## subagent-id

**What you see.** A `subagent` call is denied as invalid-arguments. The tool result names the
child id that would not fit and this entry:

```text
Denied: the arguments are not valid for this tool: child session id `sub-…` would not fit; see docs/runbook.md#subagent-id.
```

**Why.** A session id is `[a-z0-9-]`, at most 64 bytes. The child is `sub-<parent>-<n>` with
`n` the parent's next call id. A parent already near that limit, or a nested chain of
`sub-` prefixes, cannot spawn another child.

**Confirm.**

```sh
printf '%s' '<the id named in the denial>' | wc -c
```

**Fix.** Do the work in the current session, or start a shorter parent id (`bxctl chat` without
a long `--session`). Nested children share one background slot; a parallel pool is out of
scope.

**Check.** A `subagent` call from a short parent (`chat-*`, `mm-*`, `heartbeat`) completes and
writes `sessions/sub-<parent>-<n>/`.