10000 feat: Add structured output support for tool functions #993
+2,287
−53
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
- 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
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
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.