Simplify catch-all case in client session match

Replace NotImplementedError with pass since task requests are handled
earlier by _task_handlers. The catch-all satisfies pyright's exhaustiveness
check while making it clear these cases are intentionally handled elsewhere.

Author: maxisbey

src/mcp/client/experimental/task_handlers.py

@@ -15,6 +15,8 @@
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, Protocol
 
+from pydantic import TypeAdapter
+
 import mcp.types as types
 from mcp.shared.context import RequestContext
 from mcp.shared.session import RequestResponder
@@ -111,11 +113,6 @@ async def __call__(
     ) -> types.CreateTaskResult | types.ErrorData: ...  # pragma: no branch
 
 
-# =============================================================================
-# Default Handlers (return "not supported" errors)
-# =============================================================================
-
-
 async def default_get_task_handler(
     context: RequestContext["ClientSession", Any],
     params: types.GetTaskRequestParams,
@@ -259,8 +256,6 @@ async def handle_request(
 
         Call handles_request() first to check if this handler can handle the request.
         """
-        from pydantic import TypeAdapter
-
         client_response_type: TypeAdapter[types.ClientResult | types.ErrorData] = TypeAdapter(
             types.ClientResult | types.ErrorData
         )

src/mcp/client/session.py

@@ -582,8 +582,9 @@ async def _received_request(self, responder: RequestResponder[types.ServerReques
                 with responder:
                     return await responder.respond(types.ClientResult(root=types.EmptyResult()))
 
-            case _:  # pragma: no cover
-                raise NotImplementedError()
+            case _:
+                pass  # Task requests handled above by _task_handlers
+
         return None
 
     async def _handle_incoming(

src/mcp/server/experimental/request_context.py

@@ -210,10 +210,8 @@ async def work(task: ServerTaskContext) -> CallToolResult:
         # Access task_group via TaskSupport - raises if not in run() context
         task_group = support.task_group
 
-        # Create the task
         task = await support.store.create_task(self.task_metadata, task_id)
 
-        # Build ServerTaskContext with full capabilities
         task_ctx = ServerTaskContext(
             task=task,
             store=support.store,
@@ -222,21 +220,17 @@ async def work(task: ServerTaskContext) -> CallToolResult:
             handler=support.handler,
         )
 
-        # Spawn the work
         async def execute() -> None:
             try:
                 result = await work(task_ctx)
-                # Auto-complete if work returns successfully and not already terminal
                 if not is_terminal(task_ctx.task.status):
                     await task_ctx.complete(result)
             except Exception as e:
-                # Auto-fail if not already terminal
                 if not is_terminal(task_ctx.task.status):
                     await task_ctx.fail(str(e))
 
         task_group.start_soon(execute)
 
-        # Build _meta if model_immediate_response is provided
         meta: dict[str, Any] | None = None
         if model_immediate_response is not None:
             meta = {MODEL_IMMEDIATE_RESPONSE_KEY: model_immediate_response}

src/mcp/server/experimental/task_context.py

@@ -241,11 +241,9 @@ async def elicit(
         )
         request_id: RequestId = request.id
 
-        # Create resolver and register with handler for response routing
         resolver: Resolver[dict[str, Any]] = Resolver()
         self._handler._pending_requests[request_id] = resolver  # pyright: ignore[reportPrivateUsage]
 
-        # Queue the request
         queued = QueuedMessage(
             type="request",
             message=request,
@@ -315,11 +313,9 @@ async def elicit_url(
         )
         request_id: RequestId = request.id
 
-        # Create resolver and register with handler for response routing
         resolver: Resolver[dict[str, Any]] = Resolver()
         self._handler._pending_requests[request_id] = resolver  # pyright: ignore[reportPrivateUsage]
 
-        # Queue the request
         queued = QueuedMessage(
             type="request",
             message=request,
@@ -408,11 +404,9 @@ async def create_message(
         )
         request_id: RequestId = request.id
 
-        # Create resolver and register with handler for response routing
         resolver: Resolver[dict[str, Any]] = Resolver()
         self._handler._pending_requests[request_id] = resolver  # pyright: ignore[reportPrivateUsage]
 
-        # Queue the request
         queued = QueuedMessage(
             type="request",
             message=request,
@@ -469,7 +463,6 @@ async def elicit_as_task(
         # Update status to input_required
         await self._store.update_task(self.task_id, status=TASK_STATUS_INPUT_REQUIRED)
 
-        # Build request WITH task field for task-augmented elicitation
         request = self._session._build_elicit_form_request(  # pyright: ignore[reportPrivateUsage]
             message=message,
             requestedSchema=requestedSchema,
@@ -478,11 +471,9 @@ async def elicit_as_task(
         )
         request_id: RequestId = request.id
 
-        # Create resolver and register with handler for response routing
         resolver: Resolver[dict[str, Any]] = Resolver()
         self._handler._pending_requests[request_id] = resolver  # pyright: ignore[reportPrivateUsage]
 
-        # Queue the request
         queued = QueuedMessage(
             type="request",
             message=request,
@@ -586,11 +577,9 @@ async def create_message_as_task(
         )
         request_id: RequestId = request.id
 
-        # Create resolver and register with handler for response routing
         resolver: Resolver[dict[str, Any]] = Resolver()
         self._handler._pending_requests[request_id] = resolver  # pyright: ignore[reportPrivateUsage]
 
-        # Queue the request
         queued = QueuedMessage(
             type="request",
             message=request,

src/mcp/server/experimental/task_result_handler.py

@@ -109,7 +109,6 @@ async def handle(
         task_id = request.params.taskId
 
         while True:
-            # Get fresh task state each iteration
             task = await self._store.get_task(task_id)
             if task is None:
                 raise McpError(
@@ -119,7 +118,6 @@ async def handle(
                     )
                 )
 
-            # Dequeue and send all pending messages
             await self._deliver_queued_messages(task_id, session, request_id)
 
             # If task is terminal, return result
@@ -131,9 +129,7 @@ async def handle(
                 related_task = RelatedTaskMetadata(taskId=task_id)
                 related_task_meta: dict[str, Any] = {RELATED_TASK_METADATA_KEY: related_task.model_dump(by_alias=True)}
                 if result is not None:
-                    # Copy result fields and add required metadata
                     result_data = result.model_dump(by_alias=True)
-                    # Merge with existing _meta if present
                     existing_meta: dict[str, Any] = result_data.get("_meta") or {}
                     result_data["_meta"] = {**existing_meta, **related_task_meta}
                     return GetTaskPayloadResult.model_validate(result_data)

src/mcp/server/session.py

@@ -50,6 +50,7 @@ async def handle_list_prompts(ctx: RequestContext) -> list[types.Prompt]:
 from mcp.server.models import InitializationOptions
 from mcp.server.validation import validate_sampling_tools, validate_tool_use_result_messages
 from mcp.shared.experimental.tasks.capabilities import check_tasks_capability
+from mcp.shared.experimental.tasks.helpers import RELATED_TASK_METADATA_KEY
 from mcp.shared.message import ServerMessageMetadata, SessionMessage
 from mcp.shared.response_router import ResponseRouter
 from mcp.shared.session import (
@@ -520,7 +521,9 @@ def _build_elicit_form_request(
             # Defensive: model_dump() never includes _meta, but guard against future changes
             if "_meta" not in params_data:  # pragma: no cover
                 params_data["_meta"] = {}
-            params_data["_meta"]["io.modelcontextprotocol/related-task"] = {"taskId": related_task_id}
+            params_data["_meta"][RELATED_TASK_METADATA_KEY] = types.RelatedTaskMetadata(
+                taskId=related_task_id
+            ).model_dump(by_alias=True)
 
         request_id = f"task-{related_task_id}-{id(params)}" if related_task_id else self._request_id
         if related_task_id is None:
@@ -563,7 +566,9 @@ def _build_elicit_url_request(
             # Defensive: model_dump() never includes _meta, but guard against future changes
             if "_meta" not in params_data:  # pragma: no cover
                 params_data["_meta"] = {}
-            params_data["_meta"]["io.modelcontextprotocol/related-task"] = {"taskId": related_task_id}
+            params_data["_meta"][RELATED_TASK_METADATA_KEY] = types.RelatedTaskMetadata(
+                taskId=related_task_id
+            ).model_dump(by_alias=True)
 
         request_id = f"task-{related_task_id}-{id(params)}" if related_task_id else self._request_id
         if related_task_id is None:
@@ -631,7 +636,9 @@ def _build_create_message_request(
             # Defensive: model_dump() never includes _meta, but guard against future changes
             if "_meta" not in params_data:  # pragma: no cover
                 params_data["_meta"] = {}
-            params_data["_meta"]["io.modelcontextprotocol/related-task"] = {"taskId": related_task_id}
+            params_data["_meta"][RELATED_TASK_METADATA_KEY] = types.RelatedTaskMetadata(
+                taskId=related_task_id
+            ).model_dump(by_alias=True)
 
         request_id = f"task-{related_task_id}-{id(params)}" if related_task_id else self._request_id
         if related_task_id is None:

tests/experimental/tasks/test_request_context.py

@@ -16,8 +16,6 @@
     ToolExecution,
 )
 
-# --- Experimental.is_task ---
-
 
 def test_is_task_true_when_metadata_present() -> None:
     exp = Experimental(task_metadata=TaskMetadata(ttl=60000))
@@ -29,9 +27,6 @@ def test_is_task_false_when_no_metadata() -> None:
     assert exp.is_task is False
 
 
-# --- Experimental.client_supports_tasks ---
-
-
 def test_client_supports_tasks_true() -> None:
     exp = Experimental(_client_capabilities=ClientCapabilities(tasks=ClientTasksCapability()))
     assert exp.client_supports_tasks is True
@@ -47,9 +42,6 @@ def test_client_supports_tasks_false_no_capabilities() -> None:
     assert exp.client_supports_tasks is False
 
 
-# --- Experimental.validate_task_mode ---
-
-
 def test_validate_task_mode_required_with_task_is_valid() -> None:
     exp = Experimental(task_metadata=TaskMetadata(ttl=60000))
     error = exp.validate_task_mode(TASK_REQUIRED, raise_error=False)
@@ -111,9 +103,6 @@ def test_validate_task_mode_optional_without_task_is_valid() -> None:
     assert error is None
 
 
-# --- Experimental.validate_for_tool ---
-
-
 def test_validate_for_tool_with_execution_required() -> None:
     exp = Experimental(task_metadata=None)
     tool = Tool(
@@ -152,9 +141,6 @@ def test_validate_for_tool_optional_with_task() -> None:
     assert error is None
 
 
-# --- Experimental.can_use_tool ---
-
-
 def test_can_use_tool_required_with_task_support() -> None:
     exp = Experimental(_client_capabilities=ClientCapabilities(tasks=ClientTasksCapability()))
     assert exp.can_use_tool(TASK_REQUIRED) is True