The Harness

Automatic Control for anything that acts.

The Harness is Automatic Control for anything that acts. You tell an agent what to do in your own words; a language model plans and writes short programs, and The Harness runs them in an environment the agent can see and act in, while you watch, add a message or take over.

The agent thinks while it acts: between model calls, its programs, skills and reactive rules keep working, and what it learns along the way (notes, tested skills, rules) applies in the same run.

Use The Harness in the terminal, run one task from the CLI, drive it from another program with JSON lines, call it from apps, editors and other agents over MCP and ACP, or build on the Python API.

Getting started

npm install -g @lmmslab/harness   # or: pip install the-harness
harness

The Harness needs Python 3.11 or newer. harness opens a conversation in your terminal. The first time, it asks which model to use and for its key; then type a task. Any model works: hosted ones through OpenRouter, OpenAI, Anthropic and other providers, or a model you run yourself.

See Install and the Quickstart for a real task on your own computer.

Commands

harness                 # talk with the agent in the terminal
harness run "task" -p   # do one task and print the answer (--json: every event)
harness run rpc         # drive a conversation with JSON lines, from another program
harness install         # packages (also: list, remove)
harness library         # what the agent learned
harness replay          # a run again in the terminal
harness doctor          # what this computer has and lacks

Where it works

pip/npm install only the small core. Environments come in packages, installed by themselves the first time a task needs them.

Packages

Documentation

• Quickstart
• Automatic Control: the ideas behind The Harness
• Models and providers
• Interrupt and take over
• Let it learn an environment
• Write a skill, write a package, add an environment

Development

pip install -e .
harness install ./packages/computer ./packages/browser ./packages/workspace --link --yes
harness doctor

See CONTRIBUTING.md and the validation guide.

Citation

@misc{lmmslab2026harness,
  author = {{LMMs-Lab}},
  title = {The Harness: One Agent Harness for Anything that Acts},
  howpublished = {\url{https://github.com/EvolvingLMMs-Lab/the-harness}},
  year = {2026}
}

License

MIT; third-party material retains its own terms. See LICENSE and CONTRIBUTORS.md.