feat: Add structured output support for tool functions #993
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- Add support for tool functions to return structured data with validation - Functions can now use structured_output=True to enable output validation - Add outputSchema field to Tool type in MCP protocol - Implement client-side validation of structured content - Add comprehensive tests for all supported types - Add documentation and examples
- Improve readability in func_metadata.py by using helper functions and clear if-statements - Restore complex_inputs.py example to original state from main branch - Fix test_88_random_error.py by adding list_tools handler - Refactor test_lowlevel_tool_output.py to test server behavior directly without clients - Fix README formatting for structured output examples - Add type assertions to fix pyright errors in tests This completes the code improvements requested in the PR review.
Required for client-side validation of structured tool output.
- Add jsonschema==4.20.0 and its transitive dependencies - Includes jsonschema-specifications, referencing, and rpds-py - Fixes pre-commit import resolution errors
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Adds support for structured output in FastMCP tools, allowing tool functions to return well-typed, validated data that clients can easily process.
Key Changes
structured_outputparameter to@mcp.tooldecorator.structured_output=Truetriggers structured output support:outputSchemais generated from modelstructuredContentresults are validated on the server before sendingdict[str, ...]are used asstructuredContentand automatically replicated incontentblock for BC (note: FastMCP delegates to lowlevel server, so includes this support)structuredContentresults against tooloutputSchemaexamples/fastmcp/weather_structured.pyshowing various return type optionsSupported return types
@mcp.tool(structured_output=True)supports the following function return types:outputSchema{\"result\": value}Motivation and Context
Support structured output as defined in spec rev 2025-06-18
How Has This Been Tested?
New tests added
Breaking Changes
Types of changes
Checklist
Additional context
Many thanks to @davemssavage, who prototyped an initial version of this functionality.