Ruler solves the problem of maintaining AI assistant instructions across multiple tools. Write rules once in .ruler/ markdown files, then generate tool-specific config files.

How It Works

1. Author rules in .ruler/*.md files (plain markdown)
2. Run ruler apply to generate:
   • CLAUDE.md for Claude Code
   • .cursorrules for Cursor
3. Rules are injected with <!-- Source: .ruler/filename.md --> comments

Configuration

ruler.toml in each .ruler/ directory controls which agents get output:

default_agents = ["claude", "cursor"]

Rule Inventory

Root rules (.ruler/):

File	Purpose
branching.md	Branch naming (Linear format for auto-linking), PR workflow
monorepo.md	Sub-project isolation, no cross-project imports
security.md	Security scanning commands (semgrep, trivy, gitleaks)
blog-dev.md	Dev server, Playwright verify loop, pre-PR checklist

Sub-project rules:

File	Purpose
apps/blog/blog/markdown/posts/.ruler/style.md	Blog post writing style and formatting
apps/blog/blog/tests/.ruler/testing.md	Playwright test conventions
infra/ai-security-toolkit-1/.ruler/versioning.md	Toolkit image versioning
apps/games/kid-bot-battle-sim/features/.ruler/interfaces.md	Game interface contracts
apps/games/sillyapp/.ruler/build.md	SillyApp build rules

Usage

# Apply all rules (from repo root)
ruler apply

# Ruler reads .ruler/*.md and writes CLAUDE.md, .cursorrules, etc.

Related Blog Posts

• Cross-Tool AI Rules with Ruler: setup and workflow