Crimson

An open-source Ruby-based minimal coding agent made to get things done.

Features

• Multi-provider support — OpenAI, Anthropic, OpenRouter, Mistral, xAI, and custom OpenAI-compatible endpoints
• 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

Requirements

• Ruby 3.2+

Installation

git clone https://github.com/cmoiadib/crimson.git
cd crimson
bundle install

Setup

ruby bin/crimson setup

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

Usage

Start the interactive REPL:

ruby bin/crimson

Slash commands

Command	Description
/help	Show available commands
/clear	Clear conversation history
/model	Switch model (interactive selector)
/thinking	Set thinking level (off/low/medium/high)
/tools	List available tools
/save	Save conversation to file
/load	Load conversation from file
/usage	Show token usage and cost
/sessions	List sessions for current directory
/name	Set session name
/session	Show session info
/fork	Fork current session into new branch
/tree	Show conversation tree
/compact	Compact conversation history
/exit	Exit crimson

Skills

Add .md files to the skills/ directory to customize agent behavior. These are loaded into the system prompt automatically.

License

MIT