diff --git a/dd-trace-core/src/main/java/datadog/trace/common/writer/TraceProcessingWorker.java b/dd-trace-core/src/main/java/datadog/trace/common/writer/TraceProcessingWorker.java
index 9b70b09bc9e..1a6171998b9 100644
--- a/dd-trace-core/src/main/java/datadog/trace/common/writer/TraceProcessingWorker.java
+++ b/dd-trace-core/src/main/java/datadog/trace/common/writer/TraceProcessingWorker.java
@@ -85,12 +85,10 @@ public void start() {
   }
 
   public boolean flush(long timeout, TimeUnit timeUnit) {
-    CountDownLatch latch = new CountDownLatch(1);
-    FlushEvent flush = new FlushEvent(latch);
-    boolean offered;
-    do {
-      offered = primaryQueue.offer(flush);
-    } while (!offered && serializerThread.isAlive());
+    // flush both queues so sampled-out traces (routed to the secondary queue) aren't left behind
+    CountDownLatch latch = new CountDownLatch(2);
+    offer(primaryQueue, new FlushEvent(latch));
+    offer(secondaryQueue, new FlushEvent(latch));
     try {
       return latch.await(timeout, timeUnit);
     } catch (InterruptedException e) {
@@ -99,6 +97,13 @@ public boolean flush(long timeout, TimeUnit timeUnit) {
     }
   }
 
+  private void offer(MessagePassingBlockingQueue<Object> queue, FlushEvent flush) {
+    boolean offered;
+    do {
+      offered = queue.offer(flush);
+    } while (!offered && serializerThread.isAlive());
+  }
+
   @Override
   public void close() {
     spanSamplingWorker.close();
diff --git a/dd-trace-core/src/main/java/datadog/trace/common/writer/WriterFactory.java b/dd-trace-core/src/main/java/datadog/trace/common/writer/WriterFactory.java
index 73875a0e408..1a5abc85dba 100644
--- a/dd-trace-core/src/main/java/datadog/trace/common/writer/WriterFactory.java
+++ b/dd-trace-core/src/main/java/datadog/trace/common/writer/WriterFactory.java
@@ -134,6 +134,13 @@ public static Writer createWriter(
       }
     }
 
+    // In Lambda the execution environment can freeze as soon as the handler returns, before a
+    // writer's periodic flush timer next fires -- flush synchronously so buffered events (e.g.
+    // LLM Observability spans) aren't lost when that happens.
+    boolean isServerlessDefault =
+        config.isAgentConfiguredUsingDefault()
+            && ServerlessInfo.get().isRunningInServerlessEnvironment();
+
     RemoteWriter remoteWriter;
     if (DD_INTAKE_WRITER_TYPE.equals(configuredType)) {
       final TrackType trackType = DDIntakeTrackTypeResolver.resolve(config);
@@ -147,6 +154,7 @@ public static Writer createWriter(
               .healthMetrics(healthMetrics)
               .monitoring(commObjects.monitoring)
               .singleSpanSampler(singleSpanSampler)
+              .alwaysFlush(isServerlessDefault)
               .flushIntervalMilliseconds(flushIntervalMilliseconds);
 
       if (config.isCiVisibilityEnabled()) {
@@ -167,8 +175,7 @@ public static Writer createWriter(
 
     } else { // configuredType == DDAgentWriter
       boolean alwaysFlush = false;
-      if (config.isAgentConfiguredUsingDefault()
-          && ServerlessInfo.get().isRunningInServerlessEnvironment()) {
+      if (isServerlessDefault) {
         if (!ServerlessInfo.get().hasExtension()) {
           log.info(
               "Detected serverless environment. Serverless extension has not been detected, using PrintingWriter");
diff --git a/dd-trace-core/src/test/java/datadog/trace/common/writer/TraceProcessingWorkerTest.java b/dd-trace-core/src/test/java/datadog/trace/common/writer/TraceProcessingWorkerTest.java
index 86dbfa61b22..c2ee8fd3c7d 100644
--- a/dd-trace-core/src/test/java/datadog/trace/common/writer/TraceProcessingWorkerTest.java
+++ b/dd-trace-core/src/test/java/datadog/trace/common/writer/TraceProcessingWorkerTest.java
@@ -141,9 +141,10 @@ void testAFlushShouldClearThePrimaryQueue() {
       worker.start();
       boolean flushed = worker.flush(10, TimeUnit.SECONDS);
 
-      // the flush succeeds, triggers a dispatch, and the queue is empty
+      // the flush succeeds, triggers a dispatch for both the primary and secondary
+      // queue's flush events, and the primary queue is empty
       assertTrue(flushed);
-      assertEquals(1, flushCount.get());
+      assertEquals(2, flushCount.get());
       assertTrue(worker.getPrimaryQueue().isEmpty());
     }
   }