Title: fix: skip output schema validation when tool returns is_error=True by kimsehwan96 · Pull Request #2466 · modelcontextprotocol/python-sdk · GitHub
Open Graph Title: fix: skip output schema validation when tool returns is_error=True by kimsehwan96 · Pull Request #2466 · modelcontextprotocol/python-sdk
X Title: fix: skip output schema validation when tool returns is_error=True by kimsehwan96 · Pull Request #2466 · modelcontextprotocol/python-sdk
Description: Skip output schema validation for error results in convert_result(). Motivation and Context When a tool with an inferred output_schema returns CallToolResult(is_error=True), the SDK's convert_result() calls model_validate(None) unconditionally, raising a pydantic ValidationError that replaces the intended error message (e.g., "Division by zero" becomes a pydantic schema error). This was already fixed in the TypeScript SDK via modelcontextprotocol/typescript-sdk#655, but the equivalent fix has not been applied to the Python SDK. How Has This Been Tested? Added regression test test_tool_call_result_annotated_is_error_skips_validation that creates a tool with an output schema, returns CallToolResult(is_error=True) with unstructured error content, and verifies the result is returned unchanged without raising a validation error. All 34 existing test_func_metadata.py tests pass. ruff check and ruff format clean. Breaking Changes None. This only changes behavior for the error path — tools returning is_error=True were previously broken (pydantic error replaced the intended message), so no working code is affected. 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 Additional context One-line fix matching the TypeScript SDK approach: - if self.output_schema is not None: + if self.output_schema is not None and not result.is_error: A separate backport PR for v1.x will follow. Fixes #2429
Open Graph Description: Skip output schema validation for error results in convert_result(). Motivation and Context When a tool with an inferred output_schema returns CallToolResult(is_error=True), the SDK's convert_r...
X Description: Skip output schema validation for error results in convert_result(). Motivation and Context When a tool with an inferred output_schema returns CallToolResult(is_error=True), the SDK's conve...
Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2466
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