fix(replay): Set replay_id on DSC after buffer-to-session conversion

When `sendBufferedReplayOrFlush` converts from buffer to session mode,
it calls `startRecording()` directly but never updates the cached DSC
with the new replay_id. The `createDsc` hook only fires for new spans,
not for an already-cached DSC on the scope, so the replay_id was missing
from all outgoing requests until a new span was created.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: billyvg

packages/replay-internal/src/replay.ts

@@ -52,7 +52,10 @@ import { getHandleRecordingEmit } from './util/handleRecordingEmit';
 import { isExpired } from './util/isExpired';
 import { isSessionExpired } from './util/isSessionExpired';
 import { debug } from './util/logger';
-import { resetReplayIdOnDynamicSamplingContext } from './util/resetReplayIdOnDynamicSamplingContext';
+import {
+  resetReplayIdOnDynamicSamplingContext,
+  setReplayIdOnDynamicSamplingContext,
+} from './util/resetReplayIdOnDynamicSamplingContext';
 import { closestElementOfNode } from './util/rrweb';
 import { sendReplay } from './util/sendReplay';
 import { RateLimitError, ReplayDurationLimitError } from './util/sendReplayRequest';
@@ -619,6 +622,10 @@ export class ReplayContainer implements ReplayContainerInterface {
     }
 
     this.startRecording();
+
+    if (this.session) {
+      setReplayIdOnDynamicSamplingContext(this.session.id);
+    }
   }
 
   /**

packages/replay-internal/src/util/resetReplayIdOnDynamicSamplingContext.ts

@@ -18,3 +18,21 @@ export function resetReplayIdOnDynamicSamplingContext(): void {
     delete (dsc as Partial<DynamicSamplingContext>).replay_id;
   }
 }
+
+/**
+ * Set the `replay_id` field on the cached DSC.
+ * The cached DSC on the scope (set by browserTracingIntegration) persists across
+ * session boundaries, and `createDsc` won't fire when a cached DSC already exists.
+ */
+export function setReplayIdOnDynamicSamplingContext(replayId: string): void {
+  const dsc = getCurrentScope().getPropagationContext().dsc;
+  if (dsc) {
+    dsc.replay_id = replayId;
+  }
+
+  const activeSpan = getActiveSpan();
+  if (activeSpan) {
+    const dsc = getDynamicSamplingContextFromSpan(activeSpan);
+    (dsc as Partial<DynamicSamplingContext>).replay_id = replayId;
+  }
+}