diff --git a/lib/dream/conformance.sh b/lib/dream/conformance.sh
index 73c7b042..a66cfc35 100644
--- a/lib/dream/conformance.sh
+++ b/lib/dream/conformance.sh
@@ -32,6 +32,7 @@ MODULES=(
   "lib/dream/static.sx"
   "lib/dream/error.sx"
   "lib/dream/cors.sx"
+  "lib/dream/json.sx"
   "lib/dream/run.sx"
   "lib/dream/demos/hello.sx"
   "lib/dream/demos/counter.sx"
@@ -51,6 +52,7 @@ SUITES=(
   "static      dream-st-tests-run!  lib/dream/tests/static.sx"
   "error       dream-er-tests-run!  lib/dream/tests/error.sx"
   "cors        dream-co-tests-run!  lib/dream/tests/cors.sx"
+  "json        dream-js-tests-run!  lib/dream/tests/json.sx"
   "run         dream-rn-tests-run!  lib/dream/tests/run.sx"
   "demos       dream-dm-tests-run!  lib/dream/tests/demos.sx"
 )
diff --git a/lib/dream/json.sx b/lib/dream/json.sx
new file mode 100644
index 00000000..92d6cadc
--- /dev/null
+++ b/lib/dream/json.sx
@@ -0,0 +1,183 @@
+;; lib/dream/json.sx — Dream-on-SX JSON encode/parse (pure SX).
+;; The host JSON primitives live in the ocaml-on-sx runtime, not the base env, so
+;; Dream ships its own. Depends on types.sx. (number? is unreliable in this env —
+;; type-of "number" is used instead.)
+
+;; ── encoding ───────────────────────────────────────────────────────
+(define
+  dr/json-escape
+  (fn
+    (s)
+    (replace
+      (replace
+        (replace (replace (replace s "\\" "\\\\") "\"" "\\\"") "\n" "\\n")
+        "\r"
+        "\\r")
+      "\t"
+      "\\t")))
+(define dr/json-quote (fn (s) (str "\"" (dr/json-escape s) "\"")))
+
+(define
+  dream-json-encode
+  (fn
+    (v)
+    (cond
+      ((nil? v) "null")
+      ((boolean? v) (if v "true" "false"))
+      ((= (type-of v) "number") (str v))
+      ((string? v) (dr/json-quote v))
+      ((list? v) (str "[" (join "," (map dream-json-encode v)) "]"))
+      ((dict? v)
+        (str
+          "{"
+          (join
+            ","
+            (map
+              (fn
+                (k)
+                (str (dr/json-quote k) ":" (dream-json-encode (get v k))))
+              (keys v)))
+          "}"))
+      (else (dr/json-quote (str v))))))
+
+;; ── parsing (recursive descent; returns {:val :pos}) ───────────────
+(define
+  dr/json-space?
+  (fn (c) (or (= c " ") (= c "\n") (= c "\r") (= c "\t"))))
+(define
+  dr/json-ws
+  (fn
+    (s i)
+    (if
+      (and (< i (string-length s)) (dr/json-space? (char-at s i)))
+      (dr/json-ws s (+ i 1))
+      i)))
+
+(define
+  dr/json-digit?
+  (fn
+    (c)
+    (let ((n (char-code c))) (and (>= n 48) (<= n 57)))))
+(define
+  dr/json-num-char?
+  (fn
+    (c)
+    (or
+      (dr/json-digit? c)
+      (= c "-")
+      (= c "+")
+      (= c ".")
+      (= c "e")
+      (= c "E"))))
+(define
+  dr/json-num-end
+  (fn
+    (s i)
+    (if
+      (and (< i (string-length s)) (dr/json-num-char? (char-at s i)))
+      (dr/json-num-end s (+ i 1))
+      i)))
+(define
+  dr/json-to-number
+  (fn
+    (str-val)
+    (if
+      (or
+        (contains? str-val ".")
+        (contains? str-val "e")
+        (contains? str-val "E"))
+      (parse-float str-val)
+      (parse-int str-val))))
+
+(define
+  dr/json-str
+  (fn
+    (s i acc)
+    (let
+      ((c (char-at s i)))
+      (cond
+        ((= c "\"") {:val acc :pos (+ i 1)})
+        ((= c "\\")
+          (let
+            ((e (char-at s (+ i 1))))
+            (cond
+              ((= e "n") (dr/json-str s (+ i 2) (str acc "\n")))
+              ((= e "r") (dr/json-str s (+ i 2) (str acc "\r")))
+              ((= e "t") (dr/json-str s (+ i 2) (str acc "\t")))
+              (else (dr/json-str s (+ i 2) (str acc e))))))
+        (else (dr/json-str s (+ i 1) (str acc c)))))))
+
+(define
+  dr/json-num
+  (fn (s i) (let ((j (dr/json-num-end s i))) {:val (dr/json-to-number (substr s i (- j i))) :pos j})))
+
+(define
+  dr/json-arr
+  (fn
+    (s i acc)
+    (let
+      ((i (dr/json-ws s i)))
+      (if
+        (= (char-at s i) "]")
+        {:val acc :pos (+ i 1)}
+        (let
+          ((r (dr/json-val s i)))
+          (let
+            ((i2 (dr/json-ws s (get r :pos))))
+            (if
+              (= (char-at s i2) ",")
+              (dr/json-arr
+                s
+                (+ i2 1)
+                (concat acc (list (get r :val))))
+              {:val (concat acc (list (get r :val))) :pos (+ i2 1)})))))))
+
+(define
+  dr/json-obj
+  (fn
+    (s i acc)
+    (let
+      ((i (dr/json-ws s i)))
+      (if
+        (= (char-at s i) "}")
+        {:val acc :pos (+ i 1)}
+        (let
+          ((kr (dr/json-str s (+ i 1) "")))
+          (let
+            ((i2 (dr/json-ws s (get kr :pos))))
+            (let
+              ((vr (dr/json-val s (+ i2 1))))
+              (let
+                ((i3 (dr/json-ws s (get vr :pos))))
+                (if
+                  (= (char-at s i3) ",")
+                  (dr/json-obj
+                    s
+                    (+ i3 1)
+                    (assoc acc (get kr :val) (get vr :val)))
+                  {:val (assoc acc (get kr :val) (get vr :val)) :pos (+ i3 1)})))))))))
+
+(define
+  dr/json-val
+  (fn
+    (s i)
+    (let
+      ((i (dr/json-ws s i)))
+      (let
+        ((c (char-at s i)))
+        (cond
+          ((= c "{") (dr/json-obj s (+ i 1) {}))
+          ((= c "[") (dr/json-arr s (+ i 1) (list)))
+          ((= c "\"") (dr/json-str s (+ i 1) ""))
+          ((= c "t") {:val true :pos (+ i 4)})
+          ((= c "f") {:val false :pos (+ i 5)})
+          ((= c "n") {:val nil :pos (+ i 4)})
+          (else (dr/json-num s i)))))))
+
+(define dream-json-parse (fn (s) (get (dr/json-val s 0) :val)))
+
+;; ── responses ──────────────────────────────────────────────────────
+;; encode a value into a JSON response (dream-json takes a raw string body)
+(define dream-json-value (fn (v) (dream-json (dream-json-encode v))))
+;; read + parse the request body as JSON
+(define dream-json-body (fn (req) (dream-json-parse (dream-body req))))
diff --git a/lib/dream/tests/json.sx b/lib/dream/tests/json.sx
new file mode 100644
index 00000000..6deca84e
--- /dev/null
+++ b/lib/dream/tests/json.sx
@@ -0,0 +1,105 @@
+;; lib/dream/tests/json.sx — JSON encode/parse round-trips.
+
+(define dream-js-pass 0)
+(define dream-js-fail 0)
+(define dream-js-fails (list))
+
+(define
+  dream-js-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! dream-js-pass (+ dream-js-pass 1))
+      (begin
+        (set! dream-js-fail (+ dream-js-fail 1))
+        (append! dream-js-fails {:name name :actual actual :expected expected})))))
+
+;; ── encoding scalars ───────────────────────────────────────────────
+(dream-js-test "encode int" (dream-json-encode 42) "42")
+(dream-js-test "encode float" (dream-json-encode 1.5) "1.5")
+(dream-js-test "encode true" (dream-json-encode true) "true")
+(dream-js-test "encode false" (dream-json-encode false) "false")
+(dream-js-test "encode nil" (dream-json-encode nil) "null")
+(dream-js-test "encode string" (dream-json-encode "hi") "\"hi\"")
+(dream-js-test
+  "encode string escapes quote"
+  (dream-json-encode "a\"b")
+  "\"a\\\"b\"")
+(dream-js-test
+  "encode list"
+  (dream-json-encode (list 1 2 3))
+  "[1,2,3]")
+(dream-js-test
+  "encode list of strings"
+  (dream-json-encode (list "a" "b"))
+  "[\"a\",\"b\"]")
+(dream-js-test
+  "encode single-key dict"
+  (dream-json-encode {:a 1})
+  "{\"a\":1}")
+(dream-js-test "encode empty list" (dream-json-encode (list)) "[]")
+(dream-js-test "encode empty dict" (dream-json-encode {}) "{}")
+
+;; ── parsing scalars ────────────────────────────────────────────────
+(dream-js-test "parse int" (dream-json-parse "5") 5)
+(dream-js-test "parse negative" (dream-json-parse "-7") -7)
+(dream-js-test "parse float" (dream-json-parse "1.5") 1.5)
+(dream-js-test "parse true" (dream-json-parse "true") true)
+(dream-js-test "parse false" (dream-json-parse "false") false)
+(dream-js-test "parse null" (dream-json-parse "null") nil)
+(dream-js-test "parse string" (dream-json-parse "\"hello\"") "hello")
+(dream-js-test "parse string escape" (dream-json-parse "\"a\\nb\"") "a\nb")
+(dream-js-test
+  "parse array"
+  (dream-json-parse "[1,2,3]")
+  (list 1 2 3))
+(dream-js-test "parse empty array" (dream-json-parse "[]") (list))
+(dream-js-test
+  "parse with whitespace"
+  (dream-json-parse "  [ 1 , 2 ]  ")
+  (list 1 2))
+
+;; ── parsing objects ────────────────────────────────────────────────
+(define dream-js-obj (dream-json-parse "{\"x\":5,\"y\":\"hi\"}"))
+(dream-js-test "parse obj number" (get dream-js-obj "x") 5)
+(dream-js-test "parse obj string" (get dream-js-obj "y") "hi")
+(dream-js-test "parse empty obj" (dream-json-parse "{}") {})
+
+;; ── nested ─────────────────────────────────────────────────────────
+(define dream-js-nested (dream-json-parse "{\"a\":[1,{\"b\":2}],\"c\":true}"))
+(dream-js-test
+  "nested array first"
+  (first (get dream-js-nested "a"))
+  1)
+(dream-js-test
+  "nested object in array"
+  (get (nth (get dream-js-nested "a") 1) "b")
+  2)
+(dream-js-test "nested bool" (get dream-js-nested "c") true)
+
+;; ── round-trips ────────────────────────────────────────────────────
+(define dream-js-v {:name "Ada" :age 36 :tags (list "math" "engine")})
+(define dream-js-rt (dream-json-parse (dream-json-encode dream-js-v)))
+(dream-js-test "roundtrip name" (get dream-js-rt "name") "Ada")
+(dream-js-test "roundtrip age" (get dream-js-rt "age") 36)
+(dream-js-test
+  "roundtrip tags"
+  (get dream-js-rt "tags")
+  (list "math" "engine"))
+
+;; ── response + request helpers ─────────────────────────────────────
+(dream-js-test
+  "json-value content-type"
+  (dream-resp-header (dream-json-value {:ok true}) "content-type")
+  "application/json")
+(dream-js-test
+  "json-value body"
+  (dream-resp-body (dream-json-value {:ok true}))
+  "{\"ok\":true}")
+(dream-js-test
+  "json-body parses request"
+  (get (dream-json-body (dream-request "POST" "/" {} "{\"n\":9}")) "n")
+  9)
+
+(define dream-js-tests-run! (fn () {:total (+ dream-js-pass dream-js-fail) :passed dream-js-pass :failed dream-js-fail :fails dream-js-fails}))
diff --git a/plans/dream-on-sx.md b/plans/dream-on-sx.md
index b8d54a6d..2184f11a 100644
--- a/plans/dream-on-sx.md
+++ b/plans/dream-on-sx.md
@@ -104,6 +104,14 @@ with extensions + hardening below.
   short-circuits preflight `OPTIONS` with a 204 carrying Allow-Methods/Headers/Max-Age.
   `dream-cors-origin` for a specific origin, `dream-cors-with opts` for full control
   (origin/methods/headers/credentials/max-age). Composes around a router.
+- **2026-06-07 — Ext: JSON** (`lib/dream/json.sx`, 35 tests, 329 total). Host JSON
+  primitives live in the ocaml-on-sx runtime (not the base env), so Dream ships its own
+  pure-SX `dream-json-encode` (scalars/list/dict, string escaping) + `dream-json-parse`
+  (recursive-descent over chars, objects/arrays/strings/numbers/true/false/null,
+  whitespace-tolerant). `dream-json-value` (encode → application/json response) and
+  `dream-json-body` (parse request body). GOTCHA: `number?` is unreliable in this env —
+  used `(= (type-of v) "number")`; `parse-float` handles decimals. Multi-key dict
+  encode order follows `keys` (non-deterministic) so tests assert via parse round-trip.
 
 ## Extensions (post-roadmap)
 
@@ -115,7 +123,7 @@ The five-types core is complete; these harden it toward a production HTTP front
 - [x] **CORS middleware** (`dream-cors`).
 - [x] **Error-handling middleware** (`dream-catch` / custom 500 templates; `guard`-based).
 - [ ] **Signed session cookies** (the noted hardening — sign the sid).
-- [ ] **JSON helpers** (build from dict; parse to dict).
+- [x] **JSON helpers** (encode + recursive-descent parse, pure SX).
 - [ ] **Query/header convenience** (`dream-queries`, defaults).
 - [ ] **`api.sx` facade + README** — single load point listing the public surface.