diff --git a/crates/perry-hir/src/lower/lower_expr/arm_bin.rs b/crates/perry-hir/src/lower/lower_expr/arm_bin.rs
index ab7629655d..7df8bfa130 100644
--- a/crates/perry-hir/src/lower/lower_expr/arm_bin.rs
+++ b/crates/perry-hir/src/lower/lower_expr/arm_bin.rs
@@ -68,7 +68,19 @@ pub(crate) fn lower_bin_expr(ctx: &mut LoweringContext, bin: &ast::BinExpr) -> R
         let expr = Box::new(lower_expr(ctx, &bin.left)?);
         // Right side can be an identifier (ClassName) or member expression (Module.ClassName)
         let ty = match bin.right.as_ref() {
-            ast::Expr::Ident(ident) => ident.sym.to_string(),
+            // Honor a scope-local class rename: when two same-named classes
+            // collide (e.g. a bundler flattens two module factories that each
+            // declare `class l`), the SECOND is registered under a suffixed
+            // name (`l$0`) and `class_renames` maps the raw name to it. `extends`
+            // / `new` already resolve through this map, but `instanceof <name>`
+            // used the RAW ident — so `x instanceof l` resolved to the OTHER
+            // factory's `l` (class_id mismatch) and returned false even though
+            // the prototype chain was correct. This broke Auth.js's
+            // `error instanceof AuthError` guard (AuthError was the renamed
+            // class), so a `CredentialsSignin` was mis-wrapped in a
+            // `CallbackRouteError` and the login redirect fell back to
+            // `?error=Configuration` instead of `?error=CredentialsSignin`.
+            ast::Expr::Ident(ident) => ctx.resolve_class_name(ident.sym.as_ref()),
             ast::Expr::Member(member) => {
                 // Handle Module.ClassName - extract the full qualified name
                 let obj_name = if let ast::Expr::Ident(obj_ident) = member.obj.as_ref() {
diff --git a/test-files/test_gap_instanceof_flattened_class_rename.ts b/test-files/test_gap_instanceof_flattened_class_rename.ts
new file mode 100644
index 0000000000..d00e71ea37
--- /dev/null
+++ b/test-files/test_gap_instanceof_flattened_class_rename.ts
@@ -0,0 +1,27 @@
+// Two "module factories" (as a bundler emits them into one file) each declare a
+// class named `L`. Perry flattens them and scope-renames the second `L`. An
+// `x instanceof L` inside the second factory must still resolve to THAT
+// factory's `L` (its own class), not the first factory's same-named class.
+// Regression: the rename was applied to `class`/`extends`/`new` but NOT to the
+// `instanceof` operand, so `instanceof L` matched the wrong class_id and
+// returned false even though the prototype chain was correct.
+const factories: Array<(r: any) => void> = [
+  (r) => {
+    class L {
+      seal(): string { return "util"; }
+    }
+    r.Util = L;
+  },
+  (r) => {
+    class L {}
+    class D extends L {}
+    class G extends D {}
+    const g = new G();
+    r.iofL = g instanceof L;
+    r.iofD = g instanceof D;
+  },
+];
+const out: any = {};
+for (const f of factories) f(out);
+console.log("iofL=" + out.iofL);
+console.log("iofD=" + out.iofD);