fix: don't send log notification on transport error (#2257)

Author: maxisbey

src/mcp/server/lowlevel/server.py

@@ -414,11 +414,6 @@ async def _handle_message(
                         )
                 case Exception():
                     logger.error(f"Received exception from stream: {message}")
-                    await session.send_log_message(
-                        level="error",
-                        data="Internal Server Error",
-                        logger="mcp.server.exception_handler",
-                    )
                     if raise_exceptions:
                         raise message
                 case _:

src/mcp/shared/session.py

@@ -334,7 +334,7 @@ async def _receive_loop(self) -> None:
         async with self._read_stream, self._write_stream:
             try:
                 async for message in self._read_stream:
-                    if isinstance(message, Exception):  # pragma: no cover
+                    if isinstance(message, Exception):
                         await self._handle_incoming(message)
                     elif isinstance(message.message, JSONRPCRequest):
                         try:

tests/server/test_lowlevel_exception_handling.py

@@ -1,55 +1,42 @@
 from unittest.mock import AsyncMock, Mock
 
+import anyio
 import pytest
 
 from mcp import types
 from mcp.server.lowlevel.server import Server
 from mcp.server.session import ServerSession
+from mcp.shared.message import SessionMessage
 from mcp.shared.session import RequestResponder
 
 
 @pytest.mark.anyio
 async def test_exception_handling_with_raise_exceptions_true():
-    """Test that exceptions are re-raised when raise_exceptions=True"""
+    """Transport exceptions are re-raised when raise_exceptions=True."""
     server = Server("test-server")
     session = Mock(spec=ServerSession)
-    session.send_log_message = AsyncMock()
 
     test_exception = RuntimeError("Test error")
 
     with pytest.raises(RuntimeError, match="Test error"):
         await server._handle_message(test_exception, session, {}, raise_exceptions=True)
 
-    session.send_log_message.assert_called_once()
-
 
 @pytest.mark.anyio
-@pytest.mark.parametrize(
-    "exception_class,message",
-    [
-        (ValueError, "Test validation error"),
-        (RuntimeError, "Test runtime error"),
-        (KeyError, "Test key error"),
-        (Exception, "Basic error"),
-    ],
-)
-async def test_exception_handling_with_raise_exceptions_false(exception_class: type[Exception], message: str):
-    """Test that exceptions are logged when raise_exceptions=False"""
+async def test_exception_handling_with_raise_exceptions_false():
+    """Transport exceptions are logged locally but not sent to the client.
+
+    The transport that reported the error is likely broken; writing back
+    through it races with stream closure (#1967, #2064). The TypeScript,
+    Go, and C# SDKs all log locally only.
+    """
     server = Server("test-server")
     session = Mock(spec=ServerSession)
     session.send_log_message = AsyncMock()
 
-    test_exception = exception_class(message)
-
-    await server._handle_message(test_exception, session, {}, raise_exceptions=False)
-
-    # Should send log message
-    session.send_log_message.assert_called_once()
-    call_args = session.send_log_message.call_args
+    await server._handle_message(RuntimeError("Test error"), session, {}, raise_exceptions=False)
 
-    assert call_args.kwargs["level"] == "error"
-    assert call_args.kwargs["data"] == "Internal Server Error"
-    assert call_args.kwargs["logger"] == "mcp.server.exception_handler"
+    session.send_log_message.assert_not_called()
 
 
 @pytest.mark.anyio
@@ -72,3 +59,48 @@ async def test_normal_message_handling_not_affected():
 
     # Verify _handle_request was called
     server._handle_request.assert_called_once()
+
+
+@pytest.mark.anyio
+async def test_server_run_exits_cleanly_when_transport_yields_exception_then_closes():
+    """Regression test for #1967 / #2064.
+
+    Exercises the real Server.run() path with real memory streams, reproducing
+    what happens in stateless streamable HTTP when a POST handler throws:
+
+    1. Transport yields an Exception into the read stream
+       (streamable_http.py does this in its broad POST-handler except).
+    2. Transport closes the read stream (terminate() in stateless mode).
+    3. _receive_loop exits its `async with read_stream, write_stream:` block,
+       closing the write stream.
+    4. Meanwhile _handle_message(exc) was spawned via tg.start_soon and runs
+       after the write stream is closed.
+
+    Before the fix, _handle_message tried to send_log_message through the
+    closed write stream, raising ClosedResourceError inside the TaskGroup
+    and crashing server.run(). After the fix, it only logs locally.
+    """
+    server = Server("test-server")
+
+    read_send, read_recv = anyio.create_memory_object_stream[SessionMessage | Exception](1)
+    # Zero-buffer on the write stream forces send() to block until received.
+    # With no receiver, a send() sits blocked until _receive_loop exits its
+    # `async with self._read_stream, self._write_stream:` block and closes the
+    # stream, at which point the blocked send raises ClosedResourceError.
+    # This deterministically reproduces the race without sleeps.
+    write_send, write_recv = anyio.create_memory_object_stream[SessionMessage](0)
+
+    # What the streamable HTTP transport does: push the exception, then close.
+    read_send.send_nowait(RuntimeError("simulated transport error"))
+    read_send.close()
+
+    with anyio.fail_after(5):
+        # stateless=True so server.run doesn't wait for initialize handshake.
+        # Before this fix, this raised ExceptionGroup(ClosedResourceError).
+        await server.run(read_recv, write_send, server.create_initialization_options(), stateless=True)
+
+    # write_send was closed inside _receive_loop's `async with`; receive_nowait
+    # raises EndOfStream iff the buffer is empty (i.e., server wrote nothing).
+    with pytest.raises(anyio.EndOfStream):
+        write_recv.receive_nowait()
+    write_recv.close()