---
summary: "Auto-reply queue modes, defaults, and per-session overrides"
read_when:
  - Changing auto-reply execution or concurrency
  - Explaining /queue modes or message steering behavior
title: "Command queue"
---

We serialize inbound auto-reply runs (all channels) through a tiny in-process queue to prevent multiple agent runs from colliding, while still allowing safe parallelism across sessions.

## Why

- Auto-reply runs can be expensive (LLM calls) and can collide when multiple inbound messages arrive close together.
- Serializing avoids competing for shared resources (session files, logs, CLI stdin) and reduces the chance of upstream rate limits.

## How it works

- A lane-aware FIFO queue drains each lane with a configurable concurrency cap (default 1 for unconfigured lanes; main defaults to 4, subagent to 8).
- `runEmbeddedAgent` enqueues by **session key** (lane `session:<key>`) to guarantee only one active run per session.
- Each session run is then queued into a **global lane** (`main` by default) so overall parallelism is capped by `agents.defaults.maxConcurrent`.
- When verbose logging is enabled, queued runs emit a short notice if they waited more than ~2s before starting.
- Typing indicators still fire immediately on enqueue (when supported by the channel) so user experience is unchanged while we wait our turn.

## Defaults

When unset, all inbound channel surfaces use:

- `mode: "steer"`
- `debounceMs: 500`
- `cap: 20`
- `drop: "summarize"`

Same-turn steering is the default. A prompt that arrives mid-run is injected
into the active runtime when the run can accept steering, so no second session
run is started. If the active run cannot accept steering, Mnemo waits for the
active run to finish before starting the prompt.

## Queue modes

`/queue` controls what normal inbound messages do while a session already has
an active run:

- `steer`: inject messages into the active runtime. Mnemo delivers all pending steering messages **after the current assistant turn finishes executing its tool calls**, before the next LLM call; Codex app-server receives one batched `turn/steer`. If the run is not actively streaming or steering is unavailable, Mnemo waits until the active run ends before starting the prompt.
- `followup`: do not steer. Enqueue each message for a later agent turn after the current run ends.
- `collect`: do not steer. Coalesce queued messages into a **single** followup turn after the quiet window. If messages target different channels/threads, they drain individually to preserve routing.
- `interrupt`: abort the active run for that session, then run the newest message.

For runtime-specific timing and dependency behavior, see
[Steering queue](/concepts/queue-steering). For the explicit `/steer <message>`
command, see [Steer](/tools/steer).

Configure globally or per channel via `messages.queue`:

```json5
{
  messages: {
    queue: {
      mode: "steer",
      debounceMs: 500,
      cap: 20,
      drop: "summarize",
      byChannel: { discord: "collect" },
    },
  },
}
```

## Queue options

Options apply to queued delivery. `debounceMs` also sets the Codex steering
quiet window in `steer` mode:

- `debounceMs`: quiet window before draining queued followups or collect batches; in Codex `steer` mode, quiet window before sending batched `turn/steer`. Bare numbers are milliseconds; units `ms`, `s`, `m`, `h`, and `d` are accepted by `/queue` options.
- `cap`: max queued messages per session. Values below `1` are ignored.
- `drop: "summarize"`: default. Drop the oldest queued entries as needed, keep compact summaries, and inject them as a synthetic followup prompt.
- `drop: "old"`: drop the oldest queued entries as needed, without preserving summaries.
- `drop: "new"`: reject the newest message when the queue is already full.

Defaults: `debounceMs: 500`, `cap: 20`, `drop: summarize`.

## Steer and streaming

When channel streaming is `partial` or `block`, steering can look like several
short visible replies while the active run reaches runtime boundaries:

- `partial`: the preview may finalize early, then a new preview starts after
  steering is accepted.
- `block`: draft-sized blocks can create the same sequential appearance.
- Without streaming, steering falls back to a followup after the active run when
  the runtime cannot accept same-turn steering.

`steer` does not abort in-flight tools. Use `/queue interrupt` when the newest
message should abort the current run.

## Precedence

For mode selection, Mnemo resolves:

1. Inline or stored per-session `/queue` override.
2. `messages.queue.byChannel.<channel>`.
3. `messages.queue.mode`.
4. Default `steer`.

For options, inline or stored `/queue` options win over config. Then
channel-specific debounce (`messages.queue.debounceMsByChannel`), plugin
debounce defaults, global `messages.queue` options, and built-in defaults are
applied. `cap` and `drop` are global/session options, not per-channel config
keys.

## Per-session overrides

- Send `/queue <steer|followup|collect|interrupt>` as a standalone command to store the queue mode for the current session.
- Options can be combined: `/queue collect debounce:0.5s cap:25 drop:summarize`
- `/queue default` or `/queue reset` clears the session override.

## Scope and guarantees

- Applies to auto-reply agent runs across all inbound channels that use the gateway reply pipeline (WhatsApp web, Telegram, Slack, Discord, Signal, iMessage, webchat, etc.).
- Default lane (`main`) is process-wide for inbound + main heartbeats; set `agents.defaults.maxConcurrent` to allow multiple sessions in parallel.
- Additional lanes may exist (e.g. `cron`, `cron-nested`, `nested`, `subagent`) so background jobs can run in parallel without blocking inbound replies. Isolated cron agent turns hold a `cron` slot while their inner agent execution uses `cron-nested`; both use `cron.maxConcurrentRuns`. Shared non-cron `nested` flows keep their own lane behavior. These detached runs are tracked as [background tasks](/automation/tasks).
- Per-session lanes guarantee that only one agent run touches a given session at a time.
- No external dependencies or background worker threads; pure TypeScript + promises.

## Troubleshooting

- If commands seem stuck, enable verbose logs and look for "queued for ...ms" lines to confirm the queue is draining.
- If you need queue depth, enable verbose logs and watch for queue timing lines.
- Codex app-server runs that accept a turn and then stop emitting progress are interrupted by the Codex adapter so the active session lane can release instead of waiting for the outer run timeout.
- When diagnostics are enabled, sessions that remain in `processing` past `diagnostics.stuckSessionWarnMs` with no observed reply, tool, status, block, or ACP progress are classified by current activity. Active work logs as `session.long_running`; active work with no recent progress logs as `session.stalled`; `session.stuck` is reserved for recoverable stale session bookkeeping, including idle queued sessions with stale ownerless model/tool activity, and only that path can release the affected session lane so queued work drains. Repeated `session.stuck` diagnostics back off while the session remains unchanged.

## Related

- [Session management](/concepts/session)
- [Steering queue](/concepts/queue-steering)
- [Steer](/tools/steer)
- [Retry policy](/concepts/retry)
