diff --git a/packages/graalvm/src/main/kotlin/elide/runtime/plugins/js/JavaScriptConfig.kt b/packages/graalvm/src/main/kotlin/elide/runtime/plugins/js/JavaScriptConfig.kt
index 7458c4536c..809b27fd0d 100644
--- a/packages/graalvm/src/main/kotlin/elide/runtime/plugins/js/JavaScriptConfig.kt
+++ b/packages/graalvm/src/main/kotlin/elide/runtime/plugins/js/JavaScriptConfig.kt
@@ -41,7 +41,12 @@ import elide.runtime.plugins.js.JavaScriptVersion.ES2022
 
   public inner class BuiltInModulesConfig {
     /** Core module replacement map. */
-    private val moduleReplacements = mutableMapOf<String, String>()
+    private val moduleReplacements: MutableMap<String, String> = mutableMapOf(
+      "buffer" to "/__runtime__/buffer/buffer.cjs",
+      "util" to "/__runtime__/util/util.cjs",
+      "fs" to "/__runtime__/fs/fs.cjs",
+      "express" to "/__runtime__/express/express.cjs",
+    )
 
     internal fun replacements(): Map<String, String> {
       return moduleReplacements
diff --git a/packages/graalvm/src/test/kotlin/elide/runtime/plugins/js/JsCoreModulesTest.kt b/packages/graalvm/src/test/kotlin/elide/runtime/plugins/js/JsCoreModulesTest.kt
new file mode 100644
index 0000000000..93979df42d
--- /dev/null
+++ b/packages/graalvm/src/test/kotlin/elide/runtime/plugins/js/JsCoreModulesTest.kt
@@ -0,0 +1,20 @@
+package elide.runtime.plugins.js
+
+import org.junit.jupiter.api.assertDoesNotThrow
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.ValueSource
+import elide.runtime.core.DelicateElideApi
+import elide.runtime.core.PolyglotEngine
+
+@OptIn(DelicateElideApi::class)
+internal class JsCoreModulesTest {
+  @ParameterizedTest
+  @ValueSource(strings = ["buffer", "util", "fs", "express"])
+  fun testCoreModulePresent(module: String) {
+    val context = PolyglotEngine { install(JavaScript) }.acquire()
+
+    assertDoesNotThrow("core module replacement for '$module' should be present") {
+      context.javascript("""require("$module");""")
+    }
+  }
+}
\ No newline at end of file