End a subagent on its own end_turn, not the parent's tool_result, and give the expander a touch-sized row

The Agent tool runs subagents in the background, so the parent's result
arrives at launch while the subagent works on for minutes; finishing on it
read a running agent as finished with a transcript cut off at launch. A
subagent now ends on its own message_delta end_turn, and a later line for a
finished one reopens it, since a background agent can be messaged again.

The card's expander row was only the chevron's height, so a tap for it
landed on the first subcard; it is the platform's 48dp minimum now.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

server/src/session/claude/translate.rs

@@ -151,10 +151,12 @@ impl Translator {
     fn translate_child(&mut self, id: &str, message: &Value) -> Vec<Event> {
         match self.subagents.get(id) {
             Some(subagent) if !subagent.is_open() => {
-                // The Task call already ended (or this line is stale from a
-                // resumed conversation) -- see `SUBAGENTS.md`'s lifecycle.
-                tracing::debug!("dropping a line for subagent {id}, which has already finished");
-                return Vec::new();
+                // Not stale: the Task tool runs in the background by
+                // default, so a finished subagent can still be sent another
+                // message later (SendMessage) and start working again. A
+                // line arriving after `finish` means exactly that, not a
+                // conversation that is over -- see `SUBAGENTS.md`.
+                self.subagents.reopen(id);
             }
             Some(_) => {}
             None => {
@@ -178,7 +180,28 @@ impl Translator {
             .clone();
         let events = child.lock().unwrap().dispatch(message);
         for event in events {
-            self.subagents.record(id, event);
+            // The subagent's own vocabulary is Running/Exited/Unknown, never
+            // Idle -- a background Task is either working or it has ended,
+            // never merely "between turns" the way a session is. Dropped
+            // here rather than never produced, so a `result` line's own
+            // `Idle` (dispatch's ordinary end-of-turn event, for a subagent
+            // dialect that ever sends one) is caught the same way a
+            // `message_delta` would be.
+            if !matches!(
+                event,
+                Event::Status {
+                    state: SessionStatus::Idle
+                }
+            ) {
+                self.subagents.record(id, event);
+            }
+        }
+        // What actually ends a subagent's turn: not the parent's
+        // `tool_result`, which for a background Task arrives at launch
+        // ("Async agent launched...") long before the work is done -- see
+        // `SUBAGENTS.md`.
+        if ends_a_turn(message) {
+            self.subagents.finish(id);
         }
         Vec::new()
     }
@@ -678,11 +701,11 @@ impl Translator {
                 id: about.clone(),
                 output: texts.join("\n"),
             });
-            // A no-op unless `about` is a subagent's own id -- see
-            // `SUBAGENTS.md`'s lifecycle #3: the parent gets this `ToolEnd`
-            // like any other tool result, and the subagent it names (if it
-            // names one) gets its `Status::Exited`.
-            self.subagents.finish(&about);
+            // Deliberately does *not* finish a subagent `about` might name:
+            // the Task tool runs in the background by default, so this
+            // `tool_result` -- "Async agent launched..." -- arrives at
+            // launch, long before the subagent's own work is done. What
+            // ends it is its own turn ending, handled in `translate_child`.
         }
         events
     }
@@ -705,6 +728,32 @@ fn fallback_title(message: &Value) -> String {
         .to_string()
 }
 
+/// Whether this line is a subagent's *own* turn ending -- the only thing
+/// that does, per `SUBAGENTS.md`: not the parent's `tool_result`, which for
+/// a background Task arrives at launch rather than at completion.
+///
+/// Checked on the raw line rather than on what `dispatch` returns, so this
+/// never has to touch the shared `translate_stream_event`/`dispatch` code a
+/// top-level session's own turn-ending also goes through -- a subagent's
+/// idea of "ended" must not change when a real session's does.
+///
+/// `message_delta` is the raw API's own signal, carrying the stop reason:
+/// `end_turn` is genuinely done, `tool_use` means the model is about to call
+/// one and there is more coming. A `result` line is the CLI's own shape for
+/// a top-level turn; a subagent has not been observed to send one, but
+/// SUBAGENTS.md counts it too in case a future CLI version does.
+fn ends_a_turn(message: &Value) -> bool {
+    match message.get("type").and_then(Value::as_str) {
+        Some("stream_event") => {
+            let event = &message["event"];
+            event.get("type").and_then(Value::as_str) == Some("message_delta")
+                && event["delta"].get("stop_reason").and_then(Value::as_str) == Some("end_turn")
+        }
+        Some("result") => true,
+        _ => false,
+    }
+}
+
 /// Whether a failed turn failed because the account is out of quota, and when
 /// the CLI said the limit lifts.
 ///
@@ -1043,7 +1092,13 @@ mod tests {
     /// The parent's `tool_result` for the Task id is what ends the
     /// subagent -- SUBAGENTS.md's lifecycle #3 -- and nothing else does.
     #[test]
-    fn the_parents_tool_result_finishes_the_subagent() {
+    fn the_parents_tool_result_does_not_finish_the_subagent() {
+        // The Task tool runs in the background by default: this
+        // `tool_result` is "Async agent launched...", arriving the moment
+        // the subagent *starts*, while it goes on working for however long
+        // its own turn takes. Finishing it here was the bug -- a running
+        // background agent read as "finished" with its transcript truncated
+        // at launch.
         let dir = tempfile::tempdir().expect("tempdir");
         let subagents = test_subagents(&dir);
         let mut translator = Translator::new(dir.path().to_path_buf(), Arc::clone(&subagents));
@@ -1058,10 +1113,115 @@ mod tests {
         translate_lines(
             &mut translator,
             &[
-                r#"{"type":"user","message":{"role":"user","content":[{"type":"tool_result","tool_use_id":"toolu_task2","content":"done","is_error":false}]},"parent_tool_use_id":null}"#,
+                r#"{"type":"user","message":{"role":"user","content":[{"type":"tool_result","tool_use_id":"toolu_task2","content":"Async agent launched","is_error":false}]},"parent_tool_use_id":null}"#,
             ],
         );
+        assert!(subagent.is_open());
+    }
+
+    /// What actually ends a subagent: the raw API's own `message_delta`
+    /// saying its turn stopped with `end_turn`. Never written into the
+    /// subagent's own transcript as `Idle` -- its vocabulary has no such
+    /// state.
+    #[test]
+    fn the_subagents_own_end_turn_finishes_it() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let subagents = test_subagents(&dir);
+        let mut translator = Translator::new(dir.path().to_path_buf(), Arc::clone(&subagents));
+        translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"assistant","message":{"content":[{"type":"tool_use","id":"toolu_task3","name":"Task","input":{"description":"helper"}}]},"parent_tool_use_id":null}"#,
+                r#"{"type":"stream_event","event":{"type":"message_delta","delta":{"stop_reason":"end_turn"}},"parent_tool_use_id":"toolu_task3"}"#,
+            ],
+        );
+        let subagent = subagents.get("toolu_task3").expect("subagent started");
         assert!(!subagent.is_open());
+        let lines = crate::session::transcript::read_after(&subagent.transcript_path(), 0)
+            .expect("read subagent transcript");
+        assert!(
+            !lines
+                .iter()
+                .any(|entry| matches!(&entry.event, Event::Status { state } if *state == SessionStatus::Idle)),
+            "a subagent's transcript must never carry Idle: {lines:?}"
+        );
+        assert_eq!(
+            lines.last().unwrap().event,
+            Event::Status {
+                state: SessionStatus::Exited
+            }
+        );
+    }
+
+    /// `stop_reason: "tool_use"` is the model about to call a tool, with
+    /// more of the turn still coming -- not an end.
+    #[test]
+    fn a_stop_reason_of_tool_use_does_not_finish_the_subagent() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let subagents = test_subagents(&dir);
+        let mut translator = Translator::new(dir.path().to_path_buf(), Arc::clone(&subagents));
+        translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"assistant","message":{"content":[{"type":"tool_use","id":"toolu_task4","name":"Task","input":{"description":"helper"}}]},"parent_tool_use_id":null}"#,
+                r#"{"type":"stream_event","event":{"type":"message_delta","delta":{"stop_reason":"tool_use"}},"parent_tool_use_id":"toolu_task4"}"#,
+            ],
+        );
+        assert!(
+            subagents
+                .get("toolu_task4")
+                .expect("subagent started")
+                .is_open()
+        );
+    }
+
+    /// A background Task can be sent another message long after its first
+    /// turn ended -- a further child line for it reopens rather than being
+    /// dropped, and the same transcript and child translator carry on.
+    #[test]
+    fn a_line_after_finish_reopens_the_subagent_rather_than_being_dropped() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let subagents = test_subagents(&dir);
+        let mut translator = Translator::new(dir.path().to_path_buf(), Arc::clone(&subagents));
+        translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"assistant","message":{"content":[{"type":"tool_use","id":"toolu_task5","name":"Task","input":{"description":"helper"}}]},"parent_tool_use_id":null}"#,
+                r#"{"type":"stream_event","event":{"type":"message_delta","delta":{"stop_reason":"end_turn"}},"parent_tool_use_id":"toolu_task5"}"#,
+            ],
+        );
+        let subagent = subagents.get("toolu_task5").expect("subagent started");
+        assert!(!subagent.is_open());
+
+        translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"assistant","message":{"content":[{"type":"tool_use","id":"toolu_more","name":"Bash","input":{}}]},"parent_tool_use_id":"toolu_task5"}"#,
+            ],
+        );
+        assert!(subagent.is_open());
+        let lines = crate::session::transcript::read_after(&subagent.transcript_path(), 0)
+            .expect("read subagent transcript");
+        // Running, [prompt], Exited, Running (reopened), then the new line's
+        // own ToolStart -- the same transcript throughout, not a new one.
+        assert!(
+            lines.iter().any(
+                |entry| matches!(&entry.event, Event::ToolStart { tool, .. } if tool == "Bash")
+            )
+        );
+        assert_eq!(
+            lines
+                .iter()
+                .filter(|entry| matches!(
+                    &entry.event,
+                    Event::Status {
+                        state: SessionStatus::Running
+                    }
+                ))
+                .count(),
+            2,
+            "expected one Running at creation and one at the reopen: {lines:?}"
+        );
     }
 
     /// Two subagents running at once keep two separate transcripts: tool ids

server/src/session/subagent.rs

@@ -89,9 +89,9 @@ impl Subagent {
     }
 
     /// Whether this subagent's last recorded status is not `Exited` --
-    /// what decides whether a further child line still belongs in its
-    /// transcript. See `SUBAGENTS.md`'s lifecycle: "a child line whose
-    /// subagent finished already... is ignored".
+    /// what decides whether a further child line reopens it (see
+    /// `Subagents::reopen`) rather than continuing straight through. See
+    /// `SUBAGENTS.md`'s lifecycle.
     pub fn is_open(&self) -> bool {
         *self.status.lock().unwrap() != SessionStatus::Exited
     }
@@ -269,10 +269,12 @@ impl Subagents {
         }
     }
 
-    /// The parent's `tool_result` for this Task id arrived: the subagent's
-    /// own `Status::Exited`. A no-op for an id that is not a subagent's, so
-    /// callers can call this for every `tool_result` without first checking
-    /// whether it belongs to one.
+    /// The subagent's own turn ended: its `Status::Exited`. Called from
+    /// `translate_child` on the subagent's own `end_turn`, never on the
+    /// parent's `tool_result` -- a background Task's `tool_result` arrives
+    /// at launch, not at completion, so it says nothing about whether this
+    /// is over. A no-op for an id that is not a subagent's or is already
+    /// closed.
     pub fn finish(&self, id: &str) {
         if let Some(subagent) = self.live.lock().unwrap().get(id).cloned()
             && subagent.is_open()
@@ -283,6 +285,21 @@ impl Subagents {
         }
     }
 
+    /// A line arrived for a subagent that had already finished: it is
+    /// working again, not stale -- a background Task can be sent another
+    /// message long after its first turn ended. Appends `Status::Running`
+    /// so the list stops reporting it as finished; a no-op if it was not
+    /// actually closed, so a caller need not check first.
+    pub fn reopen(&self, id: &str) {
+        if let Some(subagent) = self.live.lock().unwrap().get(id).cloned()
+            && !subagent.is_open()
+        {
+            subagent.append(Event::Status {
+                state: SessionStatus::Running,
+            });
+        }
+    }
+
     /// The parent session's process is gone, so nothing still open here has
     /// a process behind it either -- see `SUBAGENTS.md`'s lifecycle #4.
     pub fn finish_all(&self) {