loopd: subagent spawn on the background slot

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

docs/runbook.md

@@ -1243,3 +1243,29 @@ session.
 
 **Check.** `sessions/heartbeat/0.jsonl` gains a new `User` whose text starts with `It is ` and an
 RFC 3339 timestamp.
+
+## 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>/`.