^{Originally posted by walsha2 October 12, 2025}

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Example Code

from enum import StrEnum

from fastapi import FastAPI
from pydantic import BaseModel, Field


class MessageEventType(StrEnum):
    alpha = "alpha"
    beta = "beta"


class MessageEvent(BaseModel):
    event_type: MessageEventType = Field(default=MessageEventType.alpha)
    output: str


class MessageOutput(BaseModel):
    body: str = ""
    events: list[MessageEvent]


class Message(BaseModel):
    id: str
    input: str
    output: MessageOutput


app = FastAPI(title="Minimal FastAPI App", version="1.0.0")


@app.post("/messages", response_model=Message)
async def create_message(input_message: str) -> Message:
    return Message(
        input=input_message,
        output=MessageOutput(body=f"Processed: {input_message}"),
    )


if __name__ == "__main__":
    openapi_spec = app.openapi()
    for c in list(openapi_spec.get("components", {}).get("schemas", {}).keys()):
        print(c)

Description

Issue Summary

Take the minimal example provided above, run with fastapi==0.118.3 and it will produce:

HTTPValidationError
Message
MessageEvent
MessageEventType
MessageOutput
ValidationError

Now run the same code with fastapi==0.119.0 and it will produce:

HTTPValidationError
Message
MessageEvent
MessageEventType
MessageOutput-Input
MessageOutput-Output
ValidationError

Note how MessageOutput is now suffixed unnecessarily as: MessageOutput-Input and MessageOutput-Output. This seems unexpected considering the changes made between v0.118.3 and v0.119.0

Minor Tweak

One minor tweak to the example and this issue goes away. Remove the enum from MessageEvent:

class MessageEvent(BaseModel):
    output: str

Run again and the issue is no longer there.

HTTPValidationError
Message
MessageEvent
MessageOutput
ValidationError

TLDR

Why does the presence of an enum value on MessageEvent create a bifurcated OpenAPI schema definition for MessageOutput and why does this issue only start happening in fastapi==0.119.0?

Operating System

macOS

Operating System Details

15.6.1 (24G90)

FastAPI Version

0.119.0

Pydantic Version

2.12.0

Python Version

Python 3.13.1

Additional Context

No response