Metadata-Version: 2.4
Name: fastapi-validation-override
Version: 1.0.0
Summary: Override FastAPI's default 422 validation error with any HTTP status code, at runtime and in the OpenAPI schema.
Project-URL: Homepage, https://github.com/mat81black/fastapi-validation-override
Project-URL: Repository, https://github.com/mat81black/fastapi-validation-override
Project-URL: Issues, https://github.com/mat81black/fastapi-validation-override/issues
Project-URL: Documentation, https://github.com/mat81black/fastapi-validation-override#readme
Project-URL: Changelog, https://github.com/mat81black/fastapi-validation-override/blob/main/RELEASE_NOTES.md
Author-email: Matteo Nieddu <mat81black@gmail.com>
License-Expression: MIT
License-File: LICENSE
Classifier: Development Status :: 5 - Production/Stable
Classifier: Environment :: Web Environment
Classifier: Framework :: FastAPI
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: MIT License
Classifier: Operating System :: OS Independent
Classifier: Programming Language :: Python :: 3 :: Only
Classifier: Programming Language :: Python :: 3.10
Classifier: Programming Language :: Python :: 3.11
Classifier: Programming Language :: Python :: 3.12
Classifier: Programming Language :: Python :: 3.13
Classifier: Programming Language :: Python :: 3.14
Classifier: Topic :: Internet :: WWW/HTTP
Classifier: Topic :: Software Development :: Libraries :: Python Modules
Classifier: Typing :: Typed
Requires-Python: >=3.10
Requires-Dist: fastapi>=0.120.0
Description-Content-Type: text/markdown

# FastAPI Validation Override

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FastAPI returns 422 Unprocessable Entity for every request validation failure. Many APIs, client teams, and HTTP standards treat 400 Bad Request as the correct status code for malformed input. Fixing this in FastAPI requires wiring a custom exception handler and updating the OpenAPI schema separately. `override_validation_error` does both in a single call.

## Features

- **Single call**: patches runtime exception handling and the OpenAPI schema at once
- **Any status code**: use 400, 409, or any valid code instead of 422
- **anyOf merge**: when a route already declares a response at the target code, the validation error schema is merged rather than overwritten
- **Independent of a custom 422**: if a route keeps its own, non-validation 422 response, it's left untouched and the validation schema is still documented at the target code
- **Custom openapi preserved**: wraps any `app.openapi` function already installed and applies the patch on top of its output
- **Bring your own handler**: `handle_exceptions=False` skips the built-in handler while still patching the schema
- **Idempotent**: safe to call multiple times on the same app instance
- **No-op guard**: `status_code=422` leaves FastAPI behavior unchanged

## Requirements

- Python 3.10+
- FastAPI 0.120.0+

## Installation

```bash
pip install fastapi-validation-override
# or
uv add fastapi-validation-override
```

## Quick start

```python
from fastapi import FastAPI
from pydantic import BaseModel

from fastapi_validation_override import override_validation_error

app = FastAPI()


class Item(BaseModel):
    name: str
    price: float


@app.post("/items")
async def create_item(item: Item) -> dict[str, object]:
    return item.model_dump()


override_validation_error(app)
# POST /items with invalid fields -> 400 Bad Request {"detail": [...]}
```

The `{"detail": [...]}` body is identical to FastAPI's default 422 response. Only the status code changes.

To use a different code, pass `status_code`:

```python
override_validation_error(app, status_code=409)
```

## Reference

### `override_validation_error`

```python
override_validation_error(app, status_code=400, handle_exceptions=True, merge_target_response=True)
```

| Parameter | Type | Default | Description |
|---|---|---|---|
| `app` | `FastAPI` | required | The FastAPI application instance to patch |
| `status_code` | `int` | `400` | HTTP status code to use instead of 422. Calling with `422` is a no-op |
| `handle_exceptions` | `bool` | `True` | When `True`, registers an exception handler that returns the custom status code at runtime. Set to `False` to patch only the OpenAPI schema and handle the exception yourself |
| `merge_target_response` | `bool` | `True` | When `True`, a response you already declared at `status_code` is merged with the validation error schema using `anyOf`. When `False`, it's left untouched |

### `patch_422_responses`

```python
patch_422_responses(schema, target_code, merge_target_response=True)
```

| Parameter | Type | Default | Description |
|---|---|---|---|
| `schema` | `dict[str, Any]` | required | An OpenAPI schema dict, such as one returned by `app.openapi()` |
| `target_code` | `str` | required | The status code to document the validation error at, e.g. `"400"` |
| `merge_target_response` | `bool` | `True` | Same as the `override_validation_error` parameter of the same name |

The schema-patching logic that `override_validation_error` uses internally, exposed as a standalone function for advanced use cases, such as patching a schema inside your own custom `app.openapi` function without going through `override_validation_error`. Mutates `schema` in place and also returns it.

```python
from fastapi_validation_override import patch_422_responses

schema = app.openapi()
patch_422_responses(schema, "400")
```

What happens to 422 and what happens to `target_code` are decided independently: a FastAPI-generated 422 validation response is removed, any other response already declared at 422 is left untouched, and routes that need validation always get it documented at `target_code`, synthesized from FastAPI's own schema components if nothing is there yet. See [Independent 422 handling](#independent-422-handling) below.

### Custom exception handler

Set `handle_exceptions=False` when you need a custom response body or additional logic. The OpenAPI schema is still patched.

```python
from fastapi import FastAPI, Request
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse

from fastapi_validation_override import override_validation_error

app = FastAPI()


@app.exception_handler(RequestValidationError)
async def validation_handler(request: Request, exc: RequestValidationError) -> JSONResponse:
    return JSONResponse(
        status_code=400,
        content={"message": "Validation failed", "errors": exc.errors()},
    )


@app.post("/items")
async def create_item(item: Item) -> dict[str, object]:
    return item.model_dump()


override_validation_error(app, status_code=400, handle_exceptions=False)
```

### Preserving a custom app.openapi

Call `override_validation_error` **after** assigning your custom openapi function. The library captures `app.openapi` at call time and wraps it, so the order matters.

```python
from typing import Any

from fastapi import FastAPI
from fastapi.openapi.utils import get_openapi

from fastapi_validation_override import override_validation_error

app = FastAPI()


def custom_openapi() -> dict[str, Any]:
    if app.openapi_schema:
        return app.openapi_schema
    schema = get_openapi(title="My API", version="1.0.0", routes=app.routes)
    schema["info"]["x-logo"] = {"url": "https://example.com/logo.png"}
    app.openapi_schema = schema
    return schema


app.openapi = custom_openapi  # type: ignore[method-assign]
override_validation_error(app)  # must come after
```

### Merging with an existing response at the target code

When a route already declares a response at the target status code, `override_validation_error` merges the schemas using `anyOf` instead of overwriting the existing one.

```python
class OutOfStockError(BaseModel):
    message: str
    item_name: str


@app.post("/items", responses={400: {"model": OutOfStockError, "description": "Out of stock"}})
async def create_item(item: Item) -> dict[str, object]:
    return item.model_dump()


override_validation_error(app)
# schema at 400: anyOf: [HTTPValidationError, OutOfStockError]
```

The merge only touches the `schema` (the JSON Schema union). It doesn't generate an OpenAPI `example`/`examples` value for either variant, so tools like Swagger UI won't show a distinct sample payload for `OutOfStockError` versus the validation error unless you declare one yourself (e.g. via a field default, or `model_config = ConfigDict(json_schema_extra={"example": ...})`) — the library can't safely invent one for a model it knows nothing about.

Set `merge_target_response=False` to leave a response you already declared at the target code completely untouched instead of merging it with `anyOf`:

```python
override_validation_error(app, merge_target_response=False)
```

### Independent 422 handling

A route can keep its own, non-validation 422 response. As long as the route has a body or parameters that FastAPI validates, it's left untouched at 422, and the validation error is still documented at the target code — independently of what occupies 422.

```python
class OutOfStockError(BaseModel):
    error_code: str
    item_name: str


@app.post("/items", responses={422: {"model": OutOfStockError, "description": "Item is out of stock"}})
async def create_item(item: Item) -> dict[str, object]:
    return item.model_dump()


override_validation_error(app)
# 422 still documents OutOfStockError, untouched.
# 400 is added automatically, documenting the validation error schema.
```

**This also applies at runtime.** If this route receives a request that fails validation, the exception handler still returns `target_code` (400 by default), not the custom 422 — `responses={422: ...}` only affects documentation, it never changes which exception handler FastAPI dispatches to. Your custom 422 response is for errors you raise yourself (like `OutOfStockError` above), not for request validation failures.

> **Note:** when the validation schema is synthesized this way, `HTTPValidationError` and `ValidationError` are added to `components.schemas` if they aren't already there. If you snapshot-test your OpenAPI schema, expect these two component definitions to appear the first time this happens.

## Examples

Runnable examples are in the [`examples/`](https://github.com/mat81black/fastapi-validation-override/tree/main/examples) directory:

| File | Description |
|---|---|
| [`basic.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/basic.py) | Minimal setup with the default 400 status code |
| [`custom_status_code.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/custom_status_code.py) | Using a custom status code (409) |
| [`handle_exceptions_false.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/handle_exceptions_false.py) | Custom exception handler with schema-only patch |
| [`custom_openapi.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/custom_openapi.py) | Preserving a custom `app.openapi` function |
| [`existing_response_at_target_code.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/existing_response_at_target_code.py) | `anyOf` merge when the target code is already declared |
| [`custom_422_response.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/custom_422_response.py) | Keeping a custom, non-validation 422 response |
| [`with_apirouter.py`](https://github.com/mat81black/fastapi-validation-override/blob/main/examples/with_apirouter.py) | Usage with multiple `APIRouter` instances |

## Release Notes

[RELEASE_NOTES](https://github.com/mat81black/fastapi-validation-override/blob/main/RELEASE_NOTES.md)

## License

[MIT](https://github.com/mat81black/fastapi-validation-override/blob/main/LICENSE)
