diff --git a/frontend/src/components/editor/CodeEditor.tsx b/frontend/src/components/editor/CodeEditor.tsx
index 6efaa17b..faa99a92 100644
--- a/frontend/src/components/editor/CodeEditor.tsx
+++ b/frontend/src/components/editor/CodeEditor.tsx
@@ -2,6 +2,7 @@ import Editor from '@monaco-editor/react';
 import { useEditorStore } from '../../store/useEditorStore';
 import { useSimulatorStore } from '../../store/useSimulatorStore';
 import { registerRetroAsm, LANGUAGE_ID as RETRO_ASM_ID } from './retroAsmLanguage';
+import { attachIntellisenseMonaco } from '../../lib/intellisenseRegistry';
 
 function getLanguage(filename: string): string {
   const ext = filename.split('.').pop()?.toLowerCase() ?? '';
@@ -62,6 +63,10 @@ export const CodeEditor = () => {
           // Register the 8080/Z80 assembly language once so Monaco knows how
           // to tokenize .s / .asm files when they're opened.
           registerRetroAsm(monaco);
+          // Hand the monaco instance to the intellisense seam. Inert in OSS;
+          // with the pro overlay loaded it registers the completion engine
+          // (idempotent per monaco instance, so per-file remounts are fine).
+          attachIntellisenseMonaco(monaco);
         }}
         onChange={(value) => {
           if (activeFileId) setFileContent(activeFileId, value || '');
diff --git a/frontend/src/lib/intellisenseRegistry.ts b/frontend/src/lib/intellisenseRegistry.ts
new file mode 100644
index 00000000..5649916d
--- /dev/null
+++ b/frontend/src/lib/intellisenseRegistry.ts
@@ -0,0 +1,45 @@
+/**
+ * Intellisense registry.
+ *
+ * Seam between the OSS editor and the pro overlay's code-completion engine
+ * (the private `velxio-intellisense` package). OSS ships no completion
+ * engine of its own, so without an overlay this registry is inert and the
+ * editor behaves exactly as before.
+ *
+ * Two-sided handshake, because arrival order is not guaranteed:
+ *
+ *   - CodeEditor's `beforeMount` calls `attachIntellisenseMonaco(monaco)`
+ *     every time an editor instance is created (which happens on every
+ *     file switch — the editor is keyed by file id).
+ *   - The overlay calls `installIntellisenseImpl(impl)` from `mountPro()`,
+ *     which lands via an async `import('@pro/index')` and may resolve
+ *     before or after the first editor mounted.
+ *
+ * Whichever side arrives last triggers the install. The impl itself is
+ * responsible for being idempotent per monaco instance (the package guards
+ * with a WeakSet), so replaying it on every mount is safe and cheap.
+ */
+
+type IntellisenseImpl = (monaco: unknown) => void;
+
+let _impl: IntellisenseImpl | null = null;
+let _monaco: unknown = null;
+
+export function installIntellisenseImpl(impl: IntellisenseImpl | null): void {
+  _impl = impl;
+  if (_impl && _monaco) run(_impl, _monaco);
+}
+
+export function attachIntellisenseMonaco(monaco: unknown): void {
+  _monaco = monaco;
+  if (_impl) run(_impl, monaco);
+}
+
+function run(impl: IntellisenseImpl, monaco: unknown): void {
+  try {
+    impl(monaco);
+  } catch (err) {
+    // eslint-disable-next-line no-console
+    console.warn('[oss] intellisense impl threw:', err);
+  }
+}