fix: surface stateful HTTP session crash cause

src/mcp/server/streamable_http.py

@@ -10,7 +10,7 @@
 import re
 from abc import ABC, abstractmethod
 from collections.abc import AsyncGenerator, Awaitable, Callable
-from contextlib import asynccontextmanager
+from contextlib import asynccontextmanager, suppress
 from dataclasses import dataclass
 from http import HTTPStatus
 from typing import Any
@@ -171,6 +171,7 @@ def __init__(
         ] = {}
         self._sse_stream_writers: dict[RequestId, MemoryObjectSendStream[dict[str, str]]] = {}
         self._terminated = False
+        self._session_run_error: BaseException | None = None
         # Idle timeout cancel scope; managed by the session manager.
         self.idle_scope: anyio.CancelScope | None = None
 
@@ -179,6 +180,16 @@ def is_terminated(self) -> bool:
         """Check if this transport has been explicitly terminated."""
         return self._terminated
 
+    def note_session_run_error(self, exc: BaseException) -> None:
+        self._session_run_error = exc
+
+    def _post_error_message(self, err: BaseException) -> str:
+        display_error = self._session_run_error or err
+        display_error_text = str(display_error) or type(display_error).__name__
+        if display_error is not err and str(display_error):
+            display_error_text = f"{type(display_error).__name__}: {display_error_text}"
+        return f"Error handling POST request: {display_error_text}"
+
     def close_sse_stream(self, request_id: RequestId) -> None:
         """Close SSE connection for a specific request without terminating the stream.
 
@@ -363,6 +374,10 @@ async def _clean_up_memory_streams(self, request_id: RequestId) -> None:
                 # Remove the request stream from the mapping
                 self._request_streams.pop(request_id, None)
 
+    async def _clean_up_post_request_stream(self, request_id: RequestId | None) -> None:
+        if request_id is not None:
+            await self._clean_up_memory_streams(request_id)
+
     async def handle_request(self, scope: Scope, receive: Receive, send: Send) -> None:
         """Application entry point that handles all HTTP requests."""
         request = Request(scope, receive)
@@ -443,6 +458,7 @@ 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.")
+        request_id: RequestId | None = None
         try:
             # Validate Accept header
             if not await self._validate_accept_header(request, scope, send):
@@ -637,15 +653,16 @@ async def sse_writer():
 
         except Exception as err:
             logger.exception("Error handling POST request")
+            await self._clean_up_post_request_stream(request_id)
             response = self._create_error_response(
-                f"Error handling POST request: {err}",
+                self._post_error_message(err),
                 HTTPStatus.INTERNAL_SERVER_ERROR,
                 INTERNAL_ERROR,
             )
             await response(scope, receive, send)
-            if writer:  # pragma: no cover
+            with suppress(anyio.BrokenResourceError, anyio.ClosedResourceError):
                 await writer.send(Exception(err))
-            return  # pragma: no cover
+            return
 
     async def _handle_get_request(self, request: Request, send: Send) -> None:
         """Handle GET request to establish SSE.

src/mcp/server/streamable_http_manager.py

@@ -274,7 +274,8 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                                 self._server_instances.pop(http_transport.mcp_session_id, None)
                                 self._session_owners.pop(http_transport.mcp_session_id, None)
                                 await http_transport.terminate()
-                        except Exception:
+                        except Exception as exc:
+                            http_transport.note_session_run_error(exc)
                             logger.exception(f"Session {http_transport.mcp_session_id} crashed")
                         finally:
                             if (  # pragma: no branch

tests/server/test_streamable_http_manager.py

@@ -209,6 +209,57 @@ async def mock_receive():  # pragma: no cover
     assert not manager._server_instances, "No sessions should be tracked after the only session crashes"
 
 
+def test_post_error_message_prefers_session_run_error():
+    transport = StreamableHTTPServerTransport(mcp_session_id="session-id", is_json_response_enabled=True)
+
+    assert transport._post_error_message(anyio.ClosedResourceError()) == (
+        "Error handling POST request: ClosedResourceError"
+    )
+
+    transport.note_session_run_error(RuntimeError("BOOM-distinctive-root-cause"))
+    assert transport._post_error_message(anyio.ClosedResourceError()) == (
+        "Error handling POST request: RuntimeError: BOOM-distinctive-root-cause"
+    )
+
+
+@pytest.mark.anyio
+async def test_stateful_json_response_includes_session_crash_cause():
+    app = Server("test-crash-cause")
+    app.run = AsyncMock(side_effect=RuntimeError("BOOM-distinctive-root-cause"))
+    manager = StreamableHTTPSessionManager(app=app, json_response=True)
+
+    sent_messages: list[Message] = []
+    response_body = b""
+
+    async def mock_send(message: Message) -> None:
+        nonlocal response_body
+        sent_messages.append(message)
+        if message["type"] == "http.response.body":
+            response_body += message.get("body", b"")
+
+    async def mock_receive() -> Message:
+        body = {
+            "jsonrpc": "2.0",
+            "id": 1,
+            "method": "initialize",
+            "params": {
+                "protocolVersion": "2025-06-18",
+                "capabilities": {},
+                "clientInfo": {"name": "test-client", "version": "1.0"},
+            },
+        }
+        return {"type": "http.request", "body": json.dumps(body).encode(), "more_body": False}
+
+    async with manager.run():
+        await manager.handle_request(_request_scope(), mock_receive, mock_send)
+        response_start = next(msg for msg in sent_messages if msg["type"] == "http.response.start")
+        assert response_start["status"] == 500
+
+        error_data = json.loads(response_body)
+        assert error_data["error"]["code"] == -32603
+        assert "RuntimeError: BOOM-distinctive-root-cause" in error_data["error"]["message"]
+
+
 @pytest.mark.anyio
 async def test_stateless_requests_memory_cleanup():
     """Test that stateless requests actually clean up resources using real transports."""