Close nested Claude subagent mirrors instead of leaving nameless ones running

[Tryanks]

Root cause

Claude Code sessions accumulated mirror sessions titled just subagent that stayed "working" forever.

When a Claude subagent itself calls the Agent tool, the grandchild's stdout lines carry the grandchild's tool_use id as parent_tool_use_id, but the spawning tool_use block only appears inside the child's stream. TranscriptMapper::map_assistant mapped every tool_use as a plain ToolCall, so the parent session never saw a Subagent item for the grandchild id. reroute_native_subagent_event then created a placeholder mirror ("subagent") for the first grandchild item and opened its turn; update_subagent had no tool item to update on task_started/task_notification, so the terminal status never came, and after a host restart the persisted turn_started with no turn_completed was never repaired. Evidence from a real store: child mirror d0e5dad2… records an Agent tool_call toolu_0115… with run_in_background: true; mirror 90ebe1d3… opens with turn_started toolu_0115…, contains every child item twice (stdout feed + transcript tail), ends with the grandchild's final message, and has no turn_completed.

Two more findings from the same trace: is_agent_tool (contains("agent")) treated ListAgents as a spawn (a subagent: mirror with an empty description), and dedupe_child_events only dropped an ItemStarted after its ItemCompleted, so every child item from the two feeds appeared twice.

Fix

Agent layer (crates/agent)

• TranscriptMapper emits Agent/Task calls as ItemContent::Subagent (same spawned_subagent snapshot as a top-level spawn), keyed by the bare tool_use id — the id the grandchild's lines name as parent_tool_use_id — and parented to the child. A foreground nested spawn completes on the child's tool_result; a background one (run_in_background, tool_use_result.status == async_launched, or the launch acknowledgement text — subagent transcripts omit toolUseResult, verified on disk) is left to its own task_notification.
• Mapper::fold_child_events registers nested spawns in tool_items (with parent_item_id) so on_task_started's tail, update_subagent, note_subagent_model and gate_subagent_terminal work unchanged for grandchildren, and emit their snapshots with the correct parent. ToolItem::Subagent carries parent_item_id.
• Child items are deduplicated per lifecycle stage (Started/Updated(content)/Completed per id): the other feed's copy of the same stage is dropped, a changed update still passes, and a stale start still cannot reopen a completed item.
• is_agent_tool matches exactly agent | task.

Runtime (crates/runtime) — option A, nested

• A Subagent item that arrives as child content is recorded in the child mirror (Codex's child-thread markers are Subagent-shaped and must stay plain content) and remembered in nested_subagent_spawns. The first grandchild item that names it as parent creates a mirror under the child mirror (parent_session_id = child mirror id) titled from the spawn, and later snapshots of the spawn update/close that mirror through the same apply_subagent_snapshot path top-level spawns use.
• find_native_subagent_mirror resolves an item id to a mirror anywhere in the session's descendant chain, so grandchild items reroute to the nested mirror across residency changes and restarts.
• interrupt_native_subagent_work (SessionClosed) and close_orchestrator_children follow the descendant chain, so nested mirrors end with the root process like direct ones. Unknown-parent placeholders are still created for a child item with no known spawn (never left open: SessionClosed interrupts them, and the repair below catches restarts).
• Load-time repair: after a mirror's timeline load (schedule_timeline_load), repair_orphaned_mirror_turn synthesizes TurnCompleted { Interrupted } when meta.native_subagent is set, the last persisted turn has no status, and native_subagent_turns does not track the mirror as running in this host — i.e. no live parent owns it. Existing zombie mirrors settle when opened after upgrade.

No user-facing strings were added.

Tests

crates/agent/src/claude.rs (fixture crates/agent/tests/fixtures/claude/subagent_nested_trace.jsonl, constructed from the on-disk shapes above):

• nested_background_subagent_settles_through_its_own_task_notification — nested Subagent item with the grandchild id parented to the child; grandchild items parented to the grandchild; terminal held until the grandchild's tail flush then emitted once with summary/model/effort; both tails' replays add nothing; tail Start/Stop for the grandchild.
• nested_foreground_subagent_completes_on_the_childs_tool_result — settles on the child's tool_result; ListAgents in the same message stays a ToolCall.
• only_spawning_tools_are_agent_tools — is_agent_tool accepts agent/task, rejects listagents/sendmessage/…; a top-level ListAgents maps to a ToolCall.

crates/runtime/src/app/tests.rs:

• nested_subagent_items_open_a_titled_mirror_under_the_child_mirror_and_close_it — spawn alone is child-mirror content; the first grandchild item creates a titled mirror under the child mirror (not on the root); the spawn's terminal closes it (Completed); a second nested mirror still running at the root's SessionClosed closes Interrupted; parent/child logs carry no parent_item_id.
• unknown_parent_mirror_closes_when_the_parent_session_closes — placeholder mirror closes Interrupted on SessionClosed. (This contract already held for direct placeholders; the test guards it. The nested case above is the regression.)
• loading_a_mirror_with_an_open_turn_and_no_live_parent_ends_it — a persisted mirror with turn_started and no completion gets TurnCompleted { Interrupted } appended on load; reloading appends nothing.

The nested runtime test and the repair test fail against the pre-fix runtime (verified by swapping in HEAD's runtime files); the nested agent test fails pre-fix because no Subagent item exists for the grandchild.

Checks

cargo fmt --all --check                                                   # ok
cargo clippy --workspace --all-targets --locked -- -D warnings            # ok
cargo nextest run --workspace --locked                                    # 913 passed, 6 skipped (pre-existing ignored)

macOS only; no live Claude probe was run — the fixture is constructed from the recorded store and ~/.claude transcripts, not a fresh capture.

Limitations

• The nested mirror is created by the first grandchild item, not by the spawn itself (Codex emits Subagent-shaped child markers that never become parents). Claude grandchildren emit their prompt as the first line, so in practice the mirror appears immediately.
• nested_subagent_spawns is in-memory; a grandchild item for a spawn recorded before a host restart falls back to a placeholder mirror on the root, which SessionClosed/repair still close.