# Integration Failure Injection v19

Canonical failure scenarios:
1. 503 -> bounded Polly retry -> durable Hangfire retry -> success.
2. 400 -> permanent failure -> no retry -> Operations Exception.
3. duplicate inbox message -> receipt conflict -> side effect once.
4. manual retry double click -> same Idempotency-Key -> one scheduled delivery.

Host CI should additionally inject timeout, DNS/network failure, 429, circuit-open and database restart between durable attempts.
