diff --git a/hosts/ocaml/bin/sx_server.ml b/hosts/ocaml/bin/sx_server.ml
index 40de7b49..5ef5298b 100644
--- a/hosts/ocaml/bin/sx_server.ml
+++ b/hosts/ocaml/bin/sx_server.ml
@@ -18,6 +18,14 @@
 
 open Sx_types
 
+(* Force-link Sx_vm_extensions so its module-init runs: installs the
+   extension dispatch fallthrough and registers the `extension-opcode-id`
+   SX primitive. Without a reference here OCaml dead-code-eliminates the
+   module from sx_server.exe (it's only otherwise reached from run_tests),
+   leaving guest-language opcode extensions (Erlang Phase 9, etc.)
+   invisible to the runtime. The applied call is a harmless lookup. *)
+let () = ignore (Sx_vm_extensions.id_of_name "")
+
 (* ====================================================================== *)
 (* Font measurement via otfm — reads OpenType/TrueType font tables         *)
 (* ====================================================================== *)