SessionEnd capture omits subagent token usage (Task tool, multi-agent workflows, looped runs) — only the main-session transcript is parsed

[madrussa]

Summary

The Claude Code SessionEnd capture only parses the single main-session transcript. Any generation that runs in a subagent — the Task tool, multi-agent workflows, and looped/iterative agent runs that spawn subagents — is silently dropped. For subagent-heavy sessions this omits the majority of token usage from PostHog.

Environment

• PostHog/ai-plugin (default branch)
• Recent Claude Code (subagent transcripts stored in a separate directory — see below)

Root cause

Recent Claude Code stores transcripts in two places:

• Main session: ~/.claude/projects/<encoded-cwd>/<session-id>.jsonl
• Subagents: ~/.claude/projects/<encoded-cwd>/<session-id>/subagents/agent-<id>.jsonl (one file per subagent, each with a .meta.json sidecar)

Subagent turns are not duplicated into the main session file (the main file contains no isSidechain: true entries). Each subagent file carries its own assistant entries with full usage (input_tokens, output_tokens, cache_read_input_tokens, cache_creation_input_tokens).

find_session_log(session_id, cwd) in posthog_llma/parser.py resolves only <session-id>.jsonl directly, then falls back to a single-level glob */{session_id}.jsonl. Neither reaches the subagent files — they're one directory deeper and named agent-<id>.jsonl, not <session-id>.jsonl. parse_session then reads that one file only. hooks/session-end-llma.py reconstructs the path via find_session_log and does not use the transcript_path provided in the hook payload.

There are no references to subagents, agent-*.jsonl, isSidechain, sidechain, agentId, or the Task tool anywhere in the capture path.

Reproduction

1. Run a Claude Code session that spawns one or more subagents (invoke the Task tool / a multi-agent workflow / a loop that delegates to subagents).
2. Let the SessionEnd hook fire.
3. Inspect ~/.claude/projects/<encoded-cwd>/<session-id>/subagents/ — the agent-*.jsonl files contain assistant usage entries.
4. Compare against the $ai_generation events that reached PostHog: none of the subagent generations appear; captured token totals match only the main <session-id>.jsonl.

Expected vs. actual

• Expected: token usage for all generations in the session, including subagents, is captured.
• Actual: only main-session generations are captured; all subagent usage is omitted.

Impact

For sessions using multi-agent workflows or Task-tool delegation — where most of the work runs in subagents — the reported spend can understate actual token usage by a large margin.

Suggested fix

After locating the main session file, also enumerate <encoded-cwd>/<session-id>/subagents/agent-*.jsonl, parse each, and merge their generations/spans into the result. Subagent entries already carry usage, the parent sessionId, and promptId; the .meta.json sidecar provides an agent slug/id that could tag events (e.g. an is_subagent / agent-name property) so main vs. subagent spend can be split in PostHog.

[F0rce]

Big 👍🏼 on this feature, because our setup runs an orchestrator and spawns a lot of subagents, and we lose observability if the subagents' logs are not transmitted in the SessionStop hook :)