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Example Code

import pytest
from fastapi import FastAPI
from starlette.testclient import TestClient


@pytest.fixture(name="client")
def get_client():
    app = FastAPI()

    from pydantic import BaseModel, computed_field

    class Rectangle(BaseModel):
        width: int
        length: int

        @computed_field
        @property
        def area(self) -> int:
            return self.width * self.length

    # A usual endpoint, works as expected.
    @app.get("/normal")
    def normal() -> Rectangle:
        return Rectangle(width=3, length=4)

    # An endpoint with response model specified.
    @app.get("/response_model", response_model=Rectangle)
    def response_model() -> Rectangle:
        return Rectangle(width=3, length=4)

    # An endpoint with responses specified.
    @app.get("/responses", responses={200: {"model": Rectangle}})
    def responses() -> Rectangle:
        return Rectangle(width=3, length=4)

    client = TestClient(app)
    return client


def test_get(client: TestClient):
    response = client.get("/openapi.json")
    data = response.json()

    # print(data)  # Uncomment this to see the full openapi.json

    assert data["paths"]["/normal"]["get"]["responses"]["200"]["content"]["application/json"]["schema"]["$ref"] == "#/components/schemas/Rectangle-Output"
    # succeeds

    assert data["paths"]["/response_model"]["get"]["responses"]["200"]["content"]["application/json"]["schema"]["$ref"] == "#/components/schemas/Rectangle-Output"
    # succeeds

    assert data["paths"]["/responses"]["get"]["responses"]["200"]["content"]["application/json"]["schema"]["$ref"] == "#/components/schemas/Rectangle-Output"
    # fails

Description

When adding the responses parameter to an end point, the generated openapi.json does not contain the output model, but the input model. This means that computed fields are not included in the schema.

For example, in the code above a Rectangle has a width and length, and a computed field area. The input model therefore only contains width and length, the output model also contains area.

The first end point returns a Rectangle as usual, and the JSON schema correctly refers to Rectangle-Output. (In the second end point, I add response_model to confirm this also works.) For the third end point, I explicitly specify that the response will contain the model Rectangle using responses. Surprisingly, the JSON schema now refers to Rectangle-Input.

Is this expected behaviour? Or am I supposed to pass the model in another way?

You can run the example code using pytest. It is based on the code from this related question. (This issue may also be related this solved issue.)

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.104.1

Pydantic Version

2.4.2

Python Version

3.11.5

Additional Context

No response