diff --git a/src/lib/auth/index.ts b/src/lib/auth/index.ts
index 45c4a630..2f786f06 100644
--- a/src/lib/auth/index.ts
+++ b/src/lib/auth/index.ts
@@ -124,6 +124,14 @@ export const auth = new Proxy({} as AuthInstance, {
   get(_target, prop) {
     return Reflect.get(getAuth(), prop);
   },
+  // Critical: better-auth's `toNextJsHandler` does `"handler" in auth`
+  // and falls back to calling `auth(req)` if false. The default `has`
+  // trap looks at the empty target (returning false), so without this
+  // we'd hit that fallback (and crash because the target isn't
+  // callable). Forward `in` checks to the real instance.
+  has(_target, prop) {
+    return Reflect.has(getAuth(), prop);
+  },
 });
 
 export type Session = typeof auth.$Infer.Session;