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Example Codefrom typing import Any
from fastapi import FastAPI
from pydantic import BaseModel, ConfigDict
# models
class CustomType:
...
@classmethod
def __get_pydantic_json_schema__(cls, *args: Any):
return {'type': 'string', 'format': 'date-time', 'description': 'ISO 8601 date-time string'}
class MyModel(BaseModel):
model_config = ConfigDict(arbitrary_types_allowed=True)
custom_field: CustomType
# app
app = FastAPI()
@app.get("/", response_model=MyModel)
def test():
return MyModel(custom_field=CustomType())DescriptionAfter upgrading to 0.119.0, It worked normally in version 0.118.3. Response on 0.118.3Error on 0.119.0Operating SystemLinux Operating System DetailsDebial bullseye FastAPI Version0.119.0 Pydantic Version2.12.1 Python Version3.11.13 Additional ContextNo response |
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Answered by
tiangolo
Dec 10, 2025
Replies: 4 comments 4 replies
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Probably related to #14177. In |
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1 reply
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#14177 is fixed but this is still an issue |
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@tiangolo Another similar example which works with 0.118.0 and fails with 0.119.0 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'}}}} |
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2 replies
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same usecase failing for us. |
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Thanks for the minimal reproducible example! I re-used it for the tests. It should be fixed by #14482 |
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It still does not work in |
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0 replies
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Thanks all for the reports and discussion! I created an issue here for completeness: #14483 It should be fixed in #14482, available in FastAPI 0.124.1, just released. 🎉 |
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1 reply
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It's been fixed perfectly! |
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Answer selected by
tiangolo
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Thanks all for the reports and discussion! I created an issue here for completeness: #14483
It should be fixed in #14482, available in FastAPI 0.124.1, just released. 🎉