Rystic
Realistic local API simulators for testing AI agents
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Rystic lets developers build and test AI agents against realistic local simulators for services like Slack, GitHub, Stripe, Linear and PayPal. Each simulator runs on your laptop with no rate limits or internet connection, matching real API behavior with up to 100% fidelity.
The simulators are stateful — every call sees the ones before it — and you can seed any initial state before testing. This means you build and test your agent end-to-end without needing real accounts on every service.
Free plans include 10 hours/month of GitHub, Linear and Slack. The Pro plan at $89.99/month unlocks the full catalog with verified UIs. From a YC F26 team that previously worked at Roblox and Google.
AI-assisted draft, human-reviewed before publishing — see how we choose & review tools.
Why we picked it
Testing AI agents against live APIs is expensive and fragile. Rystic solves this with accurate local simulators — essential infrastructure for anyone building agents across multiple services.
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