diff --git a/qa/vector/src/main/java/org/elasticsearch/test/knn/CmdLineArgs.java b/qa/vector/src/main/java/org/elasticsearch/test/knn/CmdLineArgs.java
index f51c550e5292e..85fa02aecaaef 100644
--- a/qa/vector/src/main/java/org/elasticsearch/test/knn/CmdLineArgs.java
+++ b/qa/vector/src/main/java/org/elasticsearch/test/knn/CmdLineArgs.java
@@ -52,7 +52,8 @@ record CmdLineArgs(
     int quantizeBits,
     VectorEncoding vectorEncoding,
     int dimensions,
-    boolean earlyTermination
+    boolean earlyTermination,
+    KnnIndexTester.MergePolicyType mergePolicy
 ) implements ToXContentObject {
 
     static final ParseField DOC_VECTORS_FIELD = new ParseField("doc_vectors");
@@ -79,6 +80,7 @@ record CmdLineArgs(
     static final ParseField EARLY_TERMINATION_FIELD = new ParseField("early_termination");
     static final ParseField FILTER_SELECTIVITY_FIELD = new ParseField("filter_selectivity");
     static final ParseField SEED_FIELD = new ParseField("seed");
+    static final ParseField MERGE_POLICY_FIELD = new ParseField("merge_policy");
 
     static CmdLineArgs fromXContent(XContentParser parser) throws IOException {
         Builder builder = PARSER.apply(parser, null);
@@ -112,6 +114,7 @@ static CmdLineArgs fromXContent(XContentParser parser) throws IOException {
         PARSER.declareBoolean(Builder::setEarlyTermination, EARLY_TERMINATION_FIELD);
         PARSER.declareFloat(Builder::setFilterSelectivity, FILTER_SELECTIVITY_FIELD);
         PARSER.declareLong(Builder::setSeed, SEED_FIELD);
+        PARSER.declareString(Builder::setMergePolicy, MERGE_POLICY_FIELD);
     }
 
     @Override
@@ -179,6 +182,7 @@ static class Builder {
         private boolean earlyTermination;
         private float filterSelectivity = 1f;
         private long seed = 1751900822751L;
+        private KnnIndexTester.MergePolicyType mergePolicy = null;
 
         public Builder setDocVectors(List<String> docVectors) {
             if (docVectors == null || docVectors.isEmpty()) {
@@ -304,6 +308,11 @@ public Builder setSeed(long seed) {
             return this;
         }
 
+        public Builder setMergePolicy(String mergePolicy) {
+            this.mergePolicy = KnnIndexTester.MergePolicyType.valueOf(mergePolicy.toUpperCase(Locale.ROOT));
+            return this;
+        }
+
         public CmdLineArgs build() {
             if (docVectors == null) {
                 throw new IllegalArgumentException("Document vectors path must be provided");
@@ -337,7 +346,8 @@ public CmdLineArgs build() {
                 quantizeBits,
                 vectorEncoding,
                 dimensions,
-                earlyTermination
+                earlyTermination,
+                mergePolicy
             );
         }
     }
diff --git a/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexTester.java b/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexTester.java
index c4b0ccdfe35e3..17257dcb73d59 100644
--- a/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexTester.java
+++ b/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexTester.java
@@ -15,6 +15,11 @@
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.lucene101.Lucene101Codec;
 import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsFormat;
+import org.apache.lucene.index.LogByteSizeMergePolicy;
+import org.apache.lucene.index.LogDocMergePolicy;
+import org.apache.lucene.index.MergePolicy;
+import org.apache.lucene.index.NoMergePolicy;
+import org.apache.lucene.index.TieredMergePolicy;
 import org.elasticsearch.cli.ProcessInfo;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.logging.LogConfigurator;
@@ -69,6 +74,13 @@ enum IndexType {
         IVF
     }
 
+    enum MergePolicyType {
+        TIERED,
+        LOG_BYTE,
+        NO,
+        LOG_DOC
+    }
+
     private static String formatIndexPath(CmdLineArgs args) {
         List<String> suffix = new ArrayList<>();
         if (args.indexType() == IndexType.FLAT) {
@@ -196,6 +208,7 @@ public static void main(String[] args) throws Exception {
             logger.info("Running KNN index tester with arguments: " + cmdLineArgs);
             Codec codec = createCodec(cmdLineArgs);
             Path indexPath = PathUtils.get(formatIndexPath(cmdLineArgs));
+            MergePolicy mergePolicy = getMergePolicy(cmdLineArgs);
             if (cmdLineArgs.reindex() || cmdLineArgs.forceMerge()) {
                 KnnIndexer knnIndexer = new KnnIndexer(
                     cmdLineArgs.docVectors(),
@@ -205,7 +218,8 @@ public static void main(String[] args) throws Exception {
                     cmdLineArgs.vectorEncoding(),
                     cmdLineArgs.dimensions(),
                     cmdLineArgs.vectorSpace(),
-                    cmdLineArgs.numDocs()
+                    cmdLineArgs.numDocs(),
+                    mergePolicy
                 );
                 if (cmdLineArgs.reindex() == false && Files.exists(indexPath) == false) {
                     throw new IllegalArgumentException("Index path does not exist: " + indexPath);
@@ -232,6 +246,24 @@ public static void main(String[] args) throws Exception {
         logger.info("Results: \n" + formattedResults);
     }
 
+    private static MergePolicy getMergePolicy(CmdLineArgs args) {
+        MergePolicy mergePolicy = null;
+        if (args.mergePolicy() != null) {
+            if (args.mergePolicy() == MergePolicyType.TIERED) {
+                mergePolicy = new TieredMergePolicy();
+            } else if (args.mergePolicy() == MergePolicyType.LOG_BYTE) {
+                mergePolicy = new LogByteSizeMergePolicy();
+            } else if (args.mergePolicy() == MergePolicyType.NO) {
+                mergePolicy = NoMergePolicy.INSTANCE;
+            } else if (args.mergePolicy() == MergePolicyType.LOG_DOC) {
+                mergePolicy = new LogDocMergePolicy();
+            } else {
+                throw new IllegalArgumentException("Invalid merge policy: " + args.mergePolicy());
+            }
+        }
+        return mergePolicy;
+    }
+
     static class FormattedResults {
         List<Results> indexResults = new ArrayList<>();
         List<Results> queryResults = new ArrayList<>();
diff --git a/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexer.java b/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexer.java
index f7d00c9806c8d..aa8792bb2c4a5 100644
--- a/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexer.java
+++ b/qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexer.java
@@ -31,6 +31,7 @@
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.index.MergePolicy;
 import org.apache.lucene.index.VectorEncoding;
 import org.apache.lucene.index.VectorSimilarityFunction;
 import org.apache.lucene.store.FSDirectory;
@@ -69,6 +70,7 @@ class KnnIndexer {
     private final Codec codec;
     private final int numDocs;
     private final int numIndexThreads;
+    private final MergePolicy mergePolicy;
 
     KnnIndexer(
         List<Path> docsPath,
@@ -78,7 +80,8 @@ class KnnIndexer {
         VectorEncoding vectorEncoding,
         int dim,
         VectorSimilarityFunction similarityFunction,
-        int numDocs
+        int numDocs,
+        MergePolicy mergePolicy
     ) {
         this.docsPath = docsPath;
         this.indexPath = indexPath;
@@ -88,6 +91,7 @@ class KnnIndexer {
         this.dim = dim;
         this.similarityFunction = similarityFunction;
         this.numDocs = numDocs;
+        this.mergePolicy = mergePolicy;
     }
 
     void numSegments(KnnIndexTester.Results result) {
@@ -103,7 +107,9 @@ void createIndex(KnnIndexTester.Results result) throws IOException, InterruptedE
         iwc.setCodec(codec);
         iwc.setRAMBufferSizeMB(WRITER_BUFFER_MB);
         iwc.setUseCompoundFile(false);
-
+        if (mergePolicy != null) {
+            iwc.setMergePolicy(mergePolicy);
+        }
         iwc.setMaxFullFlushMergeWaitMillis(0);
 
         iwc.setInfoStream(new PrintStreamInfoStream(System.out) {