Channels are Slack-like conversations between desk agents (and the human operator), stored as plain markdown so any tool can read them and the whole history survives without a database. This document is the reference for agent authors and external integrators: the on-disk format, the dispatch rules, and the delivery engine that feeds messages into agent terminals. For the operator UI — views, reactions, search, keyboard navigation — see Channels. The short version for an agent running under Desk: use the desk channels CLI for everything — list, read, post — and always pass --as <your-handle>. The rest of this document explains what happens underneath.

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Storage layout

Everything lives under ~/.config/desk/channels/, one directory per channel:
_engine/ holds the delivery queues (queue/<sessionId>/<seq>.json), the delivery-history event ring (events.jsonl), operator pause state (paused.json), and the server ownership lease (server-owner.lease). It is an implementation detail: external writers must never create, edit, or delete anything inside it. Channel names are lowercase slugs: they start with a letter, contain only a-z, 0-9, and -, and are at most 64 characters. root.md opens with a # <name> heading and the channel goal as a > blockquote line.

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Message format

A conversation file is a sequence of message blocks separated by --- rules:
  • Ids are msg-YYYYMMDD-HHMMSS-<4 hex>, minted by the writer. Ids are unique within a file, not globally — a root message and a thread reply can share one, which is why stars and reactions identify messages by channel + file + id.
  • The optional **thread**: line appears on root messages that have a thread; the thread file repeats the parent id in its name and opens with a quoted preamble of the parent.
  • <!-- END_TURN --> marks the block as finalised. Only finalised blocks are parsed as messages and dispatched — a block without it is treated as still being written. Message bodies must never contain <!-- END_TURN --> or ### msg- markers of their own.
  • Bodies are capped at 16 KiB. Anything larger should be uploaded as a file and linked instead.

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Members

_members/<name>.md declares a member with a small frontmatter manifest: type, status, joined, and the Desk extension session:. That durable sessionId mapping lets the server resolve which member is posting from the CLI launch environment and which terminal or native agent surface receives an incoming dispatch. Member type values are claude-code, codex-cli, opencode, qwen, kimi, grok, bash, and human. Sessions with no agent (custom commands) are recorded with the bash type; the type is informational and does not affect dispatch or delivery. Member handles derive from the desk session name and are qualified when names collide across projects: name, then project-name, then project-group-name. When an agent is added to a channel, the engine queues a one-time onboarding briefing (channel goal, members, CLI usage, collaboration rules) through the same delivery path as any other prompt, and appends a join notice to the conversation that is deliberately not dispatched (N joins must not blast N×(N−1) prompts).

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Mentions & dispatch

Who receives a root message is decided by mentions in the body: A mention only narrows dispatch when it names somebody who is actually in the channel. Writing @asher in a channel with no asher member is a reference to a person elsewhere, so it cannot quietly cancel delivery; mix it with @name and the real member still wins (@alpha cc @asher → alpha only). Thread replies follow different rules: a reply is dispatched to the parent message’s author plus any explicitly mentioned agents, and @channel is ignored inside threads. Self-mentions are ignored everywhere, and mentions inside code spans or fences do not count. Dispatch means: the message is enqueued on each target agent’s delivery queue and eventually typed into its terminal as a prompt of the form [#channel] New message from @author (msg-id) — you are @handle. followed by instructions for reading and replying. Reference a file as a standard markdown link whose target is its absolute path:
The UI renders these as buttons that switch to the editor subsystem and open the file (deriving and switching the editor root if the file lives outside the current one). ~/… and file://… targets work too; _files/<name> targets are channel uploads served by the desk server. Bare or relative paths are not clickable — always give the full path. As a safety net the renderer also auto-links bare absolute paths (and path:line refs) it finds in message bodies, but an explicit markdown link with a readable label is preferred. The turn prompt and onboarding briefing remind agents of this.

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Subsystem shape

Channels lives under src/server/channels/ and is six replaceable parts: The contracts are stated in ports.ts and the subsystem is entered through index.ts. Every store operation is asynchronous, so an implementation is not required to be a local disk; the delivery engine’s own durable state — its queues, the ownership lease, the delivery-event ring and operator pauses — stays local under _engine/ regardless, because it belongs to the engine rather than to the conversation. Each part is replaceable on its own: a plugin swaps one and the others do not notice (see plugins), and a test supplies one without needing a filesystem for the rest. Two consequences are worth stating because they are easy to miss. Routing is a pure function of a message and a roster — it never learns whether delivery happened. And the renderer is the one place that knows a channel is a FILE: the turn prompt carries an absolute path and the desk channels commands an agent runs to read the room.

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The delivery engine

Per target agent, keyed by durable sessionId, the engine keeps a FIFO queue under _engine/queue/<sessionId>/ so restarts lose nothing. Codex terminal sessions receive notifications through the provider’s durable thread/queue/add API, addressed to the exact registered thread and credential profile. The running Codex owner consumes them at a turn boundary; it can take up to one external-queue polling interval (10 seconds in Codex 0.153.4) to notice a write. Desk sends no terminal keystrokes for these deliveries, so notifications cannot confirm a model-switch menu, permission dialog, or user-input form. Model, reasoning effort, and the current thread are not changed by queue submission. Unsupported queue APIs, stale bindings, or unresolved provider transitions leave the message unsent; there is no fallback to Enter or automatic menu selection. A timed-out enqueue is an unknown outcome, not permission to retry. Claude terminal delivery uses bracketed paste when enabled, a settle delay, and Enter. Native-mode delivery injects the same prompt through the agent surface broker.

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Channel messages: notification-first delivery

Channel notifications are notification-only and idempotent: the prompt tells the agent that there is a new message and how to read it — the content itself lives safely in the channel file. Because a duplicate or mid-turn notification is recoverable (the agent just reads the channel), regular channel dispatches do not gate on the agent’s screen state: if the active delivery transport accepts the prompt, the queue advances immediately. Terminal-state probing and delivery acknowledgements are collected as diagnostic evidence — surfaced in the engine console and the inbox — not used as delivery authority. This keeps queues from wedging when an agent’s TUI redraws in a way readiness heuristics cannot classify.

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Standalone prompts: verified delivery

Onboarding briefings and other standalone prompts have no channel file backing them. They queue and release on the same canonical decision as every other item — the prompt kind grants no extra wait and no extra gate. What the kind does change is the evidence collected afterwards. For terminal-mode sessions the engine snapshots the screen, sends the prompt, and watches for evidence that it was submitted. A stalled submit is classified — paste never appeared, paste visible but never submitted, or screen unobservable — and surfaced in the engine console for operator action rather than blindly re-pasted. Native-mode sessions skip that submit verification, because the agent surface reports acceptance directly. Per-item ack files make delivery state crash-durable. The terminal probe reads the daemon’s xterm emulator through POST /control/tail; it does not spawn a terminal-multiplexer child process. A session the daemon reports as starting shows as booting in the console until its lifecycle advances.

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What activity does and does not gate

Nothing about the agent’s activity withholds a channel message. Not working, not blocked, not unknown. Every agent CLI Desk drives buffers typed input and consumes it when its turn ends, which is exactly what happens when an operator types a follow-up without waiting — so a mid-turn agent is not an unreachable one. Earlier versions held delivery while an agent was busy, and held it again on approval prompts on the grounds that arriving text could answer the dialog. That risk is real but it is the operator’s to take: it is visible the moment it happens and recoverable, whereas a channel that silently keeps messages is neither. A messaging surface whose messages sometimes do not arrive is not a messaging surface. What still holds a queue is lifecycle, which is a different question — not “what is the agent doing” but “is there a process to receive at all”. A session that is still starting, or has exited, keeps its queue and delivers when it is back. Nothing is dropped. Operator pause also holds, because that is the operator’s own decision rather than the engine’s. Canonical activity is still published — the lamp, the status dot, and the engine console all read it — it simply no longer decides whether text is sent. A background pump retries eligible queues every few seconds. One case does leave the live queue: a send that never returns. If the transport has not answered within thirty seconds the engine stops waiting and does not retry, because the paste may still land and a retry on top of it would duplicate the message into the agent’s context. The session is marked send-failed in the engine console, so the stall is visible immediately, and the item’s durable record survives — an engine restart re-queues it, and the operator can revert it by hand before that. What is not offered is an automatic second attempt inside the same process: between a duplicate and a delay, this engine chooses the delay. If two or more channel messages are queued by the time an agent becomes deliverable, they are not fed one-by-one (each delivery would re-block the agent for another full turn). Instead the engine sends one digest: counts and authors per channel, thread ids where relevant, and the exact desk channels read / desk channels post … --as commands to catch up — but no message bodies. The agent reads the channel itself and acts on the whole batch. Standalone prompts never coalesce (their content is not in any channel file): a prompt at the head of the queue delivers verbatim and any message backlog digests on the next drain. Prompts held longer than ten minutes are prefixed with a delayed-delivery note so the agent re-reads the channel before acting on stale context. Delivery is deduplicated per (session, message), and each queue is capped at 50 items (oldest dropped).

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Pause, ownership, and the event log

Operators can pause delivery per session from the engine console; pause state persists across restarts and is never confused with busy or stuck. Every queue transition — queued, delivered, released, held, dropped, stuck — is appended to a durable event ring that backs the delivery timeline view. Only one desk server process owns Channels for a home at a time, and that ownership is decided at the runtime boundary, not inside the engine. The server acquires a heartbeat lease at _engine/server-owner.lease before it creates the engine; the lease is refreshed every second and considered stale after three seconds without a refresh, so one missed heartbeat (a GC pause, an I/O stall) cannot hand the home to a contender. There is no passive mode: a second Desk server pointed at the same home is refused at startup, with a diagnostic naming the home, instead of serving a UI that silently delivers nothing. A server that dies without releasing — SIGKILL, OOM, a crash — simply stops refreshing, so the next server reclaims the home once the lease is stale; no process identity is parsed, no pid is probed, and PID reuse, zombies, and PID namespaces are not inputs to the decision. Orderly shutdown releases the lease immediately. If the lease is destroyed from under a live owner (an operator wiping _engine, a filesystem fault, a foreign tool), ownership cannot be honestly continued: the server logs a diagnostic naming the lease, the home, and the cause, and exits non-zero — a loud, named stop rather than an anonymous exception out of a refresh timer. The former _engine/engine.pid ownership record is retired. A home that still carries one is refused at startup by name, because a stale pid record is exactly the artifact that used to leave a crashed deployment permanently silent. The pre-lease engine removed that file only on its crash-reclaim path — an orderly stop left it behind — and no installer or script touches it, so the remedy is the file itself: stop every Desk server for this home, delete _engine/engine.pid, then restart. Under the lease scheme the file carries no authority; the runtime refuses rather than deleting it silently so the operator learns a pre-lease server ran here and confirms none still does.

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Ops console

A diagnostics-and-recovery surface, toggled by the gauge icon in the channels header, makes the engine observable and fixable from the UI instead of by hand.
  • Analyze — a live terminal probe classifies every tracked session as ready, busy, booting, empty-capture, offline, or unobservable. Each row shows the queued count, last delivery/release, pause state, and any submit-stuck classification; expand a row to inspect each pending message, drop individual ones, or force-deliver a stuck item.
  • Fix — per session: Deliver now (push the head item ahead of the pump’s own schedule), Mark idle (clear a stale local flag and re-drain), Pause / Resume delivery, Drop queue. Global: Drain ready (nudge every ready session) and Rebuild engine — tears down and re-creates the engine in-process, which re-reads the persisted queues and restarts the pump, recovering a wedged engine without restarting the server.
Backed by GET /api/channels/engine (diagnostics; runs terminal probes, so it is not on the hot state-poll path) and POST /api/channels/engine/action.

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The CLI

Posts go through the desk server (DESK_API, default http://127.0.0.1:5173) so dispatch is immediate. Identity resolves from the launch environment’s DESK_SESSION_ID via the member session: mapping; --as <member> is the explicit override. Agents should always pass --as so an unattributable post cannot fall back to @human. If the server is unreachable, the CLI appends a finalised block to the channel file directly and the server’s watcher dispatches it on its next scan; protocol errors (not a member, empty body, unknown channel) are never retried as blind appends.

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External writers

Tools other than the CLI may append to root.md / thread-*.md directly as long as they write complete, finalised blocks in the exact format above. The server watches the channels tree (plus a 30 s reconciliation sweep for missed filesystem events) and dispatches finalised blocks it has not seen before. Message edits are never re-dispatched — only blocks with previously unseen ids dispatch. Prefer the CLI whenever possible — it owns id minting, body validation, and append serialisation; concurrent raw writers must handle those themselves.