ai-app/SUBAGENTS.md

# Subagents

A session's subagents -- the helpers a Claude Code session starts through its
Task tool -- each get a transcript of their own, listed under the session's
card and readable in the same transcript view the session has. Designed
2026-09-05; the decisions Bryan has not yet reviewed are in `DECISIONS.md`.

## What a subagent is here

**A subagent is a second transcript owned by a session, in the same event
model, with no process and no controls.** It is not a session: it cannot be
messaged, stopped or started, and it has no setup, model or usage of its
own. Everything it shares with a session -- the transcript file format, the
paging routes, the SSE stream, the phone's cache and rendering -- is reused
by addressing, not by copying.

The CLI reports a subagent's messages on the parent's own stream-json
output, each carrying `parent_tool_use_id` = the id of the Task `tool_use`
that started it. Before this the translator dropped those lines
(`subagent_events_are_not_duplicated_into_the_transcript`); now it routes
them to that subagent's own translator and transcript. The parent's
transcript still shows only the Task call itself.

## Storage

Under the session directory:

```
<session>/subagents/<tool_use_id>/meta.json        {title, created}
<session>/subagents/<tool_use_id>/transcript.jsonl same SeqEvent lines as the session's
```

The id is the Task tool_use id (`toolu_…`), which is unique, stable across a
backend restart, and already the key everything on the parent side uses.
Only ids matching `[A-Za-z0-9_-]+` are ever created or looked up, since the
id becomes a path.

The transcript's sequence numbers are its own, starting at 1. `Transcript`,
`read_window`, `catch_up` and `read_after` work on it unchanged.

Its path out: deleting the session deletes its directory, subagents included,
and `POST /sessions/{id}/subagents/delete` removes finished ones on their own
-- all or nothing, and refused while any named one is still running, since its
transcript is still being written to and its process is the session's to stop.

## Lifecycle, as events in the subagent's transcript

1. Created on the first child line for an unseen parent id (or, when the
   parent Task call was seen, at that call). First lines written:
   `Status Running`, then `UserMessage { text: <the Task's prompt> }` when
   the prompt is known -- it genuinely is the subagent's first user turn.
2. Every child line is translated by that subagent's own `Translator`
   (one per subagent: tool ids are unique but streaming deltas are by
   content-block index, and parallel subagents interleave).
3. **The parent's `tool_result` never finishes a subagent.** The Task tool
   runs in the background by default: the `tool_result` -- "Async agent
   launched..." -- arrives the moment it *starts*, while the subagent goes
   on working for however long its own turn takes, sometimes minutes. What
   ends it is its own turn ending: the raw API's `message_delta` on its
   stream carrying `stop_reason: "end_turn"` (a `stop_reason` of `tool_use`
   is the model about to call one, not an end), or a `result` line for its
   own turn if a future CLI version ever sends one. Either maps to
   `Status Exited`; the subagent's vocabulary has no `Idle`, so the
   equivalent event `dispatch` produces for an ordinary session is dropped
   rather than written. A shipped version of this finished on the
   `tool_result` instead, which read a running background agent as
   "finished" with its transcript truncated at the moment it launched.
4. **A child line for a subagent that already finished reopens it**
   (`Status Running`) rather than being dropped: a background Task can be
   sent another message long after its first turn ended, and that is
   exactly what a further line for it means. Same transcript, same child
   `Translator`, just picking back up.
5. When the parent session's process exits (`Status Exited` on the
   session), every subagent still `Running` gets `Status Exited` too: its
   process was the parent's.

A subagent that was mid-flight when the backend restarted keeps working:
the registry reopens the existing transcript on the next child line, and
the file continues its sequence -- the same reopening #4 describes, whether
what closed it was a restart or its own `end_turn`. If its turn ended while
the backend was down nothing recorded that until the next line arrives, so
its last status stays `Running`, which the list reports as **unknown**
rather than as running (see the wire shape) until then.

Title: the Task call's `description` input, then ` (<subagent_type>)` when
one is given; falling back to the tool's name when the child arrives before
(or without) the parent call being seen.

## Server layout

- `session/subagent.rs` -- the registry: `Subagents` (per session, in
  `Shared`), `Subagent` (its `Transcript` behind a mutex plus a
  `broadcast::Sender<SeqEvent>`), `record(id, event)`, `start(id, title,
  prompt)`, `finish(id)`, `reopen(id)`, `finish_all()`, `list()` from disk, and
  `delete(ids)` -- its path out. Drivers get an
  `Arc<Subagents>` beside their `EventSink`; llama ignores it.
- `session/claude/translate.rs` -- routes child lines by parent id, holds
  one child `Translator` per subagent, remembers pending Task calls'
  description/prompt/subagent_type.
- `session/echo.rs` -- `/subagent [n]`: the test rig. Starts *n* (default 1)
  subagents at once, each named "helper k". Each writes the prompt as its
  user message, streams a few words of text, runs one `Bash` tool call, then
  finishes about three seconds after starting, and the parent's Task calls
  end when their subagent does. Three seconds so the running state can be
  seen on the phone.
- `routes.rs` -- four routes, in the doc table.

## Wire shape

```
GET /sessions/{id}                       SessionInfo gains `subagents: N` (count, 0 when none)
GET /sessions                            same field on each row
GET /sessions/{id}/subagents             [{id, title, status, created, lastActivity}], oldest first
GET /sessions/{id}/subagents/{sub}/transcript   exactly the session transcript's query and answer
GET /sessions/{id}/subagents/{sub}/events?after=N  exactly the session events stream
POST /sessions/{id}/subagents/delete     {subagents} -> 204; refused whole if one is running
```

The delete is a batch rather than a `DELETE` per id for the reason the import
list's is: the phone deletes what a reader selected, and one request per row
means a batch can half-arrive, leaving the rows that were missed looking
exactly like rows nobody picked. Unlike an import delete it is local file
removal, so it is done by the time the reply is sent and there is no per-row
state to follow afterwards. What decides "running" is
`Subagents::list`'s own rule, shared through `routes::has_a_process` so the
list and the delete cannot disagree about it.

`status` is the transcript's last `Status` event, serialised like a session's
(`running`, `exited`), except that a subagent whose session is not itself
running cannot be running: the list answers `unknown` for that one. The
phone words these as *running*, *finished* and *unknown* on the subcard.

The count on `SessionInfo` is a directory listing, so the list stays cheap.
The per-subagent status is only read when the list route is asked for.

## Phone

- The subcards are ordered **still running first, then most recently
  active** -- a display decision made on the phone (`subagentOrder`), over the
  server's stable oldest-first answer. Two keys rather than activity alone
  because a subagent that is thinking reports nothing meanwhile and would sink
  below one that just finished.
- **Holding a subcard selects it, and several at a time**, exactly as the
  import list works, with the selection bar drawn inside the session's card
  rather than at the bottom of the screen: this selection belongs to one card,
  and a bar down there would read as acting on the whole list. One card at a
  time -- picking a row in another card moves the selection rather than adding
  to it, since one Delete is one request against one parent. Delete is
  *disabled*, with the reason in words, while anything selected is still
  running. Deleting confirms first, dims the rows it is acting on
  (`BusyItem`), and on success takes them out of that card and off the
  session's count without refetching anything else. The phone's cached copy of
  a deleted subagent's transcript is purged with it.
- `SessionSummary.subagents: Int`. A card with a non-zero count ends in an
  expander row -- a full-width `Chevron(Pointing.Down)` row that flips to
  `Pointing.Up` -- collapsed by default. Expanding fetches
  `/sessions/{id}/subagents` and draws one `OutlinedCard` per subagent,
  indented inside the session card, the way dev-updater draws a project's
  components: title, then the status word and a relative time. The
  expansion state is per session id and survives a refresh of the list.
- Tapping a subcard opens `Screen.Subagent`, which is `SessionScreen` in
  **read-only** form: the same transcript, paging, cache, selection,
  images and status row, with the composer, the process button, the model
  picker, the files button, the settings cog and the usage bar left out.
  The header shows the subagent's title with the session's title beneath
  it. Back returns to the list.
- Addressing: `fetchTranscript`, `EventStream`, `TranscriptSource` and the
  cache take a transcript address rather than a session id --
  `sessions/{id}` or `sessions/{id}/subagents/{sub}` -- so the cache nests a
  subagent's copy under its session's and the same code serves both.