diff --git a/webui/src/lib/activity-timeline.ts b/webui/src/lib/activity-timeline.ts
index 0cb6ff20b..831c8e262 100644
--- a/webui/src/lib/activity-timeline.ts
+++ b/webui/src/lib/activity-timeline.ts
@@ -132,7 +132,12 @@ function pushActivityUnits(units: TurnUnit[], activityMessages: UIMessage[], vis
   for (const message of activityMessages) {
     const bucket = isFileEditActivityMessage(message) ? "file" : "other";
     const segmentId = message.activitySegmentId;
-    const segmentChanged = !!runSegmentId && !!segmentId && runSegmentId !== segmentId;
+    const segmentChanged =
+      bucket === "file"
+      && runBucket === "file"
+      && !!runSegmentId
+      && !!segmentId
+      && runSegmentId !== segmentId;
     if ((runBucket && bucket !== runBucket) || segmentChanged) {
       flushRun();
     }
diff --git a/webui/src/tests/thread-messages.test.tsx b/webui/src/tests/thread-messages.test.tsx
index 3d1839f65..5d78bb3f8 100644
--- a/webui/src/tests/thread-messages.test.tsx
+++ b/webui/src/tests/thread-messages.test.tsx
@@ -102,7 +102,7 @@ describe("ThreadMessages", () => {
     expect(units[2].type === "activity" ? units[2].messages.map((m) => m.id) : []).toEqual(["r2"]);
   });
 
-  it("splits ordinary tool activity when segment ids changed", () => {
+  it("keeps ordinary tool activity in one Thought block across segment ids", () => {
     const messages: UIMessage[] = [
       {
         id: "r1",
@@ -142,12 +142,10 @@ describe("ThreadMessages", () => {
 
     const units = buildDisplayUnits(messages);
 
-    expect(units).toHaveLength(2);
+    expect(units).toHaveLength(1);
     expect(units[0].type === "activity" ? units[0].messages.map((m) => m.id) : []).toEqual([
       "r1",
       "t1",
-    ]);
-    expect(units[1].type === "activity" ? units[1].messages.map((m) => m.id) : []).toEqual([
       "r2",
       "t2",
     ]);