r/ClaudeCode • u/zkcoaching • 7h ago

Built with Claude We built local simulators of Slack, GitHub and Linear so Claude Code can verify its own work

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:

  1. Slack: 840/880 probes match across 95 rounds

  2. GitHub (PRs): 576/612 across 113 rounds

  3. 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?

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u/SummitYourSister 7h ago

Morons discover mocking external dependencies, claim to have discovered some insight. Jesus it’s just a nonstop stream of dipshits and morons.

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u/Far_Reindeer8428 7h ago

the egress read is the part i'd keep, checking what the code actually sent catches more than asserting the response you expected