^{Originally posted by coltonevansid December 4, 2025}

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Example Code

from fastapi import FastAPI
from fastapi.openapi.utils import get_openapi
from pydantic import BaseModel, computed_field

class ModelWithComputedField(BaseModel):
    name: str

    @computed_field
    @property
    def computed_name(self) -> str:
        return f"computed_{self.name}"

class SimpleModel(BaseModel):
      value: str

app = FastAPI()

@app.get("/with-computed")
def route_with_computed() -> ModelWithComputedField:
    """Route returning a model with computed fields."""
    return ModelWithComputedField(name="test")

@app.get("/without-computed")
def route_without_computed() -> SimpleModel:
    """Route returning a simple model (no computed fields)."""
    return SimpleModel(value="test")


if __name__ == "__main__":
    # This will raise KeyError when both routes are present
    schema = get_openapi(
        title="Test App",
        version="1.0.0",
        routes=app.routes,
        separate_input_output_schemas=False,  # Bug only occurs with False
    )

Description

FastAPI 0.123.4 added support for computed fields in OpenAPI schemas (#13207) when separate_input_output_schemas=False, but the implementation has a bug where the schema generation crashes with a KeyError if the FastAPI app contains both:

1. Routes with response models that have computed fields
2. Routes with response models that don't have computed fields (e.g., dict responses or BaseModels without computed fields)

Expected Behavior

OpenAPI schema should generate successfully with computed fields included in models that have them, regardless of mixing route types.

Current Behavior

File "../python3.13/site-packages/fastapi/_compat/v2.py", line 192, in get_schema_from_model_field
  json_schema = field_mapping[(field, override_mode or field.mode)]
                ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: (ModelField(field_info=FieldInfo(annotation=SimpleModel, required=True), name='Response_route_without_computed_without_computed_get', mode='serialization'), 'validation')

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.123.4

Pydantic Version

2.12.4

Python Version

3.13.8

Additional Context

Workaround: Generate schemas separately for routes with and without computed fields, then merge them.

Looking at the PR that introduced this (#13207), the tests don't cover the scenario of mixing routes with and without computed fields, which is why this bug wasn't caught.