diff --git a/server/src/main/java/com/mirth/connect/server/userutil/ContextFactory.java b/server/src/main/java/com/mirth/connect/server/userutil/ContextFactory.java
index c3aa5c0567..d7e2a90499 100644
--- a/server/src/main/java/com/mirth/connect/server/userutil/ContextFactory.java
+++ b/server/src/main/java/com/mirth/connect/server/userutil/ContextFactory.java
@@ -51,12 +51,18 @@ public ClassLoader getClassLoader() {
     }
 
     /**
-     * Returns a classloader containing only the libraries contained in the custom resources, with
-     * no parent classloader. If no custom libraries are being used in the current JavaScript
-     * context, this will return null.
-     * 
-     * @return A classloader containing only the libraries contained in the custom resources, with
-     *         no parent classloader.
+     * Returns a classloader containing only the libraries in the custom resources assigned to the
+     * current context. Use it to load classes from those libraries in isolation, for example a
+     * specific JDBC driver version, without interference from the versions the server itself ships.
+     * <p>
+     * Core Java classes (for example {@code java.sql} or {@code javax.xml}) are visible through this
+     * classloader, but classes from the server or its plugins are not (its parent is
+     * {@link ClassLoader#getPlatformClassLoader()}). A class that exists in both a custom resource
+     * and the JDK resolves to the JDK's copy. The "Load Parent-First" option on a resource does not
+     * affect this classloader; it applies only to the classloader returned by {@link #getClassLoader()}.
+     *
+     * @return A classloader containing only the custom resource libraries, or null if the current
+     *         context has no custom resources.
      */
     public ClassLoader getIsolatedClassLoader() {
         return delegate.getIsolatedClassLoader();
diff --git a/server/src/main/java/com/mirth/connect/server/util/javascript/MirthContextFactory.java b/server/src/main/java/com/mirth/connect/server/util/javascript/MirthContextFactory.java
index 5e303757d5..6ba3bb35f1 100644
--- a/server/src/main/java/com/mirth/connect/server/util/javascript/MirthContextFactory.java
+++ b/server/src/main/java/com/mirth/connect/server/util/javascript/MirthContextFactory.java
@@ -114,7 +114,7 @@ public ClassLoader getIsolatedClassLoader() {
         if (isolatedClassLoader == null && ArrayUtils.isNotEmpty(urls)) {
             synchronized (this) {
                 if (isolatedClassLoader == null) {
-                    isolatedClassLoader = new URLClassLoader(urls, null);
+                    isolatedClassLoader = new URLClassLoader(urls, ClassLoader.getPlatformClassLoader());
                 }
             }
         }
diff --git a/server/src/test/java/com/mirth/connect/server/util/javascript/MirthContextFactoryTest.java b/server/src/test/java/com/mirth/connect/server/util/javascript/MirthContextFactoryTest.java
new file mode 100644
index 0000000000..b34ee8627d
--- /dev/null
+++ b/server/src/test/java/com/mirth/connect/server/util/javascript/MirthContextFactoryTest.java
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: MPL-2.0
+// SPDX-FileCopyrightText: Mirth Corporation
+
+package com.mirth.connect.server.util.javascript;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import java.io.File;
+import java.net.URL;
+import java.util.HashSet;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import com.google.inject.AbstractModule;
+import com.google.inject.Guice;
+import com.google.inject.Injector;
+import com.mirth.connect.server.builders.JavaScriptBuilder;
+import com.mirth.connect.server.controllers.CodeTemplateController;
+import com.mirth.connect.server.controllers.ConfigurationController;
+import com.mirth.connect.server.controllers.ControllerFactory;
+import com.mirth.connect.server.controllers.EventController;
+import com.mirth.connect.server.controllers.ExtensionController;
+
+public class MirthContextFactoryTest {
+
+    @BeforeClass
+    public static void setUpBeforeClass() {
+        // Same mocked ControllerFactory pattern as JavaScriptUtilTest, so this class is
+        // self-sufficient regardless of which test classes ran (and injected) before it.
+        ControllerFactory controllerFactory = mock(ControllerFactory.class);
+
+        EventController eventController = mock(EventController.class);
+        when(controllerFactory.createEventController()).thenReturn(eventController);
+
+        ConfigurationController configurationController = mock(ConfigurationController.class);
+        when(controllerFactory.createConfigurationController()).thenReturn(configurationController);
+
+        ExtensionController extensionController = mock(ExtensionController.class);
+        when(controllerFactory.createExtensionController()).thenReturn(extensionController);
+
+        CodeTemplateController codeTemplateController = mock(CodeTemplateController.class);
+        when(controllerFactory.createCodeTemplateController()).thenReturn(codeTemplateController);
+
+        Injector injector = Guice.createInjector(new AbstractModule() {
+            @Override
+            protected void configure() {
+                requestStaticInjection(ControllerFactory.class);
+                bind(ControllerFactory.class).toInstance(controllerFactory);
+            }
+        });
+        injector.getInstance(ControllerFactory.class);
+
+        JavaScriptBuilder.setControllersForTesting(extensionController, codeTemplateController);
+    }
+
+    /*
+     * Regression test for #338: with a null parent, the isolated classloader cannot see java.sql
+     * on Java 9+, so custom driver resources failed to deploy. The parent must be the platform
+     * classloader: JRE classes visible, server classpath not.
+     */
+    @Test
+    public void isolatedClassLoaderResolvesPlatformButNotServerClasses() throws Exception {
+        // This jar URL never gets read; it only exists because getIsolatedClassLoader() returns null on empty array.
+        URL dummyJar = new File("build/tmp/mirth-context-factory-test-dummy.jar").toURI().toURL();
+        MirthContextFactory contextFactory = new MirthContextFactory(new URL[] { dummyJar }, new HashSet<>(), false);
+
+        ClassLoader isolated = contextFactory.getIsolatedClassLoader();
+        assertNotNull(isolated);
+
+        // Fails with ClassNotFoundException if the parent ever goes back to null
+        isolated.loadClass("java.sql.Driver");
+
+        // Fails if the parent is ever widened to a loader that can see the server classpath
+        assertThrows(ClassNotFoundException.class, () -> isolated.loadClass(MirthContextFactory.class.getName()));
+    }
+
+    @Test
+    public void isolatedClassLoaderIsNullWithoutResources() {
+        MirthContextFactory contextFactory = new MirthContextFactory(new URL[0], new HashSet<>(), false);
+        assertNull(contextFactory.getIsolatedClassLoader());
+    }
+}