Dataclass structured outputs mishandle slots and InitVar fields #3711

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

Two standard dataclass configurations fail when used as structured tool outputs. With @dataclass(slots=True), schema derivation silently falls back to unstructured output: func_metadata(produce).output_schema is None. With an InitVar[str] constructor parameter, metadata construction instead raises PydanticForbiddenQualifier because the constructor-only annotation is treated as a model field.

I expected the schema and structured result to contain the dataclass's instance fields. A constructor-only InitVar should not become an output property. The ordinary dataclass control passes, and Pydantic's own TypeAdapter serializes both variants to {"value": "ok"} in the same environments. These cases matter when an existing dataclass is reused as a tool return value.

Example Code

from dataclasses import InitVar, dataclass
from mcp.server.mcpserver.utilities.func_metadata import func_metadata

@dataclass(slots=True)
class Slotted:
    value: str

def slotted() -> Slotted:
    return Slotted("ok")

print(func_metadata(slotted).output_schema)  # None

@dataclass
class ConstructorOnly:
    value: str
    token: InitVar[str]

def constructor_only() -> ConstructorOnly:
    return ConstructorOnly("ok", "constructor-only")

func_metadata(constructor_only)  # PydanticForbiddenQualifier

Two deterministic, intentionally failing tests are on this test-only branch; no production changes. Run either case independently, for example:

uv run --frozen pytest tests/server/mcpserver/test_func_metadata.py::test_slotted_dataclass_structured_output_has_instance_fields -q

Python & MCP Python SDK

Python 3.14.7; Pydantic 2.12.5; MCP 2.3.0 (tag v2.3.0, 2118f14f8a19bc158d8a1cf90af58d85d187f849). Also reproduced using main source at 754254c08fe1c3eda18b69da3d336106fe8d72d6.

AI-assisted investigation and test reproduction, with a human in the loop.