diff --git a/AgentCompany-New/tests/integration/agent-reply.test.ts b/AgentCompany-New/tests/integration/agent-reply.test.ts
new file mode 100644
index 00000000..04e3cd22
--- /dev/null
+++ b/AgentCompany-New/tests/integration/agent-reply.test.ts
@@ -0,0 +1,112 @@
+import { randomUUID } from "node:crypto";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { describe, expect, test } from "vitest";
+import { DatabaseConnection } from "../../src/main/database/connection";
+import { OnboardingService } from "../../src/main/services/onboarding-service";
+import { ProviderService } from "../../src/main/services/provider-service";
+import { providerAdapters } from "../../src/main/services/provider-adapters";
+import { TaskExecutionService } from "../../src/main/services/task-execution-service";
+import { SnapshotService } from "../../src/main/services/snapshot-service";
+import { claimNextQueuedRun } from "../../src/main/app/task-queue-worker";
+import { canonicalMentionHandle } from "../../src/main/services/mention-handles";
+import type { AgentTurnResult, ProviderToolTurnInput, ToolCall } from "../../src/main/services/provider-tool-turn";
+
+const testVault = {
+  encryptSecret: (secret: string) => Buffer.from(secret, "utf8"),
+  decryptSecret: (ciphertext: Buffer) => ciphertext.toString("utf8"),
+};
+
+function call(name: string, args: Record<string, unknown>): ToolCall {
+  return { id: `call_${randomUUID()}`, name, arguments: args };
+}
+
+/**
+ * Scripts two distinct agents off the same provider, branching on the
+ * instructions (which name the acting agent). The CEO staffs one engineer and
+ * @mentions them in #engineering; the engineer, once woken, replies in the same
+ * thread — the round trip that makes the company feel alive.
+ */
+function scriptedTeam(): (input: ProviderToolTurnInput) => Promise<AgentTurnResult> {
+  const handle = canonicalMentionHandle("Dana Lee");
+  let ceo = 0;
+  let dana = 0;
+  const ceoScript = [
+    { text: "Staffing engineering.", toolCalls: [call("hire_agent", { name: "Dana Lee", role: "Engineer", department: "Engineering" })] },
+    { text: "Delegating and pinging.", toolCalls: [
+      call("create_task", { title: "Build the booking API", description: "Ship the booking API.", assignee_agent_name: "Dana Lee" }),
+      call("send_message", { channel: "engineering", body: `@${handle} you own the booking API — kicking off here.` }),
+    ] },
+    { text: "Kickoff complete.", toolCalls: [] },
+  ];
+  return async (input: ProviderToolTurnInput): Promise<AgentTurnResult> => {
+    if ((input.instructions ?? "").includes("Dana Lee")) {
+      // Reply inside the thread the CEO started (the prompt carries its id).
+      const firstUser = input.messages.find((message) => message.role === "user");
+      const threadId = firstUser ? /thread_id="([^"]+)"/.exec(firstUser.text)?.[1] : undefined;
+      const turn = dana === 0
+        ? { text: "On it.", toolCalls: [call("send_message", { channel: "engineering", body: "On it — booking API design starting now.", ...(threadId ? { thread_id: threadId } : {}) })] }
+        : { text: "Replied.", toolCalls: [] };
+      dana += 1;
+      return { text: turn.text, toolCalls: turn.toolCalls, rawResponseJson: "{}" };
+    }
+    const turn = ceoScript[ceo] ?? { text: "done", toolCalls: [] };
+    ceo += 1;
+    return { text: turn.text, toolCalls: turn.toolCalls, rawResponseJson: "{}" };
+  };
+}
+
+describe("inter-agent reply", () => {
+  test("an @mention wakes the teammate, who replies in the same thread", async () => {
+    const connection = new DatabaseConnection(join(tmpdir(), `ac-reply-${randomUUID()}.db`));
+    const providers = new ProviderService(connection.db, providerAdapters, testVault, undefined, scriptedTeam());
+    const onboarding = new OnboardingService({ db: connection.db, saveProviderCredential: (input) => providers.saveCredential(input) });
+    const taskExecution = new TaskExecutionService(connection.db, providers);
+    const snapshot = new SnapshotService(connection.db);
+
+    let orgId = "";
+    connection.transaction(() => {
+      orgId = onboarding.bootstrap({
+        companyName: "OrbitOps AI",
+        companyDescription: "AI-run operations company.",
+        employeeName: "Yang",
+        workspacePath: "",
+        executionMode: "ai_ceo",
+        autoApproveHires: true,
+        reviewDeliverables: false,
+        goalTitle: "Stand up a service business.",
+        goalDescription: "Win the first customers.",
+        providerKey: "openai",
+        providerSecret: "sk-test-key",
+        providerModel: "gpt-5.4",
+      }).orgId;
+    });
+
+    // Run the CEO's kickoff: it hires Dana and @mentions her in #engineering.
+    const ceoRun = claimNextQueuedRun(connection);
+    await taskExecution.executeRun(ceoRun!.org_id, ceoRun!.id);
+
+    const dana = (connection.db.prepare("select id from agents where org_id = ? and name = 'Dana Lee'").get(orgId) as { id: string });
+    expect(dana).toBeTruthy();
+
+    // The @mention woke Dana: a queued chat_mention run was created for her.
+    const wake = connection.db.prepare(
+      "select id, state, trigger from task_runs where org_id = ? and agent_id = ? and trigger = 'chat_mention'",
+    ).get(orgId, dana.id) as { id: string; state: string; trigger: string } | undefined;
+    expect(wake).toMatchObject({ state: "queued", trigger: "chat_mention" });
+
+    // Execute Dana's woken run: she replies in the CEO's thread.
+    await taskExecution.executeRun(orgId, wake!.id);
+
+    const engineering = connection.db.prepare("select id from chat_spaces where org_id = ? and name = 'engineering'").get(orgId) as { id: string };
+    const messages = snapshot.load().chatMessages.filter((m) => m.orgId === orgId && m.spaceId === engineering.id);
+    const senders = new Set(messages.map((m) => m.senderId));
+    // Both the CEO and Dana have spoken in #engineering, in one shared thread.
+    expect(messages.length).toBe(2);
+    expect(senders.size).toBe(2);
+    expect(new Set(messages.map((m) => m.threadId)).size).toBe(1);
+    expect(messages.some((m) => m.senderId === dana.id && m.bodyMd.includes("On it"))).toBe(true);
+
+    connection.close();
+  });
+});