# Customizing harnesses

Harness settings live in the sandbox, so they persist like any other files. Use the file API to add instructions:

```sh
harebox new --name custom
harebox file write current --path /home/user/AGENTS.md --raw "Run the tests before finishing."
```

Bring your own harness by implementing the `Harness` port, with `ID`, `Models` and `Run`. The Shell harness is the smallest example, at `services/harness/harness.go`.