Playground

harn playground runs a pipeline against a Harn-native host module in the same process. It is intended for fast pipeline iteration without wiring a JSON-RPC host or booting a larger app shell.

Quick start

The repo ships with a minimal example:

harn playground \
  --host examples/playground/host.harn \
  --script examples/playground/echo.harn \
  --task "Explain this repository in plain English"

--task is exposed to the script through the HARN_TASK environment variable, so the example reads it with env_or("HARN_TASK", "").

If you want an offline smoke test, force the mock provider:

harn playground \
  --host examples/playground/host.harn \
  --script examples/playground/echo.harn \
  --task "Say hello" \
  --llm mock:mock

For deterministic end-to-end iteration, harn playground also accepts the same JSONL fixture flags as harn run:

harn playground \
  --host examples/playground/host.harn \
  --script examples/playground/echo.harn \
  --task "Explain this repository" \
  --llm-mock fixtures/playground.jsonl

Use --llm-mock-record <path> once to capture a replayable fixture, then switch back to --llm-mock <path> while you iterate on control flow.

Host modules

A playground host is just a .harn file that exports the functions your pipeline expects:

pub fn build_prompt(task_text) {
  return "Task: " + task_text + "\nWorkspace: " + cwd()
}

pub fn request_permission(tool_name, request_args) -> bool {
  return true
}

The playground command loads those exported functions and makes them available to the entry script during execution. If the script calls a host function that the module does not export, the command fails with a pointed error naming the missing function and the caller location.

Watch mode

Use --watch to re-run when either the host module or the script changes:

harn playground --watch --task "Refine the prompt"

The watcher tracks the host and script parent directories recursively and debounces save bursts before re-running.

Starter project

Use the built-in scaffold when you want a dedicated scratchpad:

harn new pipeline-lab-demo --template pipeline-lab
cd pipeline-lab-demo
harn playground --task "Summarize this project"