Pi Runner

Below is a concrete implementation spec for the Pi (pi-coding-agent CLI) runner shipped in Untether (v0.5.0).

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Scope

Goal

Provide the pi engine backend so Untether can:

• Run Pi non-interactively via the pi CLI (pi --print).
• Stream progress by parsing --mode json (newline-delimited JSON). Each line is a JSON object.
• Support resumable sessions via --session <token> (Untether emits a canonical resume line the user can reply with).

Non-goals (v1)

• Interactive TUI flows (session picker, prompts, etc.)
• RPC mode (requires a long-running process and JSON commands)

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UX and behavior

Engine selection

• Default: untether (auto-router uses default_engine from config)
• Override: untether pi

Resume UX (canonical line)

Untether appends a single backticked resume line at the end of the message, like:

`pi --session ccd569e0`

Notes:

• pi --resume/-r opens an interactive session picker, so Untether uses --session <token> instead.
• The resume token is the session id (short prefix), derived from the session header line ({"type":"session", ...}) emitted to stdout in --mode json. This requires pi-coding-agent >= 0.45.1.
• If the path contains spaces, the runner will quote it.

Non-interactive runs

Use --print and --mode json for headless JSONL output.

Pi does not accept -- <prompt> to protect prompts starting with -. Untether prefixes a leading space if the prompt begins with - so it is not parsed as a flag.

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Config additions

Untether config lives at ~/.untether/untether.toml.

Add a new optional [pi] section.

Recommended schema:

=== “untether config”

```sh
untether config set default_engine "pi"
untether config set pi.model "..."
untether config set pi.provider "..."
untether config set pi.extra_args "[]"
```

=== “toml”

```toml
# ~/.untether/untether.toml

default_engine = "pi"

[pi]
model = "..."               # optional; passed as --model
provider = "..."            # optional; passed as --provider
extra_args = []             # optional list of strings, appended verbatim
```

Notes:

• extra_args lets you pass new Pi flags without changing Untether.
• Session files are stored under Pi’s default session dir: ~/.pi/agent/sessions/--<cwd>-- (with path separators replaced by -).

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Code changes (by file)

1) New file: src/untether/runners/pi.py

Expose a module-level BACKEND = EngineBackend(...).

Runner invocation

The runner should launch Pi in headless JSON mode:

pi --print --mode json --session <session.jsonl> <prompt>

When resuming, <session.jsonl> is replaced by the resume token extracted from the chat.

Event translation

Pi JSONL output is AgentSessionEvent (from @mariozechner/pi-agent-core). The runner should translate:

• tool_execution_start -> action (phase: started)
• tool_execution_end -> action (phase: completed)
• agent_end -> completed

For the final answer, use the most recent assistant message text (from message_end events). For errors, if the assistant stopReason is error or aborted, emit completed(ok=false, error=...).

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Installation and auth

Install the CLI globally:

npm install -g @mariozechner/pi-coding-agent

Minimum supported pi version: 0.45.1.

Auth is stored under ~/.pi/agent/auth.json. Run pi once interactively to set up credentials before using Untether.

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Known pitfalls

• --resume is interactive; Untether uses --session <path> instead.
• Prompts that start with - are interpreted as flags by the CLI. Untether prefixes a space to make them safe.

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If you want, I can also add a sample untether.toml snippet to the README or include a small quickstart section for Pi in the onboarding panel.