Modelith - Domain Model Tooling

Tooling for authoring, validating, and rendering domain models — the canonical, plain-language expression of what a system is: its concepts, how they relate, and the rules that govern them. One model, one source of truth: everyone reads from the same picture.

The model lives as a YAML file, but you rarely write that YAML by hand. You author it by talking to an AI agent: you describe concepts in plain language, the agent drafts and validates the YAML, and it renders a Markdown version (with diagrams) that you commit alongside your code. The modelith CLI is the engine the agent and CI run for you — it's there to validate and render, not to be your starting point.

The workflow

describe concepts in plain language
   │
   ▼
Claude Code skill (author) ─▶ writes / updates the YAML
   │
   ▼
model.modelith.yaml  ─▶ canonical source (you edit this, via the agent)
   │
   ├─▶ modelith lint    : validate + completeness  ─▶ CI gate
   └─▶ modelith render  : Markdown + Mermaid        ─▶ model.modelith.md committed to the repo

The rendered Markdown is committed back next to the YAML so people and agents read the model directly, without running anything. CI regenerates it and fails on drift (modelith render --check) — like a generated-code check.

Where to start

• Authoring a model for the first time? → Getting Started — install the plugin, then build a model by conversation.
• Want to understand a model someone produced? → Understanding Your Model and Reading the Diagrams.
• See it all come together? → the Parking Garage worked example builds a real model from nothing.

The pieces

Piece	What it does
Agent authoring	The Claude Code plugin and skills — how you actually build a model
Schema	The JSON Schema that defines a valid model
modelith CLI	The lint / render engine the agent and CI run
GitHub Action	The same checks in CI