Rystic

Developers building integrations with services like Slack, GitHub, Stripe, PayPal, or Okta normally have to test against the real, live API. This means dealing with rate limits, unpredictable live data, and the risk of hitting production systems during development or CI runs. There is no standard way to run these APIs locally with realistic, changing data for testing scenarios. Rystic's site frames this as the core gap it addresses: running tests against a live API instead of a local, controllable stand-in.

Rystic ships CLI-installable simulators for APIs including Slack, GitHub, Kalshi, PayPal, Lob, Okta, Linear, Stripe, and Resend, run locally (for example on localhost:8080) with no rate limits. The simulators are stateful: actions like creating a customer persist and show up in subsequent list or delete calls, mimicking real API behavior rather than static mocks. Each simulator ships with built-in scenarios using either public data (a recorded Kalshi market, a large open-source GitHub repo) or a consistent sample company, and users can load and modify their own records mid-run. Rystic also publishes accuracy numbers, run as recurring probes comparing simulator responses to the real API before each release, with reported agreement rates like 96.7% for Slack and 100.0% for Lob and Linear.