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AI agent harnesses

AI coding agents are converging on a common shape: a harness is the program that wraps the model -- it runs the agent loop, gives the model tools (shell, file edits, search), manages sessions and context, and renders the terminal UI. For the underlying concepts (tool use, MCP, multi-agent patterns), see the AI Agents guide; for running local models instead of cloud providers, see Local & Offline Copilot Alternative.

These are the three harnesses I currently run. Like the rest of my CLI tooling, they can be installed and version-pinned with mise.

opencode​

OpenCode (GitHub) is an open-source terminal coding agent with a client-server architecture: a background service owns sessions, providers, and tool execution, while the TUI, scripts, and HTTP clients all attach to it.

  • V2 highlights: client/server split with an HTTP API, background service that survives terminal restarts, 75+ providers, agents, commands, plugins, skills, and MCP support.
  • Install: follow the V2 docs; the V2 CLI binary is opencode2 (or mise use --global opencode).
  • Usage:
    • opencode2 -- start the TUI in the current project
    • opencode2 run "explain this repo" -- one-shot print mode
    • opencode2 service status / opencode2 service restart -- manage the background service
    • Configuration lives in ~/.config/opencode/opencode.json (global) and opencode.json per project; see the config guide.

No .zshrc additions required -- it is fully self-contained.

pi​

pi (GitHub) is a minimal agent harness by Mario Zechner. Its philosophy: ship a small, solid core (agent loop, tools, sessions, provider auth) and let you extend the harness instead of adapting to it -- with TypeScript extensions, skills, prompt templates, and themes bundled as shareable packages.

  • Highlights: no MCP, sub-agents, or plan mode built in -- you add exactly what you want; tree-structured sessions with compaction; 15+ providers; four runtime modes (interactive, print/JSON, RPC, SDK).

  • Install:

    npm install -g @earendil-works/pi-coding-agent

  • Usage:

    • pi -- interactive TUI in the current project
    • pi "fix the failing test" -- one-shot prompt
    • /reload -- pick up extension changes mid-session; ask pi to modify its own extensions and it will
    • Press Enter to steer the current run, Alt+Enter to queue a follow-up

herdr​

herdr (GitHub) is an agent-aware terminal multiplexer -- tmux rebuilt for running several AI coding agents at once. A background server owns the terminals, so your agents keep working when you close the lid, drop the network, or restart the machine.

  • Highlights:
    • Every pane is classified as working, blocked, idle, or done -- the sidebar tells you which agent is waiting for you instead of you polling panes
    • Runs Claude Code, Codex, Cursor, OpenCode, pi, Grok, and 20+ others out of the box -- it does not wrap or replace them, it owns their terminals
    • Mouse-first (click, drag, right-click to split) plus tmux-style ctrl+b prefix keys
    • A CLI and socket API let scripts and agents spawn panes, prompt each other, and wait until another agent is genuinely blocked
  • Install: one binary for macOS, Linux, and Windows -- see the quick start.
  • Usage:
    • herdr -- start or reattach to your workspace
    • Start any supported agent (e.g. claude, opencode2, pi) in a pane; herdr detects it automatically
    • ctrl+b q -- detach; everything keeps running
    • herdr again -- reattach, sessions are restored
    • herdr agent list / herdr agent explain -- inspect what herdr sees