Feature Definition Language (FDL)

Define features as YAML blueprints. Generate complete implementations for any framework.

FDL is an open-source system for writing “blueprints” — YAML specifications that describe software features completely. You define the what (fields, rules, outcomes, errors, events). Any AI tool — Claude, ChatGPT, Copilot, Gemini — reads the blueprint and generates a correct, complete implementation for your chosen language and framework.

No code. No YAML knowledge needed. Eleven CLI commands handle everything through plain-language conversation.

What Problems Does This Solve?

Every developer rebuilds the same features from scratch. Login, signup, password reset, file upload — these features exist in every app. Every time they’re rebuilt, something gets missed.

When you ask AI to “build login”, it guesses. Different AI tools produce different results. There’s no shared definition of what “login” actually needs.

Business rules live in people’s heads. The expense approval policy is in a PDF somewhere. When it’s time to build software, critical rules get lost.

FDL solves all three. A blueprint is the single source of truth — what data it needs, what rules govern it, what should happen in every scenario.

How It Works

There are eleven ways to work with blueprints:

Method When to use it Command
Build a full app Describe your app in plain English /fdl-build "nextjs POS with OTP login"
Brainstorm a feature You have a problem, not a solution /fdl-brainstorm
Create from scratch You know what feature you want /fdl-create checkout payment
Extract from a document You have a BRD, policy doc, or SOP /fdl-extract docs/policy.pdf
Extract from a website API docs, developer portal /fdl-extract-web https://docs.example.com/api
Extract from code Existing codebase or git repo /fdl-extract-code ./src/auth login auth
Extract features selectively Large repo, pick only what you want /fdl-extract-code-feature https://github.com/org/repo
Generate code You have a blueprint, want code /fdl-generate login nextjs
Export for other AI tools Use blueprints with ChatGPT, Gemini, etc. /fdl-build-yaml "my app description"
Install for AI tools Set up Cursor, Windsurf, Copilot, etc. /fdl-install cursor
Auto-evolve Validate, regenerate docs, and commit /fdl-auto-evolve

Learn more about commands

Quick Start

Prerequisites

1. Clone and install 

git clone https://github.com/TheunsBarnardt/ai-fdl-kit.git
cd ai-fdl-kit
npm install

2. Create your first blueprint 

/fdl-create login auth

3. Generate the code 

/fdl-generate login nextjs

4. Validate (optional) 

node scripts/validate.js

Static API for AI Tools

Every blueprint is available as JSON for AI tools that can’t fetch raw YAML from GitHub:

GET /api/registry.json              — index of all blueprints
GET /api/blueprints/auth/login.json — complete blueprint as JSON

Browse the API registry

Explore