diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 74a7f9f..cc87ee2 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -19,3 +19,18 @@ export function inline(_: CSSObject): StylexAttrsParamter {
 export function id(_?: string): string {
   throw new Error('`id` calls should be compiled away.')
 }
+
+function createWhenAPI(errorMessage: string) {
+  return (selector: string, pseudo?: string) => {
+    throw new Error(errorMessage)
+  }
+}
+
+export const when = {
+  // a b
+  ancestor: createWhenAPI("'when.ancestor' calls should be compiled away."),
+  // a:has(b)
+  descendent: createWhenAPI("'when.descendent' calls should be compiled away."),
+  // a + b
+  sibling: createWhenAPI("'when.sibling' calls should be compiled away.")
+}