getsentry · Lms24 · Jul 21, 2026 · Jul 21, 2026 · Jul 21, 2026 · Jul 21, 2026
@@ -0,0 +1,10 @@
+import * as Sentry from '@sentry/browser';
+
+window.Sentry = Sentry;
+
+Sentry.init({
+  dsn: 'https://public@dsn.ingest.sentry.io/1337',
+  integrations: [Sentry.browserTracingIntegration()],
+  tracePropagationTargets: ['sentry-test-external.io'],
+  tracesSampleRate: 1,
+});
@@ -0,0 +1,3 @@
+Sentry.startSpan({ name: 'standalone_span', experimental: { standalone: true } }, () => {
+  fetch('http://sentry-test-external.io');
+});
@@ -0,0 +1,8 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <meta name="sentry-trace" content="12345678901234567890123456789012-1234567890123456-1" />
+    <!-- baggage meta tag omitted on purpose -->
+  </head>
+</html>
@@ -0,0 +1,48 @@
+import { expect } from '@playwright/test';
+import type { SpanEnvelope } from '@sentry/core';
+import { sentryTest } from '../../../../utils/fixtures';
+import {
+  getMultipleSentryEnvelopeRequests,
+  properFullEnvelopeRequestParser,
+  shouldSkipTracingTest,
+} from '../../../../utils/helpers';
+
+const TRACE_ID = '12345678901234567890123456789012';
+const OUTGOING_REQUEST_URL = 'http://sentry-test-external.io';
+
+sentryTest(
+  'omits the trace envelope header when a standalone span continues a trace without baggage',
+  async ({ getLocalTestUrl, page }) => {
+    sentryTest.skip(shouldSkipTracingTest());
+
+    const url = await getLocalTestUrl({ testDir: __dirname });
+    await page.route(OUTGOING_REQUEST_URL, route => route.fulfill({ status: 200, body: 'ok' }));
+    const outgoingRequestPromise = page.waitForRequest(OUTGOING_REQUEST_URL);
+
+    const [spanEnvelope] = await getMultipleSentryEnvelopeRequests<SpanEnvelope>(
+      page,
+      1,
+      { url, envelopeType: 'span' },
+      properFullEnvelopeRequestParser,
+    );
+    const outgoingRequest = await outgoingRequestPromise;
+
+    expect(spanEnvelope[0]).toEqual({
+      sent_at: expect.any(String),
+    });
+
+    // To be clear: This is _expected_ behavior, not a bug.
+    // SDKs must assume that an incoming `sentry-trace` but no `baggage` meta tag means that the
+    // trace was started from an SDK that's not yet compatible with the DSC or baggage propagation.
+    // The test demonstrates that the SDK as expected continues the trace but does not send a `trace`
+    // header, nor a baggage header.
+    expect(spanEnvelope[0].trace).toBeUndefined();
+
+    expect(spanEnvelope[1]).toHaveLength(1);
+    expect(spanEnvelope[1][0][1].trace_id).toBe(TRACE_ID);
+
+    const outgoingRequestHeaders = outgoingRequest.headers();
+    expect(outgoingRequestHeaders['sentry-trace']).toMatch(new RegExp(`^${TRACE_ID}-[\\da-f]{16}-1$`));
+    expect(outgoingRequestHeaders['baggage']).toBeUndefined();
+  },
+);
diff --git a/packages/core/src/fetch.ts b/packages/core/src/fetch.ts
@@ -222,7 +222,11 @@ export function _INTERNAL_getTracingHeadersForFetchRequest(
   const originalHeaders = fetchOptionsObj.headers || (isRequest(request) ? request.headers : undefined);
 
   if (!originalHeaders) {
-    return { ...traceHeaders };
+    return {
+      'sentry-trace': sentryTrace,
+      ...(baggage && { baggage }),
+      ...(traceparent && { traceparent }),
+    };
   } else if (isHeaders(originalHeaders)) {
     const newHeaders = new Headers(originalHeaders);
 
@@ -292,11 +296,11 @@ export function _INTERNAL_getTracingHeadersForFetchRequest(
 
     const newHeaders: {
       'sentry-trace': string;
-      baggage: string | undefined;
+      baggage?: string;
       traceparent?: string;
     } = Object.assign({}, originalHeaders, {
       'sentry-trace': (existingSentryTraceHeader as string | undefined) ?? sentryTrace,
-      baggage: newBaggageHeaders.length > 0 ? newBaggageHeaders.join(',') : undefined,
+      ...(newBaggageHeaders.length > 0 && { baggage: newBaggageHeaders.join(',') }),
     });
 
     if (propagateTraceparent && traceparent && !existingTraceparentHeader) {

diff --git a/packages/core/test/lib/fetch.test.ts b/packages/core/test/lib/fetch.test.ts
@@ -69,6 +69,21 @@ describe('_INTERNAL_getTracingHeadersForFetchRequest', () => {
         });
       });
 
+      it('omits baggage from headers object when no baggage is available', () => {
+        vi.mocked(traceData.getTraceData).mockReturnValueOnce({
+          'sentry-trace': DEFAULT_SENTRY_TRACE,
+        });
+
+        const returnedHeaders = _INTERNAL_getTracingHeadersForFetchRequest('/api/test', {
+          headers: { 'custom-header': 'custom-value' },
+        });
+
+        expect(returnedHeaders).toStrictEqual({
+          'sentry-trace': DEFAULT_SENTRY_TRACE,
+          'custom-header': 'custom-value',
+        });
+      });
+
       it('attaches sentry headers to a Headers instance', () => {
         const returnedHeaders = _INTERNAL_getTracingHeadersForFetchRequest('/api/test', {
           headers: new Headers({ 'custom-header': 'custom-value' }),