fix: handle ClosedResourceError when logging errors to disconnected clients

When a client disconnects mid-request, _handle_message catches the
resulting Exception and tries to send_log_message() back to the client.
Since the session write stream is already closed, this raises
ClosedResourceError (or BrokenResourceError), which is unhandled and
crashes the stateless session with an ExceptionGroup.

This is a different code path from PR #1384, which fixed the message
router loop. This bug is in the error recovery path:
catch exception → try to log it to client → write stream closed → crash.

Fix: catch ClosedResourceError and BrokenResourceError around the
send_log_message call in _handle_message, since failing to notify a
disconnected client is expected and harmless.

Fixes #2064

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/server/lowlevel/server.py

@@ -417,11 +417,16 @@ 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.warning(
+                            "Could not send error log to client: write stream already closed"
+                        )
                     if raise_exceptions:
                         raise message
                 case _:

tests/server/test_lowlevel_exception_handling.py

@@ -72,3 +72,49 @@ 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_exception_handling_tolerates_closed_write_stream():
+    """send_log_message should not crash when the client has already disconnected."""
+    import anyio
+
+    server = Server("test-server")
+    session = Mock(spec=ServerSession)
+    session.send_log_message = AsyncMock(side_effect=anyio.ClosedResourceError)
+
+    test_exception = RuntimeError("client went away")
+
+    # Must not raise — the ClosedResourceError should be caught internally
+    await server._handle_message(test_exception, session, {}, raise_exceptions=False)
+
+
+@pytest.mark.anyio
+async def test_exception_handling_tolerates_broken_write_stream():
+    """send_log_message should not crash when the write stream is broken."""
+    import anyio
+
+    server = Server("test-server")
+    session = Mock(spec=ServerSession)
+    session.send_log_message = AsyncMock(side_effect=anyio.BrokenResourceError)
+
+    test_exception = RuntimeError("client went away")
+
+    # Must not raise — the BrokenResourceError should be caught internally
+    await server._handle_message(test_exception, session, {}, raise_exceptions=False)
+
+
+@pytest.mark.anyio
+async def test_exception_handling_closed_stream_with_raise_exceptions():
+    """Even with raise_exceptions=True, ClosedResourceError from log should be tolerated."""
+    import anyio
+
+    server = Server("test-server")
+    session = Mock(spec=ServerSession)
+    session.send_log_message = AsyncMock(side_effect=anyio.ClosedResourceError)
+
+    test_exception = RuntimeError("client went away")
+
+    # raise_exceptions re-raises the *original* exception, not the ClosedResourceError
+    with pytest.raises(RuntimeError, match="client went away"):
+        await server._handle_message(test_exception, session, {}, raise_exceptions=True)