FlakeMonster

An async fuzzer for JavaScript

░  Your test ran too fast and left a bug behind.

░  Your test flaked!

In your development environment:

That's why your tests are flaking.

Fuzz your async timing with deterministic await delays to surface the bugs that hide within.

$ npx flake-monster test --cmd "npm test"
$ npm install flake-monster
<script type="module" src="https://unpkg.com/flake-monster@0.4.6">
$ curl -L registry.npmjs.org/flake-monster/-/flake-monster-0.4.6.tgz | tar xz
npm · GitHub · v0.4.6
Real demos! No mocks!

How It Works

FlakeMonster uses Acorn to parse your code into an AST, walks the tree to find async functions and top-level module statements, injects delay nodes, then generates the modified source.

→ → → → →
Your JavaScript source code.

Operational Jitter

See how FlakeMonster reorders async operations by injecting deterministic delays.

Without FlakeMonster

With FlakeMonster

Code Injection Intensity

Watch FlakeMonster inject delay statements into real JavaScript code.

Original

Injected

File / Seed Based Determinism

Each seed is combined with the file path and function name to derive a unique delay. Same seed, same delays, every run.

seed
src/user.jsloadUser()
src/cart.jsgetCart()
lib/auth.jscheckAuth()

  


  
  

    

      
Agent Skill

      
Uses the open Agent Skills standard. One SKILL.md works across Claude Code, Codex, Copilot, and more.

    


    

      

        

          

            
              
              
            
          

          

            Install the skill
            One command drops the skill file into your project
          

        

        

          
        

        

          

            
              
              
            
          

          

            Use the slash command
            Type /flakemonster followed by your test command
          

        

        

          
        

        

          

            
              
            
          

          

            Get a flake report
            Your agent runs multiple seeds, analyzes results, and reports flaky tests
          

        

      


      

        

          

            
            
            
            
            
            
          

          

            
$ mkdir -p .claude/commands && \
  curl -sL https://raw.githubusercontent.com/growthboot/FlakeMonster/refs/heads/main/SKILL.md \
  -o .claude/commands/flakemonster.md

            
$ mkdir -p .cursor/skills/flakemonster && \
  curl -sL https://raw.githubusercontent.com/growthboot/FlakeMonster/refs/heads/main/SKILL.md \
  -o .cursor/skills/flakemonster/SKILL.md

            
$ mkdir -p .agents/skills/flakemonster && \
  curl -sL https://raw.githubusercontent.com/growthboot/FlakeMonster/refs/heads/main/SKILL.md \
  -o .agents/skills/flakemonster/SKILL.md

            
$ mkdir -p .github/skills/flakemonster && \
  curl -sL https://raw.githubusercontent.com/growthboot/FlakeMonster/refs/heads/main/SKILL.md \
  -o .github/skills/flakemonster/SKILL.md

            
$ mkdir -p .windsurf/skills/flakemonster && \
  curl -sL https://raw.githubusercontent.com/growthboot/FlakeMonster/refs/heads/main/SKILL.md \
  -o .windsurf/skills/flakemonster/SKILL.md

            
$ mkdir -p .agent/skills/flakemonster && \
  curl -sL https://raw.githubusercontent.com/growthboot/FlakeMonster/refs/heads/main/SKILL.md \
  -o .agent/skills/flakemonster/SKILL.md

          

        


        

          

            
            Usage
          

          
// Type in any supported agent:
/flakemonster npm test
/flakemonster npx playwright test
/flakemonster --runs 20 --mode hardcore npx jest

        

      

    

  


  
  

    

      New
      
 GitHub Action

      
Catch flaky tests on every pull request. Automatically.

    


    

      
      

        

          

            

              
                
                
              
            

            

              Add one file
              Drop a YAML workflow into your repo
            

          

          

            
          

          

            

              
                
                
              
            

            

              Push code
              FlakeMonster runs 5 seeds on every PR
            

          

          

            
          

          

            

              
                
              
            

            

              See results
              A detailed comment lands on your PR
            

          

        


        
        

          

            

              
            

            

              
              retrigger: wait for npm cache
              e9bf241
              
                
              
            

          

          

            

              
            

            

              
              retrigger: action now fails on flake
              0b36d45
              
                
              
            

          

          

            

              
            

            

              
              retrigger: clean restore
              8315e73
              
                
              
            

          

        


        

          

            

              

                
              

              

                One check has failed
                1 failing check
              

            

            

              
                
              
              
              

                Flake Check / flake-monster (pull_request)
                Failing after 13s
              

            

          

          

            

              

                
              

              

                No conflicts with base branch
                Merging can be performed automatically.
              

            

          

          

            
          

        

      


      
      

        

          

            

              
            

            

              flake-monster bot 2 min ago
            

          

          

            

              
              Found 2 flaky tests across 5 seeds
            

            

              
                

                  
                  Test
                  Result
                  Failed seeds
                

              
              
                

                  
                  checkout › applies discount
                  3/5
                  17 42
                

                

                  
                  auth › refresh expired token
                  4/5
                  89
                

                

                  
                  cart › adds item
                  5/5
                  , 
                

                

                  
                  user › loads profile
                  5/5
                  , 
                

              
            

            

              Seeds: 7 17 42 63 89
              
                
                FlakeMonster
              
            

          

        

      

    

  


  
  

    

      
Pricing

      
The CLI is free forever. The GitHub Action adds automated flake detection to every pull request.

    


    

      

        

          
            
            
            
            
            
            
            
          
        

        

          
Open Source

          
$0

          forever
        

        
Full CLI with inject, test, and restore. Run locally, run in CI, run anywhere.

        
    
          
  • Full CLI & GitHub Action
    • 
          
  • 50 GitHub Action checks per month
    • 
          
  • PR comments with results
    • 
          
  • Community support
    • 
        

        
          
          Get started
        
      

      

        

          
            
            
            
            
            
            
            
            
          
        

        

          
Team

          
$29/mo

          per team
        

        
Gate every pull request with flake detection — no cap, no throttling. Stop flaky tests from reaching your main branch.

        
    
          
  • Unlimited GitHub Action checks
    • 
          
  • Everything in Free
    • 
          
  • Priority support
    • 
          
  • Cancel anytime
    • 
        

        Get started
      

    

  


  
  

    

      
Planned Features

      
FlakeMonster is just getting started. Here's what's coming next.

    


    

      

        

          
            
            
            TS
            PY
          
        

        Up next
        
More language support

        
TypeScript, Python, Go, and beyond. The adapter architecture is ready — new languages just need a parser and codegen layer.

      


      

        

          
            
            
            
            
            
          
        

        Planned
        
CI dashboard

        
A web dashboard to track flaky test trends across repos and seeds over time. See which tests are getting worse before they break.

      


      

        

          
            
            
            
            
            
            
            
          
        

        Planned
        
IDE integration

        
VS Code extension to inject and restore delays without leaving your editor. One-click flake testing from your sidebar.

      


      

        

          
            
            
            
            
          
        

        Planned
        
Monorepo support

        
First-class workspace support for monorepos. Run FlakeMonster across packages with shared seeds and unified reporting.

      


      

        

          
            
            
          
        

        Exploring
        
Timing profiles

        
Named delay profiles (network, database, filesystem) that model real-world latency patterns instead of uniform random delays.

      


      

        

          
            
            
          
        

        
Your idea here

        
Have a feature request or found a bug? We'd love to hear from you.

        
          
          Open an issue on GitHub
        
      

    

  


  
  

    

      

        
          
            
              
              
            
          

          
            
            
            

            
            
            

            
            
            
            

            
            
            
            

            
            

            
            

            
            
            
            

            
            
            
          
        
      


      

        
Our Mission

        
What if you could throw static at your code and listen for the bugs that surface?


        

          

            ~
            

              Randomness is interesting
              
Randomness is one of the most underrated primitives in software. Fuzz testing, A/B experiments, fault injection, random isolation. A coin flip in the right place reveals things that careful analysis never will. We think the surface area for this kind of thinking is way underestimated, and we're ready to start exploring it with you.

            

          


          

            >
            

              Not everything needs AI
              
AI is great at many things, but some problems have cheaper, faster, more deterministic solutions. Algorithmic tools run in milliseconds, cost nothing to operate, and give you the same answer every time. There's a huge class of bugs that can be caught without a model ever seeing your code.

            

          


          

            *
            

              Beyond delays
              
Reordering, mutation, boundary injection, symbolic constraints. Algorithmic techniques can catch race conditions, state bugs, and logic errors without understanding a single line of code. We're interested in tools that find mistakes by probing how code behaves, not by trying to read it.