HTTP status codes and checkout failure codes.
| HTTP | Meaning |
|---|---|
| 400 / 422 | Validation / business rule |
| 401 | Missing or invalid credentials |
| 402 | Insufficient credits |
| 404 | Resource not found |
| 429 | Rate limited (public verify) |
| 500 | Server error |
See Concepts for the full list (underpaid, duplicate_ref, …).