diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index 278b432d..299ed74d 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -1392,7 +1392,11 @@
 
 (define
   js-num-to-int
-  (fn (n) (if (>= n 0) (floor n) (- 0 (floor (- 0 n))))))
+  (fn
+    (v)
+    (let
+      ((n (if (number? v) v (js-to-number v))))
+      (if (>= n 0) (floor n) (- 0 (floor (- 0 n)))))))
 
 (define dict-has? (fn (d k) (contains? (keys d) k)))
 
diff --git a/lib/js/test.sh b/lib/js/test.sh
index 5e83f260..4d10d531 100755
--- a/lib/js/test.sh
+++ b/lib/js/test.sh
@@ -1053,6 +1053,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2307)
 (eval "(js-eval \"Array.from([1,2,3], (x)=>x*2).join(',')\")")
 
+;; ── Phase 11.coerce: string-to-number coercion in int params ──
+(epoch 2400)
+(eval "(js-eval \"'abcd'.charAt('2')\")")
+(epoch 2401)
+(eval "(js-eval \"'abcd'.slice('1', '3')\")")
+
 EPOCHS
 
 
@@ -1619,6 +1625,10 @@ check 2305 "Array.from array"              '3'
 check 2306 "Array.from string"             '3'
 check 2307 "Array.from w/ map"             '"2,4,6"'
 
+# ── Phase 11.coerce ───────────────────────────────────────────
+check 2400 "charAt coerces string"         '"c"'
+check 2401 "slice coerces strings"         '"bc"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"