diff --git a/Sources/GObjectGeneratorCore/CodeGen+Scaffolding.swift b/Sources/GObjectGeneratorCore/CodeGen+Scaffolding.swift
index 949fa6f..6ff97ad 100644
--- a/Sources/GObjectGeneratorCore/CodeGen+Scaffolding.swift
+++ b/Sources/GObjectGeneratorCore/CodeGen+Scaffolding.swift
@@ -149,6 +149,21 @@ extension CodeGenerator {
                 umbrellaHeader += "\n#include <gtk/gtkunixprint.h>"
             }
 
+            // Every umbrella header defines `__GI_SCANNER__` before pulling in
+            // the system header. GLib, GTK, and Graphene all gate architecture-
+            // specific `static inline` fast paths (most notably Graphene's
+            // SSE/ARM-NEON `graphene_simd4f_t` backend) behind `#ifndef
+            // __GI_SCANNER__`, falling back to a portable scalar/definition-only
+            // path when it is set - the same escape hatch upstream ships for
+            // gobject-introspection's scanner, which (like Clang's ClangImporter
+            // here) only needs declarations, never compiled SIMD bodies. Without
+            // it, a NEON-using header reachable from two different `.systemLibrary`
+            // modules (e.g. both CGraphene and CGsk textually include
+            // <graphene-simd4f.h>) is compiled with the real Clang builtin
+            // `_Builtin_intrinsics.arm.acle` module in one and not the other,
+            // and Clang's module visibility rules reject the resulting
+            // conflicting `uint32_t`/`__fsid_t` redeclarations on aarch64 -
+            // reproducible on every aarch64 Linux target, not distro-specific.
             // No `link` directives: the `.systemLibrary` target carries
             // `pkgConfig:`, so pkg-config `--libs` supplies the exact linker
             // flags. Emitting `link "glib-2"` here both duplicates that and
@@ -160,7 +175,7 @@ extension CodeGenerator {
             }
             """
             files["Sources/\(cName)/module.modulemap"] = moduleMap
-            files["Sources/\(cName)/\(cName).h"] = umbrellaHeader + "\n"
+            files["Sources/\(cName)/\(cName).h"] = "#define __GI_SCANNER__ 1\n" + umbrellaHeader + "\n"
         }
 
         // Monorepo Package.swift