fix: use SkipJsonSchema to exclude callable fields from JSON schema generation

[strawgate]

Problem

Callable fields on BaseModel classes would break JSON schema generation using Pydantic's model_json_schema() method. This prevented users from:

• Generating OpenAPI documentation
• Creating TypeScript types from Python models
• Using model_json_schema() for introspection
• Third-party tools that expect valid JSON schemas

Solution

Applied the SkipJsonSchema annotation from pydantic.json_schema to all callable fields across Tools, Resources, Prompts, and ResourceTemplates. This is the Pydantic 2 recommended approach for fields that should not appear in generated schemas, as it only affects JSON schema generation while leaving runtime behavior unchanged.

Added comprehensive test coverage to verify JSON schema generation works correctly for all affected classes.

Fixes #3046

Generated with Claude Code

[coderabbitai]

Walkthrough

This PR systematically wraps callable and serializer field types across multiple model classes with Pydantic's SkipJsonSchema annotation. Affected fields include authentication callables (auth), function references (fn), serializer functions (serializer), and parent tool references (parent_tool) in prompt, resource, tool, and template classes. This wrapping prevents these non-serializable callable types from being included in generated JSON schemas, addressing the inability to produce JSON schemas from models containing callable fields. Runtime behavior remains unchanged.

Possibly related PRs

• Add authorization checks to components and servers #2855: Directly modifies auth and function fields that are being wrapped with SkipJsonSchema in this PR, providing the authorization plumbing infrastructure these annotations support.
• Refactor prompt behavior and add meta support #2600: Modifies FunctionPrompt.fn type annotation in the same file that this PR wraps with SkipJsonSchema, causing overlapping changes to the same field signature.
• Revert "Refactor prompt behavior and add meta support" #2608: Modifies FunctionPrompt.fn and auth field type annotations in src/fastmcp/prompts/prompt.py that are wrapped with SkipJsonSchema in this PR.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The pull request title accurately and concisely summarizes the main change: using SkipJsonSchema to exclude callable fields from JSON schema generation.
Description check	✅ Passed	The description clearly explains the problem, solution, and references the linked issue. It covers the main objectives and indicates comprehensive test coverage was added.
Linked Issues check	✅ Passed	The PR addresses issue #3046 by applying SkipJsonSchema annotation to callable fields in Tools, Resources, Prompts, and ResourceTemplates, enabling JSON schema generation as required.
Out of Scope Changes check	✅ Passed	All changes are directly related to applying SkipJsonSchema wrapper to callable fields across multiple modules to fix JSON schema generation, with no out-of-scope modifications.
Docstring Coverage	✅ Passed	Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch claude/issue-3046-20260131-2057

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