How do you customise the error response format produced by ValidationPipe in NestJS?
Use the exceptionFactory option to replace the default error serialization. The factory receives the array of ValidationError objects from class-validator and must return an HttpException. This allows you to shape the errors array to match your API contract exactly.
Custom exceptionFactory for structured error responses
exceptionFactory design guidelines:
The factory receives ValidationError[] — each entry has property, value, constraints, and children.
children contains nested DTO errors — always handle them recursively for complete error reporting.
Return any HttpException subclass — UnprocessableEntityException (422) is also a common choice.
In production set validationError: { target: false, value: false } to avoid leaking request data.
Combine with stopAtFirstError: true to return only the first violation per field.