diff --git a/packages/durabletask-js/src/worker/task-hub-grpc-worker.ts b/packages/durabletask-js/src/worker/task-hub-grpc-worker.ts
index 18658db..e12464d 100644
--- a/packages/durabletask-js/src/worker/task-hub-grpc-worker.ts
+++ b/packages/durabletask-js/src/worker/task-hub-grpc-worker.ts
@@ -780,6 +780,7 @@ export class TaskHubGrpcWorker {
       const failureDetails = pbh.newFailureDetails(error);
 
       res = new pb.ActivityResponse();
+      res.setInstanceid(instanceId);
       res.setTaskid(req.getTaskid());
       res.setCompletiontoken(completionToken);
       res.setFailuredetails(failureDetails);
diff --git a/packages/durabletask-js/test/worker-activity-response.spec.ts b/packages/durabletask-js/test/worker-activity-response.spec.ts
new file mode 100644
index 0000000..b0dc347
--- /dev/null
+++ b/packages/durabletask-js/test/worker-activity-response.spec.ts
@@ -0,0 +1,151 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+/**
+ * Tests that the TaskHubGrpcWorker correctly sets instanceId on ActivityResponse
+ * in both success and failure paths.
+ *
+ * This validates the fix for a bug where the activity failure path omitted
+ * the instanceId field from the ActivityResponse, deviating from the .NET SDK
+ * behavior and potentially causing issues with sidecar response routing.
+ */
+
+import * as pb from "../src/proto/orchestrator_service_pb";
+import * as stubs from "../src/proto/orchestrator_service_grpc_pb";
+import { TaskHubGrpcWorker } from "../src/worker/task-hub-grpc-worker";
+import { NoOpLogger } from "../src/types/logger.type";
+import { ActivityContext } from "../src/task/context/activity-context";
+import { StringValue } from "google-protobuf/google/protobuf/wrappers_pb";
+
+const TEST_INSTANCE_ID = "test-instance-123";
+const TEST_TASK_ID = 42;
+const COMPLETION_TOKEN = "test-completion-token";
+
+describe("Worker Activity Response", () => {
+  /**
+   * Creates a mock gRPC stub that captures the ActivityResponse passed to
+   * completeActivityTask.
+   */
+  function createMockStub(): {
+    stub: stubs.TaskHubSidecarServiceClient;
+    capturedResponse: pb.ActivityResponse | null;
+  } {
+    let capturedResponse: pb.ActivityResponse | null = null;
+
+    const stub = {
+      completeActivityTask: (
+        response: pb.ActivityResponse,
+        metadata: any,
+        callback: (err: any, res: any) => void,
+      ) => {
+        capturedResponse = response;
+        callback(null, {});
+      },
+    } as unknown as stubs.TaskHubSidecarServiceClient;
+
+    return {
+      stub,
+      get capturedResponse() {
+        return capturedResponse;
+      },
+    };
+  }
+
+  /**
+   * Creates a minimal ActivityRequest for testing.
+   */
+  function createActivityRequest(name: string, input?: string): pb.ActivityRequest {
+    const req = new pb.ActivityRequest();
+    req.setName(name);
+    req.setTaskid(TEST_TASK_ID);
+
+    const orchInstance = new pb.OrchestrationInstance();
+    orchInstance.setInstanceid(TEST_INSTANCE_ID);
+    req.setOrchestrationinstance(orchInstance);
+
+    if (input !== undefined) {
+      const inputValue = new StringValue();
+      inputValue.setValue(input);
+      req.setInput(inputValue);
+    }
+
+    return req;
+  }
+
+  it("should set instanceId on ActivityResponse when activity succeeds", async () => {
+    // Arrange
+    const worker = new TaskHubGrpcWorker({
+      logger: new NoOpLogger(),
+    });
+
+    const successActivity = (_ctx: ActivityContext) => {
+      return "success result";
+    };
+
+    worker.addActivity(successActivity);
+
+    const mockStub = createMockStub();
+    const req = createActivityRequest("successActivity", JSON.stringify("test-input"));
+
+    // Act - call the private method directly
+    await (worker as any)._executeActivityInternal(req, COMPLETION_TOKEN, mockStub.stub);
+
+    // Assert
+    expect(mockStub.capturedResponse).not.toBeNull();
+    expect(mockStub.capturedResponse!.getInstanceid()).toBe(TEST_INSTANCE_ID);
+    expect(mockStub.capturedResponse!.getTaskid()).toBe(TEST_TASK_ID);
+    expect(mockStub.capturedResponse!.getFailuredetails()).toBeUndefined();
+  });
+
+  it("should set instanceId on ActivityResponse when activity fails", async () => {
+    // Arrange
+    const worker = new TaskHubGrpcWorker({
+      logger: new NoOpLogger(),
+    });
+
+    const failingActivity = (_ctx: ActivityContext) => {
+      throw new Error("Activity execution failed");
+    };
+
+    worker.addActivity(failingActivity);
+
+    const mockStub = createMockStub();
+    const req = createActivityRequest("failingActivity");
+
+    // Act - call the private method directly
+    await (worker as any)._executeActivityInternal(req, COMPLETION_TOKEN, mockStub.stub);
+
+    // Assert - the key assertion: instanceId MUST be set even on failure
+    expect(mockStub.capturedResponse).not.toBeNull();
+    expect(mockStub.capturedResponse!.getInstanceid()).toBe(TEST_INSTANCE_ID);
+    expect(mockStub.capturedResponse!.getTaskid()).toBe(TEST_TASK_ID);
+    expect(mockStub.capturedResponse!.getFailuredetails()).toBeDefined();
+    expect(mockStub.capturedResponse!.getFailuredetails()!.getErrormessage()).toContain(
+      "Activity execution failed",
+    );
+  });
+
+  it("should set instanceId on ActivityResponse when activity throws non-Error", async () => {
+    // Arrange
+    const worker = new TaskHubGrpcWorker({
+      logger: new NoOpLogger(),
+    });
+
+    const throwStringActivity = (_ctx: ActivityContext) => {
+      throw "string error";
+    };
+
+    worker.addActivity(throwStringActivity);
+
+    const mockStub = createMockStub();
+    const req = createActivityRequest("throwStringActivity");
+
+    // Act
+    await (worker as any)._executeActivityInternal(req, COMPLETION_TOKEN, mockStub.stub);
+
+    // Assert
+    expect(mockStub.capturedResponse).not.toBeNull();
+    expect(mockStub.capturedResponse!.getInstanceid()).toBe(TEST_INSTANCE_ID);
+    expect(mockStub.capturedResponse!.getFailuredetails()).toBeDefined();
+  });
+});