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Title: test(mcpserver): cover PEP 604 union return annotations (#2591) by Bartok9 · Pull Request #2906 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: test(mcpserver): cover PEP 604 union return annotations (#2591) by Bartok9 · Pull Request #2906 · modelcontextprotocol/python-sdk

X Title: test(mcpserver): cover PEP 604 union return annotations (#2591) by Bartok9 · Pull Request #2906 · modelcontextprotocol/python-sdk

Description: Summary Adds a regression test for tool functions whose return annotation is a multi-member PEP 604 union mixing container and scalar types (dict | list | str). Guards an already-fixed-but-untested case so it can't silently regress. Motivation Closes #2591. FastMCP/MCPServer used to crash at registration with PydanticUserError when a tool's return type was a bare PEP 604 union like dict | list | str, because the types.UnionType was passed to create_model() as a field value. This already works on current main (734746a): such a union is neither a types.GenericAlias nor a type, so _try_create_model_and_schema (func_metadata.py) falls through to the catch-all branch that calls _create_wrapped_model(...) with wrap_output=True; #2434 (5cbd259) additionally guards the schema-generation path. But there is no test covering the reported signature — the existing test_structured_output_generic_types only exercises a 2-member, all-scalar union (func_union() -> str | int), not a container+scalar mix with bare (unparametrized) generics, which is a different branch interaction (the dict member touches the RootModel dict special-casing before the union wraps). The three prior fix attempts (#2592, #2599, #2669) were all closed during backlog cleanup rather than for an approach problem, and one of them switched the repro to typed generics to satisfy pyright — which masked the exact reported case. This PR keeps the bare-generic signature (with scoped # type: ignore) so the test reproduces the issue faithfully. Verification uv run pytest tests/server/mcpserver/test_func_metadata.py -q — 34 passed uv run ruff check tests/server/mcpserver/test_func_metadata.py — clean uv run ruff format --check tests/server/mcpserver/test_func_metadata.py — already formatted uv run pyright tests/server/mcpserver/test_func_metadata.py — 0 errors Did NOT change: any source file. This is a test-only addition; the behavior it asserts already ships on main. Real behavior proof Behavior addressed: @mcp.tool() async def my_tool(flag: bool) -> dict | list | str registering without PydanticUserError, and producing a wrapped anyOf output_schema. Real environment: Python 3.12, uv run, branch off upstream/main 734746a. Command run: uv run python -c "..." invoking func_metadata(func, structured_output=True).output_schema for () -> dict | list | str. Captured output (the schema the new test asserts): { "properties": { "result": { "anyOf": [ {"additionalProperties": true, "type": "object"}, {"items": {}, "type": "array"}, {"type": "string"} ], "title": "Result" } }, "required": ["result"], "title": "func_pep604_unionOutput", "type": "object" } Regression test: tests/server/mcpserver/test_func_metadata.py::test_structured_output_pep604_union_return — asserts the exact wrapped anyOf shape above for the issue's signature. What was NOT tested: runtime conversion of an actual returned value through this model (the existing structured-output round-trip tests already cover the wrap path generically); this test is scoped to the schema-generation path that previously raised.

Open Graph Description: Summary Adds a regression test for tool functions whose return annotation is a multi-member PEP 604 union mixing container and scalar types (dict | list | str). Guards an already-fixed-but-unteste...

X Description: Summary Adds a regression test for tool functions whose return annotation is a multi-member PEP 604 union mixing container and scalar types (dict | list | str). Guards an already-fixed-but-unteste...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2906

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