The harness-md plugin

harness-md is a plugin for coding agents. It is built around one document, a project’s HARNESS.md, which records the design of the project’s harness. A harness is how a project is set up for coding agents: where the agent runs, what it reads, how it checks its work, what stops a bad change, and what it may do without asking. Every skill in the plugin either writes that document, reads it, or turns it into a working harness.

You don’t need the plugin to use a HARNESS.md. The format is designed so that an agent with nothing installed can read a file and build from it; see Using a HARNESS.md without the plugin. What the plugin adds is a fixed procedure, a validator for the format, and seven builder skills. A harness has seven layers (Environment, Knowledge, Feedback, Gates, Authority, Completion, Entropy), and each builder knows how to design, build, and check one of them.

Install

The plugin follows the Agent Plugins 1.0.0 layout and loads in three clients.

Claude Code

/plugin install harness-md --marketplace hoshiori-dev/harness.md

This is one step on Claude Code 2.1.275 or later. On earlier versions, run /plugin marketplace add hoshiori-dev/harness.md first, then install.

Codex

codex plugin marketplace add hoshiori-dev/harness.md
codex plugin add harness-md@harness-md

Antigravity

agy plugin install https://github.com/hoshiori-dev/harness.md

The scripts that ship with the skills are written in Python and need uv to run, so install uv first. uv fetches the dependencies each script declares; your project needs no Python setup of its own.

Which workflow to run

Four skills are workflows. They run only when you ask for them by name.

Your situation	Run
A new project with no harness yet	`harness-design`, then `harness-build`
An existing project that agents struggle in, with no `HARNESS.md` yet	`harness-audit` to find what is wrong, then `harness-design`, then `harness-build`
A harness that works, except that one thing keeps going wrong	`harness-audit` with the symptom, then `harness-design` to revise the design, then `harness-build`
You want a project’s harness written down, yours or one you admire	`harness-extract`
You have a `HARNESS.md` and want the project to match it	`harness-build`

The four workflows describes each one. Skills and the verifier agent covers everything else the plugin ships.