diff --git a/crates/solidcore-ffi/src/units/surface_offset_surface.rs b/crates/solidcore-ffi/src/units/surface_offset_surface.rs
index eccd5daf42..e2690834cc 100644
--- a/crates/solidcore-ffi/src/units/surface_offset_surface.rs
+++ b/crates/solidcore-ffi/src/units/surface_offset_surface.rs
@@ -1063,8 +1063,14 @@ mod tests {
             assert!(refused(&script(&off, &[Step::Extend { dist, cont }])), "{dist:e}");
             assert!(t.elapsed() < crate::offset_reference::DEADLINE);
         }
-        // a normal extension of the same surface is compared
-        assert!(script(&off, &[Step::Extend { dist: 0.1, cont: 5 }]).is_ok());
+        // a normal extension of the same surface is compared where the worker has its
+        // NLib link wrappers (GNU ld --wrap, Linux); elsewhere every extension is refused
+        let normal = script(&off, &[Step::Extend { dist: 0.1, cont: 5 }]);
+        if cfg!(target_os = "linux") {
+            assert!(normal.is_ok());
+        } else {
+            assert!(refused(&normal));
+        }
     }
 
     #[test]
diff --git a/harness/cpp/offset_worker.cpp b/harness/cpp/offset_worker.cpp
index ceb0742053..ef6d3d0226 100644
--- a/harness/cpp/offset_worker.cpp
+++ b/harness/cpp/offset_worker.cpp
@@ -115,7 +115,6 @@ extern "C" NL_FLAG __wrap_N_SrfCopy(NL_SURFACE* source, NL_SURFACE* target,
     return __real_N_SrfCopy(source, target, stack);
 }
 
-extern "C" int sc_offset_extension_guards() { return 1; }
 #endif // SC_OFFSET_NO_LINK_WRAP
 
 template <class T> bool read(T& value)
@@ -129,6 +128,9 @@ template <class T> void write(const T& value)
 
 int main()
 {
+#ifndef SC_OFFSET_NO_LINK_WRAP
+    sc_offset_extension_guards_enabled = 1;
+#endif
     int32_t op;
     uint64_t count;
     if (!read(op) || !read(count) || count == 0 || count > 1048576) return 2;
diff --git a/harness/cpp/shim/sc_offset_guard.h b/harness/cpp/shim/sc_offset_guard.h
index 48028e94aa..4f0720c0a0 100644
--- a/harness/cpp/shim/sc_offset_guard.h
+++ b/harness/cpp/shim/sc_offset_guard.h
@@ -5,9 +5,9 @@
 // offset shim and its worker-only link wrappers; unwinds into the shim boundary.
 struct ScOffsetUnsafeInput {};
 
-// Nonzero when the offset worker is linked with its NLib wrappers (GNU ld --wrap): the
-// guards for UB-107 and the HANG-6/HANG-15 extension searches. The shim's weak default
-// returns 0; the worker's strong definition overrides it where the wrappers exist.
-extern "C" int sc_offset_extension_guards();
-inline bool sc_offset_extensions_guarded() { return sc_offset_extension_guards() != 0; }
+// Set to 1 by the offset worker's main() when it is linked with its NLib wrappers (GNU
+// ld --wrap): the guards for UB-107 and the HANG-6/HANG-15 extension searches. Defined
+// as 0 in the shim (a plain variable, no weak symbols: the shim builds everywhere).
+extern "C" int sc_offset_extension_guards_enabled;
+inline bool sc_offset_extensions_guarded() { return sc_offset_extension_guards_enabled != 0; }
 #endif
diff --git a/harness/cpp/shim/surface_offset_surface.cpp b/harness/cpp/shim/surface_offset_surface.cpp
index 9247ac8771..7c28270109 100644
--- a/harness/cpp/shim/surface_offset_surface.cpp
+++ b/harness/cpp/shim/surface_offset_surface.cpp
@@ -48,6 +48,9 @@
 #include <string>
 #include <vector>
 
+// (see sc_offset_guard.h; set by the offset worker)
+extern "C" int sc_offset_extension_guards_enabled = 0;
+
 namespace
 {
 // Entry ops (keep in sync with crates/solidcore-ffi/src/units/surface_offset_surface.rs).
@@ -199,8 +202,6 @@ bool delegates_to_plane(SmOffsetSurface* s, const SmPoint2d& uv, ULONG hu, ULONG
     return SM_CAST_PTR(SmPlane, base) != NULL;
 }
 
-extern "C" __attribute__((weak)) int sc_offset_extension_guards() { return 0; }
-
 // UB-116: with bOnlyUpperHalf, EvaluatePlainOrSimple's non-zero path writes the mixed
 // derivatives at their full-layout indices, past a caller array sized (hu+1)(hv+1).
 // Mirror its checks in order: offset order above 3 and an offset nested twice fail