diff --git a/hosts/ocaml/bin/run_tests.ml b/hosts/ocaml/bin/run_tests.ml
index d12b0d85..b6cc381c 100644
--- a/hosts/ocaml/bin/run_tests.ml
+++ b/hosts/ocaml/bin/run_tests.ml
@@ -1469,7 +1469,56 @@ let run_foundation_tests () =
      Printf.printf "  FAIL: duplicate extension name should have raised\n"
    with Failure _ ->
      incr pass_count;
-     Printf.printf "  PASS: duplicate extension name rejected\n")
+     Printf.printf "  PASS: duplicate extension name rejected\n");
+
+  Printf.printf "\nSuite: extension-opcode-id primitive\n";
+  let prim = Hashtbl.find Sx_primitives.primitives "extension-opcode-id" in
+
+  (* Known opcode (registered by TestExt above). *)
+  (match prim [String "test_reg.OP_PUSH_42"] with
+   | Integer 210 ->
+     incr pass_count;
+     Printf.printf "  PASS: primitive returns Integer for registered opcode\n"
+   | other ->
+     incr fail_count;
+     Printf.printf "  FAIL: registered opcode lookup: got %s\n"
+       (Sx_types.inspect other));
+
+  (* Unknown opcode → Nil. *)
+  (match prim [String "nonexistent.OP_X"] with
+   | Nil ->
+     incr pass_count;
+     Printf.printf "  PASS: primitive returns nil for unknown opcode\n"
+   | other ->
+     incr fail_count;
+     Printf.printf "  FAIL: unknown opcode lookup: got %s\n"
+       (Sx_types.inspect other));
+
+  (* Symbol arg also accepted (compilers may pass quoted symbols). *)
+  (match prim [Symbol "test_reg.OP_DOUBLE_TOS"] with
+   | Integer 211 ->
+     incr pass_count;
+     Printf.printf "  PASS: primitive accepts Symbol args\n"
+   | other ->
+     incr fail_count;
+     Printf.printf "  FAIL: symbol arg: got %s\n" (Sx_types.inspect other));
+
+  (* Wrong arity / type raises Eval_error. *)
+  (try
+     let _ = prim [] in
+     incr fail_count;
+     Printf.printf "  FAIL: zero args should have raised\n"
+   with Sx_types.Eval_error _ ->
+     incr pass_count;
+     Printf.printf "  PASS: zero args rejected\n");
+
+  (try
+     let _ = prim [Integer 42] in
+     incr fail_count;
+     Printf.printf "  FAIL: integer arg should have raised\n"
+   with Sx_types.Eval_error _ ->
+     incr pass_count;
+     Printf.printf "  PASS: integer arg rejected\n")
 
 
 (* ====================================================================== *)
diff --git a/hosts/ocaml/lib/sx_vm_extensions.ml b/hosts/ocaml/lib/sx_vm_extensions.ml
index 8e1dd288..e301b57c 100644
--- a/hosts/ocaml/lib/sx_vm_extensions.ml
+++ b/hosts/ocaml/lib/sx_vm_extensions.ml
@@ -87,3 +87,22 @@ let install_dispatch () =
   Sx_vm.extension_dispatch_ref := dispatch
 
 let () = install_dispatch ()
+
+(** Compiler-side opcode lookup: register the [extension-opcode-id]
+    primitive. Compilers ([lib/compiler.sx]) call this to emit
+    extension opcodes by name. Returns [Integer id] when registered,
+    [Nil] otherwise — so missing extensions degrade to a fallback
+    rather than failure. *)
+let () =
+  Sx_primitives.register "extension-opcode-id" (fun args ->
+    match args with
+    | [Sx_types.String name] ->
+      (match id_of_name name with
+       | Some id -> Sx_types.Integer id
+       | None -> Sx_types.Nil)
+    | [Sx_types.Symbol name] ->
+      (match id_of_name name with
+       | Some id -> Sx_types.Integer id
+       | None -> Sx_types.Nil)
+    | _ -> raise (Sx_types.Eval_error
+        "extension-opcode-id: expected one string or symbol"))
diff --git a/plans/sx-vm-opcode-extension.md b/plans/sx-vm-opcode-extension.md
index 0eb1de91..1515bb34 100644
--- a/plans/sx-vm-opcode-extension.md
+++ b/plans/sx-vm-opcode-extension.md
@@ -279,8 +279,8 @@ compiler in `lib/compiler.sx` can call it to emit extension opcodes by name.
 Does not require any extension to actually exist — the primitive returns
 `nil` for unknown names, and the compiler falls back.
 
-- [ ] Register `extension-opcode-id` in `sx_primitives.ml`.
-- [ ] Returns `Integer id` when registered, `Nil` otherwise.
+- [x] Register `extension-opcode-id` in `sx_primitives.ml`.
+- [x] Returns `Integer id` when registered, `Nil` otherwise.
 
 **Tests:**
 - Primitive returns nil for unknown name.
@@ -457,6 +457,20 @@ familiarity.
 
 Newest first.
 
+- **2026-05-15** — Phase C done. `extension-opcode-id` SX primitive
+  registered from `sx_vm_extensions.ml` module init (avoids the
+  `sx_primitives ↔ sx_vm` cycle by registering downstream of both).
+  Accepts a string or symbol; returns `Integer id` for registered
+  opcode names, `Nil` for unknown — so a missing extension at compile
+  time degrades to a fallback rather than failure. 5 new foundation
+  tests (`extension-opcode-id primitive` suite): registered lookup,
+  unknown → nil, symbol arg, zero-arg rejection, integer-arg
+  rejection. +5 pass vs Phase B baseline (4821 vs 4816), 1111
+  pre-existing failures unchanged. Conformance suites green: erlang
+  530/530, haskell 285/285, datalog 276/276, prolog 590/590, smalltalk
+  847/847, common-lisp 487/487, apl 562/562, js 148/148, forth 632/638
+  (pre-existing), tcl 3/4 (pre-existing), ocaml-on-sx unit 607/607.
+
 - **2026-05-14** — Phase B done. Added `hosts/ocaml/lib/sx_vm_extension.ml`
   (interface: `handler` type, `extension_state` extensible variant,
   `EXTENSION` module type) and `sx_vm_extensions.ml` (registry: `register`,