Flue is a runtime-agnostic agent framework written in TypeScript, designed to provide a harness for building autonomous agents similar to Claude Code. It separates agent logic into Markdown-based skills, context, and AGENTS.md files while providing a headless programmable runtime. The framework includes runtime packages for sessions, tools, and sandboxing, plus CLI and build tooling for deploying agents across environments like Node.js, Cloudflare Workers, and CI/CD pipelines.