Crimson

A minimal Ruby-based coding agent that gets things done.

# Install
gem install crimson-code

# Configure your API key
crimson setup

# Start coding
crimson "refactor this module to use dependency injection"

• Multi-provider support — OpenAI, Anthropic, OpenRouter, Mistral, xAI, and any OpenAI-compatible endpoint
• Official SDKs — Uses the official OpenAI and Anthropic Ruby gems
• Built-in tools — Read, write, edit, list files, run commands, search code, and glob
• Streaming output — Real-time response with styled markdown rendering (headers, bold, italic, code, lists, links, blockquotes)
• Colored tool display — →Read, →Write, →Edit, $ command, ✱Search, ✱Glob, →List with per-tool colors
• Thinking indicator — Spinner while thinking, with + Thought: X.Xs timing on first token
• Run stats — Token usage, cost, and elapsed time shown at end of every run
• Skills system — Customize agent behavior with markdown files
• Session management — Save, load, fork, and name conversation sessions per directory
• Conversation compaction — Automatic and manual compaction to stay within context limits
• Cost tracking — Real-time token usage and cost tracking per run
• Interactive REPL — Conversational coding assistant with tab-completion and slash commands

• Ruby 3.3+

gem install crimson-code

git clone https://github.com/nankhor/crimson.git
cd crimson
bundle install
bundle exec exe/crimson setup

crimson setup

This walks you through selecting a provider, entering your API key, and picking a model.

Configuration is stored in ~/.crimson/config.json (600 permissions).

You can also set the API key via environment variables:

Start a conversational session:

crimson

Type your task and the agent will use its tools to read, write, and edit files in your project.

Pass a task directly as an argument:

crimson "add error handling to the database module"

The agent completes the task and exits, showing the full conversation and cost summary.

$ crimson
Crimson v0.1.0
Type /help for commands, /exit to quit

> add a health check endpoint to the Sinatra app
→Read config.ru ...
→Read app.rb ...
✱Search app.rb for "get" ...
→Write app.rb ...
Done. Added GET /health endpoint returning JSON status.
Tokens: 1,234 ↑ | Cost: $0.0123 | Time: 12.3s

Add .md files to ~/.crimson/skills/ to customize agent behavior. These are loaded into the system prompt automatically. Built-in skills are in the skills/ directory for reference.

See CONTRIBUTING.md and CODE_OF_CONDUCT.md.

See CHANGELOG.md.

MIT