Extensions

metaharness supports pluggable proposer backends through backend_plugins in metaharness.json.

This lets teams use internal or closed-source harnesses without modifying core library code.

Config Shape

{
  "backend_plugins": {
    "cursor": {
      "factory": "my_harness_plugins.cursor:create_backend",
      "options": {
        "model": "cursor-pro",
        "proposal_timeout_seconds": 180
      }
    }
  }
}

• key (cursor) becomes backend name used by CLI
• factory must be module:callable
• options are plugin defaults and can be overridden by backends.<name> and CLI overrides

Factory Contract

The factory callable can accept:

• project
• options
• backend_name

It must return a proposer backend object with:

• name
• prepare(request)
• invoke(request)
• collect(execution)

Usage

Run with your plugin backend:

uv run metaharness run ./my-project --backend cursor --budget 1

Run experiments with plugin backend names:

uv run metaharness experiment ./my-project --backend cursor --trials 3

Recommended Pattern

1. Keep provider-specific command building and parsing inside your plugin module.
2. Normalize events and changed files in your plugin collect result.
3. Keep authentication and provider environment setup outside metaharness.