Agent tool tests need sequences: creating a customer should affect subsequent listing, and deletion should remove it. Rystic’s product page demonstrates stateful local API simulation and lets users load and modify records, extending beyond isolated mock responses.
Original: Rystic — Realistic API simulators for training and testing · Rystic; its founder announced the launch on 2026-10-26.
The page reports comparing identical requests against live APIs and simulators before release: Slack reaches 96.7% agreement across 538 probes; GitHub reaches 92.1% across 468. These vendor-reported probe agreements are neither complete API compatibility nor agent task-success rates.
Resettable, manipulable state is useful for testing failure paths, but the page does not establish probe-distribution or fault-model coverage. The founder’s short claim about fault injection cannot supply that missing evidence. Local execution also differs from offline execution: Free/Pro require an account and billing connectivity; Enterprise is described as offline-capable.