# Agent Note: Web subagent catalog and human continuation

Status: implemented

English | [中文](2026-07-27-web-subagent-conversations.zh.md)

## Problem

Session-backed subagents have durable identities, persisted transcripts, and a direct-child catalog, but ordinary session lineage cannot distinguish them from forks or prove their descriptor mode and continuation authority. Generic Agent-bound Host operations can otherwise resume or drive a child outside its direct-parent continuation owner.

The browser must preserve the [continuable subagent contract](../../implemented/feature/2026-07-28-continuable-subagent-conversations.md): a continuable child has at most one process-local Activation, accepts later work only through the exact live direct parent, and uses the Agent inbox as its sole FIFO. Viewing history must not create an Activation. Once an inbox message is accepted, the HTTP caller neither owns its execution nor gains a cancellation handle.

The UI must also preserve the membership, modes, and diagnostics of the [durable catalog](../../implemented/feature/2026-07-22-durable-subagent-catalog-and-list-agents.md). The shared service reports live-preferred corpus activity, while the Web projection replaces it with the exact child Agent driver's `running` or `inactive` state. Neither activity is a durable outcome or a promise that continuation will succeed.

## Decision

The Web product exposes the selected session's direct session-backed subagents from the current-title lineage region in the header. Users can lazily expand descendant catalogs and open either mode in the existing conversation region. A one-shot child is permanently read-only. A continuable child accepts human follow-ups only while its exact direct-parent Agent is live; otherwise its persisted transcript remains readable with a recovery explanation.

Every opened child carries a catalog-derived address `{ parentSessionId, childSessionId, mode }`. The mode-bearing address, not lineage or the coarse origin marker, selects dedicated history and prompt transports. History reads the persisted session without activation. A continuable prompt calls `ctx.subagents.followup()` and succeeds at inbox acceptance with `{ messageId }`; it does not steer an open turn, expose an Activation, wait for completion, or return an outcome.

The generic Host domain preserves the same ownership boundary. `session.history` and the source side of `session.fork` read an attached Session or inspect persistence without acquiring an Agent; history folds cold projection values from that exact inspected prefix, while a fork publishes an ordinary independent session. Generic Agent-bound session, command, and goal routes return `agent-busy` for session-backed subagents, as do explicit-id `session.create` adoption and attached-only queue controls. The denial classifier accepts the coarse `origin` marker, a `subagent/descriptor` in the session's own suffix, or exact live runtime ownership by the parent; these signals only prevent generic ownership and never replace catalog mode or direct-parent authorization.

Stopping an addressed child never falls through to `session.cancel`. `SubagentRuntime.followup()` owns admission only until inbox acceptance and grants no cancellation handle; a running continuable child is stopped through the dedicated `subagent.interrupt` route under the [current-turn interrupt contract](2026-08-06-continuable-subagent-interrupt.md), which parks pending work instead of discarding it. One-shot children remain uncancellable from the Web.

This decision covers Web discovery, transcript viewing, and parent-authorized human continuation. It does not make a subagent independently user-owned; that product remains [interactive side sessions](../../proposed/feature/2026-07-08-interactive-side-sessions.md).

## Design context

The Figma [subagent list](https://www.figma.com/design/jRBBK7zBgcszdVWQ0Fh5J8/Harness?node-id=383-14602&p=f), [hierarchical expansion](https://www.figma.com/design/jRBBK7zBgcszdVWQ0Fh5J8/Harness?node-id=383-15917&p=f), and [child conversation](https://www.figma.com/design/jRBBK7zBgcszdVWQ0Fh5J8/Harness?node-id=388-18584&p=f) frames are non-normative interaction and visual references. This note owns lifecycle, wire, and failure semantics.

| Design intent | Shipped contract |
| --- | --- |
| The session header opens a compact child list. | The trigger aggregates the complete subagent-only descendant lineage; the tree shows every direct catalog entry in service order, including disabled diagnostics. |
| Selecting a row reuses the conversation UI. | Addressed history never activates the child; only a continuable row with a live parent retains the ordinary composer. |
| Nested agents expand progressively. | Each row carries a one-level `hasChildren` snapshot; disclosure reserves known direct-descendant rows immediately, then loads only that row's direct catalog and retains its own parent address. |
| Rows show labels, state, usage, and active duration without duplicating sidebar rows. | Mode and `running`/`inactive` activity are textual as well as visual; optional title, durable token usage, and active-turn duration come from the list's retained projection values. Compact duration loses smaller units above one day, while hover and accessible naming retain exact whole seconds. `SessionHeader.origin` removes duplicate navigation rows but grants no capability. |

## Product contract

The descendant-count control is absent only when a complete empty direct-catalog response agrees with the session-summary projection that no subagent descendants are known. On an ordinary session, a slash separates the current title from that count control. Every subagent breadcrumb combines its compact 12px title with a fixed double chevron: the current breadcrumb uses primary color and weight 500, while ancestors use tertiary color and weight 400. Hovering a combined control for 150ms opens its direct-parent catalog, with a short crossing grace for the portaled menu; ArrowDown remains the keyboard entry path. Clicking an ancestor cancels pending hover, closes an open catalog, and only navigates upward. Each direct-parent catalog enables sibling switching and bolds its selected row; its label overrides the session-summary title, and every missing switcher catalog refreshes automatically. Only the current subagent appends its own descendant-count control. Long titles truncate before the fixed chevron. The count trigger counts every known session-summary descendant reached through an uninterrupted `origin: 'subagent'` lineage, stops at ordinary forks, and shows ongoing activity when any counted descendant is running. Because ordinary sidebar rows hide subagent-origin sessions, the Workspace browser indexes the same uninterrupted lineage onto each visible ordinary row: any running descendant supplies its blue activity indicator and exact count in hover and assistive text without describing an idle parent as running. An ordinary fork starts a separate aggregation subtree. Pending interaction outranks parent running; either remains primary while descendant activity becomes a second hover and assistive status. With neither present, descendant activity outranks an unviewed completion reminder, which returns after the last running descendant stops. Every healthy direct-catalog row carries a read-time `hasChildren` hint derived only from direct lineage headers with durable `origin: 'subagent'`; normal healthy and diagnostic subagent candidates carry that marker, while ordinary forks do not. This lookahead reads no descendant event log, and the descriptor-backed catalog loaded after disclosure remains authoritative. When summaries establish descendants before that catalog exists or after a stale empty response, the action stays visible and exposes only disabled loading rows until opening it refreshes the catalog; summary-only rows never grant navigation. The UI omits disclosure for a known leaf before interaction; the hint does not promise that the child will remain a leaf. While an expanded direct catalog is loading, known lineage reserves one disabled loading row per direct descendant without recursively fetching descendant catalogs. The tree then presents continuable and one-shot rows, falling back to the session id when an optional one-shot label is absent. Corrupt, unsupported, and unavailable candidates remain visible as disabled diagnostic rows.

`running` means the exact child Agent driver is draining work at the Host sampling boundary; `inactive` means that driver is idle or absent. The UI does not translate either value into success, failure, cancellation, completeness, or resumability. `subagent.list` supplies the current driver-status baseline, `host/session-status` updates known activity in place, request-local replay prevents an older in-flight list response from overwriting a newer transition, and `host/session-removed` returns a known row to `inactive`; reconnect reads a fresh baseline. A `host/session-added` frame for a direct subagent immediately flips any loaded parent row to `hasChildren: true`, and that positive hint survives an older in-flight catalog response; membership, labels, mode, diagnostics, and the authoritative snapshot still require a debounced `subagent.list` refresh while the affected branch is open. A prompt response remains delivery-time authority.

Healthy rows reuse the standard session projections retained in the list mirror. The token figure sums the four disjoint `tokenUsage` buckets across the durable log. `subagentTiming` resets at every descriptor so an inherited fork seed cannot enter the child's total, accumulates completed `turn/start` → `turn/end` spans, and carries same-cut `active.since` and `active.through` bounds for an open turn. Existing session events advance `active.through` while that turn remains open; the menu adds no separate timer or log read and advances its local clock only while a known descendant is running. Below one day it formats whole seconds; longer visual values retain at most two adjacent units, using approximate 30-day months and 365-day years, while hover and accessible naming preserve the exact day/hour/minute/second duration. An inactive row bounds an interrupted open turn with `active.through`, so a stale projection never borrows newer session metadata and reopening the menu never restarts completed work. Neither metric implies a durable outcome.

Selecting a row records its exact address before opening the resident client `Session`. History pagination, event folding, tool render intents, titles, and live mux reconciliation reuse the ordinary conversation machinery. Breadcrumbs follow parent links only through `origin: 'subagent'` rows, include the first ordinary owner, and keep ordinary forks single-level. Each subagent breadcrumb gets its direct-parent sibling catalog and uses that catalog's label when available. Forking an addressed subagent creates an ordinary fork with direct source lineage and attaches it to the nearest workspace-owning ancestor. The catalog is an ARIA tree with lazy ArrowRight/ArrowLeft disclosure, linear ArrowUp/ArrowDown navigation, Home/End, Escape, and focus restoration.

A one-shot row always replaces the composer with copy explaining that the execution record is read-only. A continuable row does so only while `parentAvailable` is false and the child is not running; a running parent-offline child keeps the ordinary composer with its input and Send action disabled so independent Stop stays reachable, and the read-only takeover returns once it stops. With a live parent, Enter and Send admit another FIFO turn even while the child runs, while independent Stop routes through `subagent.interrupt` ([interrupt contract](2026-08-06-continuable-subagent-interrupt.md)). Prompt failures retain the draft through the ordinary error behavior.

Agent-bound auxiliary controls are unavailable in addressed child views. In particular, the model selector and `/model` contribution do not call ordinary `session.models` or `session.selectModel`; the Host also rejects any accidental call instead of activating persisted child history outside the direct-parent continuation path.

## Host adapter and wire contract

`@deepseek-ai/dsh-subagent` owns the browser-safe generated `subagent` Remote namespace:

- `subagent.list` takes `parentSessionId`, calls `ctx.subagents.listChildren(parentSessionId, signal)`, returns the complete ordered entries with each healthy row's boolean `hasChildren` snapshot, replaces each healthy row's corpus activity with whether its exact Agent driver is running, and includes whether the exact parent currently resolves from `ctx.agents`.
- `subagent.history` takes the full mode-bearing address plus ordinary page arguments. It verifies the child and mode against the direct catalog, reads through `ctx.sessionQuery.readSession()`, rechecks direct lineage, and returns the ordinary raw-event, render-intent, pagination, and host-computed session-projection baseline without publishing an Agent.
- `subagent.prompt` accepts only a `mode: 'continuable'` address and `ContentBlock[]`. It requires the exact live parent, revalidates the catalog address, calls `ctx.subagents.followup(parent, childId, content, { source, signal })`, and returns the accepted `MessageId`.

The gateway maps missing parent, missing or diagnostic catalog entries, not-resumable and unauthorized children, request cancellation, and temporarily unavailable continuation admission to typed RPC errors. It does not expose descriptor or provider details. A list/prompt race is normal: the prompt result, not the earlier availability or activity snapshot, is authoritative.

Viewing persisted history creates no mux subscription by itself. When a follow-up materializes a cold child Activation, the existing Host and mux streams publish its lifecycle and events. Reconnect rebuilds the addressed window through `subagent.history`.

The ordinary `session.history` route is likewise observation-only for both ordinary and subagent sessions, but it does not carry the catalog address or grant continuation authority. Every ordinary route that needs an Agent resolves through the shared ownership fence before cold resume; `session.cancel` and `session.updateQueue` apply the same check directly because they intentionally query only attached Agents.

The adapter stays behind the generated Remote namespace; `dsh-host-webserver` remains a carrier. Browser code imports the contract through the existing connection package and never reaches host `ctx`, preserving the [archived GUI RPC layering decision](../../archived/architecture/2026-07-19-gui-layering-and-rpc-protocol.md).

## Client object layer and presentation

The React-free runtime owns catalogs, single-flight refreshes, retained addresses, availability hints, transport selection, and a reference-stable map of each list row's current projection values. Re-selecting a known child retains its address so navigation cannot silently switch to ordinary session APIs. A missing intermediate breadcrumb address can be recovered from an already-loaded ancestor catalog, but it is not retained for transport and creates no scope until the user selects that breadcrumb. Restored navigation persists the full mode-bearing address.

Catalogs ride the standard `useSessions` snapshot. Component-local state owns menu visibility, expanded branches, focus, and hover timers. `ui-conversation` declares a lineage slot for the current ordinary title and every subagent breadcrumb, passes plain breadcrumb identity and display text plus an upward-navigation callback for ancestors, and retains the ordinary title as the render fallback. `@deepseek-ai/dsh-client-ui-subagent` occupies each lineage slot with direct-parent catalog navigation and elects a reason-specific read-only composer from ordinary owner props. Components receive derived props and callbacks, never `ctx`.

Every in-process subagent child stamps `SessionHeader.origin: 'subagent'` before publication. Session list summaries and incremental Host frames project it so grouped and flat sidebars omit duplicate child rows while preserving ordinary forks. The same existing `host/session-added` frame marks a loaded direct parent row expandable without introducing a catalog event stream. Descriptor mode and catalog verification remain the authority for navigation, continuation, and authorization.

The package's existing `@label` source remains separate plain-text model input. It does not resolve labels to addresses or acquire continuation semantics.

## Default Web assembly

The shipped Web composition mounts SQLite session query beside JSONL persistence and configures spawn and fork background delegation as continuable. It also mounts the model-facing `send_message` and `list_agents` adapters for coordinator parity, but the GUI calls the shared `SubagentRuntime` through the host RPC domain rather than invoking model tools. One-shot children remain catalog-visible and read-only.

## Alternatives considered

**Use ordinary session APIs for addressed children.** Rejected because generic history carries no catalog-mode verification, while Agent-bound generic controls deliberately reject subagents rather than granting direct-parent continuation authority.

**Put the adapter in the webserver.** Rejected because catalog and continuation are channel-independent client capabilities; the webserver only carries validated messages.

**Put Host-backed file and session references in this package.** Rejected because catalog and addressed-child presentation depend on subagent lineage, while combined reference discovery is a separate Host capability consumed by [`ui-reference`](../../../../packages/client/ui-reference/README.md).

**Auto-resume an absent parent.** Rejected because continuation requires the exact live direct parent. Child navigation must not mutate the parent lifecycle.

**Expose ordinary cancellation.** Rejected because the accepted inbox turn outlives its admission request and, at this decision's time, the continuation contract exposed no authority-safe cancellation handle. The later [current-turn interrupt contract](2026-08-06-continuable-subagent-interrupt.md) added that explicit authority as a dedicated subagent route; falling through to `session.cancel` remains rejected.

**Show only continuable children.** Rejected because the durable catalog deliberately describes both session-backed modes. One-shot transcripts remain useful even though they never accept follow-ups.

**Infer mode or sidebar filtering from lineage.** Rejected because ordinary forks share `parentSession`. The descriptor-backed catalog owns mode; the separate `origin` marker is only a cheap navigation classifier.

**Build an eager recursive tree or dedicated catalog stream.** Rejected for the current scale. Header-only one-level expandability lookahead preserves pre-click stability without reading descendant events, while disclosure remains a lazy authoritative direct-child read; existing Host frames update activity, restore expandable parent rows, and trigger bounded membership refreshes.

**Let a child remain independently interactive after its parent disappears.** Rejected because independent lifetime and user ownership require side-session semantics.

## Testing

- Host protocol tests pin schemas including required boolean expandability, id echoing, mode verification, non-activating history, exact-parent enforcement, FIFO admission receipts, cancellation, and sanitized failure mapping.
- Generic Host tests pin attached and cold history and forks without Agent publication, cold projection folding, descriptor/origin/runtime-owner denial, explicit-id adoption denial, and the direct queue-control fence.
- Client object tests pin retained and restored addresses, one-shot read-only and cancel rejection, history routing, continuable prompt and interrupt routing, suppression of Agent-bound model controls, live activity flips including in-flight response replay and detach fallback, subagent-parent expandability flips, and membership refresh.
- jsdom tests pin the ordinary-title separator, per-level combined subagent-title switchers, 12px nested titles, current and ancestor styling, selected-row weight, catalog-label precedence, hover delay, upward-click suppression, truncation with a retained chevron, the aggregate descendant count and activity, sidebar propagation across nested lineage and ordinary-fork boundaries, row-status precedence, token totals, second-precision running and frozen inactive durations, adaptive long-duration units with exact accessible text, the summary-backed root action across absent and stale-empty catalogs, known loading-row shape, mixed-mode rows, pre-click leaf disclosure, diagnostics, lazy descendant disclosure, direct-parent addresses, keyboard behavior, and both read-only reasons.
- The keyless assembled Web snapshot contains an inactive continuable child with durable usage, an inactive one-shot sibling with a deterministic long duration, and a persisted grandchild; it pins the three-descendant trigger across a stale empty catalog response, usage and timing rows, adaptive long-duration presentation, and the aggregate running transition, expands without activation, opens persisted history, admits a human FIFO follow-up, reconciles child mux events, and proves one-shot history remains read-only. A separate assembled scenario holds a real child Agent turn at the LLM seam while it pins the aggregate running state in both the header and visible idle owner row, then cancels the turn during teardown.
- Navigation tests pin subagent-only breadcrumbs, workspace placement for forks created from subagents, and `origin: 'subagent'` sidebar filtering without hiding ordinary forks.

## Consequences

- Catalog reads may rescan persisted lineage and each direct candidate's descriptor log, but expandability reuses only descendant headers already present in that trace; the Web activity baseline adds one Agent-registry lookup per healthy row and then uses existing live frames, while usage and duration reuse projection baselines and pushes with no per-row log read, and membership refresh stays debounced and single-flight.
- Parent availability, child activity, and `hasChildren` are snapshots. Publication, disposal, another sender, or another process may win after listing; typed prompt failure remains expected.
- A child may publish between history fetch and mux subscription, so the existing sequence reconciliation also covers the cold-to-live addressed path.
- Persisted origin adds one deliberately weak product-classification field to child headers and list projections; it cannot become an authorization shortcut.
- Beyond the current-turn Stop of a running continuable child ([interrupt contract](2026-08-06-continuable-subagent-interrupt.md)), the UI has no child cancellation, durable outcome, Activation identity, deletion, or independently interactive offline mode, and its text must not imply those capabilities. Active-turn duration measures logged work rather than Activation residency.
