Every response should follow the same structure.

Success response:

{
  "data": { ... },
  "meta": { "page": 1, "total": 100 }
}

Error response:

{
  "error": {
    "code": "validation_error",
    "message": "Email is required",
    "details": { ... }
  }
}

Clients should be able to parse any response with the same code.