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Cruel is a chaos engineering library that helps you build resilient applications by injecting failures, latency, and timeouts into any function. Test how your code handles the unexpected before your users do.

Quickstart

Get started with Cruel in under 5 minutes

Installation

Install Cruel with your package manager

Core concepts

Learn the fundamentals of chaos testing

API reference

Explore all available chaos functions

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What is Cruel?

Cruel wraps your functions and injects controlled chaos to simulate real-world failures. Inject failures, add latency, trigger timeouts, or combine multiple failure modes to test your application’s resilience. 
import { cruel } from "cruel"

const api = cruel(fetch, {
  fail: 0.1,          // 10% failure rate
  delay: [100, 500],  // random delay between 100-500ms
  timeout: 0.05,      // 5% timeout rate
})

const res = await api("https://api.example.com")

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Key features

  • Zero dependencies - No external packages required, works everywhere
  • TypeScript native - Full type safety out of the box
  • Works everywhere - Node.js, Bun, Deno, browsers, edge runtimes
  • Chaos injection - Failures, latency, timeouts, and corruption
  • Network simulation - Packet loss, disconnects, DNS failures, bandwidth throttling
  • HTTP chaos - Status codes, rate limits, slow responses, malformed bodies
  • Stream manipulation - Cut streams, pause, corrupt, truncate, reorder chunks
  • AI SDK integration - Built-in support for AI SDK v6 with specialized chaos
  • Resilience patterns - Circuit breakers, retries, bulkheads, timeouts, fallbacks
  • Fetch interception - Patch global fetch to inject chaos automatically
  • Presets and profiles - Pre-configured chaos levels from light to extreme
  • Scenarios - Define and replay complex failure scenarios
  • Deterministic mode - Reproducible chaos with seeded random number generation

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Why Cruel?

Most applications are built assuming perfect conditions. APIs always respond, networks never fail, and services are always available. Cruel helps you test the reality where things break. Zero dependencies
Cruel has no external dependencies. Install it and start testing immediately without worrying about version conflicts or supply chain security. TypeScript native
Built from the ground up with TypeScript. Get full type safety, autocomplete, and inline documentation in your editor. Works everywhere
Runs in any JavaScript environment: Node.js, Bun, Deno, browsers, Cloudflare Workers, Vercel Edge Functions, and more.

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Next steps

Ready to start testing? Follow the quickstart guide to inject your first chaos in under 5 minutes.

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