FastAPI Component name regression in OpenAPI spec for

The Fix

pip install fastapi==0.128.4

Based on closed fastapi/fastapi issue #14247 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -207,11 +207,31 @@ def get_definitions(
         None if separate_input_output_schemas else "validation"
     )
-    flat_models = get_flat_models_from_fields(fields, known_models=set())
-    flat_model_fields = [
-        ModelField(field_info=FieldInfo(annotation=model), name=model.__name__)

repro.py

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)

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install fastapi==0.128.4\nWhen NOT to use: Do not apply this fix if you rely on the previous schema naming convention for compatibility.\n\n

Why This Fix Works in Production

Trigger: Component name regression in OpenAPI spec for v0.119.0
Mechanism: Fixes the separation of schemas with nested models that was introduced in FastAPI version 0.119.0, addressing the issue where schema names were incorrectly suffixed.
Why the fix works: Fixes the separation of schemas with nested models that was introduced in FastAPI version 0.119.0, addressing the issue where schema names were incorrectly suffixed. (first fixed release: 0.128.4).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Shows up under Python 3.13.1 in real deployments (not just unit tests).
Production symptom (often without a traceback): Component name regression in OpenAPI spec for v0.119.0

Proof / Evidence

GitHub issue: #14247
Fix PR: https://github.com/fastapi/fastapi/pull/14246
First fixed release: 0.128.4
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.47

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“It should be fixed in https://github.com/fastapi/fastapi/pull/14246, just relased in FastAPI 0.120.2. :tada:”

@tiangolo · 2025-10-29 · confirmation · source

Failure Signature (Search String)

Component name regression in OpenAPI spec for v0.119.0
openapi_spec = app.openapi()

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Component name regression in OpenAPI spec for v0.119.0
openapi_spec = app.openapi()

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Component name regression in OpenAPI spec for v0.119.0
openapi_spec = app.openapi()

Minimal Reproduction

repro.py

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)

Environment

Python: 3.13.1

What Broke

Users experienced unexpected schema name changes in OpenAPI spec, leading to confusion and potential integration issues.

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install fastapi==0.128.4

When NOT to use: Do not apply this fix if you rely on the previous schema naming convention for compatibility.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/fastapi/fastapi/pull/14246

First fixed release: 0.128.4

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

Do not apply this fix if you rely on the previous schema naming convention for compatibility.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
0.128.4	Fixed

Related Issues

No related fixes found.

Cluster: fastapi:data-consistency FastAPI hub FastAPI best practices All hubs All clusters

Related clusters: Configuration error Rate limit Race condition

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

FastAPI Component name regression in OpenAPI spec for v0.119.0 (Fix)