pi-fanout

Non-blocking async agent fanout for pi.

Problem

The built-in subagent tool is powerful, but its execute() blocks until all dispatched agents finish. While parallel tasks run concurrently internally, the main pi session is frozen waiting for the final result.

Solution

pi-fanout turns subagent dispatch into a true async job queue:

• fanout_dispatch — returns immediately with job IDs; agents run as detached processes
• fanout_status — poll running/done/failed counts at any time
• fanout_collect — retrieve final output from completed jobs
• fanout_abort — kill running jobs on demand

The main pi session stays unblocked. You can do other work, dispatch more jobs, and collect results whenever they’re ready. When a background job finishes, the extension sends a followUp message into the session so you know it’s time to collect.

Architecture

pi main process
├─ pi-fanout extension
│  ├─ JobManager (in-memory + disk state in ~/.pi/fanout/jobs/)
│  └─ Poller (every 2s: check PIDs, notify on completion)
│
├─ detached pi child (Agent A) ── writes output to job dir
├─ detached pi child (Agent B) ── writes output to job dir
└─ detached pi child (Agent C) ── writes output to job dir

Jobs survive pi restarts because:

1. Child processes are detached from the parent
2. State is persisted as meta.json + output.jsonl per job
3. On startup the extension rehydrates old jobs and checks if their PIDs are still alive

Install

pi use git:git.vpsj.de/jay/pi-fanout

Or clone into your extensions directory and add it to ~/.pi/extensions.json.

Usage

> fanout_dispatch tasks=[{agent:"worker", task:"Refactor auth.ts"}, {agent:"reviewer", task:"Review auth.ts"}]
Dispatched 2 job(s). IDs:
ltv123-abc
ltv124-def

> ... do other work in the main session ...

> fanout_status
Jobs: 2 total — 0 running, 0 queued, 2 done, 0 failed/aborted

> fanout_collect jobIds=["ltv123-abc","ltv124-def"]
[ltv123-abc] done (exit 0) [claude-sonnet-4]
Refactored auth.ts to use bearer tokens...

---

[ltv124-def] done (exit 0) [claude-sonnet-4]
The refactored auth.ts looks solid. One suggestion: ...

Tools

fanout_dispatch

Parameters:

• tasks: array of { agent, task, cwd?, model?, tools? }
• agentScope: "user" | "project" | "both" (default "user")

Returns: { dispatched: string[] } — job IDs.

fanout_status

Parameters:

• jobIds?: filter to specific IDs (omit for all)
• includeDone?: include finished jobs in listing (default true)

Returns per-job: id, status, agent, task, exitCode, pid, turns, cost.

fanout_collect

Parameters:

• jobIds: array of IDs to collect

Returns per-job: id, status, output (final assistant text), exitCode, usage, modelUsed, errorMessage.

fanout_abort

Parameters:

• jobIds: array of IDs to kill

Sends SIGTERM, then SIGKILL after 5s if still running.

How it works with the agent loop

1. The LLM calls fanout_dispatch.
2. The tool returns in <50ms with job IDs.
3. The LLM is free to continue the conversation, run other tools, or ask the user.
4. Every 2 seconds the extension polls job PIDs.
5. When a job transitions to done/failed, the extension calls:

   pi.sendUserMessage(`Fanout job X completed ...`, { deliverAs: "followUp" })
   

   This injects a user-style message that triggers a follow-up turn when the agent is idle.
6. The LLM sees the notification and calls fanout_collect to retrieve outputs.
7. The LLM acts on the collected results (e.g. synthesize a final answer, dispatch follow-up jobs).

Limitations / Roadmap

• No true server push into a running turn: If pi is mid-stream when a job finishes, the notification is queued as followUp and processed when the current turn ends.
• No job result streaming into the main session yet. Jobs are collected atomically after completion.
• No automatic fanout_collect: The LLM must explicitly call it. Future versions could auto-inject a tool-call hint.
• Jobs older than 24h are auto-cleaned on startup.

License

MIT