diff --git a/host/chez/atoms.ss b/host/chez/atoms.ss
index 816d6de..da634fe 100644
--- a/host/chez/atoms.ss
+++ b/host/chez/atoms.ss
@@ -11,11 +11,36 @@
 ;; self-sufficient for atoms without the full prelude (the overlay versions, when
 ;; the full prelude loads, override these but compose the same native kernel).
 
-(define-record-type jolt-atom (fields (mutable val)) (nongenerative jolt-atom-v1))
+;; watches is an alist of (key . watch-fn); validator is a jolt fn or jolt-nil.
+;; The overlay's add-watch/set-validator! drive these via jolt.host/ref-put! on a
+;; Janet table, which a Chez atom record is not — so the peripheral ops + the
+;; notify/validate behaviour live natively here, and post-prelude.ss re-asserts
+;; them over the overlay's def-var! (jolt-mn9o).
+(define-record-type jolt-atom
+  (fields (mutable val) (mutable watches) (mutable validator))
+  (nongenerative jolt-atom-v2))
 
-;; (atom init) — extra :meta/:validator opts are accepted and ignored for now
-;; (watches/validators are overlay features layered via jolt.host/ref-put!).
-(define (jolt-atom-new v . _opts) (make-jolt-atom v))
+;; (atom init) / (atom init :validator f :meta m): scan the trailing keyword opts
+;; for :validator (the only one with runtime behaviour; :meta is accepted/ignored).
+(define (jolt-atom-new v . opts)
+  (let loop ((o opts) (validator jolt-nil))
+    (cond
+      ((or (null? o) (null? (cdr o))) (make-jolt-atom v '() validator))
+      ((and (keyword-t? (car o)) (string=? (keyword-t-name (car o)) "validator"))
+       (loop (cddr o) (cadr o)))
+      (else (loop (cddr o) validator)))))
+
+;; validate a candidate value: a non-nil validator that returns falsey rejects.
+(define (jolt-atom-validate a v)
+  (let ((vf (jolt-atom-validator a)))
+    (when (and (not (jolt-nil? vf)) (jolt-not (jolt-invoke vf v)))
+      (error #f "Invalid reference state"))))
+
+;; notify each watch (k ref old new), in insertion order (alist is reverse-built,
+;; so walk it reversed to match add order — matches the seed's :pairs iteration).
+(define (jolt-atom-notify a old new)
+  (for-each (lambda (kv) (jolt-invoke (cdr kv) (car kv) a old new))
+            (reverse (jolt-atom-watches a))))
 
 ;; deref reads an atom; it also unwraps a `reduced` (Clojure @(reduced x) => x,
 ;; which the overlay's `unreduced` relies on). The reduced record is in seq.ss.
@@ -25,31 +50,61 @@
     ((jolt-reduced? x) (jolt-reduced-val x))
     (else (error #f "deref: unsupported reference type" x))))
 
-;; (swap! a f arg*) -> (reset! a (f @a arg*)); f is invoked through jolt-invoke
-;; (a jolt fn value, keyword, or invokable collection).
+;; (swap! a f arg*) -> (reset! a (f @a arg*)); f is invoked through jolt-invoke.
+;; Validate the new value BEFORE storing, notify watches AFTER (the seed order).
 (define (jolt-swap! a f . args)
-  (let ((nv (apply jolt-invoke f (jolt-atom-val a) args)))
+  (let* ((old (jolt-atom-val a))
+         (nv (apply jolt-invoke f old args)))
+    (jolt-atom-validate a nv)
     (jolt-atom-val-set! a nv)
+    (jolt-atom-notify a old nv)
     nv))
 
-(define (jolt-reset! a v) (jolt-atom-val-set! a v) v)
+(define (jolt-reset! a v)
+  (let ((old (jolt-atom-val a)))
+    (jolt-atom-validate a v)
+    (jolt-atom-val-set! a v)
+    (jolt-atom-notify a old v)
+    v))
 
 (define (jolt-compare-and-set! a oldv newv)
   (if (jolt= (jolt-atom-val a) oldv)
-      (begin (jolt-atom-val-set! a newv) #t)
+      (begin (jolt-reset! a newv) #t)
       #f))
 
 (define (jolt-swap-vals! a f . args)
   (let* ((old (jolt-atom-val a))
          (nv (apply jolt-invoke f old args)))
-    (jolt-atom-val-set! a nv)
+    (jolt-reset! a nv)
     (jolt-vector old nv)))
 
 (define (jolt-reset-vals! a v)
   (let ((old (jolt-atom-val a)))
-    (jolt-atom-val-set! a v)
+    (jolt-reset! a v)
     (jolt-vector old v)))
 
+;; --- watches / validators (jolt-mn9o) ---------------------------------------
+;; add-watch interns (key . fn) (replacing any existing key, keeping order);
+;; remove-watch drops it; both return the atom. set-validator! installs a
+;; validator and validates the CURRENT value immediately (Clojure throws if it's
+;; already invalid); get-validator reads the slot.
+(define (jolt-add-watch a key f)
+  (jolt-atom-watches-set! a
+    (cons (cons key f)
+          (remp (lambda (kv) (jolt=2 (car kv) key)) (jolt-atom-watches a))))
+  a)
+(define (jolt-remove-watch a key)
+  (jolt-atom-watches-set! a
+    (remp (lambda (kv) (jolt=2 (car kv) key)) (jolt-atom-watches a)))
+  a)
+(define (jolt-set-validator! a f)
+  (let ((vf (if (jolt-nil? f) jolt-nil f)))
+    (when (and (not (jolt-nil? vf)) (jolt-not (jolt-invoke vf (jolt-atom-val a))))
+      (error #f "Invalid reference state"))
+    (jolt-atom-validator-set! a vf)
+    jolt-nil))
+(define (jolt-get-validator a) (jolt-atom-validator a))
+
 (def-var! "clojure.core" "atom" jolt-atom-new)
 (def-var! "clojure.core" "deref" jolt-deref)
 (def-var! "clojure.core" "swap!" jolt-swap!)
@@ -58,3 +113,9 @@
 (def-var! "clojure.core" "swap-vals!" jolt-swap-vals!)
 (def-var! "clojure.core" "reset-vals!" jolt-reset-vals!)
 (def-var! "clojure.core" "atom?" jolt-atom?)
+;; peripheral ops: the overlay (20-coll) re-defs these over jolt.host/ref-put!,
+;; which fails on a Chez atom record — post-prelude.ss re-asserts the natives.
+(def-var! "clojure.core" "add-watch" jolt-add-watch)
+(def-var! "clojure.core" "remove-watch" jolt-remove-watch)
+(def-var! "clojure.core" "set-validator!" jolt-set-validator!)
+(def-var! "clojure.core" "get-validator" jolt-get-validator)
diff --git a/host/chez/post-prelude.ss b/host/chez/post-prelude.ss
index 387895d..4f42e6e 100644
--- a/host/chez/post-prelude.ss
+++ b/host/chez/post-prelude.ss
@@ -13,6 +13,13 @@
 ;; override.)
 (def-var! "clojure.core" "char?" jolt-char-pred?)
 (def-var! "clojure.core" "atom?" jolt-atom?)
+;; atom watches/validators: the overlay drives these via jolt.host/ref-put! on a
+;; Janet table (get a :watches), which a Chez atom record is not — re-assert the
+;; native versions (defined in atoms.ss), and swap!/reset! notify+validate there.
+(def-var! "clojure.core" "add-watch" jolt-add-watch)
+(def-var! "clojure.core" "remove-watch" jolt-remove-watch)
+(def-var! "clojure.core" "set-validator!" jolt-set-validator!)
+(def-var! "clojure.core" "get-validator" jolt-get-validator)
 ;; volatiles: a Chez volatile is a jvol record, but the overlay vreset!/vswap!/
 ;; volatile? drive it via jolt.host/ref-put!+get / :jolt/type (tagged-table only).
 ;; Override with the native versions (defined in natives-xform.ss).
diff --git a/test/chez/_atomwatch.janet b/test/chez/_atomwatch.janet
new file mode 100644
index 0000000..45a4ddc
--- /dev/null
+++ b/test/chez/_atomwatch.janet
@@ -0,0 +1,51 @@
+# jolt-mn9o — atom watches + validators on Chez. The Chez atom record carries
+# watches (alist) + validator slots; swap!/reset! validate-then-set-then-notify;
+# add-watch/remove-watch/set-validator!/get-validator are native (post-prelude.ss
+# re-asserts them over the overlay's ref-put!-based versions). Oracle = build/jolt.
+#
+#   janet test/chez/_atomwatch.janet
+(def jolt-bin (or (os/getenv "JOLT_BIN") "bin/jolt-chez"))
+
+# [expr throws?] — when throws? the case is expected to exit non-zero on both
+# (the corpus :throws contract; the exception message text may differ).
+(def cases
+  [["(let [a (atom 0) seen (atom 0)] (add-watch a :k (fn [k r o n] (reset! seen 1))) (reset! a 5) @seen)" false]
+   ["(let [a (atom 0) seen (atom 0)] (add-watch a :k (fn [k r o n] (swap! seen inc))) (remove-watch a :k) (reset! a 5) @seen)" false]
+   ["(let [a (atom 0) log (atom [])] (add-watch a :k (fn [k r o n] (swap! log conj [o n]))) (reset! a 1) (reset! a 2) @log)" false]
+   ["(let [a (atom 0)] (set-validator! a number?) (reset! a 5) @a)" false]
+   ["(let [a (atom 5)] (set-validator! a pos?) (swap! a inc) @a)" false]
+   ["(let [a (atom 0)] (set-validator! a number?) (fn? (get-validator a)))" false]
+   ["(let [a (atom 0)] (nil? (get-validator a)))" false]
+   ["(let [a (atom 0)] (set-validator! a pos?) (reset! a -1))" true]
+   ["(let [a (atom 5)] (set-validator! a pos?) (swap! a (fn [_] -1)) @a)" true]
+   ["(let [a (atom 0) c (atom 0)] (add-watch a :k (fn [k r o n] (swap! c inc))) (swap! a inc) (swap! a inc) @c)" false]])
+
+(defn run-capture [bin expr]
+  (def proc (os/spawn [bin "-e" expr] :p {:out :pipe :err :pipe}))
+  (def out (ev/read (proc :out) 0x100000))
+  (def err (ev/read (proc :err) 0x100000))
+  (def code (os/proc-wait proc))
+  (def lines (filter (fn [l] (not (empty? l)))
+                     (string/split "\n" (string/trim (if out (string out) "")))))
+  [code (if (empty? lines) "" (last lines)) (string/trim (if err (string err) ""))])
+
+(var pass 0)
+(def fails @[])
+(each [expr throws?] cases
+  (def [ocode oracle _] (run-capture "build/jolt" expr))
+  (def [code got err] (run-capture jolt-bin expr))
+  (cond
+    throws?
+      (if (and (not= ocode 0) (not= code 0)) (++ pass)
+        (array/push fails [expr (string "expected both throw; oracle exit " ocode ", chez exit " code)]))
+    (not= ocode 0) (array/push fails [expr (string "ORACLE FAILED exit " ocode)])
+    (not= code 0) (array/push fails [expr (string "exit " code "; err: " err)])
+    (= got oracle) (++ pass)
+    (array/push fails [expr (string "want `" oracle "`, got `" got "`")])))
+
+(printf "\n_atomwatch parity [%s]: %d/%d passed" jolt-bin pass (length cases))
+(when (> (length fails) 0)
+  (printf "%d FAIL(s):" (length fails))
+  (each [e m] fails (printf "  FAIL %s\n    %s" e m)))
+(flush)
+(os/exit (if (empty? fails) 0 1))
diff --git a/test/chez/run-corpus-prelude.janet b/test/chez/run-corpus-prelude.janet
index 69ec7ca..8fb30be 100644
--- a/test/chez/run-corpus-prelude.janet
+++ b/test/chez/run-corpus-prelude.janet
@@ -240,8 +240,12 @@
 # map member dispatch, and the seed's universal object-methods getMessage/getCause/
 # toString/hashCode/equals. Also added getMessage/getLocalizedMessage/equals to the
 # string method surface so a thrown string / Exception. ctor answers .getMessage) 2150.
+# jolt-mn9o (atom watches/validators — the Chez atom record carries watches/
+# validator slots; swap!/reset! validate-then-set-then-notify in seed order;
+# add-watch/remove-watch/set-validator!/get-validator are native and re-asserted
+# in post-prelude.ss over the overlay's ref-put!-on-a-Janet-table versions) 2154.
 # Strided runs scale down.
-(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_PRELUDE_FLOOR") "2150")))
+(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_PRELUDE_FLOOR") "2154")))
 (def floor (if (os/getenv "JOLT_CORPUS_LIMIT") 0 base-floor))
 (when (or (> (length diverged) 0) (< pass floor))
   (printf "REGRESSION: pass %d < floor %d or %d new divergence(s)" pass floor (length diverged)))