Skip 500 after completed POST response on closed writer

Notification POSTs send 202 Accepted before forwarding to the session writer. If the writer is closed, the resulting ClosedResourceError was answered with a second response (500), which Uvicorn rejects. Track whether the POST response was already sent and log-and-drop instead. Fixes #3651.

Author: dharaneesh-sys

src/mcp/server/streamable_http.py

@@ -560,6 +560,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
         writer = self._read_stream_writer
         if writer is None:  # pragma: no cover
             raise ValueError("No read stream writer available. Ensure connect() is called first.")
+        response_sent = False
         try:
             # Validate Accept header
             if not await self._validate_accept_header(request, scope, send):
@@ -623,6 +624,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                     HTTPStatus.ACCEPTED,
                 )
                 await response(scope, receive, send)
+                response_sent = True
 
                 # Process the message after sending the response
                 session_message = SessionMessage(message, metadata=self._message_metadata(request))
@@ -716,13 +718,19 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
 
         except Exception as err:
             logger.exception("Error handling POST request")
-            response = self._create_error_response(
-                "Error handling POST request",
-                HTTPStatus.INTERNAL_SERVER_ERROR,
-                INTERNAL_ERROR,
-            )
-            await response(scope, receive, send)
-            await writer.send(Exception(err))
+            if response_sent:
+                logger.debug("Not sending error response: POST response already sent")
+            else:
+                response = self._create_error_response(
+                    "Error handling POST request",
+                    HTTPStatus.INTERNAL_SERVER_ERROR,
+                    INTERNAL_ERROR,
+                )
+                await response(scope, receive, send)
+            try:
+                await writer.send(Exception(err))
+            except (anyio.ClosedResourceError, anyio.BrokenResourceError):
+                logger.debug("Writer closed while forwarding POST error; dropping exception")
             return
 
     async def _handle_get_request(self, request: Request, send: Send) -> None:

tests/server/test_streamable_http_router.py

@@ -1,5 +1,7 @@
 """Regression coverage for the StreamableHTTP per-session response router."""
 
+import logging
+
 import anyio
 import pytest
 from mcp_types import JSONRPCMessage, JSONRPCResponse
@@ -157,3 +159,39 @@ async def test_terminated_transport_answers_404() -> None:
 
     assert post.sent[0]["type"] == "http.response.start"
     assert post.sent[0]["status"] == 404
+
+
+@pytest.mark.anyio
+async def test_closed_writer_notification_post_sends_single_202(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """A notification POST answered 202 before writer.send fails must not send a second response (#3651).
+
+    The 202 completes the ASGI response, so the closed-writer error is logged
+    and dropped instead of answered with a 500 that Uvicorn rejects.
+    """
+    transport = StreamableHTTPServerTransport(mcp_session_id="repro-session")
+    async with transport.connect():
+        pass
+    post = _AsgiPost(
+        b'{"jsonrpc": "2.0", "method": "notifications/initialized"}',
+        [
+            (b"accept", b"application/json, text/event-stream"),
+            (b"content-type", b"application/json"),
+            (b"mcp-session-id", b"repro-session"),
+        ],
+    )
+    sent: list[Message] = []
+
+    async def strict_send(message: Message) -> None:
+        if message["type"] == "http.response.start" and any(m["type"] == "http.response.start" for m in sent):
+            raise RuntimeError("Unexpected ASGI message after completed")
+        sent.append(message)
+
+    with caplog.at_level(logging.ERROR, logger="mcp.server.streamable_http"):
+        await transport.handle_request(post.scope, post.receive, strict_send)
+
+    starts = [m for m in sent if m["type"] == "http.response.start"]
+    assert len(starts) == 1
+    assert starts[0]["status"] == 202
+    assert "Error handling POST request" in caplog.text