diff --git a/libs/simdvec/src/main21/java/org/elasticsearch/simdvec/internal/IndexInputUtils.java b/libs/simdvec/src/main21/java/org/elasticsearch/simdvec/internal/IndexInputUtils.java
index 41985352a1af0..b61450427fb34 100644
--- a/libs/simdvec/src/main21/java/org/elasticsearch/simdvec/internal/IndexInputUtils.java
+++ b/libs/simdvec/src/main21/java/org/elasticsearch/simdvec/internal/IndexInputUtils.java
@@ -15,7 +15,9 @@
 import org.elasticsearch.core.DirectAccessInput;
 
 import java.io.IOException;
+import java.lang.foreign.Arena;
 import java.lang.foreign.MemorySegment;
+import java.lang.foreign.ValueLayout;
 import java.util.function.IntFunction;
 
 /**
@@ -74,6 +76,7 @@ public static <R> R withSlice(
             @SuppressWarnings("unchecked")
             R[] result = (R[]) new Object[1];
             boolean available = dai.withByteBufferSlice(offset, length, bb -> {
+                assert bb.isDirect();
                 in.skipBytes(length);
                 result[0] = action.apply(MemorySegment.ofBuffer(bb));
             });
@@ -81,7 +84,7 @@ public static <R> R withSlice(
                 return result[0];
             }
         }
-        return action.apply(copyOnHeap(in, Math.toIntExact(length), scratchSupplier));
+        return copyAndApply(in, Math.toIntExact(length), scratchSupplier, action);
     }
 
     /**
@@ -100,15 +103,29 @@ public static void checkInputType(IndexInput in) {
         }
     }
 
+    private static final boolean SUPPORTS_HEAP_SEGMENTS = Runtime.version().feature() >= 22;
+
     /**
-     * Reads the given number of bytes from the current position of the
-     * given IndexInput into a heap-backed memory segment. The returned
-     * segment is sliced to exactly {@code bytesToRead} bytes, even if
-     * the underlying array is larger.
+     * Reads bytes from the index input and applies the action to a memory
+     * segment containing the data. On Java 22+ a heap-backed segment is
+     * used directly. On Java 21, where heap segments cannot be passed to
+     * native downcalls, the data is copied into a confined arena.
      */
-    private static MemorySegment copyOnHeap(IndexInput in, int bytesToRead, IntFunction<byte[]> scratchSupplier) throws IOException {
+    private static <R> R copyAndApply(
+        IndexInput in,
+        int bytesToRead,
+        IntFunction<byte[]> scratchSupplier,
+        CheckedFunction<MemorySegment, R, IOException> action
+    ) throws IOException {
         byte[] buf = scratchSupplier.apply(bytesToRead);
         in.readBytes(buf, 0, bytesToRead);
-        return MemorySegment.ofArray(buf).asSlice(0, bytesToRead);
+        if (SUPPORTS_HEAP_SEGMENTS) {
+            return action.apply(MemorySegment.ofArray(buf).asSlice(0, bytesToRead));
+        }
+        try (Arena arena = Arena.ofConfined()) {
+            MemorySegment nativeSegment = arena.allocate(bytesToRead);
+            MemorySegment.copy(buf, 0, nativeSegment, ValueLayout.JAVA_BYTE, 0, bytesToRead);
+            return action.apply(nativeSegment);
+        }
     }
 }
diff --git a/libs/simdvec/src/test21/java/org/elasticsearch/simdvec/internal/IndexInputUtilsTests.java b/libs/simdvec/src/test21/java/org/elasticsearch/simdvec/internal/IndexInputUtilsTests.java
index 5edfaa16a7af7..683060fd3ad53 100644
--- a/libs/simdvec/src/test21/java/org/elasticsearch/simdvec/internal/IndexInputUtilsTests.java
+++ b/libs/simdvec/src/test21/java/org/elasticsearch/simdvec/internal/IndexInputUtilsTests.java
@@ -21,7 +21,9 @@
 import org.elasticsearch.test.ESTestCase;
 
 import java.io.IOException;
+import java.lang.foreign.Arena;
 import java.lang.foreign.MemorySegment;
+import java.lang.foreign.ValueLayout;
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 
@@ -72,6 +74,26 @@ public void testWithSlicePlainIndexInput() throws Exception {
         }
     }
 
+    public void testPlainInputFallbackProducesNativeSegmentOnJava21() throws Exception {
+        byte[] data = randomByteArrayOfLength(256);
+        try (Directory dir = new NIOFSDirectory(createTempDir())) {
+            writeData(dir, data);
+            try (IndexInput in = dir.openInput(FILE_NAME, IOContext.DEFAULT)) {
+                assertFalse(in instanceof MemorySegmentAccessInput);
+                assertFalse(in instanceof DirectAccessInput);
+                IndexInputUtils.withSlice(in, data.length, byte[]::new, segment -> {
+                    if (Runtime.version().feature() < 22) {
+                        assertTrue("segment should be native-backed on Java 21", segment.heapBase().isEmpty());
+                    }
+                    byte[] buf = new byte[(int) segment.byteSize()];
+                    MemorySegment.ofArray(buf).copyFrom(segment);
+                    assertArrayEquals(data, buf);
+                    return null;
+                });
+            }
+        }
+    }
+
     // -- constructor validation tests -----------------------------------------
 
     public void testES92ConstructorAcceptsPlainInput() throws Exception {
@@ -167,9 +189,12 @@ static class DirectAccessWrapper extends FilterIndexInput implements DirectAcces
 
         @Override
         public boolean withByteBufferSlice(long offset, long length, CheckedConsumer<ByteBuffer, IOException> action) throws IOException {
-            ByteBuffer bb = ByteBuffer.wrap(data, (int) offset, (int) length).asReadOnlyBuffer();
-            action.accept(bb);
-            return true;
+            try (Arena arena = Arena.ofConfined()) {
+                MemorySegment segment = arena.allocate(length);
+                MemorySegment.copy(data, (int) offset, segment, ValueLayout.JAVA_BYTE, 0, (int) length);
+                action.accept(segment.asByteBuffer().asReadOnlyBuffer());
+                return true;
+            }
         }
 
         @Override