deadline passes. The default `None` retains records
+  for the store's lifetime.
+* `clock` (not shown) injects the source of time behind the wire timestamps and
+  TTL deadlines. Inject a fixed clock for deterministic tests.
+
+## Where tasks live
+
+Records persist through a `TaskStore`, a two-method protocol:
+
+```python
+class TaskStore(Protocol):
+    async def put(self, record: TaskRecord) -> None: ...
+    async def get(self, task_id: str) -> TaskRecord | None: ...
+```
+
+The default `InMemoryTaskStore` is **per-process**: right for stdio servers and
+single-process development, wrong for a multi-worker HTTP deployment. SEP-2663
+requires a task to be durably recorded before its `CreateTaskResult` is returned,
+and a poll routed to another worker must find it — back `Tasks(store=...)` with
+shared storage there. `get` returning `None` is the whole expiry contract: unknown
+and expired ids look identical, and both answer `-32602` on the wire.
+
+Task ids are unguessable bearer capabilities (at least 128 bits of entropy): any
+caller presenting a valid id may poll the task, which is what lets a reconnecting
+client resume. Need stricter scoping or audited access? That is a custom store's
+job.
+
+## What execution actually looks like
+
+In this SDK the tool still runs **inline**, to completion, inside the `tools/call`
+request. A task is therefore born terminal:
+
+* The tool produced a result → a `completed` task, with the `CallToolResult`
+  inlined on `tasks/get`. A result with `isError: true` is still `completed`;
+  tool-level failure is a result, not a protocol error.
+* The call raised a JSON-RPC error (an `MCPError`) → a `failed` task, with the
+  error inlined on `tasks/get`. The declaring client receives a failed
+  `CreateTaskResult` instead of the JSON-RPC error, and the transparent driver
+  turns it into `TaskFailedError`.
+* `tasks/cancel` and `tasks/update` acknowledge and change nothing: cancellation
+  is cooperative in SEP-2663, and here the work has always finished before a
+  `tasks/*` request can arrive.
+
+A [multi-round-trip](multi-round-trip.md) interim (`input_required`) passes through
+un-augmented: the exchange resolves on the original `tools/call`, and only the leg
+that produces the final result becomes a task.
+
+What augmentation buys today is the wire shape and the retention: the result of an
+expensive call outlives the request that computed it, fetchable by id for `ttlMs`.
+Background execution is on the roadmap (below).
+
+## Driving the task yourself
+
+The transparent flow is a convenience, not a requirement. Drop one layer to get the
+`CreateTaskResult` and poll on your own schedule:
+
+```python title="client.py" hl_lines="21 26-27"
+--8<-- "docs_src/tasks/tutorial003.py"
+```
+
+* `session.call_tool(..., allow_create_task=True)` returns the typed
+  `CreateTaskResult` instead of polling. Without the flag, an unexpected
+  `CreateTaskResult` raises `RuntimeError` rather than leaking the widened union
+  into code that expected a `CallToolResult`.
+* The `mcp.shared.tasks` wrappers — `GetTaskRequest`, `UpdateTaskRequest`,
+  `CancelTaskRequest` — drive the `tasks/*` methods over `session.send_request`,
+  and `GetTaskResult` parses the snapshot: `result` is set on a `completed` task,
+  `error` on a `failed` one, never both.
+
+## Who sees what
+
+| Caller | `tools/call` | `tasks/*` |
+|---|---|---|
+| Declaring 2026-07-28 client | may be augmented into a task | served |
+| Non-declaring 2026-07-28 client | plain `CallToolResult`, always | `-32021` missing required client capability |
+| Legacy (2025-11-25) connection | plain `CallToolResult`, always | `-32601` method not found |
+
+The split on `tasks/*` is deliberate. A modern client could fix its declaration, so
+it gets the capability error with the machine-readable `requiredCapabilities`
+payload; a legacy client could not, so for it the methods simply don't exist. A
+declaring client naming an unknown — or expired — task id gets `-32602` (invalid
+params).
+
+Over Streamable HTTP, every `tasks/*` request carries the `Mcp-Name: <taskId>`
+routing header (SEP-2663 via SEP-2243) so intermediaries can route the poll to the
+instance holding the task's state. The SDK stamps it client-side and validates it
+server-side; you never touch it.
+
+## Roadmap
+
+This is the core SEP-2663 surface. Background execution (tasks created `working`
+and resolved later), the in-task `input_required` loop over `tasks/update`, and
+`notifications/tasks` over `subscriptions/listen` build on it as planned
+follow-ups — each needs deeper SDK plumbing, and the wire contract above is
+already shaped for them.
diff --git a/docs/migration.md b/docs/migration.md
@@ -478,10 +478,11 @@ Two reference extensions ship in their own modules:
   `TaskFailedError`/`TaskCancelledError`. Manual driving stays available —
   `client.session.call_tool(..., allow_create_task=True)` returns the typed
   `CreateTaskResult`, and the `mcp.shared.tasks` request wrappers drive
-  `tasks/get`/`tasks/update`/`tasks/cancel` over `session.send_request`. This
-  is the core SEP-2663 surface; background execution (`working` tasks), the
-  in-task `input_required` loop over `tasks/update`, `notifications/tasks`,
-  and task routing headers are deferred.
+  `tasks/get`/`tasks/update`/`tasks/cancel` over `session.send_request`, with
+  the `Mcp-Name` routing header stamped automatically over Streamable HTTP.
+  This is the core SEP-2663 surface — see [Tasks](advanced/tasks.md);
+  background execution (`working` tasks), the in-task `input_required` loop
+  over `tasks/update`, and `notifications/tasks` are deferred.
 
 Extension methods are strictly additive: a `MethodBinding` cannot name a
 spec-defined request method, and registering one whose method collides with
diff --git a/docs_src/tasks/__init__.py b/docs_src/tasks/__init__.py
diff --git a/docs_src/tasks/tutorial001.py b/docs_src/tasks/tutorial001.py
@@ -0,0 +1,18 @@
+from mcp import Client
+from mcp.server.mcpserver import MCPServer
+from mcp.server.tasks import EXTENSION_ID, Tasks
+
+mcp = MCPServer("bakery", extensions=[Tasks()])
+
+
+@mcp.tool()
+def bake(flavor: str) -> str:
+    """Bake a cake."""
+    return f"One {flavor} cake, ready."
+
+
+async def main() -> None:
+    async with Client(mcp, extensions={EXTENSION_ID: {}}) as client:
+        result = await client.call_tool("bake", {"flavor": "lemon"})
+        print(result.content)
+        # [TextContent(text='One lemon cake, ready.')]
diff --git a/docs_src/tasks/tutorial002.py b/docs_src/tasks/tutorial002.py
@@ -0,0 +1,25 @@
+from mcp_types import CallToolRequestParams
+
+from mcp.server.mcpserver import MCPServer
+from mcp.server.tasks import Tasks
+
+SLOW_TOOLS = {"transcode"}
+
+
+def augment(params: CallToolRequestParams) -> bool:
+    return params.name in SLOW_TOOLS
+
+
+mcp = MCPServer("studio", extensions=[Tasks(augment=augment, default_ttl_ms=60_000)])
+
+
+@mcp.tool()
+def transcode(clip: str) -> str:
+    """Transcode a clip to the house format."""
+    return f"{clip} transcoded."
+
+
+@mcp.tool()
+def ping() -> str:
+    """Liveness probe."""
+    return "pong"
diff --git a/docs_src/tasks/tutorial003.py b/docs_src/tasks/tutorial003.py
@@ -0,0 +1,30 @@
+from typing import cast
+
+import mcp_types as types
+
+from mcp import Client
+from mcp.server.mcpserver import MCPServer
+from mcp.server.tasks import EXTENSION_ID, CreateTaskResult, Tasks
+from mcp.shared.tasks import GetTaskRequest, GetTaskRequestParams, GetTaskResult
+
+mcp = MCPServer("bakery", extensions=[Tasks()])
+
+
+@mcp.tool()
+def bake(flavor: str) -> str:
+    """Bake a cake."""
+    return f"One {flavor} cake, ready."
+
+
+async def main() -> None:
+    async with Client(mcp, extensions={EXTENSION_ID: {}}) as client:
+        created = await client.session.call_tool("bake", {"flavor": "mocha"}, allow_create_task=True)
+        assert isinstance(created, CreateTaskResult)
+        print(created.status)
+        # completed
+
+        request = GetTaskRequest(params=GetTaskRequestParams(task_id=created.task_id))
+        polled = await client.session.send_request(cast("types.ClientRequest", request), GetTaskResult)
+        assert polled.result is not None
+        print(polled.result["content"])
+        # [{'text': 'One mocha cake, ready.', 'type': 'text'}]
diff --git a/mkdocs.yml b/mkdocs.yml
@@ -47,6 +47,7 @@ nav:
       - Middleware: advanced/middleware.md
       - Extensions: advanced/extensions.md
       - MCP Apps: advanced/apps.md
+      - Tasks: advanced/tasks.md
       - OpenTelemetry: advanced/opentelemetry.md
       - Authorization: advanced/authorization.md
       - OAuth clients: advanced/oauth-clients.md
diff --git a/tests/docs_src/test_tasks.py b/tests/docs_src/test_tasks.py
