diff --git a/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsReader.java b/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsReader.java
index dd3e59be2646..6de775c4773b 100644
--- a/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsReader.java
+++ b/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsReader.java
@@ -65,7 +65,7 @@ public class ES818BinaryQuantizedVectorsReader extends FlatVectorsReader impleme
 
     private static final long SHALLOW_SIZE = RamUsageEstimator.shallowSizeOfInstance(ES818BinaryQuantizedVectorsReader.class);
 
-    private final Map<String, FieldEntry> fields = new HashMap<>();
+    private final Map<String, FieldEntry> fields;
     private final IndexInput quantizedVectorData;
     private final FlatVectorsReader rawVectorsReader;
     private final ES818BinaryFlatVectorsScorer vectorScorer;
@@ -77,6 +77,7 @@ public class ES818BinaryQuantizedVectorsReader extends FlatVectorsReader impleme
         ES818BinaryFlatVectorsScorer vectorsScorer
     ) throws IOException {
         super(vectorsScorer);
+        this.fields = new HashMap<>();
         this.vectorScorer = vectorsScorer;
         this.rawVectorsReader = rawVectorsReader;
         int versionMeta = -1;
@@ -120,6 +121,24 @@ public class ES818BinaryQuantizedVectorsReader extends FlatVectorsReader impleme
         }
     }
 
+    private ES818BinaryQuantizedVectorsReader(ES818BinaryQuantizedVectorsReader clone, FlatVectorsReader rawVectorsReader) {
+        super(clone.vectorScorer);
+        this.rawVectorsReader = rawVectorsReader;
+        this.vectorScorer = clone.vectorScorer;
+        this.quantizedVectorData = clone.quantizedVectorData;
+        this.fields = clone.fields;
+    }
+
+    // For testing
+    FlatVectorsReader getRawVectorsReader() {
+        return rawVectorsReader;
+    }
+
+    @Override
+    public FlatVectorsReader getMergeInstance() {
+        return new ES818BinaryQuantizedVectorsReader(this, rawVectorsReader.getMergeInstance());
+    }
+
     private void readFields(ChecksumIndexInput meta, FieldInfos infos) throws IOException {
         for (int fieldNumber = meta.readInt(); fieldNumber != -1; fieldNumber = meta.readInt()) {
             FieldInfo info = infos.fieldInfo(fieldNumber);
diff --git a/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/MergeReaderWrapper.java b/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/MergeReaderWrapper.java
index 4d9d7e03848c..e74b0aad1272 100644
--- a/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/MergeReaderWrapper.java
+++ b/server/src/main/java/org/elasticsearch/index/codec/vectors/es818/MergeReaderWrapper.java
@@ -36,6 +36,11 @@ class MergeReaderWrapper extends FlatVectorsReader implements OffHeapStats {
         this.mergeReader = mergeReader;
     }
 
+    // For testing
+    FlatVectorsReader getMainReader() {
+        return mainReader;
+    }
+
     @Override
     public RandomVectorScorer getRandomVectorScorer(String field, float[] target) throws IOException {
         return mainReader.getRandomVectorScorer(field, target);
diff --git a/server/src/test/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsFormatTests.java b/server/src/test/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsFormatTests.java
index 3b187f2241cc..84fafde5af7c 100644
--- a/server/src/test/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsFormatTests.java
+++ b/server/src/test/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsFormatTests.java
@@ -23,6 +23,7 @@ import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.FilterCodec;
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.KnnVectorsReader;
+import org.apache.lucene.codecs.lucene99.Lucene99FlatVectorsReader;
 import org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat;
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field;
@@ -35,6 +36,7 @@ import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
 import org.apache.lucene.index.KnnVectorValues;
 import org.apache.lucene.index.LeafReader;
+import org.apache.lucene.index.SegmentReader;
 import org.apache.lucene.index.SoftDeletesRetentionMergePolicy;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.index.VectorSimilarityFunction;
@@ -83,6 +85,7 @@ import java.util.OptionalLong;
 import static java.lang.String.format;
 import static org.apache.lucene.index.VectorSimilarityFunction.DOT_PRODUCT;
 import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
+import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.oneOf;
 
@@ -309,6 +312,43 @@ public class ES818BinaryQuantizedVectorsFormatTests extends BaseKnnVectorsFormat
         }
     }
 
+    public void testMergeInstance() throws IOException {
+        checkDirectIOSupported();
+        float[] vector = randomVector(10);
+        VectorSimilarityFunction similarityFunction = randomSimilarity();
+        KnnFloatVectorField knnField = new KnnFloatVectorField("field", vector, similarityFunction);
+        try (Directory dir = newFSDirectory()) {
+            try (IndexWriter w = new IndexWriter(dir, newIndexWriterConfig().setUseCompoundFile(false))) {
+                Document doc = new Document();
+                knnField.setVectorValue(randomVector(10));
+                doc.add(knnField);
+                w.addDocument(doc);
+                w.commit();
+
+                try (IndexReader reader = DirectoryReader.open(w)) {
+                    SegmentReader r = (SegmentReader) getOnlyLeafReader(reader);
+                    assertThat(unwrapRawVectorReader("field", r.getVectorReader()), instanceOf(DirectIOLucene99FlatVectorsReader.class));
+                    assertThat(
+                        unwrapRawVectorReader("field", r.getVectorReader().getMergeInstance()),
+                        instanceOf(Lucene99FlatVectorsReader.class)
+                    );
+                }
+            }
+        }
+    }
+
+    private static KnnVectorsReader unwrapRawVectorReader(String fieldName, KnnVectorsReader knnReader) {
+        if (knnReader instanceof PerFieldKnnVectorsFormat.FieldsReader perField) {
+            return unwrapRawVectorReader(fieldName, perField.getFieldReader(fieldName));
+        } else if (knnReader instanceof ES818BinaryQuantizedVectorsReader bbqReader) {
+            return unwrapRawVectorReader(fieldName, bbqReader.getRawVectorsReader());
+        } else if (knnReader instanceof MergeReaderWrapper mergeReaderWrapper) {
+            return unwrapRawVectorReader(fieldName, mergeReaderWrapper.getMainReader());
+        } else {
+            return knnReader;
+        }
+    }
+
     static Directory newMMapDirectory() throws IOException {
         Directory dir = new MMapDirectory(createTempDir("ES818BinaryQuantizedVectorsFormatTests"));
         if (random().nextBoolean()) {