diff --git a/src/mcp/server/lowlevel/server.py b/src/mcp/server/lowlevel/server.py index aee644040..0ba7b2c0f 100644 --- a/src/mcp/server/lowlevel/server.py +++ b/src/mcp/server/lowlevel/server.py @@ -417,11 +417,14 @@ 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", - ) + try: + await session.send_log_message( + level="error", + data="Internal Server Error", + logger="mcp.server.exception_handler", + ) + except (anyio.ClosedResourceError, anyio.BrokenResourceError): + logger.debug("Could not send error log to client: connection already closed") if raise_exceptions: raise message case _: diff --git a/tests/server/test_lowlevel_exception_handling.py b/tests/server/test_lowlevel_exception_handling.py index 848b35b29..f2fd51c10 100644 --- a/tests/server/test_lowlevel_exception_handling.py +++ b/tests/server/test_lowlevel_exception_handling.py @@ -72,3 +72,52 @@ async def test_normal_message_handling_not_affected(): # Verify _handle_request was called server._handle_request.assert_called_once() + + +@pytest.mark.anyio +@pytest.mark.parametrize( + "error_class", + [ + pytest.param("ClosedResourceError", id="closed"), + pytest.param("BrokenResourceError", id="broken"), + ], +) +async def test_exception_handling_tolerates_closed_write_stream(error_class: str): + """send_log_message failure on a disconnected client should not crash the server. + + Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/2064. + When a client disconnects during request handling, the error handler tries to + send a log message back. If the write stream is already closed, the resulting + ClosedResourceError/BrokenResourceError should be caught silently. + """ + import anyio + + server = Server("test-server") + session = Mock(spec=ServerSession) + + exc_cls = getattr(anyio, error_class) + session.send_log_message = AsyncMock(side_effect=exc_cls()) + + test_exception = RuntimeError("Client disconnected") + + # Should NOT raise — the ClosedResourceError from send_log_message is caught + await server._handle_message(test_exception, session, {}, raise_exceptions=False) + + # send_log_message was still attempted + session.send_log_message.assert_called_once() + + +@pytest.mark.anyio +async def test_exception_handling_reraises_with_closed_stream(): + """When raise_exceptions=True AND the write stream is closed, the original + exception should still be re-raised (not the ClosedResourceError).""" + import anyio + + server = Server("test-server") + session = Mock(spec=ServerSession) + session.send_log_message = AsyncMock(side_effect=anyio.ClosedResourceError()) + + test_exception = RuntimeError("Original error") + + with pytest.raises(RuntimeError, match="Original error"): + await server._handle_message(test_exception, session, {}, raise_exceptions=True)