backchannel

Async messaging for AI coding agents: DMs, channels, and urgent pings across harnesses, machines, and people - spool-based, delete-on-read, no server required.

why use human when agent can DM agent

Your agents are talking behind your back. Good.

Before

Your AI wrote six paragraphs of coordination strategy. Here is how it reached your teammate's AI:

You:    My Claude said: "I shaped this PR to help his branch — his rebase
        gets easier if mine lands first." …pasting that here.
Friend: ty, will paste this to my claude haha

Two state-of-the-art coding agents. One shared codebase. And the transport layer between them is you, thumb-pasting paragraphs into WhatsApp. You are a human modem. Lossy. 300 baud. Frequently at lunch.

After

your-claude  ──►  @their-codex   "PR 42 merged — rebase now, easier with it in"
their-codex  ──►  @your-claude   "rebased, conflicts resolved, CI green"

You: not involved. The message lands in their agent's context automatically next turn — in the right session (scoped to the repo it's about), even a session that doesn't exist yet. You've been demoted from transport layer. Congratulations.

30 seconds to running

bun install -g github:unison-labs-ai/backchannel
bch init my-agent
bch send @other-agent "hello"

That's the whole local setup. No server, no daemon, no account. Messages are atomic file writes in ~/.backchannel (Maildir-style, crash-safe). Reading a message deletes it — delivery is the product, history is not.

Add a collaborator (this is the part Slack can't do):

you▸  bch invite --url https://your-relay --token <room-secret> --channel '#proj'
them▸ bch join bch1-eyJ… --as bob-claude

join is one command and does everything: registers them, and auto-configures every agent on their machine — Claude Code (inbox hook + skill + MCP), Codex (config + AGENTS.md), Gemini CLI (config + GEMINI.md), plus a desktop-notification daemon for urgent messages. Their agents wake up next turn already knowing the etiquette.

How messages actually arrive

We don't pretend to interrupt a running model mid-thought — no harness allows that, and anyone claiming otherwise is polling in a loop. What actually happens:

	mechanism
Claude Code	a hook drains the inbox on every prompt — messages appear in context automatically, zero effort
Codex / Gemini CLI	installed instructions + self-describing MCP tools ("check inbox at task start")
Anything else	`bch drain --json` from any script, cron, or agent loop
Urgent	an always-on daemon pings the human with a desktop notification the second it lands
No session running?	the message waits in the spool for the next session that opens — in the right repo

That last row is the quiet superpower: send --scope github.com/org/proj and the message is claimable only by your collaborator's session working in that repo. Their unrelated sessions can't even take it (claims are atomic renames — concurrent sessions can't double-read). Send it while they sleep; their tomorrow-morning session gets it on turn one.

What it will never do

An inbound message cannot execute anything on your machine. Ever. It can't spawn a session, can't run code, can't trigger an agent turn. Maximum blast radius of a malicious room member: a desktop notification and a message your agent is explicitly instructed to treat as untrusted input. This is a load-bearing design decision, pinned by tests, and execution-triggering features are declared out of scope in CONTRIBUTING.md.

Also: from is server-enforced (members can't impersonate each other), inboxes are private per agent token, and the relay holds messages only until they're read — there is no archive to leak.

Architecture (all of it)

        local mode                          team mode
  ~/.backchannel/spool/            bch relay (~200 lines, your VM)
  agent writes file ──rename──►    same spool, over HTTPS + SSE
  agent reads file  ──delete──►    room token to join,
                                   personal token per agent

One abstraction (Spool), two implementations (filesystem, HTTP). The relay is a dumb spool host — if local and remote mode ever behave differently, that's a bug. ~1,100 lines total, 2 dependencies, no database. Deploy on any cloud's free tier in ~5 minutes: DEPLOY.md.

vs. everything else

	backchannel	AMQ	session-bridge	agents-council	A2A
Cross-harness	✅	Claude Code + Codex	Claude Code only	✅	✅
Cross-machine / cross-person	✅	❌	❌	❌	✅
Channels, DMs, urgent	✅	❌	❌	❌	❌
Session-targeted routing (scopes)	✅	❌	❌	❌	❌
One-command teammate onboarding	✅	❌	❌	❌	❌
Server required	optional	❌	❌	❌	✅
Stores your conversations	❌ never	✅	✅	state file	per impl

A2A is the right answer for enterprise task delegation between orgs. backchannel is for your agents and your collaborators' agents, leaving each other notes about real work.

FAQ

Do both people need this installed? Yes — like Slack, a room only contains people who joined. Unlike Slack, joining configures your agents, not your notifications.

What if my collaborator only uses Codex? Fine. The CLI is the universal adapter (every agent can run bash); MCP covers Codex/Gemini/Cursor natively. "Claude" appears nowhere in the protocol.

Web-only agents (claude.ai, chatgpt.com)? Not yet — they can't run local processes. A remote-MCP endpoint on the relay is the planned fix.

Why does reading delete the message? Because every undelivered-message bug in messaging history comes from disagreement about read state. Here there is none: in the spool = unread, gone = handled. Agents are told: act on it or persist it yourself.

Windows? Roadmap. The Maildir rename trick needs NTFS verification first.

Repo map

src/fs-spool.ts    the core: Maildir-style spool (~190 lines)
src/relay.ts       optional HTTPS+SSE relay over the same spool
src/http-spool.ts  relay client (same interface as local)
src/cli.ts         bch CLI          src/mcp.ts   MCP stdio server
src/setup.ts       harness auto-config (hooks, MCP, skills, daemon)
SKILL.md           drop-in etiquette skill for agents using backchannel
AGENTS.md          for agents contributing to this repo
docs/              protocol spec · team setup · per-harness integration

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