# 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 `SUBAGENTS_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: ``` /subagents//meta.json {title, created} /subagents//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. There is no separate delete. ## 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: }` 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 ` ()` 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`), `record(id, event)`, `start(id, title, prompt)`, `finish(id)`, `reopen(id)`, `finish_all()`, `list()` from disk. Drivers get an `Arc` 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` -- three 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 ``` `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 - `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.