How Harness, Switchboard and Cairn fit together

Harness, Switchboard, and Cairn are three small tools that each do one job. Together they let agents pick up work, do it unattended, and leave something a person can review.

• Harness — where agents run: supervised, always-on agent sessions and scheduled sweeps, restarted when they fall over and attachable from any terminal.
• Switchboard — how work reaches agents: verified webhooks become durable todos on queues; a doorbell pushes each todo to a live session.
• Cairn — where agents put what they made: shareable artifacts (reports, diffs, logs, run traces) with comments, reactions and a TTL.

flowchart LR
  forge["GitHub / Gitea"] -- signed webhook --> sb
  cairn -- "signed webhook (artifact.created)" --> sb
  subgraph sb["Switchboard"]
    verify["verify"] --> route["route (jq rules)"] --> queue[("todo queue")]
  end
  queue -- "doorbell (MCP channel)" --> worker
  subgraph harness["Harness"]
    worker["agent worker"]
  end
  worker -- "claim / complete" --> queue
  worker -- "share artifact" --> cairn["Cairn"]
  human(["you"]) -- "read, comment" --> cairn

A handoff, end to end

1. An agent running under Harness finishes part of a job and writes a handoff prompt to Cairn with artifact_create, tagged handoff plus a lane: tag for the pool that should take it next.
2. Cairn saves the artifact and sends an outbound webhook to Switchboard.
3. Switchboard verifies the signature, checks who created the artifact, and a routing rule puts a todo on the queue the tags point at.
4. Another agent under Harness gets the doorbell, claims the todo, and reads the prompt with artifact_read.
5. It does the work, drops its report or trace in Cairn, and completes the todo with the link.
6. You open the links, comment where something's off, and the next agent reads your comments.

The artifact is the record at every step. The webhook and the doorbell are only hints that something is waiting, so a missed one delays the work without losing it.

Start with one

You don't need all three on day one.

You want to…	Start with
Share output, or read what an agent dropped for you	Cairn alone: your first share, then connect your agent
Hand work between agents yourself	Cairn: the handoff pattern needs nothing else
Turn events (a PR review request, a new artifact) into work an agent picks up	Add Switchboard: receive your first webhook, then route it with the handoff recipe
Keep agents running unattended, or on a schedule	Add Harness: the quickstart