fix(web): fold single trailing activity

apps/web/src/components/chat/MessagesTimeline.logic.test.ts

@@ -784,7 +784,7 @@ describe("deriveMessagesTimelineRows", () => {
     ).toBeDefined();
   });
 
-  it("keeps a tool group after the terminal response visible when the turn is folded", () => {
+  it("keeps a trailing tool group visible but folds a single activity", () => {
     const turnId = TurnId.make("turn-1");
     const timelineEntries = [
       {
@@ -829,19 +829,19 @@ describe("deriveMessagesTimelineRows", () => {
       })),
     ];
 
-    const rows = deriveMessagesTimelineRows({
-      timelineEntries,
+    const input = {
       latestTurn: {
         turnId,
-        state: "error",
+        state: "error" as const,
         startedAt: "2026-01-01T00:00:00Z",
         completedAt: "2026-01-01T00:00:10Z",
       },
       isWorking: false,
       activeTurnStartedAt: null,
       turnDiffSummaryByAssistantMessageId: new Map(),
       revertTurnCountByUserMessageId: new Map(),
-    });
+    };
+    const rows = deriveMessagesTimelineRows({ ...input, timelineEntries });
 
     expect(rows.map((row) => row.id)).toEqual([
       "turn-fold:turn-1",
@@ -864,6 +864,11 @@ describe("deriveMessagesTimelineRows", () => {
       showAssistantMeta: false,
       showAssistantCopyButton: false,
     });
+    expect(
+      deriveMessagesTimelineRows({ ...input, timelineEntries: timelineEntries.slice(0, 3) }).map(
+        (row) => row.id,
+      ),
+    ).toEqual(["turn-fold:turn-1", "assistant-final-entry"]);
   });
 
   it("folds all assistant messages before the terminal message", () => {

apps/web/src/components/chat/MessagesTimeline.logic.ts

@@ -540,8 +540,8 @@ function workEntryIsActiveTurnActivity(entry: WorkLogEntry): boolean {
 
 /**
  * Settled turns fold activity before their terminal assistant message behind
- * a "Worked for ..." row. Work that lands after that message stays visible so
- * failed or interrupted turns do not hide their trailing tool-call summary.
+ * a "Worked for ..." row. A single ordinary activity after that message joins
+ * the fold, while larger groups and failures stay visible as a trailing summary.
  */
 function deriveTurnFolds(input: {
   timelineEntries: ReadonlyArray<TimelineEntry>;
@@ -621,7 +621,11 @@ function deriveTurnFolds(input: {
       }
       const isCompaction =
         entry.kind === "work" && entry.entry.sourceActivityKind === "context-compaction";
-      if (!isCompaction && index > terminalEntryIndex) {
+      const isSingleTrailingActivity =
+        group.entries.length === terminalEntryIndex + 2 &&
+        entry.kind === "work" &&
+        !workEntryDisplayIndicatesToolFailure(entry.entry);
+      if (!isCompaction && index > terminalEntryIndex && !isSingleTrailingActivity) {
         continue;
       }
       // Agent-spawn CTA rows never fold: workflows outlive their launching

apps/web/src/components/chat/MessagesTimeline.test.tsx

@@ -1097,7 +1097,7 @@ describe("MessagesTimeline", () => {
     expect(markup).toContain("tool call failed");
   });
 
-  it("renders trailing tool calls as part of the terminal assistant block", () => {
+  it("renders a failed trailing tool call as part of the terminal assistant block", () => {
     const turnId = TurnId.make("turn-trailing-tools");
     const assistantMessageId = MessageId.make("assistant-trailing-tools");
     const markup = renderToStaticMarkup(
@@ -1135,7 +1135,7 @@ describe("MessagesTimeline", () => {
               label: "Ran command",
               tone: "tool",
               itemType: "command_execution",
-              toolLifecycleStatus: "completed",
+              toolLifecycleStatus: "failed",
             },
           },
         ]}