diff --git a/lib/apl/runtime.sx b/lib/apl/runtime.sx
index 7f6ba879..cff8957d 100644
--- a/lib/apl/runtime.sx
+++ b/lib/apl/runtime.sx
@@ -1108,3 +1108,47 @@
                   (make-array
                     (append frame-shape (get (first results) :shape))
                     (flatten (map (fn (r) (get r :ravel)) results))))))))))))
+
+(define
+  apl-at-replace
+  (fn
+    (vals idxs arr)
+    (let
+      ((vals-ravel (get vals :ravel))
+        (idxs-ravel (get idxs :ravel))
+        (arr-ravel (get arr :ravel))
+        (arr-shape (get arr :shape))
+        (vals-scalar? (= (len (get vals :shape)) 0)))
+      (make-array
+        arr-shape
+        (map
+          (fn
+            (i)
+            (let
+              ((pos (index-of idxs-ravel (+ i apl-io))))
+              (if
+                pos
+                (if vals-scalar? (first vals-ravel) (nth vals-ravel pos))
+                (nth arr-ravel i))))
+          (range 0 (len arr-ravel)))))))
+
+(define
+  apl-at-apply
+  (fn
+    (f idxs arr)
+    (let
+      ((idxs-ravel (get idxs :ravel))
+        (arr-ravel (get arr :ravel))
+        (arr-shape (get arr :shape)))
+      (make-array
+        arr-shape
+        (map
+          (fn
+            (i)
+            (let
+              ((pos (index-of idxs-ravel (+ i apl-io))))
+              (if
+                pos
+                (disclose (f (apl-scalar (nth arr-ravel i))))
+                (nth arr-ravel i))))
+          (range 0 (len arr-ravel)))))))
diff --git a/lib/apl/tests/operators.sx b/lib/apl/tests/operators.sx
index 39115a70..afd21895 100644
--- a/lib/apl/tests/operators.sx
+++ b/lib/apl/tests/operators.sx
@@ -698,4 +698,94 @@
       apl-tally
       1
       (make-array (list 3 4) (list 1 2 3 4 5 6 7 8 9 10 11 12))))
-  (list 4 4 4))
\ No newline at end of file
+  (list 4 4 4))
+
+(apl-test
+  "at-replace single index"
+  (rv
+    (apl-at-replace
+      (apl-scalar 99)
+      (make-array (list 1) (list 2))
+      (make-array (list 5) (list 1 2 3 4 5))))
+  (list 1 99 3 4 5))
+
+(apl-test
+  "at-replace multiple indices vector vals"
+  (rv
+    (apl-at-replace
+      (make-array (list 2) (list 99 88))
+      (make-array (list 2) (list 2 4))
+      (make-array (list 5) (list 1 2 3 4 5))))
+  (list 1 99 3 88 5))
+
+(apl-test
+  "at-replace scalar broadcast"
+  (rv
+    (apl-at-replace
+      (apl-scalar 0)
+      (make-array (list 3) (list 1 3 5))
+      (make-array (list 5) (list 10 20 30 40 50))))
+  (list 0 20 0 40 0))
+
+(apl-test
+  "at-replace preserves shape"
+  (sh
+    (apl-at-replace
+      (apl-scalar 99)
+      (make-array (list 1) (list 2))
+      (make-array (list 5) (list 1 2 3 4 5))))
+  (list 5))
+
+(apl-test
+  "at-replace last index"
+  (rv
+    (apl-at-replace
+      (apl-scalar 99)
+      (make-array (list 1) (list 5))
+      (make-array (list 5) (list 1 2 3 4 5))))
+  (list 1 2 3 4 99))
+
+(apl-test
+  "at-replace on matrix linear-index"
+  (rv
+    (apl-at-replace
+      (apl-scalar 99)
+      (make-array (list 1) (list 3))
+      (make-array (list 2 3) (list 1 2 3 4 5 6))))
+  (list 1 2 99 4 5 6))
+
+(apl-test
+  "at-apply negate at indices"
+  (rv
+    (apl-at-apply
+      apl-neg-m
+      (make-array (list 3) (list 1 3 5))
+      (make-array (list 5) (list 1 2 3 4 5))))
+  (list -1 2 -3 4 -5))
+
+(apl-test
+  "at-apply double at index 1"
+  (rv
+    (apl-at-apply
+      (fn (a) (apl-mul a (apl-scalar 2)))
+      (make-array (list 1) (list 1))
+      (make-array (list 2) (list 5 10))))
+  (list 10 10))
+
+(apl-test
+  "at-apply preserves shape"
+  (sh
+    (apl-at-apply
+      apl-neg-m
+      (make-array (list 2) (list 1 3))
+      (make-array (list 2 3) (list 1 2 3 4 5 6))))
+  (list 2 3))
+
+(apl-test
+  "at-apply on matrix linear-index"
+  (rv
+    (apl-at-apply
+      apl-neg-m
+      (make-array (list 2) (list 1 6))
+      (make-array (list 2 3) (list 1 2 3 4 5 6))))
+  (list -1 2 3 4 5 -6))
\ No newline at end of file
diff --git a/plans/apl-on-sx.md b/plans/apl-on-sx.md
index e22d3695..de380cf4 100644
--- a/plans/apl-on-sx.md
+++ b/plans/apl-on-sx.md
@@ -82,7 +82,7 @@ Core mapping:
 - [x] Compose `f∘g` — applies `g` first then `f`
 - [x] Power `f⍣n` — apply f n times; `f⍣≡` until fixed point
 - [x] Rank `f⍤k` — apply f at sub-rank k
-- [ ] At `@` — selective replace
+- [x] At `@` — selective replace
 - [ ] 40+ tests in `lib/apl/tests/operators.sx`
 
 ### Phase 5 — dfns + tradfns + control flow
@@ -118,6 +118,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-07: Phase 4 step 10 — at @ (apl-at-replace + apl-at-apply); linear-index lookup, scalar-vals broadcast; 211/211 tests
 - 2026-05-07: Phase 4 step 9 — rank f⍤k (apl-rank); cell decomposition + reassembly via frame/cell shapes; 201/201 tests
 - 2026-05-06: Phase 4 step 8 — power f⍣n (apl-power) + fixed-point f⍣≡ (apl-power-fixed); 191/191 tests
 - 2026-05-06: Phase 4 step 7 — compose f∘g (apl-compose monadic f∘g x, apl-compose-dyadic dyadic f x (g y)); 182/182 tests