Claude Code 2.1.257 lineage gaps: SubagentStart names no parent; a subagent's hooks name no parent

[ScriptedAlchemy]

Scope

Lineage only: who a subagent's parent is, what the root session is, and the parent-of-subagent chain. Operator/user identity is out of scope for agent-bundle by maintainer decision (2026-09-03, see #391); this issue tracks no identity fields.

What the host exposes (2026-09-03, live capture)

Evidence: docs/audits/2026-09-03-host-lineage-matrix.md §1–§5 and fixtures/host-lineage/claude-2.1.257.ndjson (PR #421). Real Claude Code 2.1.257, claude -p, one Agent subagent that spawned a nested Agent (depth 2).

Fact	Observed
Root id on every hook payload	session_id (also on nested subagents' events)
Subagent id on its own events	agent_id + agent_type on PreToolUse/PostToolUse/SubagentStart/SubagentStop
Parent of a subagent	absent everywhere. SubagentStart carries only the child's agent_id; the child's own events carry session_id (root) and its agent_id, never the spawning agent. agent_transcript_path is flat (<session>/subagents/agent-<id>.jsonl), so a nested agent's path does not name its parent either.
Depth	not delivered
background_tasks[] on Stop/SubagentStop	lists every running subagent (id, type: subagent, agent_type, description) — a flat set, no edges
MCP tools/call _meta	claudecode/toolUseId = the PreToolUse tool_use_id (this is what makes hook→MCP correlation exact)
PostToolUse for MCP tools	tool_response is a plain string (documented as an object)

What the framework does about it

request.lineage (#421) is the single surface for parent/root/subagent on every host. On Claude it infers the parent as the agent whose Agent/Task PreToolUse is the newest unclaimed spawn under the same root when SubagentStart fires (resolution: 'registry'); depth follows the parent chain. It is exact for sequential spawns; when two different parents have unclaimed spawns it refuses (id-not-resolvable) rather than guessing, and when several siblings from one parent are pending it places the child under that parent but withholds subagent.toolCallId (the cohort is uncertain). MCP calls correlate exactly through _meta["claudecode/toolUseId"]. The capability-table lineage reasons cite this issue and the observations record since #444 (merged as 284141958).

Host-side gap (recorded for reference; not submitted to the host — maintainer decision 2026-09-03)

parent/depth stay degraded (inferred) until a Claude Code build carries a parent_agent_id (or the spawning tool_use_id) on SubagentStart/SubagentStop and on the subagent's own hook payloads. No feedback is sent to Anthropic; the per-host write-up was removed in #467 (merged as 9d4fbd867) per maintainer decision, and docs/audits/2026-09-03-host-lineage-matrix.md plus the capability tables are the record a newer build is re-checked against.

[ScriptedAlchemy]

Final-sweep verification (2026-09-03, on main @ 21af4ce): confirmed host-side gap with reproducible evidence in-repo — fixtures/host-lineage/claude-2.1.257.ndjson and docs/audits/2026-09-03-host-lineage-matrix.md §1–§5 (PR #421). Every SubagentStart/SubagentStop and every subagent-side hook payload carries session_id + agent_id and never a parent id or depth, and the flat agent_transcript_path cannot substitute. The framework side is complete (request.lineage registry resolution, ambiguous only for concurrent spawns), and the PostToolUse string-vs-object tool_response observation is already accepted by the Claude event codec (#421, fix(events): accept any present Claude PostToolUse tool_response). Label host-integration is correct; nothing further is ours.

[ScriptedAlchemy]

2026-09-03: scope line added (lineage only; identity is out of scope per #391). Capability-table reasons now cite this issue and the refusal semantics; no feedback is submitted to the host (maintainer decision); the per-host write-up #444 kept was removed in #467 (merged as 9d4fbd867), and docs/audits/2026-09-03-host-lineage-matrix.md plus the capability tables remain the record.