boxmaker/docs/plans/M6a/files/runbook-subagent-id.md

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:

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.

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>/.