diff --git a/packages/core/src/telemetry/file-exporters.test.ts b/packages/core/src/telemetry/file-exporters.test.ts
new file mode 100644
index 000000000..40a871de1
--- /dev/null
+++ b/packages/core/src/telemetry/file-exporters.test.ts
@@ -0,0 +1,50 @@
+/**
+ * @license
+ * Copyright 2026 Qwen Team
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { FileSpanExporter } from './file-exporters.js';
+
+type SerializeAccess = { serialize: (data: unknown) => string };
+
+describe('FileExporter.serialize', () => {
+  let tmpDir: string;
+  let exporter: FileSpanExporter;
+  let serialize: (data: unknown) => string;
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'file-exporters-test-'));
+    exporter = new FileSpanExporter(path.join(tmpDir, 'out.jsonl'));
+    serialize = (exporter as unknown as SerializeAccess).serialize.bind(
+      exporter,
+    );
+  });
+
+  afterEach(async () => {
+    await exporter.shutdown();
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  // Regression for upstream PR #4689: a raw JSON.stringify on a ReadableSpan
+  // crashed because BatchSpanProcessor._shutdownOnce -> BindOnceFuture._that
+  // forms a cycle. The exporter must delegate to safeJsonStringify so cycles
+  // become "[Circular]" instead of throwing.
+  it('does not throw on BatchSpanProcessor-shaped cycle', () => {
+    const proc: Record<string, unknown> = { kind: 'BatchSpanProcessor' };
+    const future: Record<string, unknown> = { kind: 'BindOnceFuture' };
+    proc['_shutdownOnce'] = future;
+    future['_that'] = proc;
+    const span = { name: 'span-1', _spanProcessor: proc };
+
+    expect(() => serialize(span)).not.toThrow();
+    const out = serialize(span);
+    expect(out).toContain('"name": "span-1"');
+    expect(out).toContain('"[Circular]"');
+    expect(out.endsWith('\n')).toBe(true);
+  });
+});
diff --git a/packages/core/src/telemetry/file-exporters.ts b/packages/core/src/telemetry/file-exporters.ts
index 55fe64103..def6e91f4 100644
--- a/packages/core/src/telemetry/file-exporters.ts
+++ b/packages/core/src/telemetry/file-exporters.ts
@@ -17,6 +17,7 @@ import type {
   PushMetricExporter,
 } from '@opentelemetry/sdk-metrics';
 import { AggregationTemporality } from '@opentelemetry/sdk-metrics';
+import { safeJsonStringify } from '../utils/safeJsonStringify.js';
 
 class FileExporter {
   protected writeStream: fs.WriteStream;
@@ -26,7 +27,7 @@ class FileExporter {
   }
 
   protected serialize(data: unknown): string {
-    return JSON.stringify(data, null, 2) + '\n';
+    return safeJsonStringify(data, 2) + '\n';
   }
 
   shutdown(): Promise<void> {