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Simplify _is_object_schema helper #2426

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jlowin merged 2 commits into from
Nov 15, 2025
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Simplify _is_object_schema helper #2426

jlowin merged 2 commits into from
Nov 15, 2025

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@jlowin jlowin commented Nov 15, 2025 •
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Reduces the overcomplicated $ref resolution logic to a simple check. Self-referencing types always produce object schemas in our use case.

Note: this is what #2424 SHOULD have been except it was prematurely opened and merged by LLM.

Actually closes #2421

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coderabbitai bot commented Nov 15, 2025 •
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Walkthrough

The _is_object_schema function in src/fastmcp/tools/tool.py was simplified to use a more deterministic approach. The function now returns True when a schema contains a $ref that also includes a $defs, or when the schema has an explicit object type or properties. The previous recursive resolution traversal logic was removed and replaced with a single-condition check. The docstring was shortened accordingly. No exported public signatures were modified.

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❌ Failed checks (1 inconclusive)
Check name Status Explanation Resolution
Description check ❓ Inconclusive The description explains the change rationale but is missing required checklist items from the template (no checkboxes completed, no issue closure reference in standard format). Complete the Contributors and Review checklists from the template, and format the issue closure as '- [x] My change closes #2421' for clarity.
✅ Passed checks (4 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly and concisely describes the main change: simplifying the _is_object_schema helper function.
Linked Issues check ✅ Passed The PR addresses issue #2421 by simplifying the _is_object_schema logic to correctly handle self-referencing types, treating them as object schemas as intended.
Out of Scope Changes check ✅ Passed The changes are focused on simplifying the _is_object_schema helper function in src/fastmcp/tools/tool.py, which is directly related to fixing the self-referencing types issue without introducing unrelated modifications.
Docstring Coverage ✅ Passed Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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🔇 Additional comments (1)
src/fastmcp/tools/tool.py (1)

380-382: The simplification is pragmatic but test coverage for edge cases is missing.

The assumption that schemas containing both $ref and $defs at root level always represent objects is intentional and well-documented in code comments. For Pydantic/dataclass auto-generated schemas (line 499, the primary use case per issue #2421), this assumption holds. However, no unit tests were found to validate this assumption or cover edge cases for user-provided explicit schemas (line 299).

A malformed user schema like {"$ref": "#/$defs/MyStr", "$defs": {"MyStr": {"type": "string"}}} would pass this check but isn't actually an object type. While unlikely in practice, this edge case is unvalidated.

The PR intentionally chose pragmatic simplification over defensive validation. Consider whether test coverage for edge cases in user-provided explicit schemas is needed before merging, or document this as an acceptable risk given the MCP spec requirement that output schemas must be objects (which may implicitly filter such cases).

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@marvin-context-protocol marvin-context-protocol bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. server Related to FastMCP server implementation or server-side functionality. labels Nov 15, 2025
@jlowin jlowin merged commit b5f88bf into main Nov 15, 2025
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@jlowin jlowin deleted the simplify-object-schema-check branch November 15, 2025 17:39
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dhdaines commented Nov 17, 2025

Excellent, thanks!

This was referenced Dec 25, 2025
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Self-referencing types not supported correctly in structured output

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