diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index 52a27d08..d1ceec8f 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -377,6 +377,16 @@
      let chr n = _char_chr n
      let lowercase_ascii c = _string_lower c
      let uppercase_ascii c = _string_upper c
+     let is_digit c =
+       let n = _char_code c in n >= 48 && n <= 57
+     let is_lower c =
+       let n = _char_code c in n >= 97 && n <= 122
+     let is_upper c =
+       let n = _char_code c in n >= 65 && n <= 90
+     let is_alpha c = is_lower c || is_upper c
+     let is_alnum c = is_alpha c || is_digit c
+     let is_whitespace c =
+       c = \" \" || c = \"\\t\" || c = \"\\n\" || c = \"\\r\"
    end ;;
 
    module Int = struct
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 1bde32a3..b86b1c14 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1124,6 +1124,22 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4202)
 (eval "(ocaml-type-of-program \"let rec map f xs = match xs with | [] -> [] | h :: t -> f h :: map f t and length lst = match lst with | [] -> 0 | _ :: t -> 1 + length t;; map\")")
 
+;; ── Char predicate helpers ────────────────────────────────────
+(epoch 4300)
+(eval "(ocaml-run \"Char.is_digit \\\"5\\\"\")")
+(epoch 4301)
+(eval "(ocaml-run \"Char.is_digit \\\"x\\\"\")")
+(epoch 4302)
+(eval "(ocaml-run \"Char.is_alpha \\\"x\\\"\")")
+(epoch 4303)
+(eval "(ocaml-run \"Char.is_alnum \\\"5\\\"\")")
+(epoch 4304)
+(eval "(ocaml-run \"Char.is_whitespace \\\" \\\"\")")
+(epoch 4305)
+(eval "(ocaml-run \"Char.is_upper \\\"A\\\"\")")
+(epoch 4306)
+(eval "(ocaml-run \"Char.is_lower \\\"a\\\"\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1780,6 +1796,15 @@ check 4200 "let-mut x+y"                  '"Int"'
 check 4201 "let-rec-mut even 10"          '"Bool"'
 check 4202 "let-rec-mut map+length"       'list -> '
 
+# ── Char predicates ────────────────────────────────────────────
+check 4300 "Char.is_digit 5"              'true'
+check 4301 "Char.is_digit x"              'false'
+check 4302 "Char.is_alpha x"              'true'
+check 4303 "Char.is_alnum 5"              'true'
+check 4304 "Char.is_whitespace ' '"       'true'
+check 4305 "Char.is_upper A"              'true'
+check 4306 "Char.is_lower a"              'true'
+
 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 d7ccc61d..b404aad6 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,9 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 6 — Char predicates: is_digit/is_alpha/is_alnum/
+  is_whitespace/is_upper/is_lower (+7 tests, 461 total). All written
+  in OCaml in runtime.sx using Char.code + ASCII range checks.
 - 2026-05-08 Phase 5 — HM for top-level `let..and..` decls (+3
   tests, 454 total). `ocaml-type-of-program` now handles `:def-mut`
   (sequential generalization) and `:def-rec-mut` (mutual recursion