diff --git a/lib/ocaml/eval.sx b/lib/ocaml/eval.sx
index 35115a33..03a93208 100644
--- a/lib/ocaml/eval.sx
+++ b/lib/ocaml/eval.sx
@@ -58,6 +58,15 @@
       (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 "_string_ends_with"   (fn (p) (fn (s) (ends-with? s p))))
+      (list "_string_contains"    (fn (s) (fn (sub) (contains? s sub))))
+      (list "_string_trim"        (fn (s) (trim s)))
+      (list "_string_split_on_char"
+        (fn (sep) (fn (s) (split s sep))))
+      (list "_string_replace"
+        (fn (s) (fn (a) (fn (b) (replace s a b)))))
+      (list "_string_index_of"
+        (fn (s) (fn (sub) (index-of s sub))))
       (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)))
diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index 0ea5c882..e1af957d 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -326,6 +326,12 @@
      let uppercase_ascii s = _string_upper s
      let lowercase_ascii s = _string_lower s
      let starts_with prefix s = _string_starts_with prefix s
+     let ends_with suffix s = _string_ends_with suffix s
+     let contains s sub = _string_contains s sub
+     let trim s = _string_trim s
+     let split_on_char c s = _string_split_on_char c s
+     let replace_all s a b = _string_replace s a b
+     let index_of s sub = _string_index_of s sub
    end ;;
 
    module Char = struct
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 4c39a5ae..43e2da40 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -990,6 +990,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2905)
 (eval "(ocaml-run \"List.take 100 [1;2;3]\")")
 
+;; ── String extensions ──────────────────────────────────────────
+(epoch 3000)
+(eval "(ocaml-run \"String.ends_with \\\"lo\\\" \\\"hello\\\"\")")
+(epoch 3001)
+(eval "(ocaml-run \"String.contains \\\"hello\\\" \\\"ell\\\"\")")
+(epoch 3002)
+(eval "(ocaml-run \"String.trim \\\"  hi  \\\"\")")
+(epoch 3003)
+(eval "(ocaml-run \"String.split_on_char \\\" \\\" \\\"a b c\\\"\")")
+(epoch 3004)
+(eval "(ocaml-run \"String.replace_all \\\"hello\\\" \\\"l\\\" \\\"r\\\"\")")
+(epoch 3005)
+(eval "(ocaml-run \"String.index_of \\\"hello\\\" \\\"ll\\\"\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1566,6 +1580,14 @@ check 2903 "List.flat_map double"         '(1 1 2 2 3 3)'
 check 2904 "List.take 0"                  '()'
 check 2905 "List.take overflow"           '(1 2 3)'
 
+# ── String extensions ──────────────────────────────────────────
+check 3000 "String.ends_with"             'true'
+check 3001 "String.contains"              'true'
+check 3002 "String.trim"                  '"hi"'
+check 3003 "String.split_on_char"         '("a" "b" "c")'
+check 3004 "String.replace_all"           '"herro"'
+check 3005 "String.index_of"              '2'
+
 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 2d6d0a26..37288d34 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -262,9 +262,9 @@ 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.
-- [~] `String`: `length`, `get`, `sub`, `concat`, `uppercase_ascii`,
-      `lowercase_ascii`, `starts_with`. _(Pending: split_on_char, trim,
-      contains, ends_with, index_opt, replace_all.)_
+- [x] `String`: `length`, `get`, `sub`, `concat`, `uppercase_ascii`,
+      `lowercase_ascii`, `starts_with`, `ends_with`, `contains`, `trim`,
+      `split_on_char`, `replace_all`, `index_of`.
 - [~] `Char`: `code`, `chr`, `lowercase_ascii`, `uppercase_ascii`.
       _(Pending: escaped.)_
 - [~] `Int`: `to_string`, `of_string`, `abs`, `max`, `min`.
@@ -377,6 +377,9 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 6 — String extensions: ends_with/contains/trim/
+  split_on_char/replace_all/index_of (+6 tests, 406 total). Wraps host
+  primitives via `_string_*` builtins.
 - 2026-05-08 Phase 6 — `List.take/drop/filter_map/flat_map/concat_map`
   (+6 tests, 400 total). Common functional helpers, all written in
   OCaml. **400-test milestone.**