Legacy sidebar: group agent-launched threads and subagents under the thread that started them

[vaststudios]

The ask: may the legacy sidebar group threads under the thread that started them, with a fold toggle? A yes or no on direction and scope is enough for me to continue or drop it.

The problem

When an agent starts threads with t3_thread_launch or create_threads, or a provider runs subagents, each of those threads shows up in the legacy sidebar as an unrelated top-level row. With a few orchestrator sessions running, one project fills up with these rows and nothing shows which thread started which. The legacy sidebar is still the daily driver for a lot of people, mostly for its per-project grouping (#7917). #18656 and #20940 ask for the same grouping. #15812 and #20808 hide subagents instead, which makes them unreachable.

Proposal

1. Threads remember who launched them. t3_thread_launch and create_threads store the calling thread as an optional launchedByThreadId on the thread shell. Subagents already point at their parent through lineage. The field is optional and read from payload_json, so it needs no migration, and the v2 sidebar and mobile app ignore it.
2. The legacy sidebar nests them. Launched threads and subagents sit under the thread that started them, up to three levels deep, with a thin connector line.
3. Groups can be folded. A chevron on the parent row folds and unfolds its children, and the sidebar saves the choice per thread. Subagents start folded, because you can't chat in them, and launched threads start unfolded. A folded parent shows how many threads it hides and the most urgent status among them, so a folded thread waiting for approval stays visible. The branch with the open thread never folds away.

Before	After (default)	After (unfolded)

Open questions

• Is a launchedByThreadId field on the thread shell acceptable? If not, a smaller first step is nesting subagents only, through the existing lineage, with no contract change.
• Should subagents start folded, or should both kinds start the same way?

Code

The implementation is in #21267, split into a server commit and a sidebar commit. I've run it on my own desktop install with real orchestrator sessions since Oct 16. I also have a separate idea, letting "Show more" add five threads per click instead of the whole list. I'll only propose it on its own once this one is settled.

[grahamfarrar]

+1. I use the legacy sidebar for its per-project grouping, and my Claude threads spawn subagents through the native Agent tool, so each one shows up as an unrelated top-level row. The Claude desktop app now nests subagents under the session that started them, collapsed by default, and that would work well here. The parent link is already stored on each child (lineage.parentThreadId, relationshipToParent: "subagent"), so this looks like a sidebar-only change.