Raise HTTPException anywhere in a route or dependency to immediately return an error response. For domain-specific errors, define custom exception classes and register handlers with @app.exception_handler to convert them to HTTP responses.

This separation keeps route code clean: routes raise domain exceptions (DocumentNotFoundError), handlers translate them to HTTP responses (404 JSON). Override FastAPI's default RequestValidationError handler to customise the 422 response envelope for consistency across all error formats. Return JSONResponse from handlers for full control over status code and body. The pattern mirrors Python's own exception hierarchy — catch at different granularities by targeting base or specific exception classes.