## Cause
StreamableHTTPSessionManager.run() creates the anyio task group used by _handle_request; mounting only the ASGI sub-app does not start that context unless the host app lifespan enters session_manager.run().
## Steps
1. Confirm error originates from streamable_http_manager handle path with task group None.
2. Add @contextlib.asynccontextmanager lifespan on the parent Starlette/FastAPI app.
3. Inside lifespan: async with mcp.session_manager.run(): yield
4. Mount mcp.streamable_http_app(...) on the desired path with transport options as needed.
5. Restart uvicorn and retry MCP requests against the mounted endpoint.
## Expected result
MCP requests are handled without RuntimeError; session manager task group remains active for the app lifetime.
## Misleading approaches
- Mounting streamable_http_app without a parent lifespan entering session_manager.run().
- Calling mcp.run(transport='streamable-http') concurrently with a separate mounted uvicorn app on another port (overlapping servers).
- Expecting Starlette to run mounted sub-app lifespan automatically for nested mounts.
## Unknowns
- Whether newer SDK releases improve error messaging beyond the RuntimeError string.
- Exact minimum SDK version shipping the updated mounting documentation example.
## Evidence claims
- (documented_platform_behavior) StreamableHTTPSessionManager raises RuntimeError 'Task group is not initialized. Make sure to use run().' when _task_group is None during request handling. —
- (documented_platform_behavior) run() is the async context manager that initializes the anyio task group for Streamable HTTP session management. —
- (documented_workaround) Official mounting example uses parent app lifespan with async with mcp.session_manager.run() while mounting streamable_http_app. —
- (external_report) GitHub issue #1484 documents mounted Streamable HTTP failing without session manager lifecycle context on existing ASGI servers. —
Proposed approach
- Problem id
- 28dd5c0c-eaa9-4c9f-89b2-19b5d3f8a5b0
- Proposed action
- Define a parent Starlette/FastAPI lifespan that wraps async with mcp.session_manager.run(): yield while the mounted streamable HTTP app is served, following the official streamable_http_basic_mounting.py example. Do not rely on the mounted sub-app lifespan alone.
- Applicability
- State
- partial
- Text
- MCP Python SDK FastMCP/Streamable HTTP when mcp.streamable_http_app() is mounted into an existing ASGI host (FastAPI/Starlette + uvicorn). Reported with mcp 1.17.0; applies whenever session_manager.run() is not active during requests.
- Limitations
- State
- partial
- Text
- Issue #1484 was closed via documentation clarification (PR referenced on issue); behavior depends on SDK version.,run() may only be called once per manager instance per source comments.,This research did not execute a live mounted server test.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- MCP Python SDK FastMCP/Streamable HTTP when mcp.streamable_http_app() is mounted into an existin
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