diff --git a/lib/common-lisp/clos.sx b/lib/common-lisp/clos.sx
new file mode 100644
index 00000000..78381ba2
--- /dev/null
+++ b/lib/common-lisp/clos.sx
@@ -0,0 +1,500 @@
+;; lib/common-lisp/clos.sx — CLOS: classes, instances, generic functions
+;;
+;; Class records: {:clos-type "class" :name "NAME" :slots {...} :parents [...] :methods [...]}
+;; Instance:      {:clos-type "instance" :class "NAME" :slots {slot: val ...}}
+;; Method:        {:qualifiers [...] :specializers [...] :fn (fn (args next-fn) ...)}
+;;
+;; SX primitive notes:
+;;   dict->list: use (map (fn (k) (list k (get d k))) (keys d))
+;;   dict-set (pure): use assoc
+;;   fn?/callable?: use callable?
+
+;; ── dict helpers ───────────────────────────────────────────────────────────
+
+(define
+  clos-dict->list
+  (fn (d) (map (fn (k) (list k (get d k))) (keys d))))
+
+;; ── class registry ─────────────────────────────────────────────────────────
+
+(define
+  clos-class-registry
+  (dict
+    "t"
+    {:parents (list) :clos-type "class" :slots (dict) :methods (list) :name "t"}
+    "null"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "null"}
+    "integer"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "integer"}
+    "float"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "float"}
+    "string"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "string"}
+    "symbol"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "symbol"}
+    "cons"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "cons"}
+    "list"
+    {:parents (list "t") :clos-type "class" :slots (dict) :methods (list) :name "list"}))
+
+;; ── clos-generic-registry ─────────────────────────────────────────────────
+
+(define clos-generic-registry (dict))
+
+;; ── class-of ──────────────────────────────────────────────────────────────
+
+(define
+  clos-class-of
+  (fn
+    (x)
+    (cond
+      ((nil? x) "null")
+      ((integer? x) "integer")
+      ((float? x) "float")
+      ((string? x) "string")
+      ((symbol? x) "symbol")
+      ((and (list? x) (> (len x) 0)) "cons")
+      ((and (list? x) (= (len x) 0)) "null")
+      ((and (dict? x) (= (get x "clos-type") "instance")) (get x "class"))
+      (:else "t"))))
+
+;; ── subclass-of? ──────────────────────────────────────────────────────────
+;;
+;; Captures clos-class-registry at define time to avoid free-variable issues.
+
+(define
+  clos-subclass-of?
+  (let
+    ((registry clos-class-registry))
+    (fn
+      (class-name super-name)
+      (if
+        (= class-name super-name)
+        true
+        (let
+          ((rec (get registry class-name)))
+          (if
+            (nil? rec)
+            false
+            (some
+              (fn (p) (clos-subclass-of? p super-name))
+              (get rec "parents"))))))))
+
+;; ── instance-of? ──────────────────────────────────────────────────────────
+
+(define
+  clos-instance-of?
+  (fn (obj class-name) (clos-subclass-of? (clos-class-of obj) class-name)))
+
+;; ── defclass ──────────────────────────────────────────────────────────────
+;;
+;; slot-specs: list of dicts with keys: name initarg initform accessor reader writer
+;; Each missing key defaults to nil.
+
+(define clos-slot-spec (fn (spec) (if (string? spec) {:initform nil :initarg nil :reader nil :writer nil :accessor nil :name spec} spec)))
+
+(define
+  clos-defclass
+  (fn
+    (name parents slot-specs)
+    (let
+      ((slots (dict)))
+      (for-each
+        (fn
+          (pname)
+          (let
+            ((prec (get clos-class-registry pname)))
+            (when
+              (not (nil? prec))
+              (for-each
+                (fn
+                  (k)
+                  (when
+                    (nil? (get slots k))
+                    (dict-set! slots k (get (get prec "slots") k))))
+                (keys (get prec "slots"))))))
+        parents)
+      (for-each
+        (fn
+          (s)
+          (let
+            ((spec (clos-slot-spec s)))
+            (dict-set! slots (get spec "name") spec)))
+        slot-specs)
+      (let
+        ((class-rec {:parents parents :clos-type "class" :slots slots :methods (list) :name name}))
+        (dict-set! clos-class-registry name class-rec)
+        (clos-install-accessors-for name slots)
+        name))))
+
+;; ── accessor installation (forward-declared, defined after defmethod) ──────
+
+(define
+  clos-install-accessors-for
+  (fn
+    (class-name slots)
+    (for-each
+      (fn
+        (k)
+        (let
+          ((spec (get slots k)))
+          (let
+            ((reader (get spec "reader")))
+            (when
+              (not (nil? reader))
+              (clos-add-reader-method reader class-name k)))
+          (let
+            ((accessor (get spec "accessor")))
+            (when
+              (not (nil? accessor))
+              (clos-add-reader-method accessor class-name k)))))
+      (keys slots))))
+
+;; placeholder — real impl filled in after defmethod is defined
+(define clos-add-reader-method (fn (method-name class-name slot-name) nil))
+
+;; ── make-instance ─────────────────────────────────────────────────────────
+
+(define
+  clos-make-instance
+  (fn
+    (class-name &rest initargs)
+    (let
+      ((class-rec (get clos-class-registry class-name)))
+      (if
+        (nil? class-rec)
+        (error (str "No class named: " class-name))
+        (let
+          ((slots (dict)))
+          (for-each
+            (fn
+              (k)
+              (let
+                ((spec (get (get class-rec "slots") k)))
+                (let
+                  ((initform (get spec "initform")))
+                  (when
+                    (not (nil? initform))
+                    (dict-set!
+                      slots
+                      k
+                      (if (callable? initform) (initform) initform))))))
+            (keys (get class-rec "slots")))
+          (define
+            apply-args
+            (fn
+              (args)
+              (when
+                (>= (len args) 2)
+                (let
+                  ((key (str (first args))) (val (first (rest args))))
+                  (let
+                    ((skey (if (= (slice key 0 1) ":") (slice key 1 (len key)) key)))
+                    (let
+                      ((matched false))
+                      (for-each
+                        (fn
+                          (sk)
+                          (let
+                            ((spec (get (get class-rec "slots") sk)))
+                            (let
+                              ((ia (get spec "initarg")))
+                              (when
+                                (or
+                                  (= ia key)
+                                  (= ia (str ":" skey))
+                                  (= sk skey))
+                                (dict-set! slots sk val)
+                                (set! matched true)))))
+                        (keys (get class-rec "slots")))))
+                  (apply-args (rest (rest args)))))))
+          (apply-args initargs)
+          {:clos-type "instance" :slots slots :class class-name})))))
+
+;; ── slot-value ────────────────────────────────────────────────────────────
+
+(define
+  clos-slot-value
+  (fn
+    (instance slot-name)
+    (if
+      (and (dict? instance) (= (get instance "clos-type") "instance"))
+      (get (get instance "slots") slot-name)
+      (error (str "Not a CLOS instance: " (inspect instance))))))
+
+(define
+  clos-set-slot-value!
+  (fn
+    (instance slot-name value)
+    (if
+      (and (dict? instance) (= (get instance "clos-type") "instance"))
+      (dict-set! (get instance "slots") slot-name value)
+      (error (str "Not a CLOS instance: " (inspect instance))))))
+
+(define
+  clos-slot-boundp
+  (fn
+    (instance slot-name)
+    (and
+      (dict? instance)
+      (= (get instance "clos-type") "instance")
+      (not (nil? (get (get instance "slots") slot-name))))))
+
+;; ── find-class / change-class ─────────────────────────────────────────────
+
+(define clos-find-class (fn (name) (get clos-class-registry name)))
+
+(define
+  clos-change-class!
+  (fn
+    (instance new-class-name)
+    (if
+      (and (dict? instance) (= (get instance "clos-type") "instance"))
+      (dict-set! instance "class" new-class-name)
+      (error (str "Not a CLOS instance: " (inspect instance))))))
+
+;; ── defgeneric ────────────────────────────────────────────────────────────
+
+(define
+  clos-defgeneric
+  (fn
+    (name options)
+    (let
+      ((combination (or (get options "method-combination") "standard")))
+      (when
+        (nil? (get clos-generic-registry name))
+        (dict-set! clos-generic-registry name {:methods (list) :combination combination :name name}))
+      name)))
+
+;; ── defmethod ─────────────────────────────────────────────────────────────
+;;
+;; method-fn: (fn (args next-fn) body)
+;;   args = list of all call arguments
+;;   next-fn = (fn () next-method-result) or nil
+
+(define
+  clos-defmethod
+  (fn
+    (generic-name qualifiers specializers method-fn)
+    (when
+      (nil? (get clos-generic-registry generic-name))
+      (clos-defgeneric generic-name {}))
+    (let
+      ((grec (get clos-generic-registry generic-name))
+        (new-method {:fn method-fn :qualifiers qualifiers :specializers specializers}))
+      (let
+        ((kept (filter (fn (m) (not (and (= (get m "qualifiers") qualifiers) (= (get m "specializers") specializers)))) (get grec "methods"))))
+        (dict-set!
+          clos-generic-registry
+          generic-name
+          (assoc grec "methods" (append kept (list new-method))))
+        generic-name))))
+
+;; Now install the real accessor-method installer
+(set!
+  clos-add-reader-method
+  (fn
+    (method-name class-name slot-name)
+    (clos-defmethod
+      method-name
+      (list)
+      (list class-name)
+      (fn (args next-fn) (clos-slot-value (first args) slot-name)))))
+
+;; ── method specificity ─────────────────────────────────────────────────────
+
+(define
+  clos-method-matches?
+  (fn
+    (method args)
+    (let
+      ((specs (get method "specializers")))
+      (if
+        (> (len specs) (len args))
+        false
+        (define
+          check-all
+          (fn
+            (i)
+            (if
+              (>= i (len specs))
+              true
+              (let
+                ((spec (nth specs i)) (arg (nth args i)))
+                (if
+                  (= spec "t")
+                  (check-all (+ i 1))
+                  (if
+                    (clos-instance-of? arg spec)
+                    (check-all (+ i 1))
+                    false))))))
+        (check-all 0)))))
+
+;; Precedence distance: how far class-name is from spec-name up the hierarchy.
+(define
+  clos-specificity
+  (let
+    ((registry clos-class-registry))
+    (fn
+      (class-name spec-name)
+      (define
+        walk
+        (fn
+          (cn depth)
+          (if
+            (= cn spec-name)
+            depth
+            (let
+              ((rec (get registry cn)))
+              (if
+                (nil? rec)
+                nil
+                (let
+                  ((results (map (fn (p) (walk p (+ depth 1))) (get rec "parents"))))
+                  (let
+                    ((non-nil (filter (fn (x) (not (nil? x))) results)))
+                    (if
+                      (empty? non-nil)
+                      nil
+                      (reduce
+                        (fn (a b) (if (< a b) a b))
+                        (first non-nil)
+                        (rest non-nil))))))))))
+      (walk class-name 0))))
+
+(define
+  clos-method-more-specific?
+  (fn
+    (m1 m2 args)
+    (let
+      ((s1 (get m1 "specializers")) (s2 (get m2 "specializers")))
+      (define
+        cmp
+        (fn
+          (i)
+          (if
+            (>= i (len s1))
+            false
+            (let
+              ((c1 (clos-specificity (clos-class-of (nth args i)) (nth s1 i)))
+                (c2
+                  (clos-specificity (clos-class-of (nth args i)) (nth s2 i))))
+              (cond
+                ((and (nil? c1) (nil? c2)) (cmp (+ i 1)))
+                ((nil? c1) false)
+                ((nil? c2) true)
+                ((< c1 c2) true)
+                ((> c1 c2) false)
+                (:else (cmp (+ i 1))))))))
+      (cmp 0))))
+
+(define
+  clos-sort-methods
+  (fn
+    (methods args)
+    (define
+      insert
+      (fn
+        (m sorted)
+        (if
+          (empty? sorted)
+          (list m)
+          (if
+            (clos-method-more-specific? m (first sorted) args)
+            (cons m sorted)
+            (cons (first sorted) (insert m (rest sorted)))))))
+    (reduce (fn (acc m) (insert m acc)) (list) methods)))
+
+;; ── call-generic (standard method combination) ─────────────────────────────
+
+(define
+  clos-call-generic
+  (fn
+    (generic-name args)
+    (let
+      ((grec (get clos-generic-registry generic-name)))
+      (if
+        (nil? grec)
+        (error (str "No generic function: " generic-name))
+        (let
+          ((applicable (filter (fn (m) (clos-method-matches? m args)) (get grec "methods"))))
+          (if
+            (empty? applicable)
+            (error
+              (str
+                "No applicable method for "
+                generic-name
+                " with classes "
+                (inspect (map clos-class-of args))))
+            (let
+              ((primary (filter (fn (m) (empty? (get m "qualifiers"))) applicable))
+                (before
+                  (filter
+                    (fn (m) (= (get m "qualifiers") (list "before")))
+                    applicable))
+                (after
+                  (filter
+                    (fn (m) (= (get m "qualifiers") (list "after")))
+                    applicable))
+                (around
+                  (filter
+                    (fn (m) (= (get m "qualifiers") (list "around")))
+                    applicable)))
+              (let
+                ((sp (clos-sort-methods primary args))
+                  (sb (clos-sort-methods before args))
+                  (sa (clos-sort-methods after args))
+                  (sw (clos-sort-methods around args)))
+                (define
+                  make-primary-chain
+                  (fn
+                    (methods)
+                    (if
+                      (empty? methods)
+                      (fn
+                        ()
+                        (error (str "No next primary method: " generic-name)))
+                      (fn
+                        ()
+                        ((get (first methods) "fn")
+                          args
+                          (make-primary-chain (rest methods)))))))
+                (define
+                  make-around-chain
+                  (fn
+                    (around-methods inner-thunk)
+                    (if
+                      (empty? around-methods)
+                      inner-thunk
+                      (fn
+                        ()
+                        ((get (first around-methods) "fn")
+                          args
+                          (make-around-chain
+                            (rest around-methods)
+                            inner-thunk))))))
+                (for-each (fn (m) ((get m "fn") args (fn () nil))) sb)
+                (let
+                  ((primary-thunk (make-primary-chain sp)))
+                  (let
+                    ((result (if (empty? sw) (primary-thunk) ((make-around-chain sw primary-thunk)))))
+                    (for-each
+                      (fn (m) ((get m "fn") args (fn () nil)))
+                      (reverse sa))
+                    result))))))))))
+
+;; ── call-next-method / next-method-p ──────────────────────────────────────
+
+(define clos-call-next-method (fn (next-fn) (next-fn)))
+
+(define clos-next-method-p (fn (next-fn) (not (nil? next-fn))))
+
+;; ── with-slots ────────────────────────────────────────────────────────────
+
+(define
+  clos-with-slots
+  (fn
+    (instance slot-names body-fn)
+    (let
+      ((vals (map (fn (s) (clos-slot-value instance s)) slot-names)))
+      (apply body-fn vals))))
\ No newline at end of file
diff --git a/lib/common-lisp/conformance.sh b/lib/common-lisp/conformance.sh
index f8693661..da350377 100755
--- a/lib/common-lisp/conformance.sh
+++ b/lib/common-lisp/conformance.sh
@@ -91,6 +91,18 @@ run_suite "Phase 3: interactive-debugger" \
   "lib/common-lisp/runtime.sx lib/common-lisp/tests/programs/interactive-debugger.sx" \
   "debugger-passed" "debugger-failed" "debugger-failures"
 
+run_suite "Phase 4: CLOS" \
+  "lib/common-lisp/runtime.sx lib/common-lisp/clos.sx lib/common-lisp/tests/clos.sx" \
+  "passed" "failed" "failures"
+
+run_suite "Phase 4: geometry" \
+  "lib/common-lisp/runtime.sx lib/common-lisp/clos.sx lib/common-lisp/tests/programs/geometry.sx" \
+  "geo-passed" "geo-failed" "geo-failures"
+
+run_suite "Phase 4: mop-trace" \
+  "lib/common-lisp/runtime.sx lib/common-lisp/clos.sx lib/common-lisp/tests/programs/mop-trace.sx" \
+  "mop-passed" "mop-failed" "mop-failures"
+
 echo ""
 echo "=== Total: $TOTAL_PASS passed, $TOTAL_FAIL failed ==="
 
diff --git a/lib/common-lisp/eval.sx b/lib/common-lisp/eval.sx
index 7ca9f8af..10b2be4c 100644
--- a/lib/common-lisp/eval.sx
+++ b/lib/common-lisp/eval.sx
@@ -425,6 +425,55 @@
               (cl-eval-body (rest clause) env))
             (cl-eval-cond (rest clauses) env)))))))
 
+;; Dynamic variable infrastructure
+(define cl-dyn-unbound {:cl-type "dyn-unbound"})
+(define cl-specials {})
+(define cl-mark-special!
+  (fn (name) (dict-set! cl-specials name true)))
+(define cl-special?
+  (fn (name) (has-key? cl-specials name)))
+;; Apply dynamic bindings: save old global values, set new, run thunk, restore
+(define cl-apply-dyn
+  (fn (binds thunk)
+    (if (= (len binds) 0)
+      (thunk)
+      (let ((b          (nth binds 0))
+            (rest-binds (rest binds)))
+        (let ((name  (get b "name"))
+              (val   (get b "value"))
+              (gvars (get cl-global-env "vars")))
+          (let ((old (if (has-key? gvars name)
+                       (get gvars name)
+                       cl-dyn-unbound)))
+            (dict-set! gvars name val)
+            (let ((result (cl-apply-dyn rest-binds thunk)))
+              (if (and (dict? old) (= (get old "cl-type") "dyn-unbound"))
+                (dict-set! gvars name nil)
+                (dict-set! gvars name old))
+              result)))))))
+;; Sequential LET* with dynamic variable support
+(define cl-letstar-bind
+  (fn (bs e thunk)
+    (if (= (len bs) 0)
+      (thunk e)
+      (let ((b       (nth bs 0))
+            (rest-bs (rest bs)))
+        (let ((name (if (list? b) (nth b 0) b))
+              (init (if (and (list? b) (> (len b) 1)) (nth b 1) nil)))
+          (let ((val (cl-eval init e)))
+            (if (cl-special? name)
+              (let ((gvars (get cl-global-env "vars")))
+                (let ((old (if (has-key? gvars name)
+                             (get gvars name)
+                             cl-dyn-unbound)))
+                  (dict-set! gvars name val)
+                  (let ((result (cl-letstar-bind rest-bs e thunk)))
+                    (if (and (dict? old) (= (get old "cl-type") "dyn-unbound"))
+                      (dict-set! gvars name nil)
+                      (dict-set! gvars name old))
+                    result)))
+              (cl-letstar-bind rest-bs (cl-env-bind-var e name val) thunk))))))))
+
 ;; Parallel LET and sequential LET*
 (define cl-eval-let
   (fn (args env sequential)
@@ -432,17 +481,7 @@
           (body     (rest args)))
       (if sequential
         ;; LET*: each binding sees previous ones
-        (let ((new-env env))
-          (define bind-seq
-            (fn (bs e)
-              (if (= (len bs) 0)
-                e
-                (let ((b (nth bs 0)))
-                  (let ((name (if (list? b) (nth b 0) b))
-                        (init (if (and (list? b) (> (len b) 1)) (nth b 1) nil)))
-                    (bind-seq (rest bs)
-                      (cl-env-bind-var e name (cl-eval init e))))))))
-          (cl-eval-body body (bind-seq bindings env)))
+        (cl-letstar-bind bindings env (fn (new-env) (cl-eval-body body new-env)))
         ;; LET: evaluate all inits in current env, then bind
         (let ((pairs (map
                        (fn (b)
@@ -450,11 +489,14 @@
                                (init (if (and (list? b) (> (len b) 1)) (nth b 1) nil)))
                            {:name name :value (cl-eval init env)}))
                        bindings)))
-          (let ((new-env (reduce
-                           (fn (e pair)
-                             (cl-env-bind-var e (get pair "name") (get pair "value")))
-                           env pairs)))
-            (cl-eval-body body new-env)))))))
+          (let ((spec-pairs (filter (fn (p) (cl-special? (get p "name"))) pairs))
+                (lex-pairs  (filter (fn (p) (not (cl-special? (get p "name")))) pairs)))
+            (let ((new-env (reduce
+                             (fn (e pair)
+                               (cl-env-bind-var e (get pair "name") (get pair "value")))
+                             env lex-pairs)))
+              (cl-apply-dyn spec-pairs
+                (fn () (cl-eval-body body new-env))))))))))
 
 ;; SETQ / SETF (simplified: mutate nearest scope or global)
 (define cl-eval-setq
@@ -563,6 +605,7 @@
         (when (or always-assign
                   (not (cl-env-has-var? cl-global-env name)))
           (dict-set! (get cl-global-env "vars") name val))
+        (cl-mark-special! name)
         name))))
 
 ;; Function call: evaluate name → look up fns, builtins; evaluate args
diff --git a/lib/common-lisp/scoreboard.json b/lib/common-lisp/scoreboard.json
index ef70efb9..3c21a86f 100644
--- a/lib/common-lisp/scoreboard.json
+++ b/lib/common-lisp/scoreboard.json
@@ -1,14 +1,17 @@
 {
-  "generated": "2026-05-05T11:24:34Z",
-  "total_pass": 363,
+  "generated": "2026-05-05T11:37:47Z",
+  "total_pass": 437,
   "total_fail": 0,
   "suites": [
     {"name": "Phase 1: tokenizer/reader", "pass": 79, "fail": 0},
     {"name": "Phase 1: parser/lambda-lists", "pass": 31, "fail": 0},
-    {"name": "Phase 2: evaluator", "pass": 174, "fail": 0},
+    {"name": "Phase 2: evaluator", "pass": 182, "fail": 0},
     {"name": "Phase 3: condition system", "pass": 59, "fail": 0},
     {"name": "Phase 3: restart-demo", "pass": 7, "fail": 0},
     {"name": "Phase 3: parse-recover", "pass": 6, "fail": 0},
-    {"name": "Phase 3: interactive-debugger", "pass": 7, "fail": 0}
+    {"name": "Phase 3: interactive-debugger", "pass": 7, "fail": 0},
+    {"name": "Phase 4: CLOS", "pass": 41, "fail": 0},
+    {"name": "Phase 4: geometry", "pass": 12, "fail": 0},
+    {"name": "Phase 4: mop-trace", "pass": 13, "fail": 0}
   ]
 }
diff --git a/lib/common-lisp/scoreboard.md b/lib/common-lisp/scoreboard.md
index 37b8e399..dae86da3 100644
--- a/lib/common-lisp/scoreboard.md
+++ b/lib/common-lisp/scoreboard.md
@@ -1,15 +1,18 @@
 # Common Lisp on SX — Scoreboard
 
-_Generated: 2026-05-05 11:24 UTC_
+_Generated: 2026-05-05 11:37 UTC_
 
 | Suite | Pass | Fail | Status |
 |-------|------|------|--------|
 | Phase 1: tokenizer/reader | 79 | 0 | pass |
 | Phase 1: parser/lambda-lists | 31 | 0 | pass |
-| Phase 2: evaluator | 174 | 0 | pass |
+| Phase 2: evaluator | 182 | 0 | pass |
 | Phase 3: condition system | 59 | 0 | pass |
 | Phase 3: restart-demo | 7 | 0 | pass |
 | Phase 3: parse-recover | 6 | 0 | pass |
 | Phase 3: interactive-debugger | 7 | 0 | pass |
+| Phase 4: CLOS | 41 | 0 | pass |
+| Phase 4: geometry | 12 | 0 | pass |
+| Phase 4: mop-trace | 13 | 0 | pass |
 
-**Total: 363 passed, 0 failed**
+**Total: 437 passed, 0 failed**
diff --git a/lib/common-lisp/test.sh b/lib/common-lisp/test.sh
index 0068e979..85cf3f86 100755
--- a/lib/common-lisp/test.sh
+++ b/lib/common-lisp/test.sh
@@ -366,6 +366,56 @@ run_program_suite \
   "lib/common-lisp/tests/programs/interactive-debugger.sx" \
   "debugger-passed" "debugger-failed" "debugger-failures"
 
+# ── Phase 4: CLOS unit tests ─────────────────────────────────────────────────
+CLOS_FILE=$(mktemp); trap "rm -f $CLOS_FILE" EXIT
+printf '(epoch 1)\n(load "spec/stdlib.sx")\n(epoch 2)\n(load "lib/common-lisp/runtime.sx")\n(epoch 3)\n(load "lib/common-lisp/clos.sx")\n(epoch 4)\n(load "lib/common-lisp/tests/clos.sx")\n(epoch 5)\n(eval "passed")\n(epoch 6)\n(eval "failed")\n(epoch 7)\n(eval "failures")\n' > "$CLOS_FILE"
+CLOS_OUT=$(timeout 30 "$SX_SERVER" < "$CLOS_FILE" 2>/dev/null)
+rm -f "$CLOS_FILE"
+CLOS_PASSED=$(echo "$CLOS_OUT" | grep -A1 "^(ok-len 5 " | tail -1 || true)
+CLOS_FAILED=$(echo "$CLOS_OUT" | grep -A1 "^(ok-len 6 " | tail -1 || true)
+[ -z "$CLOS_PASSED" ] && CLOS_PASSED=$(echo "$CLOS_OUT" | grep "^(ok 5 " | awk '{print $3}' | tr -d ')' || true)
+[ -z "$CLOS_FAILED" ] && CLOS_FAILED=$(echo "$CLOS_OUT" | grep "^(ok 6 " | awk '{print $3}' | tr -d ')' || true)
+[ -z "$CLOS_PASSED" ] && CLOS_PASSED=0; [ -z "$CLOS_FAILED" ] && CLOS_FAILED=0
+if [ "$CLOS_FAILED" = "0" ] && [ "$CLOS_PASSED" -gt 0 ] 2>/dev/null; then
+  PASS=$((PASS + CLOS_PASSED))
+  [ "$VERBOSE" = "-v" ] && echo "  ok CLOS unit tests ($CLOS_PASSED)"
+else
+  FAIL=$((FAIL + 1))
+  ERRORS+="  FAIL [CLOS unit tests] (${CLOS_PASSED} passed, ${CLOS_FAILED} failed)
+"
+fi
+
+# ── Phase 4: CLOS classic programs ───────────────────────────────────────────
+run_clos_suite() {
+  local prog="$1" pass_var="$2" fail_var="$3" failures_var="$4"
+  local PROG_FILE=$(mktemp)
+  printf '(epoch 1)\n(load "spec/stdlib.sx")\n(epoch 2)\n(load "lib/common-lisp/runtime.sx")\n(epoch 3)\n(load "lib/common-lisp/clos.sx")\n(epoch 4)\n(load "%s")\n(epoch 5)\n(eval "%s")\n(epoch 6)\n(eval "%s")\n(epoch 7)\n(eval "%s")\n' \
+    "$prog" "$pass_var" "$fail_var" "$failures_var" > "$PROG_FILE"
+  local OUT; OUT=$(timeout 20 "$SX_SERVER" < "$PROG_FILE" 2>/dev/null)
+  rm -f "$PROG_FILE"
+  local P F
+  P=$(echo "$OUT" | grep -A1 "^(ok-len 5 " | tail -1 || true)
+  F=$(echo "$OUT" | grep -A1 "^(ok-len 6 " | tail -1 || true)
+  local ERRS; ERRS=$(echo "$OUT" | grep -A1 "^(ok-len 7 " | tail -1 || true)
+  [ -z "$P" ] && P=0; [ -z "$F" ] && F=0
+  if [ "$F" = "0" ] && [ "$P" -gt 0 ] 2>/dev/null; then
+    PASS=$((PASS + P))
+    [ "$VERBOSE" = "-v" ] && echo "  ok $prog ($P)"
+  else
+    FAIL=$((FAIL + 1))
+    ERRORS+="  FAIL [$prog] (${P} passed, ${F} failed) ${ERRS}
+"
+  fi
+}
+
+run_clos_suite \
+  "lib/common-lisp/tests/programs/geometry.sx" \
+  "geo-passed" "geo-failed" "geo-failures"
+
+run_clos_suite \
+  "lib/common-lisp/tests/programs/mop-trace.sx" \
+  "mop-passed" "mop-failed" "mop-failures"
+
 TOTAL=$((PASS+FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL lib/common-lisp tests passed"
diff --git a/lib/common-lisp/tests/clos.sx b/lib/common-lisp/tests/clos.sx
new file mode 100644
index 00000000..5535ea5d
--- /dev/null
+++ b/lib/common-lisp/tests/clos.sx
@@ -0,0 +1,334 @@
+;; lib/common-lisp/tests/clos.sx — CLOS test suite
+;;
+;; Loaded after: spec/stdlib.sx, lib/common-lisp/runtime.sx, lib/common-lisp/clos.sx
+
+(define passed 0)
+(define failed 0)
+(define failures (list))
+
+(define
+  assert-equal
+  (fn
+    (label got expected)
+    (if
+      (= got expected)
+      (set! passed (+ passed 1))
+      (begin
+        (set! failed (+ failed 1))
+        (set!
+          failures
+          (append
+            failures
+            (list
+              (str
+                "FAIL ["
+                label
+                "]: got="
+                (inspect got)
+                " expected="
+                (inspect expected)))))))))
+
+(define
+  assert-true
+  (fn
+    (label got)
+    (if
+      got
+      (set! passed (+ passed 1))
+      (begin
+        (set! failed (+ failed 1))
+        (set!
+          failures
+          (append
+            failures
+            (list
+              (str "FAIL [" label "]: expected true, got " (inspect got)))))))))
+
+(define
+  assert-nil
+  (fn
+    (label got)
+    (if
+      (nil? got)
+      (set! passed (+ passed 1))
+      (begin
+        (set! failed (+ failed 1))
+        (set!
+          failures
+          (append
+            failures
+            (list (str "FAIL [" label "]: expected nil, got " (inspect got)))))))))
+
+;; ── 1. class-of for built-in types ────────────────────────────────────────
+
+(assert-equal "class-of integer" (clos-class-of 42) "integer")
+(assert-equal "class-of float" (clos-class-of 3.14) "float")
+(assert-equal "class-of string" (clos-class-of "hi") "string")
+(assert-equal "class-of nil" (clos-class-of nil) "null")
+(assert-equal "class-of list" (clos-class-of (list 1)) "cons")
+(assert-equal "class-of empty" (clos-class-of (list)) "null")
+
+;; ── 2. subclass-of? ───────────────────────────────────────────────────────
+
+(assert-true "integer subclass-of t" (clos-subclass-of? "integer" "t"))
+(assert-true "float subclass-of t" (clos-subclass-of? "float" "t"))
+(assert-true "t subclass-of t" (clos-subclass-of? "t" "t"))
+(assert-equal
+  "integer not subclass-of float"
+  (clos-subclass-of? "integer" "float")
+  false)
+
+;; ── 3. defclass + make-instance ───────────────────────────────────────────
+
+(clos-defclass "point" (list "t") (list {:initform 0 :initarg ":x" :reader nil :writer nil :accessor "point-x" :name "x"} {:initform 0 :initarg ":y" :reader nil :writer nil :accessor "point-y" :name "y"}))
+
+(let
+  ((p (clos-make-instance "point" ":x" 3 ":y" 4)))
+  (begin
+    (assert-equal "make-instance slot x" (clos-slot-value p "x") 3)
+    (assert-equal "make-instance slot y" (clos-slot-value p "y") 4)
+    (assert-equal "class-of instance" (clos-class-of p) "point")
+    (assert-true "instance-of? point" (clos-instance-of? p "point"))
+    (assert-true "instance-of? t" (clos-instance-of? p "t"))
+    (assert-equal "instance-of? string" (clos-instance-of? p "string") false)))
+
+;; initform defaults
+(let
+  ((p0 (clos-make-instance "point")))
+  (begin
+    (assert-equal "initform default x=0" (clos-slot-value p0 "x") 0)
+    (assert-equal "initform default y=0" (clos-slot-value p0 "y") 0)))
+
+;; ── 4. slot-value / set-slot-value! ──────────────────────────────────────
+
+(let
+  ((p (clos-make-instance "point" ":x" 10 ":y" 20)))
+  (begin
+    (clos-set-slot-value! p "x" 99)
+    (assert-equal "set-slot-value! x" (clos-slot-value p "x") 99)
+    (assert-equal "slot-value y unchanged" (clos-slot-value p "y") 20)))
+
+;; ── 5. slot-boundp ────────────────────────────────────────────────────────
+
+(let
+  ((p (clos-make-instance "point" ":x" 5)))
+  (begin
+    (assert-true "slot-boundp x" (clos-slot-boundp p "x"))
+    (assert-true "slot-boundp y (initform 0)" (clos-slot-boundp p "y"))))
+
+;; ── 6. find-class ─────────────────────────────────────────────────────────
+
+(assert-equal
+  "find-class point"
+  (get (clos-find-class "point") "name")
+  "point")
+(assert-nil "find-class missing" (clos-find-class "no-such-class"))
+
+;; ── 7. inheritance ────────────────────────────────────────────────────────
+
+(clos-defclass "colored-point" (list "point") (list {:initform "white" :initarg ":color" :reader nil :writer nil :accessor nil :name "color"}))
+
+(let
+  ((cp (clos-make-instance "colored-point" ":x" 1 ":y" 2 ":color" "red")))
+  (begin
+    (assert-equal "inherited slot x" (clos-slot-value cp "x") 1)
+    (assert-equal "inherited slot y" (clos-slot-value cp "y") 2)
+    (assert-equal "own slot color" (clos-slot-value cp "color") "red")
+    (assert-true
+      "instance-of? colored-point"
+      (clos-instance-of? cp "colored-point"))
+    (assert-true "instance-of? point (parent)" (clos-instance-of? cp "point"))
+    (assert-true "instance-of? t (root)" (clos-instance-of? cp "t"))))
+
+;; ── 8. defgeneric + primary method ───────────────────────────────────────
+
+(clos-defgeneric "describe-obj" {})
+
+(clos-defmethod
+  "describe-obj"
+  (list)
+  (list "point")
+  (fn
+    (args next-fn)
+    (let
+      ((p (first args)))
+      (str "(" (clos-slot-value p "x") "," (clos-slot-value p "y") ")"))))
+
+(clos-defmethod
+  "describe-obj"
+  (list)
+  (list "t")
+  (fn (args next-fn) (str "object:" (inspect (first args)))))
+
+(let
+  ((p (clos-make-instance "point" ":x" 3 ":y" 4)))
+  (begin
+    (assert-equal
+      "primary method for point"
+      (clos-call-generic "describe-obj" (list p))
+      "(3,4)")
+    (assert-equal
+      "fallback t method"
+      (clos-call-generic "describe-obj" (list 42))
+      "object:42")))
+
+;; ── 9. method inheritance + specificity ───────────────────────────────────
+
+(clos-defmethod
+  "describe-obj"
+  (list)
+  (list "colored-point")
+  (fn
+    (args next-fn)
+    (let
+      ((cp (first args)))
+      (str
+        (clos-slot-value cp "color")
+        "@("
+        (clos-slot-value cp "x")
+        ","
+        (clos-slot-value cp "y")
+        ")"))))
+
+(let
+  ((cp (clos-make-instance "colored-point" ":x" 5 ":y" 6 ":color" "blue")))
+  (assert-equal
+    "most specific method wins"
+    (clos-call-generic "describe-obj" (list cp))
+    "blue@(5,6)"))
+
+;; ── 10. :before / :after / :around qualifiers ─────────────────────────────
+
+(clos-defgeneric "logged-action" {})
+
+(clos-defmethod
+  "logged-action"
+  (list "before")
+  (list "t")
+  (fn (args next-fn) (set! action-log (append action-log (list "before")))))
+
+(clos-defmethod
+  "logged-action"
+  (list)
+  (list "t")
+  (fn
+    (args next-fn)
+    (set! action-log (append action-log (list "primary")))
+    "result"))
+
+(clos-defmethod
+  "logged-action"
+  (list "after")
+  (list "t")
+  (fn (args next-fn) (set! action-log (append action-log (list "after")))))
+
+(define action-log (list))
+(clos-call-generic "logged-action" (list 1))
+(assert-equal
+  ":before/:after order"
+  action-log
+  (list "before" "primary" "after"))
+
+;; :around
+(define around-log (list))
+
+(clos-defgeneric "wrapped-action" {})
+
+(clos-defmethod
+  "wrapped-action"
+  (list "around")
+  (list "t")
+  (fn
+    (args next-fn)
+    (set! around-log (append around-log (list "around-enter")))
+    (let
+      ((r (next-fn)))
+      (set! around-log (append around-log (list "around-exit")))
+      r)))
+
+(clos-defmethod
+  "wrapped-action"
+  (list)
+  (list "t")
+  (fn
+    (args next-fn)
+    (set! around-log (append around-log (list "primary")))
+    42))
+
+(let
+  ((r (clos-call-generic "wrapped-action" (list nil))))
+  (begin
+    (assert-equal ":around result" r 42)
+    (assert-equal
+      ":around log"
+      around-log
+      (list "around-enter" "primary" "around-exit"))))
+
+;; ── 11. call-next-method ─────────────────────────────────────────────────
+
+(clos-defgeneric "chain-test" {})
+
+(clos-defmethod
+  "chain-test"
+  (list)
+  (list "colored-point")
+  (fn (args next-fn) (str "colored:" (clos-call-next-method next-fn))))
+
+(clos-defmethod
+  "chain-test"
+  (list)
+  (list "point")
+  (fn (args next-fn) "point-base"))
+
+(let
+  ((cp (clos-make-instance "colored-point" ":x" 0 ":y" 0 ":color" "green")))
+  (assert-equal
+    "call-next-method chains"
+    (clos-call-generic "chain-test" (list cp))
+    "colored:point-base"))
+
+;; ── 12. accessor methods ──────────────────────────────────────────────────
+
+(let
+  ((p (clos-make-instance "point" ":x" 7 ":y" 8)))
+  (begin
+    (assert-equal
+      "accessor point-x"
+      (clos-call-generic "point-x" (list p))
+      7)
+    (assert-equal
+      "accessor point-y"
+      (clos-call-generic "point-y" (list p))
+      8)))
+
+;; ── 13. with-slots ────────────────────────────────────────────────────────
+
+(let
+  ((p (clos-make-instance "point" ":x" 3 ":y" 4)))
+  (assert-equal
+    "with-slots"
+    (clos-with-slots p (list "x" "y") (fn (x y) (* x y)))
+    12))
+
+;; ── 14. change-class ─────────────────────────────────────────────────────
+
+(clos-defclass "special-point" (list "point") (list {:initform "" :initarg ":label" :reader nil :writer nil :accessor nil :name "label"}))
+
+(let
+  ((p (clos-make-instance "point" ":x" 1 ":y" 2)))
+  (begin
+    (clos-change-class! p "special-point")
+    (assert-equal
+      "change-class updates class"
+      (clos-class-of p)
+      "special-point")))
+
+;; ── summary ────────────────────────────────────────────────────────────────
+
+(if
+  (= failed 0)
+  (print (str "ok " passed "/" (+ passed failed) " CLOS tests passed"))
+  (begin
+    (for-each (fn (f) (print f)) failures)
+    (print
+      (str "FAIL " passed "/" (+ passed failed) " passed, " failed " failed"))))
\ No newline at end of file
diff --git a/lib/common-lisp/tests/eval.sx b/lib/common-lisp/tests/eval.sx
index 0b8e54d3..2a58146e 100644
--- a/lib/common-lisp/tests/eval.sx
+++ b/lib/common-lisp/tests/eval.sx
@@ -436,3 +436,31 @@
 (cl-test "values: truthy primary in if"
   (ev "(if (values 42 nil) 'yes 'no)")
   "YES")
+
+;; --- Dynamic variables ---
+(cl-test "defvar marks special"
+  (do (ev "(defvar *dv* 10)")
+    (cl-special? "*DV*"))
+  true)
+(cl-test "defvar: let rebinds dynamically"
+  (ev "(progn (defvar *x* 1) (defun get-x () *x*) (let ((*x* 99)) (get-x)))")
+  99)
+(cl-test "defvar: binding restores after let"
+  (ev "(progn (defvar *yrst* 5) (let ((*yrst* 42)) *yrst*) *yrst*)")
+  5)
+(cl-test "defparameter marks special"
+  (do (ev "(defparameter *dp* 0)")
+    (cl-special? "*DP*"))
+  true)
+(cl-test "defparameter: let rebinds dynamically"
+  (ev "(progn (defparameter *z* 10) (defun get-z () *z*) (let ((*z* 77)) (get-z)))")
+  77)
+(cl-test "defparameter: always assigns"
+  (ev "(progn (defparameter *p* 1) (defparameter *p* 2) *p*)")
+  2)
+(cl-test "dynamic binding: nested lets"
+  (ev "(progn (defvar *n* 0) (let ((*n* 1)) (let ((*n* 2)) *n*)))")
+  2)
+(cl-test "dynamic binding: restores across nesting"
+  (ev "(progn (defvar *m* 10) (let ((*m* 20)) (let ((*m* 30)) nil)) *m*)")
+  10)
diff --git a/lib/common-lisp/tests/programs/geometry.sx b/lib/common-lisp/tests/programs/geometry.sx
new file mode 100644
index 00000000..a7e17188
--- /dev/null
+++ b/lib/common-lisp/tests/programs/geometry.sx
@@ -0,0 +1,291 @@
+;; geometry.sx — Multiple dispatch with CLOS
+;;
+;; Demonstrates generic functions dispatching on combinations of
+;; geometric types: point, line, plane.
+;;
+;; Depends on: lib/common-lisp/runtime.sx, lib/common-lisp/clos.sx
+
+;; ── geometric classes ──────────────────────────────────────────────────────
+
+(clos-defclass "geo-point" (list "t") (list {:initform 0 :initarg ":px" :reader nil :writer nil :accessor nil :name "px"} {:initform 0 :initarg ":py" :reader nil :writer nil :accessor nil :name "py"}))
+
+(clos-defclass "geo-line" (list "t") (list {:initform nil :initarg ":p1" :reader nil :writer nil :accessor nil :name "p1"} {:initform nil :initarg ":p2" :reader nil :writer nil :accessor nil :name "p2"}))
+
+(clos-defclass "geo-plane" (list "t") (list {:initform nil :initarg ":normal" :reader nil :writer nil :accessor nil :name "normal"} {:initform 0 :initarg ":d" :reader nil :writer nil :accessor nil :name "d"}))
+
+;; ── helpers ────────────────────────────────────────────────────────────────
+
+(define geo-point-x (fn (p) (clos-slot-value p "px")))
+(define geo-point-y (fn (p) (clos-slot-value p "py")))
+
+(define
+  geo-make-point
+  (fn (x y) (clos-make-instance "geo-point" ":px" x ":py" y)))
+
+(define
+  geo-make-line
+  (fn (p1 p2) (clos-make-instance "geo-line" ":p1" p1 ":p2" p2)))
+
+(define
+  geo-make-plane
+  (fn
+    (nx ny d)
+    (clos-make-instance "geo-plane" ":normal" (list nx ny) ":d" d)))
+
+;; ── describe generic ───────────────────────────────────────────────────────
+
+(clos-defgeneric "geo-describe" {})
+
+(clos-defmethod
+  "geo-describe"
+  (list)
+  (list "geo-point")
+  (fn
+    (args next-fn)
+    (let
+      ((p (first args)))
+      (str "P(" (geo-point-x p) "," (geo-point-y p) ")"))))
+
+(clos-defmethod
+  "geo-describe"
+  (list)
+  (list "geo-line")
+  (fn
+    (args next-fn)
+    (let
+      ((l (first args)))
+      (str
+        "L["
+        (clos-call-generic "geo-describe" (list (clos-slot-value l "p1")))
+        "-"
+        (clos-call-generic "geo-describe" (list (clos-slot-value l "p2")))
+        "]"))))
+
+(clos-defmethod
+  "geo-describe"
+  (list)
+  (list "geo-plane")
+  (fn
+    (args next-fn)
+    (let
+      ((pl (first args)))
+      (str "Plane(d=" (clos-slot-value pl "d") ")"))))
+
+;; ── intersect: multi-dispatch generic ─────────────────────────────────────
+;;
+;; Returns a string description of the intersection result.
+
+(clos-defgeneric "intersect" {})
+
+;; point ∩ point: same if coordinates match
+(clos-defmethod
+  "intersect"
+  (list)
+  (list "geo-point" "geo-point")
+  (fn
+    (args next-fn)
+    (let
+      ((p1 (first args)) (p2 (first (rest args))))
+      (if
+        (and
+          (= (geo-point-x p1) (geo-point-x p2))
+          (= (geo-point-y p1) (geo-point-y p2)))
+        "point"
+        "empty"))))
+
+;; point ∩ line: check if point lies on line (cross product = 0)
+(clos-defmethod
+  "intersect"
+  (list)
+  (list "geo-point" "geo-line")
+  (fn
+    (args next-fn)
+    (let
+      ((pt (first args)) (ln (first (rest args))))
+      (let
+        ((lp1 (clos-slot-value ln "p1")) (lp2 (clos-slot-value ln "p2")))
+        (let
+          ((dx (- (geo-point-x lp2) (geo-point-x lp1)))
+            (dy (- (geo-point-y lp2) (geo-point-y lp1)))
+            (ex (- (geo-point-x pt) (geo-point-x lp1)))
+            (ey (- (geo-point-y pt) (geo-point-y lp1))))
+          (if (= (- (* dx ey) (* dy ex)) 0) "point" "empty"))))))
+
+;; line ∩ line: parallel (same slope = empty) or point
+(clos-defmethod
+  "intersect"
+  (list)
+  (list "geo-line" "geo-line")
+  (fn
+    (args next-fn)
+    (let
+      ((l1 (first args)) (l2 (first (rest args))))
+      (let
+        ((p1 (clos-slot-value l1 "p1"))
+          (p2 (clos-slot-value l1 "p2"))
+          (p3 (clos-slot-value l2 "p1"))
+          (p4 (clos-slot-value l2 "p2")))
+        (let
+          ((dx1 (- (geo-point-x p2) (geo-point-x p1)))
+            (dy1 (- (geo-point-y p2) (geo-point-y p1)))
+            (dx2 (- (geo-point-x p4) (geo-point-x p3)))
+            (dy2 (- (geo-point-y p4) (geo-point-y p3))))
+          (let
+            ((cross (- (* dx1 dy2) (* dy1 dx2))))
+            (if (= cross 0) "parallel" "point")))))))
+
+;; line ∩ plane: general case = point (or parallel if line ⊥ normal)
+(clos-defmethod
+  "intersect"
+  (list)
+  (list "geo-line" "geo-plane")
+  (fn
+    (args next-fn)
+    (let
+      ((ln (first args)) (pl (first (rest args))))
+      (let
+        ((p1 (clos-slot-value ln "p1"))
+          (p2 (clos-slot-value ln "p2"))
+          (n (clos-slot-value pl "normal")))
+        (let
+          ((dx (- (geo-point-x p2) (geo-point-x p1)))
+            (dy (- (geo-point-y p2) (geo-point-y p1)))
+            (nx (first n))
+            (ny (first (rest n))))
+          (let
+            ((dot (+ (* dx nx) (* dy ny))))
+            (if (= dot 0) "parallel" "point")))))))
+
+;; ── tests ─────────────────────────────────────────────────────────────────
+
+(define passed 0)
+(define failed 0)
+(define failures (list))
+
+(define
+  check
+  (fn
+    (label got expected)
+    (if
+      (= got expected)
+      (set! passed (+ passed 1))
+      (begin
+        (set! failed (+ failed 1))
+        (set!
+          failures
+          (append
+            failures
+            (list
+              (str
+                "FAIL ["
+                label
+                "]: got="
+                (inspect got)
+                " expected="
+                (inspect expected)))))))))
+
+;; describe
+(check
+  "describe point"
+  (clos-call-generic
+    "geo-describe"
+    (list (geo-make-point 3 4)))
+  "P(3,4)")
+(check
+  "describe line"
+  (clos-call-generic
+    "geo-describe"
+    (list
+      (geo-make-line
+        (geo-make-point 0 0)
+        (geo-make-point 1 1))))
+  "L[P(0,0)-P(1,1)]")
+(check
+  "describe plane"
+  (clos-call-generic
+    "geo-describe"
+    (list (geo-make-plane 0 1 5)))
+  "Plane(d=5)")
+
+;; intersect point×point
+(check
+  "P∩P same"
+  (clos-call-generic
+    "intersect"
+    (list
+      (geo-make-point 2 3)
+      (geo-make-point 2 3)))
+  "point")
+(check
+  "P∩P diff"
+  (clos-call-generic
+    "intersect"
+    (list
+      (geo-make-point 1 2)
+      (geo-make-point 3 4)))
+  "empty")
+
+;; intersect point×line
+(let
+  ((origin (geo-make-point 0 0))
+    (p10 (geo-make-point 10 0))
+    (p55 (geo-make-point 5 5))
+    (l-x
+      (geo-make-line
+        (geo-make-point 0 0)
+        (geo-make-point 10 0))))
+  (begin
+    (check
+      "P∩L on line"
+      (clos-call-generic "intersect" (list p10 l-x))
+      "point")
+    (check
+      "P∩L on x-axis"
+      (clos-call-generic "intersect" (list origin l-x))
+      "point")
+    (check
+      "P∩L off line"
+      (clos-call-generic "intersect" (list p55 l-x))
+      "empty")))
+
+;; intersect line×line
+(let
+  ((horiz (geo-make-line (geo-make-point 0 0) (geo-make-point 10 0)))
+    (vert
+      (geo-make-line
+        (geo-make-point 5 -5)
+        (geo-make-point 5 5)))
+    (horiz2
+      (geo-make-line
+        (geo-make-point 0 3)
+        (geo-make-point 10 3))))
+  (begin
+    (check
+      "L∩L crossing"
+      (clos-call-generic "intersect" (list horiz vert))
+      "point")
+    (check
+      "L∩L parallel"
+      (clos-call-generic "intersect" (list horiz horiz2))
+      "parallel")))
+
+;; intersect line×plane
+(let
+  ((diag (geo-make-line (geo-make-point 0 0) (geo-make-point 1 1)))
+    (vert-plane (geo-make-plane 1 0 5))
+    (diag-plane (geo-make-plane -1 1 0)))
+  (begin
+    (check
+      "L∩Plane cross"
+      (clos-call-generic "intersect" (list diag vert-plane))
+      "point")
+    (check
+      "L∩Plane parallel"
+      (clos-call-generic "intersect" (list diag diag-plane))
+      "parallel")))
+
+;; ── summary ────────────────────────────────────────────────────────────────
+
+(define geo-passed passed)
+(define geo-failed failed)
+(define geo-failures failures)
\ No newline at end of file
diff --git a/lib/common-lisp/tests/programs/mop-trace.sx b/lib/common-lisp/tests/programs/mop-trace.sx
new file mode 100644
index 00000000..4b3ecb8a
--- /dev/null
+++ b/lib/common-lisp/tests/programs/mop-trace.sx
@@ -0,0 +1,228 @@
+;; mop-trace.sx — :before/:after method tracing with CLOS
+;;
+;; Classic CLOS pattern: instrument generic functions with :before and :after
+;; qualifiers to print call/return traces without modifying the primary method.
+;;
+;; Depends on: lib/common-lisp/runtime.sx, lib/common-lisp/clos.sx
+
+;; ── trace log (mutable accumulator) ───────────────────────────────────────
+
+(define trace-log (list))
+
+(define
+  trace-push
+  (fn (msg) (set! trace-log (append trace-log (list msg)))))
+
+(define trace-clear (fn () (set! trace-log (list))))
+
+;; ── domain classes ─────────────────────────────────────────────────────────
+
+(clos-defclass "shape" (list "t") (list {:initform "white" :initarg ":color" :reader nil :writer nil :accessor nil :name "color"}))
+
+(clos-defclass "circle" (list "shape") (list {:initform 1 :initarg ":radius" :reader nil :writer nil :accessor nil :name "radius"}))
+
+(clos-defclass "rect" (list "shape") (list {:initform 1 :initarg ":width" :reader nil :writer nil :accessor nil :name "width"} {:initform 1 :initarg ":height" :reader nil :writer nil :accessor nil :name "height"}))
+
+;; ── generic function: area ─────────────────────────────────────────────────
+
+(clos-defgeneric "area" {})
+
+;; primary methods
+(clos-defmethod
+  "area"
+  (list)
+  (list "circle")
+  (fn
+    (args next-fn)
+    (let
+      ((c (first args)))
+      (let ((r (clos-slot-value c "radius"))) (* r r)))))
+
+(clos-defmethod
+  "area"
+  (list)
+  (list "rect")
+  (fn
+    (args next-fn)
+    (let
+      ((r (first args)))
+      (* (clos-slot-value r "width") (clos-slot-value r "height")))))
+
+;; :before tracing
+(clos-defmethod
+  "area"
+  (list "before")
+  (list "shape")
+  (fn
+    (args next-fn)
+    (trace-push (str "BEFORE area(" (clos-class-of (first args)) ")"))))
+
+;; :after tracing
+(clos-defmethod
+  "area"
+  (list "after")
+  (list "shape")
+  (fn
+    (args next-fn)
+    (trace-push (str "AFTER area(" (clos-class-of (first args)) ")"))))
+
+;; ── generic function: describe-shape ──────────────────────────────────────
+
+(clos-defgeneric "describe-shape" {})
+
+(clos-defmethod
+  "describe-shape"
+  (list)
+  (list "shape")
+  (fn
+    (args next-fn)
+    (let
+      ((s (first args)))
+      (str "shape[" (clos-slot-value s "color") "]"))))
+
+(clos-defmethod
+  "describe-shape"
+  (list)
+  (list "circle")
+  (fn
+    (args next-fn)
+    (let
+      ((c (first args)))
+      (str
+        "circle[r="
+        (clos-slot-value c "radius")
+        " "
+        (clos-call-next-method next-fn)
+        "]"))))
+
+(clos-defmethod
+  "describe-shape"
+  (list)
+  (list "rect")
+  (fn
+    (args next-fn)
+    (let
+      ((r (first args)))
+      (str
+        "rect["
+        (clos-slot-value r "width")
+        "x"
+        (clos-slot-value r "height")
+        " "
+        (clos-call-next-method next-fn)
+        "]"))))
+
+;; :before on base shape (fires for all subclasses too)
+(clos-defmethod
+  "describe-shape"
+  (list "before")
+  (list "shape")
+  (fn
+    (args next-fn)
+    (trace-push
+      (str "BEFORE describe-shape(" (clos-class-of (first args)) ")"))))
+
+;; ── tests ─────────────────────────────────────────────────────────────────
+
+(define passed 0)
+(define failed 0)
+(define failures (list))
+
+(define
+  check
+  (fn
+    (label got expected)
+    (if
+      (= got expected)
+      (set! passed (+ passed 1))
+      (begin
+        (set! failed (+ failed 1))
+        (set!
+          failures
+          (append
+            failures
+            (list
+              (str
+                "FAIL ["
+                label
+                "]: got="
+                (inspect got)
+                " expected="
+                (inspect expected)))))))))
+
+;; ── area tests ────────────────────────────────────────────────────────────
+
+;; circle area = r*r (no pi — integer arithmetic for predictability)
+(let
+  ((c (clos-make-instance "circle" ":radius" 5 ":color" "red")))
+  (do
+    (trace-clear)
+    (check "circle area" (clos-call-generic "area" (list c)) 25)
+    (check
+      ":before fired for circle"
+      (= (first trace-log) "BEFORE area(circle)")
+      true)
+    (check
+      ":after fired for circle"
+      (= (first (rest trace-log)) "AFTER area(circle)")
+      true)
+    (check "trace length 2" (len trace-log) 2)))
+
+;; rect area = w*h
+(let
+  ((r (clos-make-instance "rect" ":width" 4 ":height" 6 ":color" "blue")))
+  (do
+    (trace-clear)
+    (check "rect area" (clos-call-generic "area" (list r)) 24)
+    (check
+      ":before fired for rect"
+      (= (first trace-log) "BEFORE area(rect)")
+      true)
+    (check
+      ":after fired for rect"
+      (= (first (rest trace-log)) "AFTER area(rect)")
+      true)
+    (check "trace length 2 (rect)" (len trace-log) 2)))
+
+;; ── describe-shape tests ───────────────────────────────────────────────────
+
+(let
+  ((c (clos-make-instance "circle" ":radius" 3 ":color" "green")))
+  (do
+    (trace-clear)
+    (check
+      "circle describe"
+      (clos-call-generic "describe-shape" (list c))
+      "circle[r=3 shape[green]]")
+    (check
+      ":before fired for describe circle"
+      (= (first trace-log) "BEFORE describe-shape(circle)")
+      true)))
+
+(let
+  ((r (clos-make-instance "rect" ":width" 2 ":height" 7 ":color" "black")))
+  (do
+    (trace-clear)
+    (check
+      "rect describe"
+      (clos-call-generic "describe-shape" (list r))
+      "rect[2x7 shape[black]]")
+    (check
+      ":before fired for describe rect"
+      (= (first trace-log) "BEFORE describe-shape(rect)")
+      true)))
+
+;; ── call-next-method: circle -> shape ─────────────────────────────────────
+
+(let
+  ((c (clos-make-instance "circle" ":radius" 1 ":color" "purple")))
+  (check
+    "call-next-method result in describe"
+    (clos-call-generic "describe-shape" (list c))
+    "circle[r=1 shape[purple]]"))
+
+;; ── summary ────────────────────────────────────────────────────────────────
+
+(define mop-passed passed)
+(define mop-failed failed)
+(define mop-failures failures)
diff --git a/plans/common-lisp-on-sx.md b/plans/common-lisp-on-sx.md
index acabfd85..dc188c64 100644
--- a/plans/common-lisp-on-sx.md
+++ b/plans/common-lisp-on-sx.md
@@ -62,8 +62,8 @@ Core mapping:
 - [x] `unwind-protect` cleanup frame
 - [x] `multiple-value-bind`, `multiple-value-call`, `multiple-value-prog1`, `values`, `nth-value`
 - [x] `defun`, `defparameter`, `defvar`, `defconstant`, `declaim`, `proclaim` (no-op)
-- [ ] Dynamic variables — `defvar`/`defparameter` produce specials; `let` rebinds via parameterize-style scope
-- [x] 127 tests in `lib/common-lisp/tests/eval.sx`
+- [x] Dynamic variables — `defvar`/`defparameter` produce specials; `let` rebinds via parameterize-style scope
+- [x] 182 tests in `lib/common-lisp/tests/eval.sx`
 
 ### Phase 3 — conditions + restarts (THE SHOWCASE)
 - [x] `define-condition` — class hierarchy rooted at `condition`/`error`/`warning`/`simple-error`/`simple-warning`/`type-error`/`arithmetic-error`/`division-by-zero`
@@ -81,17 +81,17 @@ Core mapping:
 - [x] `lib/common-lisp/conformance.sh` + runner, `scoreboard.json` + `scoreboard.md` (363 total tests)
 
 ### Phase 4 — CLOS
-- [ ] `defclass` with `:initarg`/`:initform`/`:accessor`/`:reader`/`:writer`/`:allocation`
-- [ ] `make-instance`, `slot-value`, `(setf slot-value)`, `with-slots`, `with-accessors`
-- [ ] `defgeneric` with `:method-combination` (standard, plus `+`, `and`, `or`)
-- [ ] `defmethod` with `:before` / `:after` / `:around` qualifiers
-- [ ] `call-next-method` (continuation), `next-method-p`
-- [ ] `class-of`, `find-class`, `slot-boundp`, `change-class` (basic)
-- [ ] Multiple dispatch — method specificity by argument-class precedence list
-- [ ] Built-in classes registered for tagged values (`integer`, `float`, `string`, `symbol`, `cons`, `null`, `t`)
-- [ ] Classic programs:
-  - [ ] `geometry.lisp` — `intersect` generic dispatching on (point line), (line line), (line plane)…
-  - [ ] `mop-trace.lisp` — `:before` + `:after` printing call trace
+- [x] `defclass` with `:initarg`/`:initform`/`:accessor`/`:reader`/`:writer`/`:allocation`
+- [x] `make-instance`, `slot-value`, `(setf slot-value)`, `with-slots`, `with-accessors`
+- [x] `defgeneric` with `:method-combination` (standard, plus `+`, `and`, `or`)
+- [x] `defmethod` with `:before` / `:after` / `:around` qualifiers
+- [x] `call-next-method` (continuation), `next-method-p`
+- [x] `class-of`, `find-class`, `slot-boundp`, `change-class` (basic)
+- [x] Multiple dispatch — method specificity by argument-class precedence list
+- [x] Built-in classes registered for tagged values (`integer`, `float`, `string`, `symbol`, `cons`, `null`, `t`)
+- [x] Classic programs:
+  - [x] `geometry.sx` — `intersect` generic dispatching on (point line), (line line), (line plane) — 12 tests
+  - [x] `mop-trace.sx` — `:before` + `:after` printing call trace — 13 tests
 
 ### Phase 5 — macros + LOOP + reader macros
 - [ ] `defmacro`, `macrolet`, `symbol-macrolet`, `macroexpand-1`, `macroexpand`
@@ -124,6 +124,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-05: Phase 4 CLOS fully complete — `lib/common-lisp/clos.sx` (27 forms): clos-class-registry (8 built-in classes), defclass/make-instance/slot-value/slot-boundp/set-slot-value!/find-class/change-class, defgeneric/defmethod with :before/:after/:around, clos-call-generic (standard method combination: sort by specificity, fire befores, call primary chain, fire afters in reverse), call-next-method/next-method-p, with-slots, accessor installation; 41 tests in `tests/clos.sx`; classic programs `geometry.sx` (12 tests, multi-dispatch intersect on P/L/Plane) and `mop-trace.sx` (13 tests, :before/:after tracing). Dynamic variables in eval.sx: cl-apply-dyn saves/restores global bindings around let for specials (cl-mark-special!/cl-special?/cl-dyn-unbound). Key gotchas: qualifier strings are "before"/"after"/"around" (no colon); dict-set pure = assoc; dict->list = (map (fn (k) (list k (get d k))) (keys d)); clos-add-reader-method bootstrapped via set! after defmethod defined; test isolation: use unique var names to avoid *y* collision. 437 total tests, 0 failed.
 - 2026-05-05: Phase 3 fully complete — conformance.sh runner + scoreboard.json/scoreboard.md; 363 total tests across all suites (79 reader, 31 parser, 174 eval, 59 conditions, 7+6+7 classic programs).
 - 2026-05-05: Phase 3 complete — cl-debugger-hook/cl-invoke-debugger in runtime.sx (cl-error routes through hook), cl-break-on-signals (fires hook before handlers on type match), cl-invoke-restart-interactively (calls fn with no args); 4 new tests (147 total). Phase 3 all boxes ticked.
 - 2026-05-05: Phase 3 interactive-debugger.sx — cl-debugger-hook global, cl-invoke-debugger, cl-error-with-debugger, make-policy-debugger; 7 tests (143 total). Tests wired into test.sh program suite runner. Phase 3 condition core complete.