diff --git a/lib/ocaml/eval.sx b/lib/ocaml/eval.sx
index 3bb16651..ab67a841 100644
--- a/lib/ocaml/eval.sx
+++ b/lib/ocaml/eval.sx
@@ -47,7 +47,26 @@
       ;; can pattern-match them.
       (list "raise" (fn (e) (raise e)))
       (list "failwith" (fn (msg) (raise (list "Failure" msg))))
-      (list "invalid_arg" (fn (msg) (raise (list "Invalid_argument" msg)))))))
+      (list "invalid_arg" (fn (msg) (raise (list "Invalid_argument" msg)))
+      )
+      ;; Host primitives exposed for the OCaml stdlib (lib/ocaml/runtime.sx).
+      ;; Underscore-prefixed to avoid clashing with user names.
+      (list "_string_length" (fn (s) (len s)))
+      (list "_string_get"    (fn (s) (fn (i) (nth s i))))
+      (list "_string_sub"    (fn (s) (fn (i) (fn (n) (slice s i (+ i n))))))
+      (list "_string_concat" (fn (sep) (fn (xs) (join sep xs))))
+      (list "_string_upper"  (fn (s) (upper s)))
+      (list "_string_lower"  (fn (s) (lower s)))
+      (list "_string_starts_with" (fn (p) (fn (s) (starts-with? s p))))
+      (list "_int_of_string" (fn (s) (parse-number s)))
+      (list "_string_of_int" (fn (i) (str i)))
+      (list "_string_of_float" (fn (f) (str f)))
+      (list "_char_code" (fn (c) (char-code c)))
+      (list "_char_chr"  (fn (n) (char-from-code n)))
+      ;; Print: prints to host stdout via println.
+      (list "print_string"  (fn (s) (begin (print s) nil)))
+      (list "print_endline" (fn (s) (begin (println s) nil)))
+      (list "print_int"     (fn (i) (begin (print (str i)) nil))))))
 
 (define ocaml-env-lookup
   (fn (env name)
diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index 47b8bbb5..52ea6cf1 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -136,6 +136,40 @@
        match r with
        | Ok _ -> false
        | Error _ -> true
+   end ;;
+
+   module String = struct
+     let length s = _string_length s
+     let get s i = _string_get s i
+     let sub s i n = _string_sub s i n
+     let concat sep xs = _string_concat sep xs
+     let uppercase_ascii s = _string_upper s
+     let lowercase_ascii s = _string_lower s
+     let starts_with prefix s = _string_starts_with prefix s
+   end ;;
+
+   module Char = struct
+     let code c = _char_code c
+     let chr n = _char_chr n
+     let lowercase_ascii c = _string_lower c
+     let uppercase_ascii c = _string_upper c
+   end ;;
+
+   module Int = struct
+     let to_string i = _string_of_int i
+     let of_string s = _int_of_string s
+     let abs n = if n < 0 then 0 - n else n
+     let max a b = if a > b then a else b
+     let min a b = if a < b then a else b
+   end ;;
+
+   module Float = struct
+     let to_string f = _string_of_float f
+   end ;;
+
+   module Printf = struct
+     let sprintf fmt = fmt
+     let printf fmt = print_string fmt
    end")
 
 (define ocaml-stdlib-loaded false)
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 0b8da7cf..5b87ac87 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -672,6 +672,36 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 913)
 (eval "(ocaml-type-of \"not true\")")
 
+;; ── Phase 6 expanded: String / Char / Int / Float modules ─────
+(epoch 950)
+(eval "(ocaml-run \"String.length \\\"hello\\\"\")")
+(epoch 951)
+(eval "(ocaml-run \"String.uppercase_ascii \\\"hi\\\"\")")
+(epoch 952)
+(eval "(ocaml-run \"String.lowercase_ascii \\\"HI\\\"\")")
+(epoch 953)
+(eval "(ocaml-run \"String.sub \\\"hello\\\" 1 3\")")
+(epoch 954)
+(eval "(ocaml-run \"String.starts_with \\\"he\\\" \\\"hello\\\"\")")
+(epoch 955)
+(eval "(ocaml-run \"String.concat \\\",\\\" [\\\"a\\\"; \\\"b\\\"; \\\"c\\\"]\")")
+
+(epoch 960)
+(eval "(ocaml-run \"Char.code \\\"A\\\"\")")
+(epoch 961)
+(eval "(ocaml-run \"Char.chr 65\")")
+
+(epoch 970)
+(eval "(ocaml-run \"Int.to_string 42\")")
+(epoch 971)
+(eval "(ocaml-run \"Int.of_string \\\"123\\\"\")")
+(epoch 972)
+(eval "(ocaml-run \"Int.abs (-5)\")")
+(epoch 973)
+(eval "(ocaml-run \"Int.max 7 3\")")
+(epoch 974)
+(eval "(ocaml-run \"Int.min 7 3\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1065,6 +1095,23 @@ check 911 "type twice"                   ' -> '
 check 912 "type bool branch"             '"Bool -> Int"'
 check 913 "type not true"                '"Bool"'
 
+# ── Phase 6 String / Char / Int ─────────────────────────────────
+check 950 "String.length"                '5'
+check 951 "String.uppercase_ascii"       '"HI"'
+check 952 "String.lowercase_ascii"       '"hi"'
+check 953 "String.sub"                   '"ell"'
+check 954 "String.starts_with"           'true'
+check 955 "String.concat"                '"a,b,c"'
+
+check 960 "Char.code A"                  '65'
+check 961 "Char.chr 65"                  '"A"'
+
+check 970 "Int.to_string"                '"42"'
+check 971 "Int.of_string"                '123'
+check 972 "Int.abs -5"                   '5'
+check 973 "Int.max"                      '7'
+check 974 "Int.min"                      '3'
+
 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 06e288d5..a7b756d1 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -229,6 +229,16 @@ SX CEK evaluator (both JS and OCaml hosts)
       _(Pending: fold/join/iter/to_list/to_result.)_
 - [~] `Result`: `map`, `bind`, `is_ok`, `is_error`. _(Pending:
       fold/get_ok/get_error/map_error/to_option.)_
+- [~] `String`: `length`, `get`, `sub`, `concat`, `uppercase_ascii`,
+      `lowercase_ascii`, `starts_with`. _(Pending: split_on_char, trim,
+      contains, ends_with, index_opt, replace_all.)_
+- [~] `Char`: `code`, `chr`, `lowercase_ascii`, `uppercase_ascii`.
+      _(Pending: escaped.)_
+- [~] `Int`: `to_string`, `of_string`, `abs`, `max`, `min`.
+      _(Pending: arithmetic helpers, min_int/max_int.)_
+- [~] `Float`: `to_string`. _(Pending: of_string, arithmetic helpers.)_
+- [~] `Printf`: stub `sprintf`/`printf`. _(Real format-string
+      interpretation pending.)_
 - [ ] `String`: `length`, `get`, `sub`, `concat`, `split_on_char`, `trim`,
       `uppercase_ascii`, `lowercase_ascii`, `contains`, `starts_with`, `ends_with`,
       `index_opt`, `replace_all` (non-stdlib but needed).
@@ -333,6 +343,14 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — extended stdlib slice (+13 tests, 278 total).
+  Host primitives exposed via `_string_*`, `_char_*`, `_int_*`,
+  `_string_of_*` underscore-prefixed builtins so the OCaml-side
+  `lib/ocaml/runtime.sx` modules can wrap them: String (length, get,
+  sub, concat, uppercase_ascii, lowercase_ascii, starts_with), Char
+  (code, chr, lowercase_ascii, uppercase_ascii), Int (to_string,
+  of_string, abs, max, min), Float.to_string, Printf stubs. Also added
+  `print_string`/`print_endline`/`print_int` builtins.
 - 2026-05-08 Phase 5 — Hindley-Milner type inference, paired-sequencing
   consumer of `lib/guest/hm.sx` (algebra) and `lib/guest/match.sx`
   (unify). `lib/ocaml/infer.sx` ships Algorithm W rules for OCaml AST: