fix(server): terminate streamable HTTP sessions on manager shutdown by baiyuxi930826 · Pull Request #3218 · modelcontextprotocol/python-sdk

baiyuxi930826 · 2026-07-30T02:19:21Z

Summary

Fixes #2150 on current main.

When shutting down a Streamable HTTP server (CTRL+C / lifespan exit) with live SSE clients, uvicorn logs:

ERROR: ASGI callable returned without completing response.

Root causes (still present on `main` @ `a4f4ccd`)

StreamableHTTPSessionManager.run() finally cancelled the task group and cleared _server_instances without await transport.terminate() on active sessions.
StreamableHTTPServerTransport.terminate() closed request/read/write streams but left _sse_stream_writers open, so EventSourceResponse never completed.

Fix

Manager shutdown: terminate each non-terminated transport before cancelling the task group.
terminate(): close all SSE writers first via close_sse_stream (covers POST request streams, GET standalone, and provisional replay keys).
Register the GET standalone SSE writer immediately; use a provisional writer key during event replay so mid-replay shutdown can still close the ASGI response.

Relation to other work

Earlier Atlas PR fix(server): terminate streamable HTTP sessions on manager shutdown #3125 landed the same approach with tests, then was closed unmerged during v2 backlog cleanup with a note to track via fix: terminate active StreamableHTTP sessions during shutdown #2253 and reopen if still relevant on main.
fix: terminate active StreamableHTTP sessions during shutdown #2253 targets v1.x, has no regression tests, and is stale (last update 2026-04-25). This PR is the main + tests line maintainers invited.
Happy to close/adjust if maintainers prefer a different landing path.

Verification

uv run pytest tests/server/test_streamable_http_manager.py::test_terminate_closes_active_sse_stream_writers \
  tests/server/test_streamable_http_manager.py::test_manager_shutdown_terminates_active_sessions \
  tests/server/test_streamable_http_manager.py::test_terminate_closes_standalone_get_sse_writer_when_registered \
  tests/server/test_streamable_http_manager.py::test_terminate_closes_provisional_replay_sse_writer -q
# 4 passed

uv run pytest tests/server/test_streamable_http_manager.py -q
# 34 passed

Test plan

New unit regressions for terminate + manager shutdown
Full test_streamable_http_manager.py suite green locally (34)
CI on this PR

Why: On CTRL+C / lifespan exit with live SSE clients, StreamableHTTPSessionManager cancelled the task group without terminating transports, and terminate() left _sse_stream_writers open — uvicorn then logs "ASGI callable returned without completing response" (modelcontextprotocol#2150). - Manager run() finally: await terminate() on active transports before cancel - terminate(): close all SSE writers first (incl. GET + provisional replay keys) - Register GET standalone writer immediately; provisional key during replay - Regression tests for writer close + manager shutdown path Rebased onto current main after modelcontextprotocol#3125 was closed unmerged during v2 backlog cleanup (maintainers pointed at modelcontextprotocol#2253 on v1.x; this targets main with tests). Signed-off-by: Atlas/luccc-grok-4.5 <atlas@agent-studio.local>

cubic-dev-ai

2 issues found across 3 files

Prompt for AI agents (unresolved issues)


Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/mcp/server/streamable_http_manager.py">

<violation number="1" location="src/mcp/server/streamable_http_manager.py:158">
P1: A session created while this loop awaits an earlier `terminate()` is omitted from the snapshot and reaches task-group cancellation with its SSE response still open. Serialize creation against shutdown (or set/check a shutdown flag before creating transports) so every accepted session is terminated before cancellation.</violation>
</file>

<file name="src/mcp/server/streamable_http.py">

<violation number="1" location="src/mcp/server/streamable_http.py:738">
P1: Concurrent standalone GETs can still both pass the one-stream check because it only observes `_request_streams`, which is populated later by the writer task. The second request overwrites this newly registered writer, and either response's cleanup can then remove the other registration; that leaves a live SSE response that `terminate()` cannot close. Consider making the duplicate-stream guard use the writer registration (or reserving the GET key atomically) and only removing a mapping when it still refers to that handler's writer.</violation>
</file>

_{Reply with feedback, questions, or to request a fix.

Re-trigger cubic}

cubic-dev-ai · 2026-07-30T02:27:50Z

                logger.info("StreamableHTTP session manager shutting down")
+                # Terminate active transports before cancelling the task group so
+                # in-flight SSE responses can complete cleanly (issue #2150).
+                active_transports = list(self._server_instances.values())


P1: A session created while this loop awaits an earlier terminate() is omitted from the snapshot and reaches task-group cancellation with its SSE response still open. Serialize creation against shutdown (or set/check a shutdown flag before creating transports) so every accepted session is terminated before cancellation.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/mcp/server/streamable_http_manager.py, line 158: <comment>A session created while this loop awaits an earlier `terminate()` is omitted from the snapshot and reaches task-group cancellation with its SSE response still open. Serialize creation against shutdown (or set/check a shutdown flag before creating transports) so every accepted session is terminated before cancellation.</comment> <file context> @@ -153,6 +153,18 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]: logger.info("StreamableHTTP session manager shutting down") + # Terminate active transports before cancelling the task group so + # in-flight SSE responses can complete cleanly (issue #2150). + active_transports = list(self._server_instances.values()) + for transport in active_transports: + if not transport.is_terminated: # pragma: no branch </file context>

cubic-dev-ai · 2026-07-30T02:27:50Z

        # Create SSE stream
        sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[SSEEvent](0)
+        # Register immediately so session terminate() can close this ASGI response.
+        self._sse_stream_writers[GET_STREAM_KEY] = sse_stream_writer


P1: Concurrent standalone GETs can still both pass the one-stream check because it only observes _request_streams, which is populated later by the writer task. The second request overwrites this newly registered writer, and either response's cleanup can then remove the other registration; that leaves a live SSE response that terminate() cannot close. Consider making the duplicate-stream guard use the writer registration (or reserving the GET key atomically) and only removing a mapping when it still refers to that handler's writer.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/mcp/server/streamable_http.py, line 738: <comment>Concurrent standalone GETs can still both pass the one-stream check because it only observes `_request_streams`, which is populated later by the writer task. The second request overwrites this newly registered writer, and either response's cleanup can then remove the other registration; that leaves a live SSE response that `terminate()` cannot close. Consider making the duplicate-stream guard use the writer registration (or reserving the GET key atomically) and only removing a mapping when it still refers to that handler's writer.</comment> <file context> @@ -734,6 +734,8 @@ async def _handle_get_request(self, request: Request, send: Send) -> None: # Create SSE stream sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[SSEEvent](0) + # Register immediately so session terminate() can close this ASGI response. + self._sse_stream_writers[GET_STREAM_KEY] = sse_stream_writer async def standalone_sse_writer(): </file context>

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fix(server): terminate streamable HTTP sessions on manager shutdown - #3218

fix(server): terminate streamable HTTP sessions on manager shutdown#3218
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baiyuxi930826:atlas/fix-2150-shutdown-sessions-main

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baiyuxi930826 commented Jul 30, 2026

Summary

Root causes (still present on main @ a4f4ccd)

Fix

Relation to other work

Verification

Test plan

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Root causes (still present on `main` @ `a4f4ccd`)