diff --git a/src/config/sessions/transcript.test.ts b/src/config/sessions/transcript.test.ts
index 50abe30bfac9..fae9db58757f 100644
--- a/src/config/sessions/transcript.test.ts
+++ b/src/config/sessions/transcript.test.ts
@@ -736,6 +736,46 @@ describe("appendAssistantMessageToSessionTranscript", () => {
     }
   });
 
+  it("dedupes concurrent exact assistant appends by idempotency key", async () => {
+    writeTranscriptStore();
+    const idempotencyKey = "mirror:concurrent-assistant";
+
+    const results = await Promise.all(
+      Array.from({ length: 8 }, () =>
+        appendExactAssistantMessageToSessionTranscript({
+          sessionKey,
+          storePath: fixture.storePath(),
+          idempotencyKey,
+          updateMode: "none",
+          message: createExactAssistantMessage({
+            text: "Mirrored reply",
+            provider: "openclaw",
+            model: "delivery-mirror",
+          }),
+        }),
+      ),
+    );
+
+    expect(results.every((result) => result.ok)).toBe(true);
+    const messageIds = results.map((result) => (result.ok ? result.messageId : ""));
+    expect(new Set(messageIds).size).toBe(1);
+
+    const firstOk = results.find((result) => result.ok);
+    if (!firstOk?.ok) {
+      throw new Error("expected exact assistant append to succeed");
+    }
+    const records = fs
+      .readFileSync(firstOk.sessionFile, "utf-8")
+      .trim()
+      .split("\n")
+      .map((line) => JSON.parse(line) as { message?: { role?: string; idempotencyKey?: string } })
+      .filter(
+        (record) =>
+          record.message?.role === "assistant" && record.message.idempotencyKey === idempotencyKey,
+      );
+    expect(records).toHaveLength(1);
+  });
+
   it("can emit file-only transcript refresh events for exact assistant appends", async () => {
     writeTranscriptStore();
     const emitSpy = vi.spyOn(transcriptEvents, "emitSessionTranscriptUpdate");
diff --git a/src/config/sessions/transcript.ts b/src/config/sessions/transcript.ts
index ab96abc52f73..8aece357ee5a 100644
--- a/src/config/sessions/transcript.ts
+++ b/src/config/sessions/transcript.ts
@@ -353,7 +353,6 @@ export async function appendExactAssistantMessageToSessionTranscript(params: {
             return await appendSessionTranscriptMessage({
               transcriptPath: sessionFile,
               message,
-              idempotencyLookup: explicitIdempotencyKey ? "caller-checked" : "scan",
               config: params.config,
             });
           },