bitterbot-desktop - docs tools subagents
---
summary: "Sub-agents: spawning isolated agent runs that announce results back to the requester chat"
read_when:
- You want background/parallel work via the agent
- You are changing sessions_spawn or sub-agent tool policy
title: "Sub-Agents"
---
# Sub-agents
Sub-agents are background agent runs spawned from an existing agent run. They run in their own session (`agent:<agentId>:subagent:<uuid>`) and, when finished, **announce** their result back to the requester chat channel.
## Slash command
Use `/subagents` to inspect or control sub-agent runs for the **current session**:
- `/subagents list`
- `/subagents kill <id|#|all>`
- `/subagents log <id|#> [limit] [tools]`
- `/subagents info <id|#>`
- `/subagents send <id|#> <message>`
`/subagents info` shows run metadata (status, timestamps, session id, transcript path, cleanup).
Primary goals:
- Parallelize "research / long task / slow tool" work without blocking the main run.
- Keep sub-agents isolated by default (session separation + optional sandboxing).
- Keep the tool surface hard to misuse: sub-agents do **not** get session tools by default.
- Support configurable nesting depth for orchestrator patterns.
Cost note: each sub-agent has its **own** context and token usage. For heavy or repetitive
tasks, set a cheaper model for sub-agents and keep your main agent on a higher-quality model.
You can configure this via `agents.defaults.subagents.model` or per-agent overrides.
## Tool
Use `sessions_spawn`:
- Starts a sub-agent run (`deliver: false`, global lane: `subagent`)
- Then runs an announce step and posts the announce reply to the requester chat channel
- Default model: inherits the caller unless you set `agents.defaults.subagents.model` (or per-agent `agents.list[].subagents.model`); an explicit `sessions_spawn.model` still wins.
- Default thinking: inherits the caller unless you set `agents.defaults.subagents.thinking` (or per-agent `agents.list[].subagents.thinking`); an explicit `sessions_spawn.thinking` still wins.
Tool params:
- `task` (required)
- `handoff?` (optional structured envelope; see below)
- `label?` (optional)
- `agentId?` (optional; spawn under another agent id if allowed)
- `model?` (optional; overrides the sub-agent model; invalid values are skipped and the sub-agent runs on the default model with a warning in the tool result)
- `thinking?` (optional; overrides thinking level for the sub-agent run)
- `runTimeoutSeconds?` (default `0`; when set, the sub-agent run is aborted after N seconds)
- `cleanup?` (`delete|keep`, default `keep`)
### Handoff envelope (recommended)
Spawning with just a free-form `task` is the most common cause of
shallow sub-agent work: the child receives an ambiguous brief, asks
redundant clarification questions, and produces output the parent
has to redo. The optional `handoff` envelope forces the parent to
articulate intent up front, and the structure is rendered as a
Markdown block at the top of the child's system prompt.
```json
{
"tool": "sessions_spawn",
"task": "Audit the wallet onboarding flow for the 30s timeout reported last night.",
"handoff": {
"goal": "Identify the root cause of the wallet view stalling for 30s on a fresh install.",
"inputs": ["src/agents/usage.ts", "session-key chat-1234"],
"success_criteria": ["Root cause identified", "Reproducer documented"],
"out_of_scope": ["Don't change the Stripe integration"],
"parent_context": ["User reported 30s timeout last night.", "Stripe Onramp itself is healthy."]
}
}
```
Validation:
- `goal` is required, must be ≥ 8 characters, and rejects vague phrases ("do the thing", "stuff", "figure it out").
- Each list field accepts up to 12 entries, 3-400 chars each.
- A malformed envelope rejects the spawn with a structured error so the parent can see which field is wrong.
When `handoff` is omitted, plain `task` continues to work unchanged.
Allowlist:
- `agents.list[].subagents.allowAgents`: list of agent ids that can be targeted via `agentId` (`["*"]` to allow any). Default: only the requester agent.
Discovery:
- Use `agents_list` to see which agent ids are currently allowed for `sessions_spawn`.
Auto-archive:
- Sub-agent sessions are automatically archived after `agents.defaults.subagents.archiveAfterMinutes` (default: 60).
- Archive uses `sessions.delete` and renames the transcript to `*.deleted.<timestamp>` (same folder).
- `cleanup: "delete"` archives immediately after announce (still keeps the transcript via rename).
- Auto-archive is best-effort; pending timers are lost if the gateway restarts.
- `runTimeoutSeconds` does **not** auto-archive; it only stops the run. The session remains until auto-archive.
- Auto-archive applies equally to depth-1 and depth-2 sessions.
## Nested Sub-Agents
By default, sub-agents cannot spawn their own sub-agents (`maxSpawnDepth: 1`). You can enable one level of nesting by setting `maxSpawnDepth: 2`, which allows the **orchestrator pattern**: main → orchestrator sub-agent → worker sub-sub-agents.
### How to enable
```json5
{
agents: {
defaults: {
subagents: {
maxSpawnDepth: 2, // allow sub-agents to spawn children (default: 1)
maxChildrenPerAgent: 5, // max active children per agent session (default: 5)
maxConcurrent: 8, // global concurrency lane cap (default: 8)
},
},
},
}
```
### Depth levels
| Depth | Session key shape | Role | Can spawn? |
| ----- | -------------------------------------------- | --------------------------------------------- | ---------------------------- |
| 0 | `agent:<id>:main` | Main agent | Always |
| 1 | `agent:<id>:subagent:<uuid>` | Sub-agent (orchestrator when depth 2 allowed) | Only if `maxSpawnDepth >= 2` |
| 2 | `agent:<id>:subagent:<uuid>:subagent:<uuid>` | Sub-sub-agent (leaf worker) | Never |
### Announce chain
Results flow back up the chain:
1. Depth-2 worker finishes → announces to its parent (depth-1 orchestrator)
2. Depth-1 orchestrator receives the announce, synthesizes results, finishes → announces to main
3. Main agent receives the announce and delivers to the user
Each level only sees announces from its direct children.
### Tool policy by depth
- **Depth 1 (orchestrator, when `maxSpawnDepth >= 2`)**: Gets `sessions_spawn`, `subagents`, `sessions_list`, `sessions_history` so it can manage its children. Other session/system tools remain denied.
- **Depth 1 (leaf, when `maxSpawnDepth == 1`)**: No session tools (current default behavior).
- **Depth 2 (leaf worker)**: No session tools — `sessions_spawn` is always denied at depth 2. Cannot spawn further children.
### Per-agent spawn limit
Each agent session (at any depth) can have at most `maxChildrenPerAgent` (default: 5) active children at a time. This prevents runaway fan-out from a single orchestrator.
### Cascade stop
Stopping a depth-1 orchestrator automatically stops all its depth-2 children:
- `/stop` in the main chat stops all depth-1 agents and cascades to their depth-2 children.
- `/subagents kill <id>` stops a specific sub-agent and cascades to its children.
- `/subagents kill all` stops all sub-agents for the requester and cascades.
## Authentication
Sub-agent auth is resolved by **agent id**, not by session type:
- The sub-agent session key is `agent:<agentId>:subagent:<uuid>`.
- The auth store is loaded from that agent's `agentDir`.
- The main agent's auth profiles are merged in as a **fallback**; agent profiles override main profiles on conflicts.
Note: the merge is additive, so main profiles are always available as fallbacks. Fully isolated auth per agent is not supported yet.
## Announce
Sub-agents report back via an announce step:
- The announce step runs inside the sub-agent session (not the requester session).
- If the sub-agent replies exactly `ANNOUNCE_SKIP`, nothing is posted.
- Otherwise the announce reply is posted to the requester chat channel via a follow-up `agent` call (`deliver=true`).
- Announce replies preserve thread/topic routing when available (Slack threads, Telegram topics, Matrix threads).
- Announce messages are normalized to a stable template:
- `Status:` derived from the run outcome (`success`, `error`, `timeout`, or `unknown`).
- `Result:` the summary content from the announce step (or `(not available)` if missing).
- `Notes:` error details and other useful context.
- `Status` is not inferred from model output; it comes from runtime outcome signals.
Announce payloads include a stats line at the end (even when wrapped):
- Runtime (e.g., `runtime 5m12s`)
- Token usage (input/output/total)
- Estimated cost when model pricing is configured (`models.providers.*.models[].cost`)
- `sessionKey`, `sessionId`, and transcript path (so the main agent can fetch history via `sessions_history` or inspect the file on disk)
## Tool Policy (sub-agent tools)
By default, sub-agents get **all tools except session tools** and system tools:
- `sessions_list`
- `sessions_history`
- `sessions_send`
- `sessions_spawn`
When `maxSpawnDepth >= 2`, depth-1 orchestrator sub-agents additionally receive `sessions_spawn`, `subagents`, `sessions_list`, and `sessions_history` so they can manage their children.
Override via config:
```json5
{
agents: {
defaults: {
subagents: {
maxConcurrent: 1,
},
},
},
tools: {
subagents: {
tools: {
// deny wins
deny: ["gateway", "cron"],
// if allow is set, it becomes allow-only (deny still wins)
// allow: ["read", "exec", "process"]
},
},
},
}
```
## Concurrency
Sub-agents use a dedicated in-process queue lane:
- Lane name: `subagent`
- Concurrency: `agents.defaults.subagents.maxConcurrent` (default `8`)
## Stopping
- Sending `/stop` in the requester chat aborts the requester session and stops any active sub-agent runs spawned from it, cascading to nested children.
- `/subagents kill <id>` stops a specific sub-agent and cascades to its children.
## Limitations
- Sub-agent announce is **best-effort**. If the gateway restarts, pending "announce back" work is lost.
- Sub-agents still share the same gateway process resources; treat `maxConcurrent` as a safety valve.
- `sessions_spawn` is always non-blocking: it returns `{ status: "accepted", runId, childSessionKey }` immediately.
- Sub-agent context only injects `TOOLS.md` (no `GENOME.md`, `PROTOCOLS.md`, `HEARTBEAT.md`, `MEMORY.md`, or `BOOTSTRAP.md`).
- Maximum nesting depth is 5 (`maxSpawnDepth` range: 1–5). Depth 2 is recommended for most use cases.
- `maxChildrenPerAgent` caps active children per session (default: 5, range: 1–20).