Sidebar: show child threads nested under the thread that opened them

[acovot]

When an agent opens other threads (its own subagents, delegate_task children), T3 already records who opened whom, but the sidebar hides those children and shows them only in the parent's Lineage panel. Threads an agent opens with t3_thread_launch aren't linked at all and show up as loose top-level threads.

What I'd like: the parent row in the sidebar with its children indented below it, collapsible, each child with its own status dot — the way the Claude desktop app shows sessions opened from another session. With that tree, a top "director" thread can open department threads, those can open specialists, and I can see and enter any of them from the sidebar.

Related: #31144 (notifications) and our comment in #30329 (linking threads across local projects).

cc @t3dotgg

[Entrpi]

I built this in my fork, on web and Android. In case it helps the design, here's how it behaves.

• Two kinds of children, kept apart:

  • provider-native subagents (for example Claude's Agent tool), which run inside the parent's turn;
  • subsessions (delegate_task children), which are full threads.

  Each kind has its own sidebar setting: Hidden, Count, Working and waiting, or All. A row can show both counts.

• Nesting:

  • A thread lists its subsessions under a caret, and subsessions of subsessions nest under their own caret.
  • Each row has its own status. Settled subsessions are listed in the Settled section.
  • The same tree works in the boring sidebar's project folders.

• Leaving the tree: right-click a subsession to detach it to top level and to reattach it later. It keeps the same id, history and provider session. An agent can do the same through new detach and reattach actions on t3_thread_organize.

Before (main)	After	After (boring sidebar)

Cost:

• Shell stream: it now carries each thread's subagent counts and the names of its working subagents. The thread list's JSON grows 2.3%.
• Sidebar, measured with 2,000 threads (300 subsessions, up to 3 deep): grouping costs about 1 ms more per thread update than on main.
• Renders: a status change in one subsession re-renders only its own row, not the other 300 parent rows.

Not covered: threads opened with t3_thread_launch are still not linked to their opener. That part of this idea would need a lineage field on launch.

The branches are on my fork (Entrpi/t3code) if a maintainer wants to look before deciding scope.

[saphid]

Note

🤖 Claude Opus 5.5 responding on behalf of Alex

Julius asked for maintainer agreement on this direction before three closed PRs of mine can be reconsidered. This is the open thread for it, so I'm adding them here instead of starting another:

• #12213: a running/finished subagent count in the thread row that expands the child tree in place (web, desktop, mobile)
• #13709: working/done/failed subagent counts on a waiting thread's row (web)
• #14530: stacked on #13709; the counts toggle a list of that batch's subagents under the card (web)

The open question, from Julius on #12213: which sidebar summary should own the overlap between #12213 and #13709. #13709 and #14530 were closed on the same unresolved decision.

Earlier requests to nest subagents, #18656 and #21266, were closed as outdated after the legacy sidebar was removed in #21039. They weren't declined on the idea itself.

Decisions for maintainers:

1. Should child threads (provider subagents and delegate_task children) be reachable from the sidebar, or stay in the Lineage panel?
2. If reachable, which form: a nested, collapsible tree under the parent (this proposal and #12213), a count or progress summary on the parent row (#13709), or both? If both, which one owns the overlap?

I'll only rebuild what you approve, on current main, and link it here.