Return 422 Unprocessable Entity with detailed field-level errors:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Validation failed",
    "details": [
      {
        "field": "email",
        "code": "INVALID_FORMAT",
        "message": "Email must be valid format"
      },
      {
        "field": "password",
        "code": "TOO_SHORT",
        "message": "Password must be at least 8 characters"
      }
    ]
  }
}

This helps clients understand exactly what needs to be fixed.