diff --git a/.changeset/shared-sync-cloneable-abort.md b/.changeset/shared-sync-cloneable-abort.md
new file mode 100644
index 000000000..2e2ffd3e6
--- /dev/null
+++ b/.changeset/shared-sync-cloneable-abort.md
@@ -0,0 +1,5 @@
+---
+"@powersync/web": patch
+---
+
+Use a cloneable abort reason when closing shared sync client ports to avoid DataCloneError across Comlink.
diff --git a/packages/web/src/worker/sync/SharedSyncImplementation.ts b/packages/web/src/worker/sync/SharedSyncImplementation.ts
index 5c4f5b683..c3979a3fb 100644
--- a/packages/web/src/worker/sync/SharedSyncImplementation.ts
+++ b/packages/web/src/worker/sync/SharedSyncImplementation.ts
@@ -1,5 +1,4 @@
 import {
-  AbortOperation,
   BaseObserver,
   ConnectionManager,
   createLogger,
@@ -313,11 +312,10 @@ export class SharedSyncImplementation extends BaseObserver<SharedSyncImplementat
        * The port might currently be in use. Any active functions might
        * not resolve. Abort them here.
        */
+      const abortReason = 'Closing pending requests after client port is removed';
       [this.fetchCredentialsController, this.uploadDataController].forEach((abortController) => {
         if (abortController?.activePort == port) {
-          abortController!.controller.abort(
-            new AbortOperation('Closing pending requests after client port is removed')
-          );
+          abortController!.controller.abort(abortReason);
         }
       });
 
diff --git a/packages/web/tests/shared_sync_abort_reason.test.ts b/packages/web/tests/shared_sync_abort_reason.test.ts
new file mode 100644
index 000000000..0ca5ab339
--- /dev/null
+++ b/packages/web/tests/shared_sync_abort_reason.test.ts
@@ -0,0 +1,96 @@
+import * as Comlink from 'comlink';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { SharedSyncImplementation, type WrappedSyncPort } from '../src/worker/sync/SharedSyncImplementation';
+import type { AbstractSharedSyncClientProvider } from '../src/worker/sync/AbstractSharedSyncClientProvider';
+
+describe('Shared sync abort reasons', { sequential: true }, () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('uses a cloneable abort reason when closing a client port', async () => {
+    const implementation = new SharedSyncImplementation();
+    const abortController = new AbortController();
+    const { port1 } = new MessageChannel();
+
+    const clientProvider = {
+      fetchCredentials: vi.fn(async () => null),
+      invalidateCredentials: vi.fn(),
+      uploadCrud: vi.fn(async () => {}),
+      statusChanged: vi.fn(),
+      getDBWorkerPort: vi.fn(async () => port1),
+      trace: vi.fn(),
+      debug: vi.fn(),
+      info: vi.fn(),
+      log: vi.fn(),
+      warn: vi.fn(),
+      error: vi.fn(),
+      time: vi.fn(),
+      timeEnd: vi.fn(),
+      [Comlink.releaseProxy]: vi.fn()
+    } as unknown as Comlink.Remote<AbstractSharedSyncClientProvider>;
+
+    const wrappedPort = {
+      port: port1,
+      clientProvider,
+      currentSubscriptions: [],
+      closeListeners: []
+    } satisfies WrappedSyncPort;
+
+    (implementation as any).ports.push(wrappedPort);
+    (implementation as any).fetchCredentialsController = {
+      controller: abortController,
+      activePort: wrappedPort
+    };
+
+    const abortSpy = vi.spyOn(abortController, 'abort');
+    await implementation.removePort(wrappedPort);
+
+    expect(abortSpy).toHaveBeenCalled();
+    const reason = abortSpy.mock.calls[0]?.[0];
+    expect(typeof reason).toBe('string');
+  });
+
+  it('uses a cloneable abort reason when closing a client port with uploads in-flight', async () => {
+    const implementation = new SharedSyncImplementation();
+    const abortController = new AbortController();
+    const { port1 } = new MessageChannel();
+
+    const clientProvider = {
+      fetchCredentials: vi.fn(async () => null),
+      invalidateCredentials: vi.fn(),
+      uploadCrud: vi.fn(async () => {}),
+      statusChanged: vi.fn(),
+      getDBWorkerPort: vi.fn(async () => port1),
+      trace: vi.fn(),
+      debug: vi.fn(),
+      info: vi.fn(),
+      log: vi.fn(),
+      warn: vi.fn(),
+      error: vi.fn(),
+      time: vi.fn(),
+      timeEnd: vi.fn(),
+      [Comlink.releaseProxy]: vi.fn()
+    } as unknown as Comlink.Remote<AbstractSharedSyncClientProvider>;
+
+    const wrappedPort = {
+      port: port1,
+      clientProvider,
+      currentSubscriptions: [],
+      closeListeners: []
+    } satisfies WrappedSyncPort;
+
+    (implementation as any).ports.push(wrappedPort);
+    (implementation as any).uploadDataController = {
+      controller: abortController,
+      activePort: wrappedPort
+    };
+
+    const abortSpy = vi.spyOn(abortController, 'abort');
+    await implementation.removePort(wrappedPort);
+
+    expect(abortSpy).toHaveBeenCalled();
+    const reason = abortSpy.mock.calls[0]?.[0];
+    expect(typeof reason).toBe('string');
+  });
+});