MethodArgumentNotValidException: Validation failed for argument [0]
Bean Validation (@Valid with @NotNull, @Email, @Size…) rejected the request body.
Exact message you may see
Validation failed for argument
Seen on:
Java
Meaning
Spring returns 400 by default. The exception lists each field error — expose them to clients instead of a generic error.
Common causes
- Missing required fields
- Constraint violations (format, size)
- Client/server field name mismatch
⚡ Quick fix
- Return field errors from an @ControllerAdvice
- Align DTO constraints with the client
- Check the defaultMessage per field
Detailed fix by platform
Java
@ExceptionHandler(MethodArgumentNotValidException.class) ResponseEntity<?> handle(MethodArgumentNotValidException e) { return ResponseEntity.badRequest().body(e.getBindingResult().getFieldErrors()); }
How to diagnose
- Fields — Which field errors are listed?
- Recent change — Did it start after a deploy, upgrade, or configuration change? Compare with the last working version.
🔧 Still not fixed?
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Where this error sits
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Last updated 13 Oct 2026