Local, stateful simulators of third-party APIs like Slack, GitHub, Stripe and Kalshi, for testing code and agents without hitting live services.
At a Glance
Try every simulator for 10 free hours per month, with every feature but latency, fault injection and verified UIs. Requires sign-up.
One-week trial before an Enterprise contract.
Engagement
Available On
Listed Oct 2026
About Rystic
Rystic builds local, stateful simulators of the APIs that application code depends on, such as Slack, GitHub, Kalshi, PayPal, Okta, Linear, Stripe and Resend. It is made by a two-person team in Y Combinator's F26 batch. A CLI runs each simulator on a laptop or CI runner, so tests and agents can run against a simulator instead of the live API.
What It Is
Rystic is an API simulation tool. Each simulator keeps state across calls: add a customer, list customers and the new one appears; delete it and it disappears. Simulators ship with a seeded world, either from public data or a sample mid-sized design studio company. Users can load their own records and change them mid-run.
How It Works
The CLI is installed with a shell script, then simulators are started with commands like rystic run slack-twin. Client code is pointed at a localhost base URL. Docs describe several further workflows:
- Multi-simulator environments defined in a rystic.yaml, with export to Compose.
- Intercepting calls to vendor hosts without code changes.
- Webhook delivery and world events.
- Shared simulators with an admin token.
- Integration tests in CI such as GitHub Actions.
- Kalshi backtesting with captured market tapes.
- Harbor agent environments for Slack MCP.
Fidelity Measurement
Rystic states that before each release the same requests are sent to the live API and the simulator, and the agreement rate is published per simulator. The homepage lists figures such as 96.7% for Slack across 538 probes and 100% for Resend across 64 probes.
Catalog
Documented simulators include Slack, GitHub pulls, Kalshi, PayPal, Lob, Okta, Linear, Stripe, Resend, Clerk, Dropbox, Gmail, Google Drive, Notion, S3, Shopify Storefront and Word. OpenAI, Anthropic, ElevenLabs, Shopify, NetSuite, QuickBooks and Square are marked as coming soon.
Current Status
Free and Pro plans need an account and a network connection for billing; Enterprise runs offline.
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Pricing
Free
Try every simulator for 10 free hours per month, with every feature but latency, fault injection and verified UIs. Requires sign-up.
- Every simulator
- 1 seat, 10 hours a month
- Every scenario
- Settable state
- No latency or fault injection
Enterprise trial
One-week trial before an Enterprise contract.
Pro
Per seat per month, starting with $100 in credit per seat. Adds fault injection (429, 503, latency) and verified UIs.
- The full catalog
- $0.012 a simulator-minute past the pool
- Latency and fault injection
- Verified UIs
- Any number of seats
- Starting with $100 in credit per seat
Enterprise
RL environments and agent evals at scale. A one-week trial, then a contract.
- Every simulator, plus custom builds
- Unlimited seats, local and CI
- Run fully offline
- Resale inside RL environments
Capabilities
Key Features
- Stateful API simulators where calls see prior state
- Published fidelity measurements against live APIs
- Local runs on a laptop or CI runner
- Seeded realistic worlds and bring-your-own records
- Multi-simulator environments via rystic.yaml and Compose export
- Intercept vendor API calls without code changes
- Webhook delivery and world events
- Shared simulators with admin token
- Kalshi captured-day backtesting
- Docker support and docker builder
- Agent-readable docs and llms.txt