diff --git a/lib/smalltalk/runtime.sx b/lib/smalltalk/runtime.sx
new file mode 100644
index 00000000..1f66e5ec
--- /dev/null
+++ b/lib/smalltalk/runtime.sx
@@ -0,0 +1,291 @@
+;; Smalltalk runtime — class table, bootstrap hierarchy, type→class mapping,
+;; instance construction. Method dispatch / eval-ast live in a later layer.
+;;
+;; Class record shape:
+;;   {:name "Foo"
+;;    :superclass "Object"          ; or nil for Object itself
+;;    :ivars (list "x" "y")          ; instance variable names declared on this class
+;;    :methods   (dict selector→method-record)
+;;    :class-methods (dict selector→method-record)}
+;;
+;; A method record is the AST returned by st-parse-method, plus a :defining-class
+;; field so super-sends can resolve from the right place. (Methods are registered
+;; via runtime helpers that fill the field.)
+;;
+;; The class table is a single dict keyed by class name. Bootstrap installs the
+;; canonical hierarchy. Test code resets it via (st-bootstrap-classes!).
+
+(define st-class-table {})
+
+(define st-class-table-clear! (fn () (set! st-class-table {})))
+
+(define
+  st-class-define!
+  (fn
+    (name superclass ivars)
+    (begin
+      (set!
+        st-class-table
+        (assoc
+          st-class-table
+          name
+          {:name name
+           :superclass superclass
+           :ivars ivars
+           :methods {}
+           :class-methods {}}))
+      name)))
+
+(define
+  st-class-get
+  (fn (name) (if (has-key? st-class-table name) (get st-class-table name) nil)))
+
+(define
+  st-class-exists?
+  (fn (name) (has-key? st-class-table name)))
+
+(define
+  st-class-superclass
+  (fn
+    (name)
+    (let
+      ((c (st-class-get name)))
+      (cond ((= c nil) nil) (else (get c :superclass))))))
+
+;; Walk class chain root-to-leaf? No, follow superclass chain leaf-to-root.
+;; Returns list of class names starting at `name` and ending with the root.
+(define
+  st-class-chain
+  (fn
+    (name)
+    (let ((acc (list)) (cur name))
+      (begin
+        (define
+          ch-loop
+          (fn
+            ()
+            (when
+              (and (not (= cur nil)) (st-class-exists? cur))
+              (begin
+                (append! acc cur)
+                (set! cur (st-class-superclass cur))
+                (ch-loop)))))
+        (ch-loop)
+        acc))))
+
+;; Inherited + own ivars in declaration order from root to leaf.
+(define
+  st-class-all-ivars
+  (fn
+    (name)
+    (let ((chain (reverse (st-class-chain name))) (out (list)))
+      (begin
+        (for-each
+          (fn
+            (cn)
+            (let
+              ((c (st-class-get cn)))
+              (when
+                (not (= c nil))
+                (for-each (fn (iv) (append! out iv)) (get c :ivars)))))
+          chain)
+        out))))
+
+;; Method install. The defining-class field is stamped on the method record
+;; so super-sends look up from the right point in the chain.
+(define
+  st-class-add-method!
+  (fn
+    (cls-name selector method-ast)
+    (let
+      ((cls (st-class-get cls-name)))
+      (cond
+        ((= cls nil) (error (str "st-class-add-method!: unknown class " cls-name)))
+        (else
+          (let
+            ((m (assoc method-ast :defining-class cls-name)))
+            (begin
+              (set!
+                st-class-table
+                (assoc
+                  st-class-table
+                  cls-name
+                  (assoc
+                    cls
+                    :methods
+                    (assoc (get cls :methods) selector m))))
+              selector)))))))
+
+(define
+  st-class-add-class-method!
+  (fn
+    (cls-name selector method-ast)
+    (let
+      ((cls (st-class-get cls-name)))
+      (cond
+        ((= cls nil) (error (str "st-class-add-class-method!: unknown class " cls-name)))
+        (else
+          (let
+            ((m (assoc method-ast :defining-class cls-name)))
+            (begin
+              (set!
+                st-class-table
+                (assoc
+                  st-class-table
+                  cls-name
+                  (assoc
+                    cls
+                    :class-methods
+                    (assoc (get cls :class-methods) selector m))))
+              selector)))))))
+
+;; Method lookup: walk superclass chain starting at `cls-name`.
+;; class-side? = true searches :class-methods, false searches :methods.
+;; Returns the method record (with :defining-class) or nil.
+(define
+  st-method-lookup
+  (fn
+    (cls-name selector class-side?)
+    (let
+      ((found nil))
+      (begin
+        (define
+          ml-loop
+          (fn
+            (cur)
+            (when
+              (and (= found nil) (not (= cur nil)) (st-class-exists? cur))
+              (let
+                ((c (st-class-get cur)))
+                (let
+                  ((dict (if class-side? (get c :class-methods) (get c :methods))))
+                  (cond
+                    ((has-key? dict selector) (set! found (get dict selector)))
+                    (else (ml-loop (get c :superclass)))))))))
+        (ml-loop cls-name)
+        found))))
+
+;; SX value → Smalltalk class name. Native types are not boxed.
+(define
+  st-class-of
+  (fn
+    (v)
+    (cond
+      ((= v nil) "UndefinedObject")
+      ((= v true) "True")
+      ((= v false) "False")
+      ((integer? v) "SmallInteger")
+      ((number? v) "Float")
+      ((string? v) "String")
+      ((symbol? v) "Symbol")
+      ((list? v) "Array")
+      ((and (dict? v) (has-key? v :type) (= (get v :type) "st-instance"))
+        (get v :class))
+      ((and (dict? v) (has-key? v :type) (= (get v :type) "block"))
+        "BlockClosure")
+      ((and (dict? v) (has-key? v :st-block?) (get v :st-block?))
+        "BlockClosure")
+      ((dict? v) "Dictionary")
+      ((lambda? v) "BlockClosure")
+      (else "Object"))))
+
+;; Construct a fresh instance of cls-name. Ivars (own + inherited) start as nil.
+(define
+  st-make-instance
+  (fn
+    (cls-name)
+    (cond
+      ((not (st-class-exists? cls-name))
+        (error (str "st-make-instance: unknown class " cls-name)))
+      (else
+        (let
+          ((iv-names (st-class-all-ivars cls-name)) (ivars {}))
+          (begin
+            (for-each (fn (n) (set! ivars (assoc ivars n nil))) iv-names)
+            {:type "st-instance" :class cls-name :ivars ivars}))))))
+
+(define
+  st-instance?
+  (fn
+    (v)
+    (and (dict? v) (has-key? v :type) (= (get v :type) "st-instance"))))
+
+(define
+  st-iv-get
+  (fn
+    (inst name)
+    (let ((ivs (get inst :ivars)))
+      (if (has-key? ivs name) (get ivs name) nil))))
+
+(define
+  st-iv-set!
+  (fn
+    (inst name value)
+    (let
+      ((new-ivars (assoc (get inst :ivars) name value)))
+      (assoc inst :ivars new-ivars))))
+
+;; Inherits-from check: is `descendant` either equal to `ancestor` or a subclass?
+(define
+  st-class-inherits-from?
+  (fn
+    (descendant ancestor)
+    (let ((found false) (cur descendant))
+      (begin
+        (define
+          ih-loop
+          (fn
+            ()
+            (when
+              (and (not found) (not (= cur nil)) (st-class-exists? cur))
+              (cond
+                ((= cur ancestor) (set! found true))
+                (else
+                  (begin
+                    (set! cur (st-class-superclass cur))
+                    (ih-loop)))))))
+        (ih-loop)
+        found))))
+
+;; Bootstrap the canonical class hierarchy. Reset and rebuild.
+(define
+  st-bootstrap-classes!
+  (fn
+    ()
+    (begin
+      (st-class-table-clear!)
+      ;; Root
+      (st-class-define! "Object" nil (list))
+      ;; Class side machinery
+      (st-class-define! "Behavior" "Object" (list "superclass" "methodDict" "format"))
+      (st-class-define! "ClassDescription" "Behavior" (list "instanceVariables" "organization"))
+      (st-class-define! "Class" "ClassDescription" (list "name" "subclasses"))
+      (st-class-define! "Metaclass" "ClassDescription" (list "thisClass"))
+      ;; Pseudo-variable types
+      (st-class-define! "UndefinedObject" "Object" (list))
+      (st-class-define! "Boolean" "Object" (list))
+      (st-class-define! "True" "Boolean" (list))
+      (st-class-define! "False" "Boolean" (list))
+      ;; Magnitudes
+      (st-class-define! "Magnitude" "Object" (list))
+      (st-class-define! "Number" "Magnitude" (list))
+      (st-class-define! "Integer" "Number" (list))
+      (st-class-define! "SmallInteger" "Integer" (list))
+      (st-class-define! "LargePositiveInteger" "Integer" (list))
+      (st-class-define! "Float" "Number" (list))
+      (st-class-define! "Character" "Magnitude" (list "value"))
+      ;; Collections
+      (st-class-define! "Collection" "Object" (list))
+      (st-class-define! "SequenceableCollection" "Collection" (list))
+      (st-class-define! "ArrayedCollection" "SequenceableCollection" (list))
+      (st-class-define! "Array" "ArrayedCollection" (list))
+      (st-class-define! "String" "ArrayedCollection" (list))
+      (st-class-define! "Symbol" "String" (list))
+      (st-class-define! "OrderedCollection" "SequenceableCollection" (list "array" "firstIndex" "lastIndex"))
+      (st-class-define! "Dictionary" "Collection" (list))
+      ;; Blocks / contexts
+      (st-class-define! "BlockClosure" "Object" (list))
+      "ok")))
+
+;; Initialise on load. Tests can re-bootstrap to reset state.
+(st-bootstrap-classes!)
diff --git a/lib/smalltalk/test.sh b/lib/smalltalk/test.sh
index 90c01854..afe6ef3b 100755
--- a/lib/smalltalk/test.sh
+++ b/lib/smalltalk/test.sh
@@ -59,10 +59,12 @@ EPOCHS
 (epoch 2)
 (load "lib/smalltalk/parser.sx")
 (epoch 3)
-(load "lib/smalltalk/tests/tokenize.sx")
+(load "lib/smalltalk/runtime.sx")
 (epoch 4)
-(load "$FILE")
+(load "lib/smalltalk/tests/tokenize.sx")
 (epoch 5)
+(load "$FILE")
+(epoch 6)
 (eval "(list st-test-pass st-test-fail)")
 EPOCHS
   fi
@@ -108,10 +110,12 @@ EPOCHS
 (epoch 2)
 (load "lib/smalltalk/parser.sx")
 (epoch 3)
-(load "lib/smalltalk/tests/tokenize.sx")
+(load "lib/smalltalk/runtime.sx")
 (epoch 4)
-(load "$FILE")
+(load "lib/smalltalk/tests/tokenize.sx")
 (epoch 5)
+(load "$FILE")
+(epoch 6)
 (eval "(map (fn (f) (get f :name)) st-test-fails)")
 EPOCHS
     fi
diff --git a/lib/smalltalk/tests/runtime.sx b/lib/smalltalk/tests/runtime.sx
new file mode 100644
index 00000000..3561132e
--- /dev/null
+++ b/lib/smalltalk/tests/runtime.sx
@@ -0,0 +1,182 @@
+;; Smalltalk runtime tests — class table, type→class mapping, instances.
+;;
+;; Reuses helpers (st-test, st-deep=?) from tokenize.sx. Counters reset
+;; here so this file's summary covers runtime tests only.
+
+(set! st-test-pass 0)
+(set! st-test-fail 0)
+(set! st-test-fails (list))
+
+;; Fresh hierarchy for every test file.
+(st-bootstrap-classes!)
+
+;; ── 1. Bootstrap installed expected classes ──
+(st-test "Object exists" (st-class-exists? "Object") true)
+(st-test "Behavior exists" (st-class-exists? "Behavior") true)
+(st-test "Metaclass exists" (st-class-exists? "Metaclass") true)
+(st-test "True/False/UndefinedObject"
+  (and
+    (st-class-exists? "True")
+    (st-class-exists? "False")
+    (st-class-exists? "UndefinedObject"))
+  true)
+(st-test "SmallInteger / Float / Symbol exist"
+  (and
+    (st-class-exists? "SmallInteger")
+    (st-class-exists? "Float")
+    (st-class-exists? "Symbol"))
+  true)
+(st-test "BlockClosure exists" (st-class-exists? "BlockClosure") true)
+
+;; ── 2. Superclass chain ──
+(st-test "Object has no superclass" (st-class-superclass "Object") nil)
+(st-test "Behavior super = Object" (st-class-superclass "Behavior") "Object")
+(st-test "True super = Boolean" (st-class-superclass "True") "Boolean")
+(st-test "Symbol super = String" (st-class-superclass "Symbol") "String")
+(st-test
+  "String chain"
+  (st-class-chain "String")
+  (list "String" "ArrayedCollection" "SequenceableCollection" "Collection" "Object"))
+(st-test
+  "SmallInteger chain"
+  (st-class-chain "SmallInteger")
+  (list "SmallInteger" "Integer" "Number" "Magnitude" "Object"))
+
+;; ── 3. inherits-from? ──
+(st-test "True inherits from Boolean" (st-class-inherits-from? "True" "Boolean") true)
+(st-test "True inherits from Object" (st-class-inherits-from? "True" "Object") true)
+(st-test "True inherits from True" (st-class-inherits-from? "True" "True") true)
+(st-test
+  "True does not inherit from Number"
+  (st-class-inherits-from? "True" "Number")
+  false)
+(st-test
+  "Object does not inherit from Number"
+  (st-class-inherits-from? "Object" "Number")
+  false)
+
+;; ── 4. type→class mapping ──
+(st-test "class-of nil" (st-class-of nil) "UndefinedObject")
+(st-test "class-of true" (st-class-of true) "True")
+(st-test "class-of false" (st-class-of false) "False")
+(st-test "class-of int" (st-class-of 42) "SmallInteger")
+(st-test "class-of zero" (st-class-of 0) "SmallInteger")
+(st-test "class-of negative int" (st-class-of -3) "SmallInteger")
+(st-test "class-of float" (st-class-of 3.14) "Float")
+(st-test "class-of string" (st-class-of "hi") "String")
+(st-test "class-of symbol" (st-class-of (quote foo)) "Symbol")
+(st-test "class-of list" (st-class-of (list 1 2)) "Array")
+(st-test "class-of empty list" (st-class-of (list)) "Array")
+(st-test "class-of lambda" (st-class-of (fn (x) x)) "BlockClosure")
+(st-test "class-of dict" (st-class-of {:a 1}) "Dictionary")
+
+;; ── 5. User class definition ──
+(st-class-define! "Account" "Object" (list "balance" "owner"))
+(st-class-define! "SavingsAccount" "Account" (list "rate"))
+
+(st-test "Account exists" (st-class-exists? "Account") true)
+(st-test "Account super = Object" (st-class-superclass "Account") "Object")
+(st-test
+  "SavingsAccount chain"
+  (st-class-chain "SavingsAccount")
+  (list "SavingsAccount" "Account" "Object"))
+(st-test
+  "SavingsAccount own ivars"
+  (get (st-class-get "SavingsAccount") :ivars)
+  (list "rate"))
+(st-test
+  "SavingsAccount inherited+own ivars"
+  (st-class-all-ivars "SavingsAccount")
+  (list "balance" "owner" "rate"))
+
+;; ── 6. Instance construction ──
+(define a1 (st-make-instance "Account"))
+(st-test "instance is st-instance" (st-instance? a1) true)
+(st-test "instance class" (get a1 :class) "Account")
+(st-test "instance ivars start nil" (st-iv-get a1 "balance") nil)
+(st-test
+  "instance has all expected ivars"
+  (sort (keys (get a1 :ivars)))
+  (sort (list "balance" "owner")))
+(define a2 (st-iv-set! a1 "balance" 100))
+(st-test "iv-set! returns updated copy" (st-iv-get a2 "balance") 100)
+(st-test "iv-set! does not mutate original" (st-iv-get a1 "balance") nil)
+(st-test "class-of instance" (st-class-of a1) "Account")
+
+(define s1 (st-make-instance "SavingsAccount"))
+(st-test
+  "subclass instance has all inherited ivars"
+  (sort (keys (get s1 :ivars)))
+  (sort (list "balance" "owner" "rate")))
+
+;; ── 7. Method install + lookup ──
+(st-class-add-method!
+  "Account"
+  "balance"
+  (st-parse-method "balance ^ balance"))
+(st-class-add-method!
+  "Account"
+  "deposit:"
+  (st-parse-method "deposit: amount balance := balance + amount. ^ self"))
+
+(st-test
+  "method registered"
+  (has-key? (get (st-class-get "Account") :methods) "balance")
+  true)
+
+(st-test
+  "method lookup direct"
+  (= (st-method-lookup "Account" "balance" false) nil)
+  false)
+
+(st-test
+  "method lookup walks superclass"
+  (= (st-method-lookup "SavingsAccount" "deposit:" false) nil)
+  false)
+
+(st-test
+  "method lookup unknown selector"
+  (st-method-lookup "Account" "frobnicate" false)
+  nil)
+
+(st-test
+  "method lookup records defining class"
+  (get (st-method-lookup "SavingsAccount" "balance" false) :defining-class)
+  "Account")
+
+;; SavingsAccount overrides deposit:
+(st-class-add-method!
+  "SavingsAccount"
+  "deposit:"
+  (st-parse-method "deposit: amount ^ super deposit: amount + 1"))
+
+(st-test
+  "subclass override picked first"
+  (get (st-method-lookup "SavingsAccount" "deposit:" false) :defining-class)
+  "SavingsAccount")
+
+(st-test
+  "Account still finds its own deposit:"
+  (get (st-method-lookup "Account" "deposit:" false) :defining-class)
+  "Account")
+
+;; ── 8. Class-side methods ──
+(st-class-add-class-method!
+  "Account"
+  "new"
+  (st-parse-method "new ^ super new"))
+(st-test
+  "class-side lookup"
+  (= (st-method-lookup "Account" "new" true) nil)
+  false)
+(st-test
+  "instance-side does not find class method"
+  (st-method-lookup "Account" "new" false)
+  nil)
+
+;; ── 9. Re-bootstrap resets table ──
+(st-bootstrap-classes!)
+(st-test "after re-bootstrap Account gone" (st-class-exists? "Account") false)
+(st-test "after re-bootstrap Object stays" (st-class-exists? "Object") true)
+
+(list st-test-pass st-test-fail)
diff --git a/plans/smalltalk-on-sx.md b/plans/smalltalk-on-sx.md
index dec0cd01..e6f32b20 100644
--- a/plans/smalltalk-on-sx.md
+++ b/plans/smalltalk-on-sx.md
@@ -56,7 +56,7 @@ Core mapping:
 - [x] Unit tests in `lib/smalltalk/tests/parse.sx`
 
 ### Phase 2 — object model + sequential eval
-- [ ] Class table + bootstrap: `Object`, `Behavior`, `Class`, `Metaclass`, `UndefinedObject`, `Boolean`/`True`/`False`, `Number`/`Integer`/`Float`, `String`, `Symbol`, `Array`, `Block`
+- [x] Class table + bootstrap (`lib/smalltalk/runtime.sx`): canonical hierarchy installed (`Object`, `Behavior`, `ClassDescription`, `Class`, `Metaclass`, `UndefinedObject`, `Boolean`/`True`/`False`, `Magnitude`/`Number`/`Integer`/`SmallInteger`/`Float`/`Character`, `Collection`/`SequenceableCollection`/`ArrayedCollection`/`Array`/`String`/`Symbol`/`OrderedCollection`/`Dictionary`, `BlockClosure`). User class definition via `st-class-define!`, methods via `st-class-add-method!` (stamps `:defining-class` for super), method lookup walks chain, ivars accumulated through superclass chain, native SX value types map to Smalltalk classes via `st-class-of`.
 - [ ] `smalltalk-eval-ast`: literals, variable reference, assignment, message send, cascade, sequence, return
 - [ ] Method lookup: walk class → superclass; cache hit-class on `(class, selector)`
 - [ ] `doesNotUnderstand:` fallback constructing `Message` object
@@ -108,6 +108,7 @@ Core mapping:
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-25: class table + bootstrap (`lib/smalltalk/runtime.sx`, `lib/smalltalk/tests/runtime.sx`). Canonical hierarchy, type→class mapping for native SX values, instance construction, ivar inheritance, method install with `:defining-class` stamp, instance- and class-side method lookup walking the superclass chain. 54 new tests, 185/185 total.
 - 2026-04-25: chunk-stream parser + pragmas + 21 chunk/pragma tests (`lib/smalltalk/tests/parse_chunks.sx`). `st-read-chunks` (with `!!` doubling), `st-parse-chunks` state machine for `methodsFor:` batches incl. class-side. Pragmas with multiple keyword pairs, signed numeric / string / symbol args, in either pragma-then-temps or temps-then-pragma order. 131/131 tests pass.
 - 2026-04-25: expression-level parser + 47 parse tests (`lib/smalltalk/parser.sx`, `lib/smalltalk/tests/parse.sx`). Full message precedence (unary > binary > keyword), cascades, blocks with params/temps, literal/byte arrays, assignment chain, method headers (unary/binary/keyword). Chunk-format `! !` driver deferred to a follow-up box. 110/110 tests pass.
 - 2026-04-25: tokenizer + 63 tests (`lib/smalltalk/tokenizer.sx`, `lib/smalltalk/tests/tokenize.sx`, `lib/smalltalk/test.sh`). All token types covered except scaled decimals `1.5s2` (deferred). `#(` and `#[` emit open tokens; literal-array contents lexed as ordinary tokens for the parser to interpret.