| Network timeout on read | Retry with backoff. |
| Timeout on idempotent mutation | Retry identical payload with the same idempotency key. |
| 400/413/415/422 | Change the request; do not retry unchanged. |
| 401/403 | Replace credentials or authorization; do not probe. |
| 404 | Verify ID/type; wrong-owner and unknown are indistinguishable. |
| 409 state conflict | Reread and choose an allowed action. |
| 409 idempotency conflict | Use original payload or a new key for genuinely new intent. |
| 412 | Reread, reconcile, and use the new ETag. |
| 429/503 | Honor Retry-After and add jitter. |
| 500 | Retry reads or idempotent writes cautiously; retain request ID. |