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Fall back to unstructured output when output model selection fails (#3573) - #3587
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Picking the output model builds throwaway pydantic models for the return type, which raises PydanticSchemaGenerationError for annotations pydantic cannot model, such as Iterator[str] or AsyncIterator[str]. That call sat outside the try/except that routes expected schema failures to the unstructured fallback, so the error escaped func_metadata and a properly annotated generator tool could not register at all. With structured_output=True the existing flow raises InvalidSignature instead. Fixes modelcontextprotocol#3573
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Fixes #3573
Disclosure: this fix was developed with AI assistance; I have reviewed the diff, run the tests locally, and can explain the change.
Summary
func_metadata raises an uncaught pydantic.errors.PydanticSchemaGenerationError when a tool's return annotation is Iterator[...] / AsyncIterator[...] (either the typing or the collections.abc spelling), so a correctly annotated generator tool cannot register at all. The fallback machinery for unserializable return types already exists — the call to _create_output_model() simply sits outside the try that routes those failures to it.
This wraps that call in the same failure path: on PydanticSchemaGenerationError the tool degrades to unstructured output (output_schema=None), and with structured_output=True the existing flow raises InvalidSignature. Generator[...] is untouched — pydantic treats it as a sequence and its structured array schema keeps working.
Motivation and Context
-> Iterator[str] is the PEP 484 spelling for generator functions, but registering one currently crashes at decoration time instead of taking the documented unstructured fallback (#3573). Fixing it at the model-selection step reuses the existing fallback semantics in one place rather than special-casing iterator types.
How Has This Been Tested?
Breaking Changes
None. Registrations that previously crashed now succeed as unstructured tools (or raise InvalidSignature under structured_output=True, matching the documented contract for unserializable return types).
Types of changes
Checklist