fix(core): preserve span links through beforeSendSpan

CHANGELOG.md

@@ -49,6 +49,10 @@ Work in this release was contributed by @adityareddy-dev, @jpdefo, @Jxxunnn, @ph
 
   `denoServeIntegration` now records a request session for each incoming request, as `denoHttpIntegration` does for `node:http`. This gives release health and crash-free rates for `Deno.serve` apps. Sessions are sent only when a `release` is set. To turn them off, set `denoServeIntegration({ sessions: false })`. `sessionFlushingDelayMS` sets how long the SDK waits before it sends the session aggregate.
 
+- **fix(core): Preserve span links through `beforeSendSpan`**
+
+  A transaction's `contexts.trace.links` (including the `previous_trace` link) was dropped whenever `beforeSendSpan` was set, even for an identity callback, because the transaction-event/span-JSON round trip did not carry `links`. The conversion now preserves them.
+
 ## 11.8.0
 
 ### Important Changes

packages/core/src/utils/transactionEvent.ts

@@ -6,7 +6,7 @@ import type { SpanJSON } from '../types/span';
  * Converts a transaction event to a span JSON object.
  */
 export function convertTransactionEventToSpanJson(event: TransactionEvent): SpanJSON {
-  const { trace_id, parent_span_id, span_id, status, origin, data, op } = event.contexts?.trace ?? {};
+  const { trace_id, parent_span_id, span_id, status, origin, data, op, links } = event.contexts?.trace ?? {};
 
   return {
     data: data ?? {},
@@ -19,6 +19,7 @@ export function convertTransactionEventToSpanJson(event: TransactionEvent): Span
     timestamp: event.timestamp,
     trace_id: trace_id ?? '',
     origin,
+    links,
     profile_id: data?.[SENTRY_PROFILE_ID] as string | undefined,
     exclusive_time: data?.[SENTRY_EXCLUSIVE_TIME] as number | undefined,
     measurements: event.measurements,
@@ -43,6 +44,7 @@ export function convertSpanJsonToTransactionEvent(span: SpanJSON): TransactionEv
         op: span.op,
         status: span.status,
         origin: span.origin,
+        links: span.links,
         data: {
           ...span.data,
           ...(span.profile_id && { [SENTRY_PROFILE_ID]: span.profile_id }),

packages/core/test/lib/client.test.ts

@@ -1596,6 +1596,37 @@ describe('Client', () => {
       });
     });
 
+    test('keeps root span links on the sent transaction with `beforeSendSpan`', () => {
+      const beforeSendSpan = withStaticSpan(vi.fn((span: SpanJSON) => span));
+      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSendSpan });
+      const client = new TestClient(options);
+
+      const links = [
+        {
+          trace_id: '12312012123120121231201212312012',
+          span_id: '1231201212312012',
+          sampled: true,
+          attributes: { 'sentry.link.type': 'previous_trace' },
+        },
+      ];
+
+      client.captureEvent({
+        transaction: '/animals/are/great',
+        type: 'transaction',
+        spans: [],
+        contexts: {
+          trace: {
+            span_id: '9e15bf99fbe4bc80',
+            trace_id: '86f39e84263a4de99c326acab3bfe3bd',
+            links,
+          },
+        },
+      });
+
+      expect(beforeSendSpan).toHaveBeenCalledTimes(1);
+      expect(TestClient.instance!.event!.contexts?.trace?.links).toEqual(links);
+    });
+
     test('calls `beforeSendTransaction` and uses the modified event', () => {
       expect.assertions(2);
 

packages/core/test/lib/utils/transactionEvent.test.ts

@@ -48,6 +48,14 @@ describe('convertTransactionEventToSpanJson', () => {
             [SENTRY_EXCLUSIVE_TIME]: 123.45,
             other: 'value',
           },
+          links: [
+            {
+              span_id: 'linked-span',
+              trace_id: 'linked-trace',
+              sampled: true,
+              attributes: { 'sentry.link.type': 'previous_trace' },
+            },
+          ],
         },
       },
       start_timestamp: 1234567800,
@@ -74,6 +82,14 @@ describe('convertTransactionEventToSpanJson', () => {
       origin: 'manual',
       profile_id: 'profile123',
       exclusive_time: 123.45,
+      links: [
+        {
+          span_id: 'linked-span',
+          trace_id: 'linked-trace',
+          sampled: true,
+          attributes: { 'sentry.link.type': 'previous_trace' },
+        },
+      ],
       measurements: {
         fp: { value: 123, unit: 'millisecond' },
       },
@@ -142,6 +158,14 @@ describe('convertSpanJsonToTransactionEvent', () => {
       origin: 'manual',
       profile_id: 'profile123',
       exclusive_time: 123.45,
+      links: [
+        {
+          span_id: 'linked-span',
+          trace_id: 'linked-trace',
+          sampled: true,
+          attributes: { 'sentry.link.type': 'previous_trace' },
+        },
+      ],
       measurements: {
         fp: { value: 123, unit: 'millisecond' },
       },
@@ -160,6 +184,14 @@ describe('convertSpanJsonToTransactionEvent', () => {
           op: 'http',
           status: 'ok',
           origin: 'manual',
+          links: [
+            {
+              span_id: 'linked-span',
+              trace_id: 'linked-trace',
+              sampled: true,
+              attributes: { 'sentry.link.type': 'previous_trace' },
+            },
+          ],
           data: {
             other: 'value',
             [SENTRY_PROFILE_ID]: 'profile123',