diff --git a/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIT.java b/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIT.java
index 5dc45a0669e31..932f44d69a66d 100644
--- a/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIT.java
+++ b/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIT.java
@@ -10,6 +10,7 @@
 package org.elasticsearch.xpack.logsdb;
 
 import org.elasticsearch.client.Request;
+import org.elasticsearch.index.IndexFeatures;
 import org.elasticsearch.test.rest.ObjectPath;
 
 import java.io.IOException;
@@ -22,11 +23,13 @@
 
 public class TsdbIT extends AbstractLogsdbRollingUpgradeTestCase {
 
-    static final String TEMPLATE = """
+    private static final String SYNTHETIC_ID_PLACEHOLDER = "$SYNTHETIC_ID_SETTING";
+    // Do not access directly, use getTemplate(Boolean useSyntheticId)
+    private static final String TEMPLATE = """
         {
             "settings":{
                 "index": {
-                    "mode": "time_series"
+                    "mode": "time_series"%s
                 }
             },
             "mappings":{
@@ -80,7 +83,17 @@ public class TsdbIT extends AbstractLogsdbRollingUpgradeTestCase {
                 }
             }
         }
-        """;
+        """.formatted(SYNTHETIC_ID_PLACEHOLDER);
+
+    /**
+     * Returns the template with optional synthetic_id setting. When {@code useSyntheticId} is null the setting is omitted;
+     * when true/false, adds {@code "mapping": { "synthetic_id": true/false }} to index settings.
+     */
+    static String getTemplate(Boolean useSyntheticId) {
+        String replacement = useSyntheticId == null ? "" : ", \"mapping\": { \"synthetic_id\": " + useSyntheticId + " }";
+        return TEMPLATE.replace(SYNTHETIC_ID_PLACEHOLDER, replacement);
+    }
+
     private static final String BULK =
         """
             {"create": {}}
@@ -120,6 +133,9 @@ public class TsdbIT extends AbstractLogsdbRollingUpgradeTestCase {
         """;
 
     public void testTsdbDataStream() throws Exception {
+        boolean hasSupport = oldClusterHasFeature(IndexFeatures.TIME_SERIES_SYNTHETIC_ID);
+        Boolean useSyntheticId = hasSupport ? randomBoolean() : null;
+
         String dataStreamName = "k8s";
         final String INDEX_TEMPLATE = """
             {
@@ -131,7 +147,9 @@ public void testTsdbDataStream() throws Exception {
         // Add composable index template
         String templateName = "1";
         var putIndexTemplateRequest = new Request("POST", "/_index_template/" + templateName);
-        putIndexTemplateRequest.setJsonEntity(INDEX_TEMPLATE.replace("$TEMPLATE", TEMPLATE).replace("$PATTERN", dataStreamName));
+        putIndexTemplateRequest.setJsonEntity(
+            INDEX_TEMPLATE.replace("$TEMPLATE", getTemplate(useSyntheticId)).replace("$PATTERN", dataStreamName)
+        );
         assertOK(client().performRequest(putIndexTemplateRequest));
 
         performOldClustertOperations(templateName, dataStreamName);
diff --git a/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIndexingRollingUpgradeIT.java b/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIndexingRollingUpgradeIT.java
index 6f66b8caddff0..469d4d3fc7fb5 100644
--- a/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIndexingRollingUpgradeIT.java
+++ b/x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/xpack/logsdb/TsdbIndexingRollingUpgradeIT.java
@@ -14,6 +14,10 @@
 import org.elasticsearch.client.RestClient;
 import org.elasticsearch.common.network.NetworkAddress;
 import org.elasticsearch.common.xcontent.XContentHelper;
+import org.elasticsearch.index.IndexFeatures;
+import org.elasticsearch.index.IndexMode;
+import org.elasticsearch.index.IndexSettings;
+import org.elasticsearch.index.mapper.SourceFieldMapper;
 import org.elasticsearch.test.rest.ObjectPath;
 import org.elasticsearch.xcontent.XContentType;
 
@@ -25,8 +29,8 @@
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicReference;
 
-import static org.elasticsearch.xpack.logsdb.TsdbIT.TEMPLATE;
 import static org.elasticsearch.xpack.logsdb.TsdbIT.formatInstant;
+import static org.elasticsearch.xpack.logsdb.TsdbIT.getTemplate;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
@@ -40,16 +44,27 @@ public class TsdbIndexingRollingUpgradeIT extends AbstractLogsdbRollingUpgradeTe
             """;
 
     public void testIndexing() throws Exception {
+        // If cluster has support for synthetic id, randomly set index.mapping.synthetic_id to true/false
+        // If cluster doesn't have support, don't set index.mapping.synthetic_id at all (null),
+        // indicated by oldClusterHasFeature(IndexFeatures.TIME_SERIES_SYNTHETIC_ID)==false
+        Boolean useSyntheticId = oldClusterHasFeature(IndexFeatures.TIME_SERIES_SYNTHETIC_ID) ? randomBoolean() : null;
+
         String dataStreamName = "k9s";
-        createTemplate(dataStreamName, getClass().getSimpleName().toLowerCase(Locale.ROOT), TEMPLATE);
+        createTemplate(dataStreamName, getClass().getSimpleName().toLowerCase(Locale.ROOT), getTemplate(useSyntheticId));
 
         Instant startTime = Instant.now().minusSeconds(60 * 60);
         bulkIndex(dataStreamName, 4, 1024, startTime, TsdbIndexingRollingUpgradeIT::docSupplier);
 
         String firstBackingIndex = getDataStreamBackingIndexNames(dataStreamName).getFirst();
         var settings = (Map<?, ?>) getIndexSettings(firstBackingIndex, true).get(firstBackingIndex);
-        assertThat(((Map<?, ?>) settings.get("settings")).get("index.mode"), equalTo("time_series"));
-        assertThat(((Map<?, ?>) settings.get("defaults")).get("index.mapping.source.mode"), equalTo("SYNTHETIC"));
+        assertThat(((Map<?, ?>) settings.get("settings")).get(IndexSettings.MODE.getKey()), equalTo(IndexMode.TIME_SERIES.getName()));
+        assertThat(
+            ((Map<?, ?>) settings.get("defaults")).get(IndexSettings.INDEX_MAPPER_SOURCE_MODE_SETTING.getKey()),
+            equalTo(SourceFieldMapper.Mode.SYNTHETIC.toString())
+        );
+        if (useSyntheticId != null) {
+            assertThat(((Map<?, ?>) settings.get("settings")).get(IndexSettings.SYNTHETIC_ID.getKey()), equalTo(useSyntheticId.toString()));
+        }
 
         var mapping = getIndexMappingAsMap(firstBackingIndex);
         assertThat(