FastAPI Since FastAPI 0.119.0, using

The Fix

pip install fastapi==0.128.4

Based on closed fastapi/fastapi issue #14483 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -1,7 +1,7 @@
 import warnings
 from copy import copy, deepcopy
-from dataclasses import dataclass
+from dataclasses import dataclass, is_dataclass
 from enum import Enum

repro.py

from typing import Annotated

import numpy as np
from fastapi import FastAPI
from pydantic import BaseModel, ConfigDict, WithJsonSchema, TypeAdapter

type MyNumpyArray = Annotated[
    np.ndarray, WithJsonSchema(TypeAdapter(list[float]).json_schema(), mode="serialization"),
]


class MyModel(BaseModel):
    model_config = ConfigDict(arbitrary_types_allowed=True)
    custom_field: MyNumpyArray

app = FastAPI()

@app.get("/")
def test() -> MyModel:
    raise NotImplementedError()

print(app.openapi())
# {'openapi': '3.1.0', 'info': {'title': 'FastAPI', 'version': '0.1.0'}, 'paths': {'/': {'get': {'summary': 'Test', 'operationId': 'test__get', 'responses': {'200': {'description': 'Successful Response', 'content': {'application/json': {'schema': {'$ref': '#/components/schemas/MyModel'}}}}}}}}, 'components': {'schemas': {'MyModel': {'properties': {'custom_field': {'$ref': '#/components/schemas/MyNumpyArray'}}, 'type': 'object', 'required': ['custom_field'], 'title': 'MyModel'}, 'MyNumpyArray': {'items': {'type': 'number'}, 'type': 'array'}}}}

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 use this fix if arbitrary_types_allowed is not required.\n\n

Why This Fix Works in Production

Trigger: Since FastAPI 0.119.0, using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI
Mechanism: Using arbitrary_types_allowed=True with custom types breaks OpenAPI generation
Why the fix works: Fix handling arbitrary types when using `arbitrary_types_allowed=True` to prevent issues with OpenAPI generation. (first fixed release: 0.128.4).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Using arbitrary_types_allowed=True with custom types breaks OpenAPI generation
Production symptom (often without a traceback): Since FastAPI 0.119.0, using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI

Proof / Evidence

GitHub issue: #14483
Fix PR: https://github.com/fastapi/fastapi/pull/14482
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: 0.41

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/14482, available in FastAPI 0.124.1, just released. :tada:”

@tiangolo · 2025-12-10 · confirmation · source

Failure Signature (Search String)

Since FastAPI 0.119.0, using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI
Using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Since FastAPI 0.119.0, using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI
Using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Since FastAPI 0.119.0, using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI
Using `arbitrary_types_allowed=True` with custom types that define their serialization and JSON Schema breaks when generating OpenAPI.

Minimal Reproduction

repro.py

from typing import Annotated

import numpy as np
from fastapi import FastAPI
from pydantic import BaseModel, ConfigDict, WithJsonSchema, TypeAdapter

type MyNumpyArray = Annotated[
    np.ndarray, WithJsonSchema(TypeAdapter(list[float]).json_schema(), mode="serialization"),
]


class MyModel(BaseModel):
    model_config = ConfigDict(arbitrary_types_allowed=True)
    custom_field: MyNumpyArray

app = FastAPI()

@app.get("/")
def test() -> MyModel:
    raise NotImplementedError()

print(app.openapi())
# {'openapi': '3.1.0', 'info': {'title': 'FastAPI', 'version': '0.1.0'}, 'paths': {'/': {'get': {'summary': 'Test', 'operationId': 'test__get', 'responses': {'200': {'description': 'Successful Response', 'content': {'application/json': {'schema': {'$ref': '#/components/schemas/MyModel'}}}}}}}}, 'components': {'schemas': {'MyModel': {'properties': {'custom_field': {'$ref': '#/components/schemas/MyNumpyArray'}}, 'type': 'object', 'required': ['custom_field'], 'title': 'MyModel'}, 'MyNumpyArray': {'items': {'type': 'number'}, 'type': 'array'}}}}

Why It Broke

Using arbitrary_types_allowed=True with custom types breaks OpenAPI generation

Fix Options (Details)

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

pip install fastapi==0.128.4

When NOT to use: Do not use this fix if arbitrary_types_allowed is not required.

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/14482

First fixed release: 0.128.4

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

When NOT to Use This Fix

Do not use this fix if arbitrary_types_allowed is not required.

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:configuration-error FastAPI hub FastAPI best practices All hubs All clusters

Related clusters: Data consistency Rate limit Race condition

Sources

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

FastAPI Since FastAPI 0.119.0, using `arbitrary_types_allowed=True` with custom types that define (Fix)