IdemCheck CLI Exposes Idempotency-Key Race Conditions in Black-Box HTTP Testing

A Go command-line tool called IdemCheck has been released to help engineers detect race conditions in APIs that use idempotency keys. The core problem it addresses is that sequential retries typically pass idempotency checks, while concurrent requests sharing the same key can each complete the same operation independently before either stores its result. IdemCheck works by bursting ten simultaneous requests under one key, waiting briefly, replaying the original request, and then comparing all responses to determine if they converged on a single outcome. Verdicts — PASS, FAIL, ERROR, or INCONCLUSIVE — are mapped to exit codes so the tool can integrate directly into CI pipelines. The tool is limited to observable HTTP responses and cannot verify whether internal side effects such as database writes, emails, or payment captures occurred exactly once.