diff --git a/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/adapters/ResourceOperationTypeAdapter.java b/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/adapters/ResourceOperationTypeAdapter.java
index 2c661ee6a..9ecf7ddf3 100644
--- a/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/adapters/ResourceOperationTypeAdapter.java
+++ b/org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/adapters/ResourceOperationTypeAdapter.java
@@ -1,12 +1,12 @@
 /******************************************************************************
  * Copyright (c) 2018 Microsoft Corporation and others.
- * 
+ *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License v. 2.0 which is available at
  * http://www.eclipse.org/legal/epl-2.0,
  * or the Eclipse Distribution License v. 1.0 which is available at
  * http://www.eclipse.org/org/documents/edl-v10.php.
- * 
+ *
  * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
  ******************************************************************************/
 
@@ -46,9 +46,9 @@ public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
 
 	private static class InnerResourceOperationTypeAdapter extends TypeAdapter<ResourceOperation> {
 
-		TypeAdapter<CreateFile> createFileAdapter;
-		TypeAdapter<DeleteFile> deleteFileAdapter;
-		TypeAdapter<RenameFile> renameFileAdapter;
+		final TypeAdapter<CreateFile> createFileAdapter;
+		final TypeAdapter<DeleteFile> deleteFileAdapter;
+		final TypeAdapter<RenameFile> renameFileAdapter;
 
 		InnerResourceOperationTypeAdapter(TypeAdapterFactory factory, Gson gson) {
 			createFileAdapter = gson.getDelegateAdapter(factory, TypeToken.get(CreateFile.class));
@@ -58,12 +58,14 @@ private static class InnerResourceOperationTypeAdapter extends TypeAdapter<Resou
 
 		@Override
 		public void write(JsonWriter out, ResourceOperation value) throws IOException {
-			if (value.getClass().isAssignableFrom(CreateFile.class)) {
+			if (value instanceof CreateFile) {
 				createFileAdapter.write(out, (CreateFile) value);
-			} else if (value.getClass().isAssignableFrom(DeleteFile.class)) {
+			} else if (value instanceof DeleteFile) {
 				deleteFileAdapter.write(out, (DeleteFile) value);
-			} else if (value.getClass().isAssignableFrom(RenameFile.class)) {
+			} else if (value instanceof RenameFile) {
 				renameFileAdapter.write(out, (RenameFile) value);
+			} else {
+				throw new IllegalArgumentException("Unknown ResourceOperation subtype: " + value.getClass().getName());
 			}
 		}
 
diff --git a/org.eclipse.lsp4j/src/test/java/org/eclipse/lsp4j/test/adapters/ResourceOperationTypeAdapterTest.java b/org.eclipse.lsp4j/src/test/java/org/eclipse/lsp4j/test/adapters/ResourceOperationTypeAdapterTest.java
new file mode 100644
index 000000000..869244ce8
--- /dev/null
+++ b/org.eclipse.lsp4j/src/test/java/org/eclipse/lsp4j/test/adapters/ResourceOperationTypeAdapterTest.java
@@ -0,0 +1,69 @@
+/******************************************************************************
+ * Copyright (c) 2025 Vegard IT GmbH and others.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0,
+ * or the Eclipse Distribution License v. 1.0 which is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+ ******************************************************************************/
+package org.eclipse.lsp4j.test.adapters;
+
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+
+import org.eclipse.lsp4j.CreateFile;
+import org.eclipse.lsp4j.ResourceOperation;
+import org.eclipse.lsp4j.adapters.ResourceOperationTypeAdapter;
+import org.junit.Test;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+
+/**
+ * Tests for {@link ResourceOperationTypeAdapter}.
+ */
+public class ResourceOperationTypeAdapterTest {
+
+	static class CustomCreateFile extends CreateFile {
+		CustomCreateFile(String uri) {
+			super(uri);
+		}
+	}
+
+	/**
+	 * Verify that a subclass of {@link CreateFile} serializes like {@link CreateFile} itself.
+	 */
+	@Test
+	public void write_subclassOfCreateFile_serializes() {
+		Gson gson = new GsonBuilder().create();
+		ResourceOperation op = new CustomCreateFile("file:///tmp/x");
+		String json = gson.toJson(op, ResourceOperation.class);
+		// Expected: kind=create and uri present in JSON
+		assertTrue("Expected kind=create in JSON but was: " + json, json.contains("\"kind\":\"create\""));
+		assertTrue("Expected uri in JSON but was: " + json, json.contains("\"uri\":\"file:///tmp/x\""));
+	}
+
+	/**
+	 * An unknown {@link ResourceOperation} subtype to demonstrate that the adapter
+	 * should fail fast instead of producing empty/invalid output.
+	 */
+	static class UnknownOp extends ResourceOperation {
+		UnknownOp() {
+			super("unknown");
+		}
+	}
+
+	/**
+	 * Verify that an unknown subtype of {@link ResourceOperation}
+	 * causes an {@link IllegalArgumentException} during writing.
+	 */
+	@Test
+	public void write_unknownSubtype_throws() {
+		Gson gson = new GsonBuilder().create();
+		ResourceOperation op = new UnknownOp();
+		assertThrows(IllegalArgumentException.class, () -> gson.toJson(op, ResourceOperation.class));
+	}
+}