diff --git a/lib/persist/conformance.sh b/lib/persist/conformance.sh
index 1ce9c1e3..50c502c8 100755
--- a/lib/persist/conformance.sh
+++ b/lib/persist/conformance.sh
@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view recovery)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas recovery)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
diff --git a/lib/persist/kv.sx b/lib/persist/kv.sx
index 50d57167..e85483cf 100644
--- a/lib/persist/kv.sx
+++ b/lib/persist/kv.sx
@@ -24,3 +24,21 @@
   (fn
     (b key dflt f)
     (persist/kv-put b key (f (persist/kv-get-or b key dflt)))))
+
+; compare-and-swap: set key to new ONLY if its current value equals expected.
+; Returns new on success, or a conflict value {:conflict true :expected :actual}
+; the caller can re-read and retry on. The kv analogue of log append-expect.
+(define
+  persist/kv-cas
+  (fn
+    (b key expected new)
+    (let
+      ((actual (persist/kv-get b key)))
+      (if (equal? actual expected) (persist/kv-put b key new) {:actual actual :expected expected :conflict true}))))
+
+; create-only: put a value only if the key is absent; conflict if it exists
+(define
+  persist/kv-put-new
+  (fn
+    (b key val)
+    (if (persist/kv-has? b key) {:actual (persist/kv-get b key) :conflict true :reason "exists"} (persist/kv-put b key val))))
diff --git a/lib/persist/scoreboard.json b/lib/persist/scoreboard.json
index 4c3650f0..25e612b6 100644
--- a/lib/persist/scoreboard.json
+++ b/lib/persist/scoreboard.json
@@ -11,9 +11,10 @@
     "durable": {"pass": 15, "fail": 0},
     "blob": {"pass": 14, "fail": 0},
     "view": {"pass": 11, "fail": 0},
+    "cas": {"pass": 11, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 122,
+  "total_pass": 133,
   "total_fail": 0,
-  "total": 122
+  "total": 133
 }
diff --git a/lib/persist/scoreboard.md b/lib/persist/scoreboard.md
index 41ad6a35..b4bfb284 100644
--- a/lib/persist/scoreboard.md
+++ b/lib/persist/scoreboard.md
@@ -15,5 +15,6 @@ _Generated by `lib/persist/conformance.sh`_
 | durable | 15 | 0 | 15 |
 | blob | 14 | 0 | 14 |
 | view | 11 | 0 | 11 |
+| cas | 11 | 0 | 11 |
 | recovery | 6 | 0 | 6 |
-| **Total** | **122** | **0** | **122** |
+| **Total** | **133** | **0** | **133** |
diff --git a/lib/persist/tests/cas.sx b/lib/persist/tests/cas.sx
new file mode 100644
index 00000000..130584ea
--- /dev/null
+++ b/lib/persist/tests/cas.sx
@@ -0,0 +1,96 @@
+; Extension — kv compare-and-swap: atomic current-state updates. Uses
+; persist/conflict? from concurrency.sx.
+
+(persist-test
+  "cas on absent key with nil expected succeeds"
+  (let ((b (persist/open))) (persist/kv-cas b "k" nil 1))
+  1)
+(persist-test
+  "cas with matching expected succeeds"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "k" 5)
+      (persist/kv-cas b "k" 5 6)
+      (persist/kv-get b "k")))
+  6)
+(persist-test
+  "cas with stale expected returns a conflict"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "k" 5)
+      (persist/conflict? (persist/kv-cas b "k" 4 6))))
+  true)
+(persist-test
+  "a conflicting cas does not write"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "k" 5)
+      (persist/kv-cas b "k" 4 6)
+      (persist/kv-get b "k")))
+  5)
+(persist-test
+  "cas conflict carries expected and actual"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "k" 5)
+      (let
+        ((r (persist/kv-cas b "k" 4 6)))
+        (list (persist/conflict-expected r) (persist/conflict-actual r)))))
+  (list 4 5))
+(persist-test
+  "two cas racers: first wins, second conflicts"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "stock" 10)
+      (persist/kv-cas b "stock" 10 9)
+      (persist/conflict? (persist/kv-cas b "stock" 10 9))))
+  true)
+(persist-test
+  "retry after cas conflict succeeds"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "stock" 10)
+      (persist/kv-cas b "stock" 10 9)
+      (let
+        ((r (persist/kv-cas b "stock" 10 9)))
+        (if
+          (persist/conflict? r)
+          (persist/kv-cas b "stock" (persist/conflict-actual r) 8)
+          r))))
+  8)
+(persist-test
+  "put-new on absent key succeeds"
+  (let ((b (persist/open))) (persist/kv-put-new b "k" 1))
+  1)
+(persist-test
+  "put-new on existing key conflicts"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "k" 1)
+      (persist/conflict? (persist/kv-put-new b "k" 2))))
+  true)
+(persist-test
+  "put-new does not overwrite"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/kv-put b "k" 1)
+      (persist/kv-put-new b "k" 2)
+      (persist/kv-get b "k")))
+  1)
+(persist-test
+  "cas works on the durable backend"
+  (let
+    ((db (persist/mock-durable (persist/mem-backend))))
+    (begin
+      (persist/kv-put db "k" 1)
+      (persist/kv-cas db "k" 1 2)
+      (persist/kv-get db "k")))
+  2)
diff --git a/plans/persist-on-sx.md b/plans/persist-on-sx.md
index 8ebb7427..13a288a8 100644
--- a/plans/persist-on-sx.md
+++ b/plans/persist-on-sx.md
@@ -42,7 +42,7 @@ read models (feeds, indices, audit logs) update incrementally.
 
 ## Status (rolling)
 
-`bash lib/persist/conformance.sh` → **122/122** (Phases 1–4 complete + extensions)
+`bash lib/persist/conformance.sh` → **133/133** (Phases 1–4 complete + extensions)
 
 ## Ground rules
 
@@ -150,11 +150,20 @@ over an in-process disk (the mock-IO harness).
   O(1) read. The consumer-facing read-model abstraction (feed indices, audit
   rollups, search counters).
 
+- [x] `kv.sx` CAS — `persist/kv-cas` (compare-and-swap) + `persist/kv-put-new`
+  (create-only): atomic current-state updates, conflict as a real value (kv
+  analogue of log `append-expect`). For sessions, acl grants, stock counts.
+
 ## Consumers (post-foundation, not in scope here)
 feed/-log, flow store, mod/audit, search index, acl grants, identity sessions all
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
 ## Progress log
+- **Ext: kv compare-and-swap (133/133).** `persist/kv-cas` sets a key only if
+  its current value equals expected, else returns `{:conflict :expected
+  :actual}`; `persist/kv-put-new` is create-only. The kv analogue of log
+  `append-expect` — atomic current-state for sessions/acl/stock. 11 tests incl.
+  racer + retry + durable backend.
 - **Ext: materialized views (122/122).** `view.sx` — `persist/view` bundles
   stream + step + seed + snapshot name; `view-attach` subscribes it to a hub so
   every publish refreshes the snapshot incrementally; `view-peek` is then an