diff --git a/extensions/qa-lab/src/live-transports/slack/scenario-runtime.test.ts b/extensions/qa-lab/src/live-transports/slack/scenario-runtime.test.ts
new file mode 100644
index 000000000000..9e98f9e77bb0
--- /dev/null
+++ b/extensions/qa-lab/src/live-transports/slack/scenario-runtime.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it, vi } from "vitest";
+import { runSlackScenario } from "./scenario-runtime.js";
+import { slackQaMpimAppMentionDedupeScenario } from "./slack-live.scenario-implementations.js";
+
+function createMpimRuntime(
+  capturedMessages: Array<{ channelId: string; text: string; ts: string }>,
+) {
+  const builtRun = slackQaMpimAppMentionDedupeScenario.buildRun("U_SUT");
+  if (
+    builtRun.kind === "approval" ||
+    builtRun.kind === "codex-approval" ||
+    builtRun.kind === "direct-transport"
+  ) {
+    throw new Error("expected Slack MPIM message scenario");
+  }
+  const markerText = `${builtRun.matchText}_BOT_TESTNONCE`;
+  const run = {
+    ...builtRun,
+    afterReply: undefined,
+    beforeRun: undefined,
+    cleanup: undefined,
+    settleObservedMs: undefined,
+  };
+  const postMessage = vi.fn().mockResolvedValue({ channel: "C_MPIM", ts: "1.000000" });
+  const history = vi.fn().mockResolvedValue({
+    messages: [
+      {
+        bot_id: "B_SUT",
+        text: markerText,
+        thread_ts: "1.000000",
+        ts: "2.000000",
+        user: "U_SUT",
+      },
+    ],
+  });
+  const environment = {
+    channelId: "C_MPIM",
+    configureScenario: vi.fn().mockResolvedValue({
+      cfg: {},
+      primaryModel: "mock-openai/gpt-5.6-luna",
+      run,
+    }),
+    context: {
+      driverClient: { chat: { postMessage } },
+      sutReadClient: { conversations: { history } },
+    },
+    getMessageWriteCursor: () => 0,
+    observedMessages: [],
+    readMessageWrites: vi.fn().mockResolvedValue(capturedMessages),
+    scenario: {
+      id: "slack-mpim-app-mention-dedupe",
+      timeoutMs: 1_000,
+      title: "Slack MPIM app mention dispatches once with thread context",
+    },
+    sutIdentity: { botId: "B_SUT", userId: "U_SUT" },
+  };
+  return { env: environment as never, marker: markerText, writes: environment.readMessageWrites };
+}
+
+describe("Slack scenario runtime capture merge", () => {
+  it.each([
+    ["ignores off-channel captured commentary", "C_OTHER", "commentary", "1.500000", undefined],
+    [
+      "rejects a second same-channel non-marker response",
+      "C_MPIM",
+      "commentary",
+      "2.500000",
+      "1 marker match(es)",
+    ],
+    ["rejects two marker responses", "C_MPIM", "MARKER", "2.500000", "2 marker match(es)"],
+    ["deduplicates the same Slack timestamp", "C_MPIM", "MARKER", "2.000000", undefined],
+  ])("%s", async (_label, channelId, capturedText, ts, expectedMarkerCount) => {
+    const runtime = createMpimRuntime([]);
+    runtime.writes.mockResolvedValue([
+      {
+        channelId,
+        text: capturedText === "MARKER" ? runtime.marker : capturedText,
+        ts,
+      },
+    ]);
+    const result = runSlackScenario(runtime.env, slackQaMpimAppMentionDedupeScenario);
+    if (expectedMarkerCount) {
+      await expect(result).rejects.toThrow(`got 2 response(s) and ${expectedMarkerCount}`);
+      return;
+    }
+    await expect(result).resolves.toEqual(
+      expect.objectContaining({ details: expect.stringContaining("one MPIM reply observed") }),
+    );
+  });
+});
diff --git a/extensions/qa-lab/src/live-transports/slack/scenario-runtime.ts b/extensions/qa-lab/src/live-transports/slack/scenario-runtime.ts
index 78a632088f3c..f0439db557ea 100644
--- a/extensions/qa-lab/src/live-transports/slack/scenario-runtime.ts
+++ b/extensions/qa-lab/src/live-transports/slack/scenario-runtime.ts
@@ -117,7 +117,7 @@ async function runSlackMessageScenario(params: {
       finalMessage: reply.message,
       messages: [
         ...params.environment.observedMessages.slice(observedMessageStartIndex),
-        ...capturedMessages,
+        ...capturedMessages.filter((message) => message.channelId === channelId),
       ],
     });
     const afterReplyDetails = await params.run.afterReply?.(reply.message, {