Web subagent feature owner: contributes the lazily expandable catalog tree to conversation.session.header.actions, reason-specific read-only replacements to the conversation composer chain, and the existing @ reference source to ctx.slash.

The header action reads subagentsByParent and session summaries through the standard useSessions hook. After a non-empty catalog arrives it shows the healthy direct-child count and a compact tree in service order. Continuable and one-shot rows display mode plus running/inactive activity, an optional log-backed title, and session-summary activity time; an unlabeled one-shot row falls back to its session id. Corrupt, unsupported, or unavailable rows remain readable but disabled. Expanding a row lazily opens that child's direct catalog and reports every visible branch to the runtime so membership frames cause a debounced refresh only where the tree is being consumed. Selecting any depth calls SessionsService.openSubagent() with the row's exact {parentSessionId, childSessionId, mode} address. Component-local state owns tree visibility, expanded branches, and keyboard focus. ArrowRight/ArrowLeft expand and collapse branches; ArrowUp/ArrowDown, Home, End, and Escape navigate or close the tree; closing returns focus to the trigger. Styling uses tokens only.

A one-shot child always elects a read-only composer that identifies the transcript as a completed execution record. A continuable child does so only when its exact parent is unavailable, with copy explaining the recovery path. A continuable child with a live parent keeps the ordinary input chrome, whose Session routes through subagent.prompt; running input remains Send because every follow-up joins the child's FIFO inbox, and addressed sessions never expose Stop. This package never receives host context or calls a model-facing tool. The catalog and composer behavior are specified by the Web subagent conversations Agent Note.

Subagent-origin Session rows are omitted from the ordinary sidebar, so the parent header catalog is their navigation entry point. Ordinary forks remain in the sidebar.

The @ source remains deliberately separate and inert. Candidates are zero-RPC running children from ctx.sessions.list; picking one inserts literal @label text, and the codec projects @label. It has no command-adjudication hooks and does not resolve labels into continuation addresses.

Model Experience

Subagent label text in the user prompt

What the model sees

Only the legacy @ reference source affects model input: a picked candidate reaches the ordinary user message as literal @label, without a dedicated block or host-side resolution. Catalog browsing, child navigation, and persisted transcript viewing add no prompt section; accepted continuation content becomes a normal FIFO user message through the host subagent adapter.

Token effect

Conditional and append-only: the literal @label or a human follow-up adds tokens only to its new user message. Catalog and transcript operations add zero model tokens.

KV Cache effect

Append-only. This package never edits earlier request tokens.

Known Limitations and Deferred Work

The catalog has coarse activity only — it cannot show durable outcome, elapsed time, Activation identity, or an authority-safe cancel button.
@ references remain display-title text — duplicate or renamed labels are ambiguous, so they intentionally do not acquire continuation semantics.