diff --git a/lucene/core/src/java/org/apache/lucene/codecs/HnswGraphProvider.java b/lucene/core/src/java/org/apache/lucene/codecs/HnswGraphProvider.java
new file mode 100644
index 00000000000..000432aff3f
--- /dev/null
+++ b/lucene/core/src/java/org/apache/lucene/codecs/HnswGraphProvider.java
@@ -0,0 +1,37 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.codecs;
+
+import java.io.IOException;
+import org.apache.lucene.util.hnsw.HnswGraph;
+
+/**
+ * An interface that provides an HNSW graph. This interface is useful when gathering multiple HNSW
+ * graphs to bootstrap segment merging. The graph may be off the JVM heap.
+ *
+ * @lucene.experimental
+ */
+public interface HnswGraphProvider {
+  /**
+   * Return the stored HnswGraph for the given field.
+   *
+   * @param field the field containing the graph
+   * @return the HnswGraph for the given field if found
+   * @throws IOException when reading potentially off-heap graph fails
+   */
+  HnswGraph getGraph(String field) throws IOException;
+}
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java
index b16f874e5c3..becc58f6268 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsReader.java
@@ -24,6 +24,7 @@ import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 import org.apache.lucene.codecs.CodecUtil;
+import org.apache.lucene.codecs.HnswGraphProvider;
 import org.apache.lucene.codecs.KnnVectorsReader;
 import org.apache.lucene.index.ByteVectorValues;
 import org.apache.lucene.index.CorruptIndexException;
@@ -55,7 +56,7 @@ import org.apache.lucene.util.packed.DirectMonotonicReader;
  *
  * @lucene.experimental
  */
-public final class Lucene95HnswVectorsReader extends KnnVectorsReader {
+public final class Lucene95HnswVectorsReader extends KnnVectorsReader implements HnswGraphProvider {
 
   private static final long SHALLOW_SIZE =
       RamUsageEstimator.shallowSizeOfInstance(Lucene95HnswVectorsFormat.class);
@@ -308,6 +309,7 @@ public final class Lucene95HnswVectorsReader extends KnnVectorsReader {
   }
 
   /** Get knn graph values; used for testing */
+  @Override
   public HnswGraph getGraph(String field) throws IOException {
     FieldInfo info = fieldInfos.fieldInfo(field);
     if (info == null) {
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsWriter.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsWriter.java
index fb94833c9fe..01f06d9cadd 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsWriter.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene95/Lucene95HnswVectorsWriter.java
@@ -30,6 +30,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import org.apache.lucene.codecs.CodecUtil;
+import org.apache.lucene.codecs.HnswGraphProvider;
 import org.apache.lucene.codecs.KnnFieldVectorsWriter;
 import org.apache.lucene.codecs.KnnVectorsReader;
 import org.apache.lucene.codecs.KnnVectorsWriter;
@@ -506,7 +507,7 @@ public final class Lucene95HnswVectorsWriter extends KnnVectorsWriter {
       throws IOException {
     // Find the KnnVectorReader with the most docs that meets the following criteria:
     //  1. Does not contain any deleted docs
-    //  2. Is a Lucene95HnswVectorsReader/PerFieldKnnVectorReader
+    //  2. Is a HnswGraphProvider/PerFieldKnnVectorReader
     // If no readers exist that meet this criteria, return -1. If they do, return their index in
     // merge state
     int maxCandidateVectorCount = 0;
@@ -520,21 +521,23 @@ public final class Lucene95HnswVectorsWriter extends KnnVectorsWriter {
       }
 
       if (!allMatch(mergeState.liveDocs[i])
-          || !(currKnnVectorsReader instanceof Lucene95HnswVectorsReader candidateReader)) {
+          || !(currKnnVectorsReader instanceof HnswGraphProvider)) {
         continue;
       }
 
       int candidateVectorCount = 0;
       switch (fieldInfo.getVectorEncoding()) {
         case BYTE -> {
-          ByteVectorValues byteVectorValues = candidateReader.getByteVectorValues(fieldInfo.name);
+          ByteVectorValues byteVectorValues =
+              currKnnVectorsReader.getByteVectorValues(fieldInfo.name);
           if (byteVectorValues == null) {
             continue;
           }
           candidateVectorCount = byteVectorValues.size();
         }
         case FLOAT32 -> {
-          FloatVectorValues vectorValues = candidateReader.getFloatVectorValues(fieldInfo.name);
+          FloatVectorValues vectorValues =
+              currKnnVectorsReader.getFloatVectorValues(fieldInfo.name);
           if (vectorValues == null) {
             continue;
           }
@@ -553,13 +556,12 @@ public final class Lucene95HnswVectorsWriter extends KnnVectorsWriter {
   private HnswGraph getHnswGraphFromReader(String fieldName, KnnVectorsReader knnVectorsReader)
       throws IOException {
     if (knnVectorsReader instanceof PerFieldKnnVectorsFormat.FieldsReader perFieldReader
-        && perFieldReader.getFieldReader(fieldName)
-            instanceof Lucene95HnswVectorsReader fieldReader) {
+        && perFieldReader.getFieldReader(fieldName) instanceof HnswGraphProvider fieldReader) {
       return fieldReader.getGraph(fieldName);
     }
 
-    if (knnVectorsReader instanceof Lucene95HnswVectorsReader) {
-      return ((Lucene95HnswVectorsReader) knnVectorsReader).getGraph(fieldName);
+    if (knnVectorsReader instanceof HnswGraphProvider provider) {
+      return provider.getGraph(fieldName);
     }
 
     // We should not reach here because knnVectorsReader's type is checked in