RequestValidationError handler scope conflict when using APIRouter

Direct answer

No. In standard FastAPI, RequestValidationError handling is application-level. APIRouter has no public add_exception_handler() method, so a router-level handler cannot replace the global handler for routes under that router. If a global handler exists, it is the effective handler for validation failures from every included router unless the handler branches by request path or earlier middleware intercepts the exception.

Confirmed facts and assumptions

Assumption: a standard FastAPI installation on Starlette, approximately recent 0.x to 1.x releases. Exception handlers are stored on the FastAPI/Starlette application instance, while APIRouter groups routes, dependencies, tags, and prefixes. Calling router.add_exception_handler() raises AttributeError. Path-prefix matching is a workaround, not true scope isolation, so adjust it for nested or multiply mounted routers. Verify installed FastAPI and Starlette versions if behavior differs.

Steps for per-router formatting without duplicating the handler

1. Register one global RequestValidationError handler on the FastAPI app.
2. Inside that handler, inspect request.url.path and match the prefix used by the target router.
3. Return a tailored response for matched paths and the default response otherwise.
4. Include all routers in the same app; do not attempt router-level handler registration.

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

app = FastAPI()

def validation_payload(exc: RequestValidationError, code: str = 'validation_error') -> dict:
    return {'code': code, 'errors': exc.errors()}

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    if request.url.path.startswith('/special-api'):
        return JSONResponse(
            status_code=422,
            content=validation_payload(exc, code='special_api_validation_error')
        )
    return JSONResponse(
        status_code=422,
        content=validation_payload(exc)
    )

Verification

• Confirm hasattr(router, 'add_exception_handler') is False.
• Include two routers with different prefixes, trigger a 422 error under each, and assert that only the matched prefix receives the custom code.
• If you use middleware that catches exceptions before FastAPI's handler, provide its position in the middleware stack; that can change which handler runs.