diff --git a/tests/runtime/duplicate-in-flight-task.test.ts b/tests/runtime/duplicate-in-flight-task.test.ts
new file mode 100644
index 0000000..bb3c515
--- /dev/null
+++ b/tests/runtime/duplicate-in-flight-task.test.ts
@@ -0,0 +1,79 @@
+import { describe, expect, it } from "vitest";
+import { AgentRuntime } from "../../src/runtime/agent-runtime.js";
+
+describe("AgentRuntime duplicate in-flight task IDs", () => {
+  it("rejects a duplicate active task without emitting a failed lifecycle event", async () => {
+    const runtime = new AgentRuntime({ runtimeId: "duplicate-id-runtime" });
+    let release!: () => void;
+
+    runtime.registerTool({
+      name: "blocking",
+      description: "waits for release",
+      execute: async () => {
+        await new Promise<void>((resolve) => {
+          release = resolve;
+        });
+        return { ok: true };
+      }
+    });
+
+    const failedEvents: unknown[] = [];
+    runtime.getDependencies().eventBus.on("runtime.task.failed", (event) => {
+      failedEvents.push(event);
+    });
+
+    await runtime.start();
+    const first = runtime.executeTask({
+      taskId: "same-id",
+      agentId: "agent-a",
+      toolName: "blocking",
+      input: "first",
+      payload: {}
+    });
+
+    expect(runtime.getInFlightTaskCount()).toBe(1);
+
+    await expect(
+      runtime.executeTask({
+        taskId: "same-id",
+        agentId: "agent-b",
+        toolName: "blocking",
+        input: "duplicate",
+        payload: {}
+      })
+    ).rejects.toMatchObject({
+      code: "DUPLICATE_IN_FLIGHT_TASK",
+      details: { taskId: "same-id" }
+    });
+
+    expect(runtime.getInFlightTaskCount()).toBe(1);
+    expect(failedEvents).toHaveLength(0);
+
+    release();
+    await first;
+    expect(runtime.getInFlightTaskCount()).toBe(0);
+  });
+
+  it("allows the same task ID again after the previous execution settles", async () => {
+    const runtime = new AgentRuntime({ runtimeId: "task-id-reuse-runtime" });
+    runtime.registerTool({
+      name: "echo",
+      description: "returns success",
+      execute: async () => ({ ok: true })
+    });
+
+    await runtime.start();
+
+    const task = {
+      taskId: "reusable-id",
+      agentId: "agent-a",
+      toolName: "echo",
+      input: "echo",
+      payload: { value: 1 }
+    };
+
+    await expect(runtime.executeTask(task)).resolves.toBeDefined();
+    await expect(runtime.executeTask(task)).resolves.toBeDefined();
+    expect(runtime.getInFlightTaskCount()).toBe(0);
+  });
+});