diff --git a/tests/runtime/reproduce-issue-259.test.ts b/tests/runtime/reproduce-issue-259.test.ts
new file mode 100644
index 0000000..1079cf3
--- /dev/null
+++ b/tests/runtime/reproduce-issue-259.test.ts
@@ -0,0 +1,179 @@
+import { describe, it, expect, vi } from "vitest";
+import { AgentRuntime } from "../../src/runtime/agent-runtime.js";
+
+describe("Issue #259 Reproduction — Await in-flight task promises in AgentRuntime.stop() instead of busy-polling", () => {
+  it("does not use sleep-based busy-polling loop during task draining when tasks complete", async () => {
+    const runtime = new AgentRuntime({
+      runtimeId: "rt-reproduce-polling",
+      logger: {
+        level: "error",
+        info: vi.fn(),
+        warn: vi.fn(),
+        error: vi.fn(),
+        debug: vi.fn()
+      }
+    });
+
+    const sleepSpy = vi.spyOn(runtime as any, "sleep");
+
+    runtime.registerTool({
+      name: "delayed-work",
+      description: "Work that completes after a brief delay",
+      execute: async () => {
+        await new Promise((resolve) => setTimeout(resolve, 30));
+        return { completed: true };
+      }
+    });
+
+    await runtime.start();
+
+    const taskPromise = runtime.executeTask({
+      taskId: "task-polling-1",
+      agentId: "agent-1",
+      toolName: "delayed-work",
+      input: "run delayed work",
+      payload: {}
+    });
+
+    await runtime.stop({ drainTimeoutMs: 1000 });
+    await taskPromise;
+
+    // Issue #259: drainInFlightTasks in current implementation uses a busy-wait loop:
+    // while (this.inFlightTasks.size > 0) { await this.sleep(Math.min(5, remaining)); }
+    // When tasks are in-flight, sleep(5) is invoked repeatedly every 5ms.
+    // The expected behavior is event-driven draining that awaits task promises directly
+    // without executing a sleep-polling loop.
+    const shortSleepPolls = sleepSpy.mock.calls.filter(
+      ([ms]) => typeof ms === "number" && ms <= 5
+    );
+    expect(shortSleepPolls).toHaveLength(0);
+    expect(sleepSpy).not.toHaveBeenCalledWith(5);
+  });
+
+  it("awaits in-flight task promises directly and unblocks immediately upon task completion", async () => {
+    const runtime = new AgentRuntime({
+      runtimeId: "rt-reproduce-promise-await",
+      logger: {
+        level: "error",
+        info: vi.fn(),
+        warn: vi.fn(),
+        error: vi.fn(),
+        debug: vi.fn()
+      }
+    });
+
+    let resolveTask!: (value: unknown) => void;
+    const taskExecutionPromise = new Promise((resolve) => {
+      resolveTask = resolve;
+    });
+
+    runtime.registerTool({
+      name: "controlled-task",
+      description: "Task controlled by external promise resolution",
+      execute: () => taskExecutionPromise
+    });
+
+    await runtime.start();
+
+    const taskPromise = runtime.executeTask({
+      taskId: "task-controlled-1",
+      agentId: "agent-1",
+      toolName: "controlled-task",
+      input: "run controlled task",
+      payload: {}
+    });
+
+    // Interface Contract from PROJECT.md:
+    // Runtime must track in-flight task promises (inFlightPromises: Map<string, Promise<unknown>>)
+    // so stop() can await them directly via Promise.allSettled instead of polling a Set of strings.
+    const inFlightPromises = (runtime as any).inFlightPromises;
+    expect(inFlightPromises).toBeDefined();
+    expect(
+      inFlightPromises instanceof Map || inFlightPromises instanceof Set
+    ).toBe(true);
+    expect(inFlightPromises.size).toBe(1);
+
+    let stopSettled = false;
+    const stopPromise = runtime.stop({ drainTimeoutMs: 5000 }).then(() => {
+      stopSettled = true;
+    });
+
+    // Flush immediate microtasks
+    await new Promise((resolve) => setTimeout(resolve, 20));
+
+    // While task execution promise is pending, stop should remain in-flight
+    expect(stopSettled).toBe(false);
+    expect(runtime.getInFlightTaskCount()).toBe(1);
+
+    // Resolve task execution promise directly
+    resolveTask({ status: "success" });
+
+    await taskPromise;
+    await stopPromise;
+
+    expect(stopSettled).toBe(true);
+    expect(runtime.getInFlightTaskCount()).toBe(0);
+    expect((runtime as any).inFlightPromises.size).toBe(0);
+  });
+
+  it("tracks multiple concurrent in-flight task promises and drains without busy-polling", async () => {
+    const runtime = new AgentRuntime({
+      runtimeId: "rt-reproduce-concurrent",
+      logger: {
+        level: "error",
+        info: vi.fn(),
+        warn: vi.fn(),
+        error: vi.fn(),
+        debug: vi.fn()
+      }
+    });
+
+    const sleepSpy = vi.spyOn(runtime as any, "sleep");
+
+    runtime.registerTool<{ delayMs: number }, { delayed: number }>({
+      name: "variable-delay",
+      description: "Task taking variable delay",
+      execute: async ({ payload }) => {
+        await new Promise((resolve) => setTimeout(resolve, payload.delayMs));
+        return { delayed: payload.delayMs };
+      }
+    });
+
+    await runtime.start();
+
+    const task1 = runtime.executeTask<{ delayMs: number }, { delayed: number }>(
+      {
+        taskId: "task-concurrent-1",
+        agentId: "agent-1",
+        toolName: "variable-delay",
+        input: "delay 15",
+        payload: { delayMs: 15 }
+      }
+    );
+
+    const task2 = runtime.executeTask<{ delayMs: number }, { delayed: number }>(
+      {
+        taskId: "task-concurrent-2",
+        agentId: "agent-2",
+        toolName: "variable-delay",
+        input: "delay 35",
+        payload: { delayMs: 35 }
+      }
+    );
+
+    // Verify task promises are tracked concurrently
+    const inFlightPromises = (runtime as any).inFlightPromises;
+    expect(inFlightPromises).toBeDefined();
+    expect(inFlightPromises.size).toBe(2);
+
+    await runtime.stop({ drainTimeoutMs: 2000 });
+    await Promise.all([task1, task2]);
+
+    const shortSleepPolls = sleepSpy.mock.calls.filter(
+      ([ms]) => typeof ms === "number" && ms <= 5
+    );
+    expect(shortSleepPolls).toHaveLength(0);
+    expect(sleepSpy).not.toHaveBeenCalledWith(5);
+    expect((runtime as any).inFlightPromises.size).toBe(0);
+  });
+});