takopi/docs/runner/pi/pi-runner.md

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

---

Scope

Goal

Provide the pi engine backend so Takopi 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 <path> (Takopi 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)

---

UX and behavior

Engine selection

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

Resume UX (canonical line)

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

`pi --session /home/user/.pi/agent/sessions/--repo--/2026-01-02T12-34-56-789Z_abcd.jsonl`

Notes:

• pi --resume/-r opens an interactive session picker, so Takopi uses --session <path> instead.
• The resume token is the session file path (JSONL), treated as an opaque string.
• 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 -. Takopi prefixes a leading space if the prompt begins with - so it is not parsed as a flag.

---

Config additions

Takopi config lives at ~/.takopi/takopi.toml.

Add a new optional [pi] section.

Recommended schema:

# ~/.takopi/takopi.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 Takopi.
• Session files are stored under Pi's default session dir: ~/.pi/agent/sessions/--<cwd>-- (with path separators replaced by -).

---

Code changes (by file)

1) New file: src/takopi/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 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=...).

---

Installation and auth

Install the CLI globally:

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

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

---

Known pitfalls

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

---

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