Claude Code writes code faster than we can verify it. We’re Evan and Zach, the two founders of Rystic, so weigh this accordingly. The problem: unit tests cover single functions, not the whole system. Sandboxes are slow, rate-limited, and still need a human to check the result. A simulator fixes both, but keeping a faithful copy of someone else’s API is a lot of work, and you end up debugging the copy instead of your code. What we built: local, stateful simulators of third-party APIs. Each one is a Go binary with state you can seed and inspect, multiple actors, injectable latency and faults, and thousands of concurrent requests with no rate limits. Runs on a laptop or in CI. How Claude Code uses it: We give it one CLAUDE.md line: “Integration tests run against the local Rystic simulators; set GITHUB_API_URL to the simulator, and read rystic docs <product> before guessing.” In the loop it seeds state with POST /_rystic/scenario , runs the code, then reads the simulator’s state and GET /_rystic/egress to see what the code actually sent. Then it fixes the code and runs again. How we know it matches: we run every simulator against the real API and compare responses each round. Latest: Slack: 840/880 probes match across 95 rounds GitHub (PRs): 576/612 across 113 rounds Linear: 134/134 across 26 rounds Fidelity cards with scores: https://www.rystic.ai/simulators Try it: Slack, GitHub and Linear are free, no card. brew install rystic-inc/tap/rystic or curl -fsSL https://www.rystic.ai/install.sh | sh (read it first). Which API would you most want Claude Code to have a simulator for? submitted by /u/zkcoaching
Originally posted by u/zkcoaching on r/ClaudeCode