HTTP Status 422 - Unprocessable Entity

POST /api/orders HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"product_id": 42, "quantity": -5, "shipping_address": ""}

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/json

{
  "error": "validation_failed",
  "message": "Request contains semantic errors",
  "errors": [
    {
      "field": "quantity",
      "value": -5,
      "message": "Must be a positive integer",
      "code": "positive_integer_required"
    },
    {
      "field": "shipping_address",
      "value": "",
      "message": "Required for physical products",
      "code": "required_field"
    }
  ]
}

POST /api/events HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"title": "Meeting", "start": "2025-10-01T15:00:00Z", "end": "2025-10-01T14:00:00Z"}

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/json

{
  "error": "invalid_time_range",
  "message": "Event end time must be after start time",
  "start": "2025-10-01T15:00:00Z",
  "end": "2025-10-01T14:00:00Z"
}

POST /api/invoices/inv-123/pay HTTP/1.1
Host: api.example.com

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/json

{
  "error": "invalid_state",
  "message": "Cannot pay invoice in 'cancelled' state",
  "current_state": "cancelled",
  "valid_states_for_payment": ["pending", "overdue"]
}