Codex review caught an observe-only violation: the subagent/end emit fires from a
detached `.then` registered BEFORE start() returns — so before the caller's own
`await run.result` continuation runs. Carrying `result.output` by reference let a
mutating subagent/end listener corrupt the SubagentResult.output the caller/tool
then consumes. structuredClone() makes the event a read-only snapshot. Added a
regression test that mutates the event's array and asserts the caller's result is
untouched; proven to fail red without the clone. Updated the RFC + READMEs to note
the clone is load-bearing for the observe-only guarantee.
A hooks bridge translating SubagentStart/SubagentStop needs to know WHICH kind of
subagent ran and WHAT it produced — Claude Code's hooks carry subagent_type and the
child's final message. Enrich the existing lifecycle emits to match, observe-only:
- agentType: an optional caller-supplied subagent-kind label (CC's subagent_type),
added to SubagentStartRequest and carried VERBATIM onto both subagent/start
(SubagentRunInfo) and subagent/end (SubagentRunEndInfo). The seam never interprets
it. dsh-tool-subagent threads it from a new optional Config.agentType, so a
deployment exposing multiple subagent kinds (one tool load per kind) labels each.
- lastAssistantMessage: the child's final output (SubagentResult.output), added to
SubagentRunEndInfo on the settle path so an observer sees what the subagent
produced without holding the run. Absent on the reject path (no result produced).
Strictly observe-only: both events stay plain emits (subagent/end fires from a
detached .then and awaits no listener). A control-flow subagent/end (awaited
waterfall returning a decision) would need the emit→waterfall reshape, awaiting
listeners before settling, and a provider resume capability — deferred to the
background/steering redesign (FIXME(subagent-continuation) anchors it). RFC:
implemented/feature/2026-06-30-subagent-observe-enrich.md.
Tianyi Cui