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Headjack

Headjack is a macOS CLI tool for spawning isolated CLI-based LLM coding agents in container environments. Each agent runs in its own VM-isolated container with a dedicated git worktree, enabling safe parallel development across multiple branches.

Why Headjack?​

Running multiple AI coding agents simultaneously presents challenges: they can interfere with each other's work, create conflicting changes, and consume shared resources unpredictably. Headjack solves this by giving each agent its own isolated environment.

  • Isolation: Each agent runs in a VM-isolated container with its own filesystem
  • Branch-based workflows: Every instance is tied to a git branch via dedicated worktrees
  • Parallel development: Run multiple agents on different features simultaneously
  • Supported agents: Claude Code, Gemini CLI, and Codex CLI

Key Concepts​

Instance​

An instance is a running container environment tied to a specific git branch. When you create an instance, Headjack:

  1. Creates a git worktree for the branch
  2. Spawns a container with the worktree mounted at /workspace
  3. Tracks the instance in a local catalog

Instances persist across sessions and can be stopped, started, or removed as needed.

Session​

A session is a terminal multiplexer pane running inside an instance. Each instance can have multiple sessions, allowing you to run an agent alongside a shell for debugging or run multiple agents with different prompts.

Sessions can run in attached mode (interactive) or detached mode (background).

Agent​

An agent is one of the supported CLI-based LLM coding tools:

  • claude - Claude Code from Anthropic
  • gemini - Gemini CLI from Google
  • codex - Codex CLI from OpenAI

Agents are authenticated via the hjk auth command before first use.

Quick Example​

# Start Claude on a feature branch with a prompt
hjk run feat/auth --agent claude "Implement JWT authentication"

# Run another agent in the background on the same branch
hjk run feat/auth --agent claude -d "Write tests for the auth module"

# List all running instances
hjk ls

# Attach to an existing session
hjk attach feat/auth
tip

The hjk run command is idempotent for instances: if an instance already exists for the branch, it creates a new session within it rather than a new instance.

Next Steps​