Errors
| Status | Meaning |
|---|---|
| 402 | Payment required — settle and retry identically |
| 404 | Unknown path (not on the gateway allowlist) |
| 409 | Payment already used |
| 429 | Upstream daily budget exhausted (operator capacity) |
| 501 | Receiver or facilitator not configured |
| 502 | Upstream proxy failure |
| 503 | Budget store unavailable in production |
Error bodies are typically JSON { "error": "…" }. On settle failure you may receive a fresh 402 instead of a plain error — always prefer the latest challenge amounts.