Validation and Field Errors

Validate request syntax (400) and semantics (422) before touching business logic, using schema validation generated from your OpenAPI/JSON Schema or typed models. Return all field errors at once with JSON-pointer-like paths (items[0].quantity) and machine-readable codes (too_small, invalid_format), so UIs can map errors to inputs. Keep business-rule violations (insufficient funds, wrong state) as separate problem types, usually 409/422.

Validation layers

HTTP/1.1 422 Unprocessable Content
Content-Type: application/problem+json

{
  "type": "https://api.example.com/problems/validation",
  "title": "Your request has invalid fields",
  "status": 422,
  "errors": [
    {"pointer": "/items/0/quantity", "code": "too_small", "detail": "must be at least 1"},
    {"pointer": "/shipping/country", "code": "invalid_enum", "detail": "must be an ISO 3166-1 alpha-2 code"}
  ]
}

Common pitfalls

• Stopping at the first error.
• Only client-side validation.
• Error paths that don't map to request fields.

Key points

• Validate early, return all errors
• Pointers + codes for UIs
• Separate schema vs business-rule errors

Further reading: Understanding JSON Schema