Rystic
Local, stateful simulators of third-party APIs for testing and training software agents
Rystic provides local, stateful simulators of third-party APIs such as Stripe, GitHub, Kalshi, Slack, PayPal, Lob, Okta, Linear, and Resend. The simulators run on a laptop or CI runner without rate limits, and each call sees the state left by previous calls. Before every release, the same requests are sent to the live API and to the simulator, and the resulting agreement percentage is published from CI per commit.
Simulators ship with pre-seeded scenarios drawn from publicly available data or a sample company, and Pro users can load their own records and change them mid-run. Fault injection covers 429, 503, and latency errors, and simulator state can be set directly. The CLI installs via a shell script, and clients point at a local base URL.
Rystic is packaged in three editions: a free plan with one seat and a monthly allotment of simulator hours with one scenario per simulator; a seat-based Pro plan with fault injection, settable state, and usage-based simulator minutes past an included pool; and an Enterprise edition with unlimited seats, custom simulator builds, a license file for offline use, and resale inside RL environments, offered after a one-week trial and contract.
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