Hooks: PreCompact/PostCompact/SessionStart(compact) fire for a subagent's compaction without agent fields; SubagentStop fires for internal summarizer calls with a never-created agent_transcript_path

Summary

Three related hook-payload gaps around subagent compaction and the harness's internal summarizer calls. Observed on Claude Code 2.1.259, Linux, with project hooks registered for PreCompact, PostCompact, SessionStart (matcher compact) and SubagentStop.

1. PreCompact and SessionStart(compact) fire for a background subagent's auto-compaction with the main loop's payload shape

When a background subagent (Agent tool) auto-compacts, the PreCompact hook and the SessionStart hook with matcher: compact both run. The payload carries the parent session's session_id, transcript_path and cwd, and nothing that identifies the subagent: no agent_id, no agent_type, no agent_transcript_path, no flag that says the compaction is a subagent's. A hook cannot tell a subagent's compaction from the main loop's without parsing the transcript tail itself.

Consequence: a PreCompact hook that checkpoints state, or holds a compaction with {"decision":"block"}, applies to every subagent's compaction as well. A SessionStart(compact) hook that re-injects resume state injects the main loop's state into the subagent. The workaround is reading the transcript tail for isSidechain records, which is a heuristic.

Ask: carry agent_id, agent_type and agent_transcript_path on PreCompact, PostCompact and SessionStart(compact) when the compaction belongs to a subagent, matching what SubagentStop already carries. A boolean is_subagent would also do.

2. PostCompact fires for the subagent compaction with the same gap

PostCompact fires roughly 35 ms after SessionStart(compact) for a subagent's compaction, with the parent's fields plus compact_summary, and no agent identity. Same ask as above.

3. SubagentStop fires for internal model calls that are not subagents

The activity summarizer (the short progress strings in the task display) and the compaction summarizer (the <analysis> output) each fire SubagentStop with an empty agent_type, a fresh agent_id, and an agent_transcript_path that is never created on disk. In one long session, 342 of the 381 distinct agent_transcript_path values a SubagentStop hook recorded were such phantoms. Only 39 were real dispatches with a transcript file.

Consequence: a SubagentStop count is not a dispatch count, and any hook that opens agent_transcript_path reports hundreds of missing files. The workaround is discriminating on agent_type == "" and on the presence of the agent-<id>.meta.json sibling, which only real subagent transcripts have.

Ask: either do not fire SubagentStop for internal summarizer calls, or mark them (for example internal: true, or agent_type: "internal:activity" / "internal:compaction"), and do not populate agent_transcript_path with a path that will never exist.

4. Minor

• SubagentStop fires before the subagent transcript's final record is appended, so a hook that reads the file at stop time sees an incomplete transcript. A flag on the payload once the file is final, or a post-stop event, would let hooks stop polling.
• A project-scope settings.json with env.TMPDIR logs "project-scoped settings can't set this key" on every start since 2.1.257. The docs still list env as project-settable without naming the excluded keys.

Environment

• Claude Code 2.1.259
• Linux
• Hooks: PreCompact, PostCompact, SessionStart (compact), SubagentStop, registered in project settings.json

async function*NZ(e,n,r=Cp,s=!1,d,m,_,E,A="turn_end"){let P=d?"SubagentStop":"Stop"; …
  me=d?{...re,hook_event_name:"SubagentStop",stop_hook_active:s,agent_id:d,agent_transcript_path:yp(d),agent_type:E??"",…}

// HTn(parentCtx, overrides) → forked toolUseContext
agentId:n?.agentId??ly(),  agentType:n?.agentType,   // FE never passes agentType → undefined → ""