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An SDK MCP tool handler that raises an empty-message exception (e.g. 'raise TimeoutError()') produced an error result whose text was an empty string, so the model could see that the tool failed but not why. Fall back to the exception type name, matching the existing fallback in Query._handle_sdk_mcp_request, so create_sdk_mcp_server's 'isError results the model can read' note holds.
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The empty-message MCP tool error fallback is green through |
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When an SDK MCP tool raises an exception with an empty message, such as
TimeoutError(), its error result contains an empty text block. The model receivesisError: truewithout any explanation.Fall back to the exception type name when
str(e)is empty, matchingQuery._handle_sdk_mcp_request. Exceptions with a message keep their existing text.The regression calls the tool through
Query, the in-memory JSON-RPC connection and the real MCP server. Against main1ea8cb74, it fails on both asyncio and Trio:[''] != ['TimeoutError']. With this one-line fix,python -m pytest tests/test_sdk_mcp_integration.py -qpasses 144 tests with both MCP 2.3.0 and the supported 1.23.0 floor (Python 3.13.3).Also passed
ruff check src/ tests/ scripts/andruff format --check src/ tests/ scripts/.mypy src/ scripts/passed with both MCP versions. No live Claude API or CLI was used. Upstream CI has not run yet.