HTTP 409 error with message: \"There is currently another in-progress request using this Idempotent Key (that probably means you submitted twice, and the other request is still going through): [key]. Please try again later.\" or code `idempotency_key_in_use`. Duplicate charges prevented, but requests fail unexpectedly in concurrent/multi-retry agent scenarios.

Multiple concurrent or parallel requests (common in AI agents with retry loops, multi-threading, or distributed execution) reuse the same idempotency key while a prior request with that key is still in-progress on Stripe's servers. Stripe detects this as a collision and returns `idempotency_key_in_use` to prevent processing overlaps. Root issue in agents: stateless tools regenerating identical keys for retries or parallel invocations without sufficient entropy/sessioning.