Title: fix: skip outputSchema validation when CallToolResult.is_error=True by RanjithHiremath · Pull Request #2508 · modelcontextprotocol/python-sdk · GitHub
Open Graph Title: fix: skip outputSchema validation when CallToolResult.is_error=True by RanjithHiremath · Pull Request #2508 · modelcontextprotocol/python-sdk
X Title: fix: skip outputSchema validation when CallToolResult.is_error=True by RanjithHiremath · Pull Request #2508 · modelcontextprotocol/python-sdk
Description: Resolves #2429. When a tool function with an output_schema returns CallToolResult(is_error=True), convert_result() was unconditionally calling output_model.model_validate(result.structured_content). For an error result, structured_content is None, so model_validate(None) raises a Pydantic ValidationError that replaces the tool's actual error message before it reaches the client. Skipping schema validation when is_error is true matches the resolution that landed in typescript-sdk#655. Motivation and Context Closes #2429. How Has This Been Tested? Added test_tool_call_result_with_is_error_skips_output_schema_validation in tests/server/mcpserver/test_func_metadata.py. The test registers a function with an inferred output schema, returns CallToolResult(content=[], is_error=True), and asserts convert_result() returns the result unchanged. Without the fix the test fails with pydantic.ValidationError (because model_validate(None) runs against a model with required fields); with the fix it passes. Verified locally: uv run --frozen ruff check src tests — clean uv run --frozen ruff format --check src tests — clean uv run --frozen pyright — 0 errors ./scripts/test — 100.00% coverage, strict-no-cover passes Breaking Changes None. Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update Checklist I have read the MCP Documentation My code follows the repository's style guidelines New and existing tests pass locally I have added appropriate error handling I have added or updated documentation as needed (no doc change needed for a behavioral fix) Additional context While reviewing, I noticed src/mcp/client/session.py:336 has the same bug pattern client-side: it raises RuntimeError("Tool ... has an output schema but did not return structured content") without first checking result.is_error. I'm scoping that out of this PR (different surface, separate code path) — happy to file a follow-up issue once this is in. A v1.x backport will follow as a separate PR after this lands (src/mcp/server/fastmcp/utilities/func_metadata.py:115, same change with the v1.x camelCase fields).
Open Graph Description: Resolves #2429. When a tool function with an output_schema returns CallToolResult(is_error=True), convert_result() was unconditionally calling output_model.model_validate(result.structured_content)...
X Description: Resolves #2429. When a tool function with an output_schema returns CallToolResult(is_error=True), convert_result() was unconditionally calling output_model.model_validate(result.structured_content)...
Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2508
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| og:image:alt | Resolves #2429. When a tool function with an output_schema returns CallToolResult(is_error=True), convert_result() was unconditionally calling output_model.model_validate(result.structured_content)... |
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