diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index df5757ca..0d9c3957 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -1636,6 +1636,257 @@
   parseFloat
   (fn (&rest args) (if (= (len args) 0) 0 (js-to-number (nth args 0)))))
 
+(define
+  js-json-stringify
+  (fn
+    (&rest args)
+    (if
+      (= (len args) 0)
+      js-undefined
+      (js-json-stringify-value (nth args 0)))))
+
+(define
+  js-json-stringify-value
+  (fn
+    (v)
+    (cond
+      ((= v nil) "null")
+      ((js-undefined? v) js-undefined)
+      ((= (type-of v) "boolean") (if v "true" "false"))
+      ((number? v) (js-number-to-string v))
+      ((= (type-of v) "string") (js-json-escape-string v))
+      ((list? v)
+        (let
+          ((parts (list)))
+          (for-each
+            (fn
+              (x)
+              (let
+                ((s (js-json-stringify-value x)))
+                (if
+                  (js-undefined? s)
+                  (append! parts "null")
+                  (append! parts s))))
+            v)
+          (str "[" (join "," parts) "]")))
+      ((dict? v)
+        (let
+          ((parts (list)))
+          (for-each
+            (fn
+              (k)
+              (let
+                ((val (get v k)))
+                (let
+                  ((vs (js-json-stringify-value val)))
+                  (if
+                    (not (js-undefined? vs))
+                    (append! parts (str (js-json-escape-string k) ":" vs))))))
+            (keys v))
+          (str "{" (join "," parts) "}")))
+      (else "null"))))
+
+(define
+  js-json-escape-string
+  (fn (s) (str "\"" (js-json-escape-loop s 0 "") "\"")))
+
+(define
+  js-json-escape-loop
+  (fn
+    (s i acc)
+    (if
+      (>= i (len s))
+      acc
+      (let
+        ((c (char-at s i)))
+        (cond
+          ((= c "\"") (js-json-escape-loop s (+ i 1) (str acc "\\\"")))
+          ((= c "\\") (js-json-escape-loop s (+ i 1) (str acc "\\\\")))
+          ((= c "\n") (js-json-escape-loop s (+ i 1) (str acc "\\n")))
+          ((= c "\r") (js-json-escape-loop s (+ i 1) (str acc "\\r")))
+          ((= c "\t") (js-json-escape-loop s (+ i 1) (str acc "\\t")))
+          (else (js-json-escape-loop s (+ i 1) (str acc c))))))))
+
+(define
+  js-json-parse
+  (fn
+    (&rest args)
+    (if
+      (= (len args) 0)
+      js-undefined
+      (let
+        ((st (dict)))
+        (dict-set! st "s" (js-to-string (nth args 0)))
+        (dict-set! st "i" 0)
+        (js-json-parse-value st)))))
+
+(define
+  js-json-skip-ws!
+  (fn
+    (st)
+    (let
+      ((s (get st "s")) (i (get st "i")))
+      (cond
+        ((>= i (len s)) nil)
+        ((or (= (char-at s i) " ") (= (char-at s i) "\t") (= (char-at s i) "\n") (= (char-at s i) "\r"))
+          (begin (dict-set! st "i" (+ i 1)) (js-json-skip-ws! st)))
+        (else nil)))))
+
+(define
+  js-json-parse-value
+  (fn
+    (st)
+    (js-json-skip-ws! st)
+    (let
+      ((s (get st "s")) (i (get st "i")))
+      (cond
+        ((>= i (len s)) (error "JSON: unexpected end"))
+        ((= (char-at s i) "\"") (js-json-parse-string st))
+        ((= (char-at s i) "[") (js-json-parse-array st))
+        ((= (char-at s i) "{") (js-json-parse-object st))
+        ((= (char-at s i) "t") (begin (dict-set! st "i" (+ i 4)) true))
+        ((= (char-at s i) "f") (begin (dict-set! st "i" (+ i 5)) false))
+        ((= (char-at s i) "n") (begin (dict-set! st "i" (+ i 4)) nil))
+        (else (js-json-parse-number st))))))
+
+(define
+  js-json-parse-string
+  (fn
+    (st)
+    (let
+      ((s (get st "s")))
+      (dict-set! st "i" (+ (get st "i") 1))
+      (let
+        ((buf (list)))
+        (js-json-parse-string-loop st s buf)
+        (dict-set! st "i" (+ (get st "i") 1))
+        (join "" buf)))))
+
+(define
+  js-json-parse-string-loop
+  (fn
+    (st s buf)
+    (let
+      ((i (get st "i")))
+      (cond
+        ((>= i (len s)) nil)
+        ((= (char-at s i) "\"") nil)
+        ((= (char-at s i) "\\")
+          (begin
+            (when
+              (< (+ i 1) (len s))
+              (let
+                ((e (char-at s (+ i 1))))
+                (cond
+                  ((= e "n") (append! buf "\n"))
+                  ((= e "t") (append! buf "\t"))
+                  ((= e "r") (append! buf "\r"))
+                  ((= e "\"") (append! buf "\""))
+                  ((= e "\\") (append! buf "\\"))
+                  ((= e "/") (append! buf "/"))
+                  (else (append! buf e)))))
+            (dict-set! st "i" (+ i 2))
+            (js-json-parse-string-loop st s buf)))
+        (else
+          (begin
+            (append! buf (char-at s i))
+            (dict-set! st "i" (+ i 1))
+            (js-json-parse-string-loop st s buf)))))))
+
+(define
+  js-json-parse-number
+  (fn
+    (st)
+    (let
+      ((s (get st "s")) (i (get st "i")))
+      (let
+        ((start i))
+        (js-json-parse-number-loop st s)
+        (js-to-number (js-string-slice s start (get st "i")))))))
+
+(define
+  js-json-parse-number-loop
+  (fn
+    (st s)
+    (let
+      ((i (get st "i")))
+      (cond
+        ((>= i (len s)) nil)
+        ((or (js-is-digit? (char-at s i)) (= (char-at s i) "-") (= (char-at s i) "+") (= (char-at s i) ".") (= (char-at s i) "e") (= (char-at s i) "E"))
+          (begin
+            (dict-set! st "i" (+ i 1))
+            (js-json-parse-number-loop st s)))
+        (else nil)))))
+
+(define
+  js-json-parse-array
+  (fn
+    (st)
+    (let
+      ((result (list)))
+      (dict-set! st "i" (+ (get st "i") 1))
+      (js-json-skip-ws! st)
+      (cond
+        ((and (< (get st "i") (len (get st "s"))) (= (char-at (get st "s") (get st "i")) "]"))
+          (begin (dict-set! st "i" (+ (get st "i") 1)) result))
+        (else (begin (js-json-parse-array-loop st result) result))))))
+
+(define
+  js-json-parse-array-loop
+  (fn
+    (st result)
+    (append! result (js-json-parse-value st))
+    (js-json-skip-ws! st)
+    (let
+      ((c (char-at (get st "s") (get st "i"))))
+      (cond
+        ((= c ",")
+          (begin
+            (dict-set! st "i" (+ (get st "i") 1))
+            (js-json-skip-ws! st)
+            (js-json-parse-array-loop st result)))
+        ((= c "]") (dict-set! st "i" (+ (get st "i") 1)))
+        (else (error "JSON: expected , or ]"))))))
+
+(define
+  js-json-parse-object
+  (fn
+    (st)
+    (let
+      ((result (dict)))
+      (dict-set! st "i" (+ (get st "i") 1))
+      (js-json-skip-ws! st)
+      (cond
+        ((and (< (get st "i") (len (get st "s"))) (= (char-at (get st "s") (get st "i")) "}"))
+          (begin (dict-set! st "i" (+ (get st "i") 1)) result))
+        (else (begin (js-json-parse-object-loop st result) result))))))
+
+(define
+  js-json-parse-object-loop
+  (fn
+    (st result)
+    (js-json-skip-ws! st)
+    (let
+      ((k (js-json-parse-string st)))
+      (js-json-skip-ws! st)
+      (when
+        (not (= (char-at (get st "s") (get st "i")) ":"))
+        (error "JSON: expected :"))
+      (dict-set! st "i" (+ (get st "i") 1))
+      (let ((v (js-json-parse-value st))) (dict-set! result k v))
+      (js-json-skip-ws! st)
+      (let
+        ((c (char-at (get st "s") (get st "i"))))
+        (cond
+          ((= c ",")
+            (begin
+              (dict-set! st "i" (+ (get st "i") 1))
+              (js-json-parse-object-loop st result)))
+          ((= c "}") (dict-set! st "i" (+ (get st "i") 1)))
+          (else (error "JSON: expected , or }")))))))
+
+(define JSON {:parse js-json-parse :stringify js-json-stringify})
+
 (define
   js-promise-flush-callbacks!
   (fn
@@ -2050,4 +2301,4 @@
         (str "/" (get rx "source") "/" (get rx "flags")))
       (else js-undefined))))
 
-(define js-global {:isFinite js-global-is-finite :console console :Number Number :Math Math :Array Array :String String :NaN 0 :Infinity inf :isNaN js-global-is-nan :Object Object :undefined js-undefined})
+(define js-global {:isFinite js-global-is-finite :console console :Number Number :parseFloat parseFloat :Math Math :Array Array :String String :NaN 0 :Infinity inf :isNaN js-global-is-nan :Object Object :parseInt parseInt :JSON JSON :undefined js-undefined})
diff --git a/lib/js/test.sh b/lib/js/test.sh
index ad5a835d..ed680169 100755
--- a/lib/js/test.sh
+++ b/lib/js/test.sh
@@ -993,6 +993,28 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1904)
 (eval "(js-eval \"parseFloat('3.14')\")")
 
+;; ── Phase 11.json: JSON.stringify / JSON.parse ────────────────
+(epoch 2000)
+(eval "(js-eval \"JSON.stringify(42)\")")
+(epoch 2001)
+(eval "(js-eval \"JSON.stringify('hi')\")")
+(epoch 2002)
+(eval "(js-eval \"JSON.stringify([1,2,3])\")")
+(epoch 2003)
+(eval "(js-eval \"JSON.stringify({a:1})\")")
+(epoch 2004)
+(eval "(js-eval \"JSON.stringify(true)\")")
+(epoch 2005)
+(eval "(js-eval \"JSON.parse('42')\")")
+(epoch 2006)
+(eval "(js-eval \"JSON.parse('true')\")")
+(epoch 2007)
+(eval "(js-eval \"JSON.parse('\\\"hello\\\"')\")")
+(epoch 2008)
+(eval "(js-eval \"JSON.parse('[1,2,3]').length\")")
+(epoch 2009)
+(eval "(js-eval \"JSON.parse('{\\\"a\\\":1}').a\")")
+
 EPOCHS
 
 
@@ -1525,6 +1547,18 @@ check 1902 "parseInt('42')"                 '42'
 check 1903 "parseInt(3.7)"                  '3'
 check 1904 "parseFloat('3.14')"             '3.14'
 
+# ── Phase 11.json ──────────────────────────────────────────────
+check 2000 "stringify(42)"                  '"42"'
+check 2001 "stringify('hi')"                '"\"hi\""'
+check 2002 "stringify array"                '"[1,2,3]"'
+check 2003 "stringify object"               '"{\"a\":1}"'
+check 2004 "stringify true"                 '"true"'
+check 2005 "parse 42"                       '42'
+check 2006 "parse true"                     'true'
+check 2007 "parse string"                   '"hello"'
+check 2008 "parse array length"             '3'
+check 2009 "parse object.a"                 '1'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"
diff --git a/plans/js-on-sx.md b/plans/js-on-sx.md
index 123909c3..d576ac8b 100644
--- a/plans/js-on-sx.md
+++ b/plans/js-on-sx.md
@@ -187,6 +187,8 @@ Append-only record of completed iterations. Loop writes one line per iteration:
 
 - 2026-04-23 — **`String.fromCharCode`, `parseInt`, `parseFloat`.** `String` global with `fromCharCode` (variadic, loops through args and concatenates via `js-code-to-char`). `parseInt` truncates toward zero via `js-math-trunc`; `parseFloat` delegates to `js-to-number`. Wired into `js-global`. 5 new tests, **381/383** (376→+5). Conformance unchanged.
 
+- 2026-04-23 — **JSON.stringify + JSON.parse.** Shipped a recursive-descent parser and serializer in SX. `js-json-stringify` dispatches on `type-of` for primitives, lists, dicts. `js-json-parse` uses a state dict `{:s src :i idx}` mutated in-place by helpers (`js-json-skip-ws!`, `js-json-parse-value`, `-string`, `-number`, `-array`, `-object`). String parser handles `\n \t \r \" \\ \/` escapes. Number parser collects digits/signs/e+E/. then delegates to `js-to-number`. Array and object loops recursively call parse-value. JSON wired into `js-global`. 10 new tests (stringify primitives/arrays/objects, parse primitives/string/array/object), **391/393** (381→+10). Conformance unchanged.
+
 ## Phase 3-5 gotchas
 
 Worth remembering for later phases: