# Rystic

> Local, stateful simulators of third-party APIs like Slack, GitHub, Stripe and Kalshi, for testing code and agents without hitting live services.

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.

## 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

## Integrations
Slack, GitHub, Kalshi, PayPal, Lob, Okta, Linear, Stripe, Resend, Clerk, Dropbox, Gmail, Google Drive, Notion, Amazon S3, Shopify Storefront, Microsoft Word, Docker, GitHub Actions, Vercel, Harbor

## Platforms
MACOS, LINUX, WEB, API, CLI

## Pricing
Freemium — Free tier available with paid upgrades

## Links
- Website: https://www.rystic.ai
- Documentation: https://www.rystic.ai/docs
- EveryDev.ai: https://www.everydev.ai/tools/rystic