lemmafit

Make agents prove that their code is correct.

Read our launch post: Introducing lemmafit: A Verifier in the AI Loop.

Lemmafit integrates Dafny formal verification into your development workflow via Claude Code. Business logic, state machines, and other logic are written in Dafny, mathematically verified, then auto-compiled to TypeScript for use in your React app.

Quick Start

# Install lemmafit globally
npm install -g lemmafit

# Create a new project
lemmafit init PROJECT_NAME
cd PROJECT_NAME

# Install deps (downloads Dafny automatically)
npm install

# In one terminal, start the verification daemon
npm run daemon

# In another terminal, start the Vite dev server
npm run dev

# In a third terminal, open Claude Code
claude

Use Cases / Considerations

• lemmafit works with greenfield projects. You typically begin a project with lemmafit init though lemmafit add provides rudimentary support for existing codebases.

• lemmafit compiles Dafny to Javascript/Typescript which then hooks into a runtime like a React app. In the future, we will support other languages.

• lemmafit is optimized to work with Claude Code. In the future, lemmafit will be agent-agnostic.

How It Works

1. Prompt Claude Code as you normally would. You may use a simple starting prompt or a structured prompting system. Example: "Create a pomodoro app I can use personally and locally."
2. The agent will write a SPEC.yaml and write verified logic in lemmafit/dafny/Domain.dfy
3. The daemon watches .dfy files, runs dafny verify, and on success compiles to src/dafny/Domain.cjs + src/dafny/app.ts
4. The agent will hook the generated TypeScript API into a React app — the logic is proven correct
5. After proofs complete, run the /guarantees skill to activate claimcheck and generate a guarantees report

Project Structure

my-app/
├── SPEC.yaml                    # Your requirements
├── lemmafit/
│   ├── dafny/
│   │   └── Domain.dfy           # Your verified Dafny logic
│   │   └── Replay.dfy           # Generic Replay kernel
│   ├── .vibe/
│   │   ├── config.json           # Project config
│   │   ├── modules.json          # Module registry (for multi-module projects)
│   │   ├── status.json           # Verification status (generated)
│   │   └── claims.json           # Proof obligations (generated)
│   └── reports/
│       └── guarantees.md         # Guarantee report (generated)
├── src/
│   ├── dafny/
│   │   ├── Domain.cjs            # Compiled JS (generated)
│   │   └── app.ts                # TypeScript API (generated - DO NOT EDIT)
│   ├── App.tsx                   # Your React app
│   └── main.tsx
├── .claude/                      # Hooks & settings (managed by lemmafit)
└── package.json

CLI

lemmafit init [dir]                # Create project from template
lemmafit add [Name]                # Add a verified module to an existing project
lemmafit sync [dir]                # Re-sync system files (.claude/, hooks)
lemmafit daemon [dir]              # Run verification daemon standalone
lemmafit logs [dir]                # View daemon log
lemmafit logs --clear [dir]        # Clear daemon log

Updating

System files sync automatically on install:

npm update lemmafit
# postinstall re-syncs .claude/settings.json, hooks, and instructions

Requirements

• Node.js 18+
• Claude Code CLI
• claimcheck (npm install -g claimcheck) — needed for the /guarantees skill

Dafny and dafny2js are downloaded automatically during npm install to ~/.lemmafit/.