diff --git a/lib/ocaml/parser.sx b/lib/ocaml/parser.sx
index 70e3ff25..cfbff29f 100644
--- a/lib/ocaml/parser.sx
+++ b/lib/ocaml/parser.sx
@@ -787,7 +787,7 @@
                     (list :try expr cases)))))))
         (define parse-function
           (fn ()
-            ;; `function | pat -> body | …` ≡ fun x -> match x with | pat -> body
+            ;; `function | pat [when GUARD] -> body | …`
             (let ()
               (begin
                 (when (at-op? "|") (advance-tok!))
@@ -795,11 +795,18 @@
                   (begin
                     (define one
                       (fn ()
-                        (let ((p (parse-pattern)))
+                        (let ((p (parse-pattern)) (guard nil))
                           (begin
+                            (when (at-kw? "when")
+                              (begin (advance-tok!)
+                                     (set! guard (parse-expr-no-seq))))
                             (consume! "op" "->")
                             (let ((body (parse-expr)))
-                              (append! cases (list :case p body)))))))
+                              (cond
+                                ((= guard nil)
+                                  (append! cases (list :case p body)))
+                                (else
+                                  (append! cases (list :case-when p guard body)))))))))
                     (one)
                     (define loop
                       (fn ()
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index b86b1c14..1501d238 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1140,6 +1140,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4306)
 (eval "(ocaml-run \"Char.is_lower \\\"a\\\"\")")
 
+;; ── function with `when` guard ────────────────────────────────
+(epoch 4400)
+(eval "(ocaml-run \"(function | n when n > 0 -> 1 | _ -> 0) 5\")")
+(epoch 4401)
+(eval "(ocaml-run \"(function | n when n > 0 -> 1 | _ -> 0) (-3)\")")
+(epoch 4402)
+(eval "(ocaml-run \"(function | n when n > 0 -> 1 | n when n < 0 -> -1 | _ -> 0) 0\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1805,6 +1813,11 @@ check 4304 "Char.is_whitespace ' '"       'true'
 check 4305 "Char.is_upper A"              'true'
 check 4306 "Char.is_lower a"              'true'
 
+# ── function with `when` guard ─────────────────────────────────
+check 4400 "function when 5"              '1'
+check 4401 "function when -3"             '0'
+check 4402 "function sign 0"              '0'
+
 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 b1fa4cb7..7d0d292e 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 1+3 — `function | pat when GUARD -> body | …`
+  guard support (+3 tests, 464 total). `parse-function` mirrors the
+  match-clause when-handling.
 - 2026-05-08 Phase 5.1 — anagrams.ml baseline (18/18 pass). Counts
   anagram-equivalence groups via Hashtbl + List.sort + String.get +
   for-loop. `["eat";"tea";"tan";"ate";"nat";"bat"]` → 3 groups.