diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index e1af957d..ce31974f 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -366,6 +366,18 @@
      let printf fmt = print_string fmt
    end ;;
 
+   module Sys = struct
+     let os_type = \"SX\"
+     let word_size = 64
+     let max_array_length = 4611686018427387903
+     let max_string_length = 4611686018427387903
+     let executable_name = \"ocaml-on-sx\"
+     let big_endian = false
+     let unix = true
+     let win32 = false
+     let cygwin = false
+   end ;;
+
    module Hashtbl = struct
      let create n = _hashtbl_create n
      let add t k v = _hashtbl_add t k v
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index edda595a..a9eec931 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1012,6 +1012,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3102)
 (eval "(ocaml-type-of-program \"type t = A of bool | B of float;; A true\")")
 
+;; ── Sys module stubs ──────────────────────────────────────────
+(epoch 3200)
+(eval "(ocaml-run \"Sys.os_type\")")
+(epoch 3201)
+(eval "(ocaml-run \"Sys.word_size\")")
+(epoch 3202)
+(eval "(ocaml-run \"Sys.unix\")")
+(epoch 3203)
+(eval "(ocaml-run \"Sys.win32\")")
+(epoch 3204)
+(eval "(ocaml-run \"Sys.executable_name\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1601,6 +1613,13 @@ check 3100 "shape -> Int"                 '"shape -> Int"'
 check 3101 "TStr 'hi' : tag"              '"tag"'
 check 3102 "A true : t"                   '"t"'
 
+# ── Sys stubs ──────────────────────────────────────────────────
+check 3200 "Sys.os_type"                  '"SX"'
+check 3201 "Sys.word_size"                '64'
+check 3202 "Sys.unix"                     'true'
+check 3203 "Sys.win32"                    'false'
+check 3204 "Sys.executable_name"          '"ocaml-on-sx"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 32e5853f..bdec70c7 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -264,6 +264,10 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [~] `Hashtbl`: `create`, `add`, `find`, `find_opt`, `replace`, `mem`,
       `length`. Backed by a one-element list cell holding a SX dict;
       keys coerced to strings via `str` for polymorphic-key support.
+- [~] `Sys`: `os_type` (`"SX"`), `word_size`, `max_array_length`,
+      `max_string_length`, `executable_name`, `big_endian`, `unix`,
+      `win32`, `cygwin`. Constants only; `argv`/`getenv_opt`/`command`
+      pending (would need host platform integration).
 - [x] `String`: `length`, `get`, `sub`, `concat`, `uppercase_ascii`,
       `lowercase_ascii`, `starts_with`, `ends_with`, `contains`, `trim`,
       `split_on_char`, `replace_all`, `index_of`.
@@ -379,6 +383,10 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — `Sys` module constants (+5 tests, 414 total).
+  os_type, word_size, max_array_length, max_string_length,
+  executable_name, big_endian, unix, win32, cygwin. Constants-only
+  for now; `argv`/`getenv_opt`/`command` need host platform integration.
 - 2026-05-08 Phase 5 — parse simple type sources in user type-defs
   (+3 tests, 409 total). `ocaml-hm-parse-type-src` recognises
   primitive type names, tyvars `'a`, and `T list`/`T option`-style