diff --git a/nanobot/utils/webui_transcript.py b/nanobot/utils/webui_transcript.py
index 31d10380b..819804a09 100644
--- a/nanobot/utils/webui_transcript.py
+++ b/nanobot/utils/webui_transcript.py
@@ -106,9 +106,9 @@ def tool_trace_lines_from_events(events: Any) -> list[str]:
         if event.get("phase") not in {"start", "end", "error"}:
             continue
         call_id = event.get("call_id")
-        if call_id is not None and call_id in seen:
-            continue
-        if call_id is not None:
+        if isinstance(call_id, str) and call_id:
+            if call_id in seen:
+                continue
             seen.add(call_id)
         t = _format_tool_call_trace(event)
         if t:
@@ -116,6 +116,22 @@ def tool_trace_lines_from_events(events: Any) -> list[str]:
     return lines
 
 
+def _merge_unique_tool_trace_lines(
+    previous_traces: list[str],
+    lines: list[str],
+) -> tuple[list[str], bool]:
+    seen_lines = set(previous_traces)
+    traces = list(previous_traces)
+    added = False
+    for line in lines:
+        if line in seen_lines:
+            continue
+        seen_lines.add(line)
+        traces.append(line)
+        added = True
+    return traces, added
+
+
 def replay_transcript_to_ui_messages(
     lines: list[dict[str, Any]],
     *,
@@ -448,13 +464,19 @@ def replay_transcript_to_ui_messages(
                     and (last.get("activitySegmentId") in (None, segment))
                 ):
                     prev_traces = list(last.get("traces") or [last.get("content")])
-                    merged_traces = prev_traces + trace_lines
-                    messages[-1] = {
+                    if structured:
+                        merged_traces, added = _merge_unique_tool_trace_lines(prev_traces, structured)
+                        if not added:
+                            continue
+                    else:
+                        merged_traces = prev_traces + trace_lines
+                    merged = {
                         **last,
                         "traces": merged_traces,
-                        "content": trace_lines[-1],
+                        "content": merged_traces[-1],
                         "activitySegmentId": last.get("activitySegmentId") or segment,
                     }
+                    messages[-1] = merged
                 else:
                     messages.append(
                         {
diff --git a/tests/utils/test_webui_transcript.py b/tests/utils/test_webui_transcript.py
index f13380f46..4ed9c132d 100644
--- a/tests/utils/test_webui_transcript.py
+++ b/tests/utils/test_webui_transcript.py
@@ -98,6 +98,53 @@ def test_replay_file_edit_event_creates_file_activity(tmp_path, monkeypatch) ->
     assert msgs[2]["activitySegmentId"] != msgs[1]["activitySegmentId"]
 
 
+def test_replay_tool_events_dedupes_finish_after_start() -> None:
+    msgs = replay_transcript_to_ui_messages([
+        {
+            "event": "message",
+            "chat_id": "t-tool",
+            "text": 'exec({"cmd":"ls"})',
+            "kind": "tool_hint",
+            "tool_events": [
+                {
+                    "phase": "start",
+                    "call_id": "call-exec",
+                    "name": "exec",
+                    "arguments": {"cmd": "ls"},
+                },
+            ],
+        },
+        {
+            "event": "message",
+            "chat_id": "t-tool",
+            "text": "",
+            "kind": "progress",
+            "tool_events": [
+                {
+                    "phase": "end",
+                    "call_id": "call-exec",
+                    "name": "exec",
+                    "arguments": {"cmd": "ls"},
+                    "result": "ok",
+                },
+                {
+                    "phase": "end",
+                    "call_id": "call-read",
+                    "name": "read_file",
+                    "arguments": {"path": "notes.md"},
+                    "result": "done",
+                },
+            ],
+        },
+    ])
+
+    assert len(msgs) == 1
+    assert msgs[0]["traces"] == [
+        'exec({"cmd": "ls"})',
+        'read_file({"path": "notes.md"})',
+    ]
+
+
 def test_build_response_schema(monkeypatch, tmp_path) -> None:
     from nanobot.utils.webui_transcript import build_webui_thread_response
 
diff --git a/webui/src/hooks/useNanobotStream.ts b/webui/src/hooks/useNanobotStream.ts
index 2ee113227..6ed236422 100644
--- a/webui/src/hooks/useNanobotStream.ts
+++ b/webui/src/hooks/useNanobotStream.ts
@@ -2,7 +2,7 @@ import { useCallback, useEffect, useRef, useState } from "react";
 
 import { useClient } from "@/providers/ClientProvider";
 import { toMediaAttachment } from "@/lib/media";
-import { toolTraceLinesFromEvents } from "@/lib/tool-traces";
+import { mergeUniqueToolTraceLines, toolTraceLinesFromEvents } from "@/lib/tool-traces";
 import type { StreamError } from "@/lib/nanobot-client";
 import type {
   InboundEvent,
@@ -652,10 +652,16 @@ export function useNanobotStream(
                 : last.content
                   ? [last.content]
                   : [];
+              const mergedLines = structuredLines.length > 0
+                ? mergeUniqueToolTraceLines(previousTraces, structuredLines)
+                : null;
+              if (mergedLines && !mergedLines.added) return prev;
               const merged: UIMessage = {
                 ...last,
-                traces: [...previousTraces, ...lines],
-                content: lines[lines.length - 1],
+                traces: mergedLines ? mergedLines.traces : [...previousTraces, ...lines],
+                content: mergedLines
+                  ? mergedLines.traces[mergedLines.traces.length - 1]
+                  : lines[lines.length - 1],
                 activitySegmentId: last.activitySegmentId ?? segmentId,
               };
               return [...prev.slice(0, -1), merged];
diff --git a/webui/src/lib/tool-traces.ts b/webui/src/lib/tool-traces.ts
index a3949caae..a9272b6e9 100644
--- a/webui/src/lib/tool-traces.ts
+++ b/webui/src/lib/tool-traces.ts
@@ -44,20 +44,35 @@ const VALID_PHASES = new Set(["start", "end", "error"]);
 export function toolTraceLinesFromEvents(events: unknown): string[] {
   if (!Array.isArray(events)) return [];
   const seen = new Set<string>();
-  return events
-    .filter((event) => {
-      if (!event || typeof event !== "object") return false;
-      const phase = (event as { phase?: unknown }).phase;
-      if (!(phase && typeof phase === "string" && VALID_PHASES.has(phase))) {
-        return false;
-      }
-      const callId = (event as { call_id?: unknown }).call_id;
-      if (callId && typeof callId === "string") {
-        if (seen.has(callId)) return false;
-        seen.add(callId);
-      }
-      return true;
-    })
-    .map(formatToolCallTrace)
-    .filter((trace): trace is string => !!trace);
+  const lines: string[] = [];
+  for (const event of events) {
+    if (!event || typeof event !== "object") continue;
+    const phase = (event as { phase?: unknown }).phase;
+    if (!(phase && typeof phase === "string" && VALID_PHASES.has(phase))) continue;
+    const callId = (event as { call_id?: unknown }).call_id;
+    if (callId && typeof callId === "string") {
+      if (seen.has(callId)) continue;
+      seen.add(callId);
+    }
+    const line = formatToolCallTrace(event);
+    if (!line) continue;
+    lines.push(line);
+  }
+  return lines;
+}
+
+export function mergeUniqueToolTraceLines(
+  previousTraces: string[],
+  lines: string[],
+): { traces: string[]; added: boolean } {
+  const seen = new Set(previousTraces);
+  const traces = [...previousTraces];
+  let added = false;
+  for (const line of lines) {
+    if (seen.has(line)) continue;
+    seen.add(line);
+    traces.push(line);
+    added = true;
+  }
+  return { traces, added };
 }
diff --git a/webui/src/tests/useNanobotStream.test.tsx b/webui/src/tests/useNanobotStream.test.tsx
index 925102dad..b221a6552 100644
--- a/webui/src/tests/useNanobotStream.test.tsx
+++ b/webui/src/tests/useNanobotStream.test.tsx
@@ -308,6 +308,56 @@ describe("useNanobotStream", () => {
     );
   });
 
+  it("dedupes finish-phase tool events after their start trace", () => {
+    const fake = fakeClient();
+    const { result } = renderHook(() => useNanobotStream("chat-tool-finish", EMPTY_MESSAGES), {
+      wrapper: wrap(fake.client),
+    });
+
+    act(() => {
+      fake.emit("chat-tool-finish", {
+        event: "message",
+        chat_id: "chat-tool-finish",
+        text: 'exec({"cmd":"ls"})',
+        kind: "tool_hint",
+        tool_events: [{
+          phase: "start",
+          call_id: "call-exec",
+          name: "exec",
+          arguments: { cmd: "ls" },
+        }],
+      });
+      fake.emit("chat-tool-finish", {
+        event: "message",
+        chat_id: "chat-tool-finish",
+        text: "",
+        kind: "progress",
+        tool_events: [
+          {
+            phase: "end",
+            call_id: "call-exec",
+            name: "exec",
+            arguments: { cmd: "ls" },
+            result: "ok",
+          },
+          {
+            phase: "error",
+            call_id: "call-read",
+            name: "read_file",
+            arguments: { path: "notes.md" },
+            error: "missing",
+          },
+        ],
+      });
+    });
+
+    expect(result.current.messages).toHaveLength(1);
+    expect(result.current.messages[0].traces).toEqual([
+      'exec({"cmd":"ls"})',
+      'read_file({"path":"notes.md"})',
+    ]);
+  });
+
   it("renders live file_edit events as their own activity trace", () => {
     const fake = fakeClient();
     const { result } = renderHook(() => useNanobotStream("chat-file-edit", EMPTY_MESSAGES), {