Fix duplicate reasoning display

webui/src/components/thread/ThreadMessages.tsx

@@ -49,12 +49,47 @@ export function buildDisplayUnits(messages: UIMessage[]): DisplayUnit[] {
       out.push({ type: "cluster", messages: cluster });
       continue;
     }
+    const previous = out[out.length - 1];
+    if (previous?.type === "cluster" && assistantHasInlineReasoning(m)) {
+      previous.messages.push(reasoningOnlyMessageFromAnswer(m));
+      out.push({ type: "single", message: stripInlineReasoning(m) });
+      i += 1;
+      continue;
+    }
     out.push({ type: "single", message: m });
     i += 1;
   }
   return out;
 }
 
+function assistantHasInlineReasoning(message: UIMessage): boolean {
+  return (
+    message.role === "assistant"
+    && message.kind !== "trace"
+    && message.content.trim().length > 0
+    && (!!message.reasoning?.trim() || !!message.reasoningStreaming)
+  );
+}
+
+function reasoningOnlyMessageFromAnswer(message: UIMessage): UIMessage {
+  return {
+    id: `${message.id}-reasoning`,
+    role: "assistant",
+    content: "",
+    createdAt: message.createdAt,
+    reasoning: message.reasoning,
+    reasoningStreaming: message.reasoningStreaming,
+    isStreaming: message.reasoningStreaming,
+  };
+}
+
+function stripInlineReasoning(message: UIMessage): UIMessage {
+  const next = { ...message };
+  delete next.reasoning;
+  delete next.reasoningStreaming;
+  return next;
+}
+
 export function assistantCopyFlags(units: DisplayUnit[]): boolean[] {
   const flags = new Array<boolean>(units.length).fill(true);
   let hasLaterUnitBeforeUser = false;

webui/src/tests/thread-messages.test.tsx

@@ -55,6 +55,59 @@ describe("ThreadMessages", () => {
     expect(rows[1]).toHaveClass("mt-4");
   });
 
+  it("folds final answer reasoning into the preceding activity cluster", () => {
+    const messages: UIMessage[] = [
+      {
+        id: "r1",
+        role: "assistant",
+        content: "",
+        reasoning: "search plan",
+        reasoningStreaming: false,
+        createdAt: 1,
+      },
+      {
+        id: "t1",
+        role: "tool",
+        kind: "trace",
+        content: "web_search()",
+        traces: ["web_search()"],
+        createdAt: 2,
+      },
+      {
+        id: "a1",
+        role: "assistant",
+        content: "final answer",
+        reasoning: "summarize results",
+        reasoningStreaming: false,
+        createdAt: 3,
+      },
+    ];
+
+    const units = buildDisplayUnits(messages);
+
+    expect(units).toHaveLength(2);
+    expect(units[0]).toMatchObject({ type: "cluster" });
+    expect(units[0].type === "cluster" ? units[0].messages.map((m) => m.id) : []).toEqual([
+      "r1",
+      "t1",
+      "a1-reasoning",
+    ]);
+    expect(units[1]).toMatchObject({
+      type: "single",
+      message: {
+        id: "a1",
+        content: "final answer",
+      },
+    });
+    if (units[1].type === "single") {
+      expect(units[1].message).not.toHaveProperty("reasoning");
+    }
+
+    render(<ThreadMessages messages={messages} isStreaming={false} />);
+    expect(screen.queryByRole("button", { name: /^thinking$/i })).not.toBeInTheDocument();
+    expect(screen.getByText("final answer")).toBeInTheDocument();
+  });
+
   it("shows copy only on the last assistant slice before the next user turn", () => {
     const messages: UIMessage[] = [
       {