Fix joining active inference

internal/conversation/conversation.go

@@ -155,6 +155,7 @@ type EventType string
 const (
 	EventAssistantUpdate EventType = "assistant_update"
 	EventDone            EventType = "done"
+	EventInferenceActive EventType = "inference_active"
 	EventMessageDeleted  EventType = "message_deleted"
 	EventMessageSaved    EventType = "message_saved"
 )
@@ -197,6 +198,22 @@ type Done struct {
 
 func (Done) Type() EventType { return EventDone }
 
+// InferenceActive reports that an [Inference] is reserved or in progress.
+// [Conversation.Subscribe] sends this event one time, after the cache diff
+// and before the live events of the run.
+//
+// Without this event, two cases look the same to the client:
+//   - No run is active. The stream closes after the diff.
+//   - A run is active, but it sends no events yet. For example, the model
+//     has not sent its first token, or a slow tool is running.
+//
+// A client that did not start the run uses this event to show the stop
+// control. The client also uses this event to separate saved history
+// (before the event) from live events (after the event).
+type InferenceActive struct{}
+
+func (InferenceActive) Type() EventType { return EventInferenceActive }
+
 // Inference is one reserved inference call and its events.
 // [Conversation.Reserve] creates it, and [Inference.Run] runs it.
 // [Inference.Events] and [Conversation.Subscribe] use its events to send
@@ -323,11 +340,11 @@ func (inf *Inference) follow(
 //  1. Cache diff: [MessageSaved] for each saved message missing from or
 //     newer than manifest, followed by [MessageDeleted] for each ID in
 //     manifest that is no longer saved.
-//  2. Active inference: if an [Inference] is reserved or running when
-//     Subscribe loads the database, Subscribe streams its
-//     [MessageSaved] and [AssistantUpdate] events, then ends with [Done].
-//     If no inference is reserved or running, Subscribe sends no [Done]
-//     event.
+//  2. Active inference: if an [Inference] is reserved or in progress when
+//     this method reads the database, the stream sends [InferenceActive],
+//     then the [MessageSaved] and [AssistantUpdate] events of the run, and
+//     then [Done]. If no run is active, the stream closes after the diff and
+//     does not send [InferenceActive] or [Done].
 //
 // The channel closes when delivery finishes or ctx is canceled. Canceling
 // ctx stops delivery to this subscriber; it does not stop an ongoing
@@ -372,6 +389,9 @@ func (c *Conversation) Subscribe(
 		if inf == nil {
 			return
 		}
+		if !send(ctx, ch, InferenceActive{}) {
+			return
+		}
 		// Skip events for messages that the initial diff already
 		// sent. If the client's manifest already had a message from
 		// this call, it is not in sent and will be re-sent here; the

internal/conversation/conversation_test.go

@@ -1474,17 +1474,21 @@ func TestSubscribeInference(t *testing.T) {
 	close(release)
 	got := collectEvents(t, ch)
 
-	// The diff delivers the user message first.
+	// The cache diff sends the user message first. Then, the active marker
+	// shows that the live events of the run start.
 	saved, ok := got[0].(MessageSaved)
 	if !ok || saved.Message.Role != db.MessageRoleUser {
 		t.Fatalf("first event = %v, want the user MessageSaved", got[0])
 	}
+	if _, ok := got[1].(InferenceActive); !ok {
+		t.Fatalf("second event = %v, want InferenceActive", got[1])
+	}
 	var (
 		chunkTexts []string
 		chunkIDs   []string
 		asst       *MessageSaved
 	)
-	for _, e := range got[1:] {
+	for _, e := range got[2:] {
 		switch e := e.(type) {
 		case AssistantUpdate:
 			if asst != nil {
@@ -1589,6 +1593,10 @@ func TestSubscribeInferenceError(t *testing.T) {
 	close(release)
 	got := collectEvents(t, ch)
 
+	// The active marker comes after the user message from the cache diff.
+	if _, ok := got[1].(InferenceActive); !ok {
+		t.Fatalf("second event = %v, want InferenceActive", got[1])
+	}
 	done, ok := got[len(got)-1].(Done)
 	if !ok {
 		t.Fatalf("last event = %v, want Done", got[len(got)-1])
@@ -1709,12 +1717,18 @@ func TestSubscribeToolResult(t *testing.T) {
 					t.Errorf("event %d role = %q, want the user first", i, e.Message.Role)
 				}
 			}
+		case InferenceActive:
+			// This event separates the cache diff from the live events
+			// of the run.
 		case Done:
 			if e.Error != "" {
 				t.Errorf("Done.Error = %q, want empty", e.Error)
 			}
 		default:
-			t.Errorf("event %d = %v, want MessageSaved or Done", i, e)
+			t.Errorf(
+				"event %d = %v, want MessageSaved, InferenceActive, or Done",
+				i, e,
+			)
 		}
 	}
 	if aId == "" {