diff --git a/lib/guest/reflective/env.sx b/lib/guest/reflective/env.sx
index b83aba28..595f80fe 100644
--- a/lib/guest/reflective/env.sx
+++ b/lib/guest/reflective/env.sx
@@ -42,6 +42,9 @@
 ;;   (refl-env-has?-with   CFG SCOPE NAME)
 ;;   (refl-env-lookup-with CFG SCOPE NAME)
 ;;   (refl-env-lookup-or-nil-with CFG SCOPE NAME)
+;;   (refl-env-find-frame-with CFG SCOPE NAME)
+;;     — returns the scope in the chain that contains NAME (or nil).
+;;       Consumers needing source-frame mutation use this.
 ;;
 ;;   (refl-canonical-cfg) — the default cfg, exposed so consumers
 ;;                          can compare or extend it.
@@ -131,6 +134,24 @@
       (:else
         (refl-env-lookup-or-nil-with cfg ((get cfg :parent-of) scope) name)))))
 
+;; Returns the SCOPE in the chain that contains NAME, or nil if no
+;; scope binds it. Consumers (e.g. Smalltalk) use this to mutate the
+;; binding at its source frame rather than introducing a new shadow
+;; binding at the current frame. Pairs with `refl-env-lookup-with`
+;; for callers that need both the value and the defining scope.
+
+(define refl-env-find-frame-with
+  (fn (cfg scope name)
+    (cond
+      ((nil? scope) nil)
+      ((not ((get cfg :env?) scope)) nil)
+      ((dict-has? ((get cfg :bindings-of) scope) name) scope)
+      (:else
+        (refl-env-find-frame-with cfg ((get cfg :parent-of) scope) name)))))
+
+(define refl-env-find-frame
+  (fn (env name) (refl-env-find-frame-with refl-canonical-cfg env name)))
+
 ;; ── Default canonical cfg ───────────────────────────────────────
 ;; Exposed so consumers can use it explicitly, compose with it, or
 ;; check adapter-correctness against the canonical implementation.
diff --git a/lib/smalltalk/compare.sh b/lib/smalltalk/compare.sh
index d28c883a..1db73b6d 100755
--- a/lib/smalltalk/compare.sh
+++ b/lib/smalltalk/compare.sh
@@ -41,6 +41,7 @@ run_sx () {
 (load "lib/smalltalk/tokenizer.sx")
 (load "lib/smalltalk/parser.sx")
 (load "lib/smalltalk/runtime.sx")
+(load "lib/guest/reflective/env.sx")
 (load "lib/smalltalk/eval.sx")
 (epoch 2)
 (eval "(begin (st-bootstrap-classes!) (smalltalk-load \"Object subclass: #B instanceVariableNames: ''! !B methodsFor: 'x'! fib: n n < 2 ifTrue: [^ n]. ^ (self fib: n - 1) + (self fib: n - 2)! !\") (smalltalk-eval-program \"^ B new fib: 22\"))")
diff --git a/lib/smalltalk/eval.sx b/lib/smalltalk/eval.sx
index 500ae5a3..9c049566 100644
--- a/lib/smalltalk/eval.sx
+++ b/lib/smalltalk/eval.sx
@@ -60,16 +60,34 @@
   st-class-ref?
   (fn (v) (and (dict? v) (has-key? v :type) (= (get v :type) "st-class"))))
 
-;; Walk the frame chain looking for a local binding.
+;; Smalltalk-side adapter for lib/guest/reflective/env.sx. The
+;; Smalltalk frame carries language-specific metadata (:self,
+;; :method-class, :return-k, :active-cell) but the parent-walk for
+;; local-binding lookup is the same algorithm Kernel and Tcl use.
+;; Third consumer of the env kit; cfg routes through :locals and
+;; :parent and uses mutable dict-set! for binding.
+(define st-frame-cfg
+  {:bindings-of (fn (f) (get f :locals))
+   :parent-of   (fn (f) (get f :parent))
+   :extend      (fn (f) (st-make-frame nil nil f nil nil))
+   :bind!       (fn (f n v)
+                  (dict-set! (get f :locals) n v) f)
+   :env?        (fn (v) (and (dict? v) (dict? (get v :locals))))})
+
+;; Walk the frame chain looking for a local binding. Returns the
+;; Smalltalk-flavoured {:found :value :frame} shape callers expect;
+;; the parent-walk delegates to refl-env-find-frame-with.
 (define
   st-lookup-local
   (fn
     (frame name)
-    (cond
-      ((= frame nil) {:found false :value nil :frame nil})
-      ((has-key? (get frame :locals) name)
-        {:found true :value (get (get frame :locals) name) :frame frame})
-      (else (st-lookup-local (get frame :parent) name)))))
+    (let ((src (refl-env-find-frame-with st-frame-cfg frame name)))
+      (cond
+        ((nil? src) {:found false :value nil :frame nil})
+        (:else
+          {:found true
+           :value (get (get src :locals) name)
+           :frame src})))))
 
 ;; Walk the frame chain looking for the frame whose self has this ivar.
 (define
diff --git a/lib/smalltalk/test.sh b/lib/smalltalk/test.sh
index ce782993..45d5b905 100755
--- a/lib/smalltalk/test.sh
+++ b/lib/smalltalk/test.sh
@@ -61,6 +61,7 @@ EPOCHS
 (epoch 3)
 (load "lib/smalltalk/runtime.sx")
 (epoch 4)
+(load "lib/guest/reflective/env.sx")
 (load "lib/smalltalk/eval.sx")
 (epoch 5)
 (load "lib/smalltalk/sunit.sx")
@@ -116,6 +117,7 @@ EPOCHS
 (epoch 3)
 (load "lib/smalltalk/runtime.sx")
 (epoch 4)
+(load "lib/guest/reflective/env.sx")
 (load "lib/smalltalk/eval.sx")
 (epoch 5)
 (load "lib/smalltalk/sunit.sx")