fix(core): Propagate sample_rand when continuing a trace without incoming baggage

packages/browser/test/tracing/browserTracingIntegration.test.ts

@@ -1132,7 +1132,9 @@ describe('browserTracingIntegration', () => {
       expect(spanIsSampled(idleSpan)).toBe(false);
 
       expect(dynamicSamplingContext).toBeDefined();
-      expect(dynamicSamplingContext).toStrictEqual({});
+      // Continuing a trace without an incoming Sentry DSC does not populate a new one, but the
+      // scope's `sample_rand` is still propagated so downstream sampling decisions stay consistent.
+      expect(dynamicSamplingContext).toStrictEqual({ sample_rand: propagationContext.sampleRand.toString() });
 
       // Propagation context keeps the meta tag trace data for later events on the same route to add them to the trace
       expect(propagationContext.traceId).toEqual('12312012123120121231201212312012');

packages/core/src/tracing/trace.ts

@@ -433,7 +433,7 @@ function createChildOrRootSpan({
 
     freezeDscOnSpan(span, dsc);
   } else {
-    const { traceId, dsc, parentSpanId, sampled: parentSampled } = scope.getPropagationContext();
+    const { traceId, dsc, parentSpanId, sampled: parentSampled, sampleRand } = scope.getPropagationContext();
 
     span = _startRootSpan(
       {
@@ -447,7 +447,13 @@ function createChildOrRootSpan({
     );
 
     if (dsc) {
-      freezeDscOnSpan(span, dsc);
+      // A trace continued without incoming baggage carries an empty DSC (we are not the head of
+      // trace). Fold in the scope's `sample_rand` so it still propagates downstream and sampling
+      // decisions stay consistent across the trace. A populated frozen DSC (e.g. an OTel remote
+      // parent whose DSC came in via baggage/trace state) is left untouched.
+      const dscWithSampleRand =
+        Object.keys(dsc).length === 0 && sampleRand !== undefined ? { sample_rand: sampleRand.toString() } : dsc;
+      freezeDscOnSpan(span, dscWithSampleRand);
     }
   }
 

packages/sveltekit/test/server-common/handle.test.ts

@@ -1,6 +1,7 @@
 import type { EventEnvelopeHeaders, Span } from '@sentry/core';
 import { HTTP_ROUTE } from '@sentry/conventions/attributes';
 import {
+  getCapturedScopesOnSpan,
   getRootSpan,
   getSpanDescendants,
   SEMANTIC_ATTRIBUTE_SENTRY_SOURCE,
@@ -265,7 +266,11 @@ describe('sentryHandle', () => {
       expect(_span!.spanContext().traceId).toEqual('1234567890abcdef1234567890abcdef');
       expect(spanToJSON(_span!).parent_span_id).toEqual('1234567890abcdef');
       expect(spanIsSampled(_span!)).toEqual(true);
-      expect(envelopeHeaders!.trace).toEqual({});
+      // Continuing a trace without incoming baggage does not populate a new DSC, but the scope's
+      // `sample_rand` is still propagated so downstream sampling decisions stay consistent. Assert it
+      // matches the scope value rather than just any number, so a freshly-minted rand would fail.
+      const scopeSampleRand = getCapturedScopesOnSpan(_span!).scope!.getPropagationContext().sampleRand;
+      expect(envelopeHeaders!.trace).toEqual({ sample_rand: scopeSampleRand.toString() });
     });
 
     it('creates a transaction with dynamic sampling context from baggage header', async () => {