diff --git a/lib/build.ml b/lib/build.ml
index e4f62ac..e0726d3 100644
--- a/lib/build.ml
+++ b/lib/build.ml
@@ -879,6 +879,25 @@ let run what cmd =
 
 let shared ?(opts = default) ~ir ~out () : timing =
   let opts = if opts.debug then { opts with opt = "-O0" } else opts in
+  (* An [--x86] host must get [--x86] modules, and this is the one place that
+     can say so cheaply. The two backends' conventions agree on every scalar
+     and disagree on every aggregate, so a crossed pair links, loads, and then
+     dies at the first call into a redefined function that takes or returns a
+     struct — measured as SIGSEGV, in test_reload.ml's aggregate section. The
+     option record is where the backend choice lives, so a builder handed the
+     other backend's record refuses by name rather than producing that object.
+
+     This is a guard and not a proof: a caller holding two option records can
+     still pick the wrong one, and the loaded object carries no mark saying
+     which backend made it. The complete answer is a marker symbol the host
+     defines and a module references, so the loader refuses the pair at dlopen
+     rather than the processor refusing it at a call. See
+     HANDOFF-x86-aggregates.md. *)
+  if opts.x86 then
+    failwith
+      "--x86: Build.shared is the LLVM redefinition path, and an --x86 host \
+       must get --x86 modules — the two calling conventions disagree on every \
+       aggregate. Use Build.shared_x86 with X86.redefinition.";
   if wasm_target opts then
     failwith
       ((if web_target opts then "web" else "wasm32")
@@ -928,6 +947,14 @@ let shared ?(opts = default) ~ir ~out () : timing =
 let assembler = try Sys.getenv "FLAN_AS" with Not_found -> "as"
 
 let shared_x86 ?(opts = default) ~asm ~out () : timing =
+  (* The other half of the same guard, and it is the half that costs nothing to
+     get right: a module built here for a host that was built by LLVM is the
+     same mismatch seen from the other side. *)
+  if not opts.x86 then
+    failwith
+      "Build.shared_x86 is the --x86 redefinition path and was handed an LLVM \
+       option record — an --x86 host must get --x86 modules, and an LLVM host \
+       must get LLVM ones. Set [x86] on the options, or use Build.shared.";
   let dir = workdir () in
   let base = Filename.remove_extension (Filename.basename out) in
   let src = Filename.concat dir (base ^ ".s") in
diff --git a/test/test_reload.ml b/test/test_reload.ml
index b5ed8a3..f690f5f 100644
--- a/test/test_reload.ml
+++ b/test/test_reload.ml
@@ -347,6 +347,35 @@ let () =
       in
       ignore (Build.shared_x86 ~opts:x86 ~asm ~out:o ());
       o);
+    (* The mismatch, which is the same measurement run crossed. An --x86 host
+       given LLVM-built modules dies with SIGSEGV on the first call into a
+       redefined aggregate body — measured, not argued:
+
+         got: "a1\nhost 54063108\na1\n" (exit 139)
+
+       That is not asserted here. It is undefined behaviour and what it prints
+       is a property of whichever LLVM is installed; a test that pins it would
+       be pinning the shape of a crash. What is asserted is that the two
+       builders refuse to be crossed when the option record says which backend
+       is in play — see below. *)
+
+    (* Which is the only thing that stands between a future caller and that
+       segfault. [Build.opts] is where the backend choice lives, so a builder
+       handed an option record belonging to the other backend refuses rather
+       than producing an object that links, loads, and then dies at a call
+       site. It is not a complete defence and must not be read as one: a caller
+       that keeps two option records and picks the wrong one by hand defeats
+       it, and the CLI can still do exactly that — see
+       HANDOFF-x86-aggregates.md. *)
+    (match Build.shared ~opts:x86 ~ir:"" ~out:(tmp "never.so") () with
+     | _ -> fail "Build.shared accepted an --x86 option record"
+     | exception Failure m when has m "--x86" -> ()
+     | exception Failure m -> fail "Build.shared refused for the wrong reason: %s" m);
+    (match Build.shared_x86 ~opts:dev ~asm:"" ~out:(tmp "never.so") () with
+     | _ -> fail "Build.shared_x86 accepted an LLVM option record"
+     | exception Failure m when has m "--x86" -> ()
+     | exception Failure m ->
+       fail "Build.shared_x86 refused for the wrong reason: %s" m);
 
     (* The layout-drift guard, which needs a process of its own because what it
        does is abort one. [extra] does not exist in the host: v3 introduced it