fix(node-core): Pass rejection reason instead of Promise as originalException (#20366)

This PR passes the rejection reason to `captureException` as
`originalException`. Previously it was passing the promise which was
useless to users.

This aligns Node with browser/onUncaughtException behavior as well.

closes #20325

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: logaretm

packages/node-core/src/integrations/onunhandledrejection.ts

@@ -85,7 +85,7 @@ export function makeUnhandledPromiseHandler(
   client: Client,
   options: OnUnhandledRejectionOptions,
 ): (reason: unknown, promise: unknown) => void {
-  return function sendUnhandledPromise(reason: unknown, promise: unknown): void {
+  return function sendUnhandledPromise(reason: unknown, _promise: unknown): void {
     // Only handle for the active client
     if (getClient() !== client) {
       return;
@@ -109,7 +109,7 @@ export function makeUnhandledPromiseHandler(
 
     activeSpanWrapper(() => {
       captureException(reason, {
-        originalException: promise,
+        originalException: reason,
         captureContext: {
           extra: { unhandledPromiseRejection: true },
           level,

packages/node-core/test/integrations/onunhandledrejection.test.ts

@@ -0,0 +1,54 @@
+import * as SentryCore from '@sentry/core';
+import type { Client } from '@sentry/core';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import {
+  makeUnhandledPromiseHandler,
+  onUnhandledRejectionIntegration,
+} from '../../src/integrations/onunhandledrejection';
+
+// don't log the test errors we're going to throw, so at a quick glance it doesn't look like the test itself has failed
+global.console.warn = () => null;
+global.console.error = () => null;
+
+describe('unhandled promises', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('installs a global listener', () => {
+    const client = { getOptions: () => ({}) } as unknown as Client;
+    SentryCore.setCurrentClient(client);
+
+    const beforeListeners = process.listeners('unhandledRejection').length;
+
+    const integration = onUnhandledRejectionIntegration();
+    integration.setup!(client);
+
+    expect(process.listeners('unhandledRejection').length).toBe(beforeListeners + 1);
+  });
+
+  it('passes the rejection reason (not the promise) as originalException', () => {
+    const client = { getOptions: () => ({}) } as unknown as Client;
+    SentryCore.setCurrentClient(client);
+
+    const reason = new Error('boom');
+    const promise = Promise.reject(reason);
+    // swallow the rejection so it does not leak into the test runner
+    promise.catch(() => {});
+
+    const captureException = vi.spyOn(SentryCore, 'captureException').mockImplementation(() => 'test');
+
+    const handler = makeUnhandledPromiseHandler(client, { mode: 'warn', ignore: [] });
+    handler(reason, promise);
+
+    expect(captureException).toHaveBeenCalledTimes(1);
+    const [capturedReason, hint] = captureException.mock.calls[0]!;
+    expect(capturedReason).toBe(reason);
+    expect(hint?.originalException).toBe(reason);
+    expect(hint?.originalException).not.toBe(promise);
+    expect(hint?.mechanism).toEqual({
+      handled: false,
+      type: 'auto.node.onunhandledrejection',
+    });
+  });
+});