diff --git a/jolt-core/clojure/core/30-macros.clj b/jolt-core/clojure/core/30-macros.clj
index 05f9217..9df03ca 100644
--- a/jolt-core/clojure/core/30-macros.clj
+++ b/jolt-core/clojure/core/30-macros.clj
@@ -295,6 +295,15 @@
                                         (and mt (:num mt)) "num"
                                         :else nil)))
                         fields)
+        ;; per-field MUTABILITY (jolt-c3q): ^:unsynchronized-mutable / ^:volatile-
+        ;; mutable marks a field set!-able. A type with any mutable field opts out
+        ;; of the immutable shape-rec layout and uses the mutable table form, so
+        ;; set! can mutate it (the ctor reads this vector). Spliced as a literal.
+        field-muts (map (fn [f] (let [mt (meta f)]
+                                  (if (and mt (or (:unsynchronized-mutable mt)
+                                                  (:volatile-mutable mt)))
+                                    true false)))
+                        fields)
         impl (fn [proto specs]
                `(extend-type ~tname ~proto
                   ~@(map (fn [spec]
@@ -304,7 +313,7 @@
                              `(~(first spec) ~argv (let [~@binds] ~@(drop 2 spec)))))
                          specs)))]
     `(do
-       (def ~tname (make-deftype-ctor (quote ~tname) [~@field-kws] [~@field-tags]))
+       (def ~tname (make-deftype-ctor (quote ~tname) [~@field-kws] [~@field-tags] [~@field-muts]))
        (def ~arrow ~tname)
        ~@(map (fn [g] (impl (first g) (rest g))) (group-by-head body))
        ~tname)))
diff --git a/src/jolt/eval_runtime.janet b/src/jolt/eval_runtime.janet
index 6e4caef..0e20298 100644
--- a/src/jolt/eval_runtime.janet
+++ b/src/jolt/eval_runtime.janet
@@ -385,20 +385,26 @@
   instances carry a stable tag matching what extend-type registers methods under.
   field-kws is the [:f1 :f2 …] keyword vector; the ctor maps positional args to
   those keys. A ctx-capturing closure (make-deftype-ctor) is the public handle."
-  [ctx type-name-sym field-kws &opt field-tags]
+  [ctx type-name-sym field-kws &opt field-tags field-muts]
   (def type-tag (string (ctx-current-ns ctx) "." (type-name-sym :name)))
   (def kws (d-realize field-kws))
   # per-field type hints (jolt-3ko): a tuple parallel to kws — "Vec3" (a record
   # type name), "num", or nil. The inference resolves these to the field's exact
   # type so reading a field back carries it (a nested record stays typed).
   (def tags (if field-tags (d-realize field-tags) (array/new-filled (length kws))))
+  # jolt-c3q: a type with any ^:unsynchronized-mutable / ^:volatile-mutable field
+  # is set!-able, so it CAN'T be an immutable shape-rec tuple. Such a type uses
+  # the mutable :jolt/deftype table form regardless of :shapes? (set! mutates it,
+  # field reads route through the tagged-table path), and is NOT registered as a
+  # shape so the inference never emits a bare-index read against the table.
+  (def mutable? (and field-muts (some |(identity $) (d-realize field-muts))))
   # The ctor closure itself. Built FIRST so it can be indexed by value below.
   # Records are shape-recs when shapes are active (:shapes? = direct-link, where
   # the inference proves the reads) — the whole field-access pipeline handles
-  # them; otherwise the original :jolt/deftype tables. Read at ctor-BUILD time so
-  # a type is consistently one representation or the other.
+  # them; otherwise (or when mutable) the original :jolt/deftype tables. Read at
+  # ctor-BUILD time so a type is consistently one representation or the other.
   (def the-ctor
-    (if (get (ctx :env) :shapes?)
+    (if (and (get (ctx :env) :shapes?) (not mutable?))
       (fn [& args] (make-record type-tag kws args))
       (fn [& args]
         (var inst @{:jolt/deftype type-tag})
@@ -408,7 +414,9 @@
   # the inference types (->Name ...) as a struct of these fields and field reads
   # on the result bare-index. Keyed by the ctor var-key "ns/->Name" to match how
   # the IR names the call head. Harmless when records aren't shaped (sidx gated).
-  (let [rs (or (get (ctx :env) :record-shapes)
+  # Skipped for mutable types — they're tables, not shape-recs (jolt-c3q).
+  (unless mutable?
+   (let [rs (or (get (ctx :env) :record-shapes)
                (let [t @{}] (put (ctx :env) :record-shapes t) t))
         # ctor-value index: maps each ctor closure to its rs key, so a ^Type hint
         # in another namespace can resolve home through the type var's root value
@@ -428,7 +436,7 @@
                       tags)]
     (put rs (string (ctx-current-ns ctx) "/->" (type-name-sym :name))
          {:fields (tuple ;kws) :type type-tag :tags (tuple ;resolved)})
-    (put cix the-ctor (string (ctx-current-ns ctx) "/->" (type-name-sym :name))))
+    (put cix the-ctor (string (ctx-current-ns ctx) "/->" (type-name-sym :name)))))
   the-ctor)
 
 (defn install-stateful-fns!
@@ -504,7 +512,7 @@
   (ns-intern core "refer-clojure" (fn [& args] (refer-clojure-impl ctx ;args)))
   (ns-intern core "defmulti-setup" (fn [name-sym dispatch & opts] (defmulti-setup ctx name-sym dispatch ;opts)))
   (ns-intern core "defmethod-setup" (fn [mm-sym dval impl] (defmethod-setup ctx mm-sym dval impl)))
-  (ns-intern core "make-deftype-ctor" (fn [name-sym field-kws &opt field-tags] (make-deftype-ctor-impl ctx name-sym field-kws field-tags)))
+  (ns-intern core "make-deftype-ctor" (fn [name-sym field-kws &opt field-tags field-muts] (make-deftype-ctor-impl ctx name-sym field-kws field-tags field-muts)))
   # Var/namespace lookups that need the ctx (the rest of the var fns — var-get/
   # var-set/var?/alter-var-root/alter-meta!/reset-meta! — are plain core-bindings).
   (ns-intern core "find-var" (fn [sym] (find-var ctx sym)))
diff --git a/test/integration/mutable-deftype-test.janet b/test/integration/mutable-deftype-test.janet
new file mode 100644
index 0000000..261cee2
--- /dev/null
+++ b/test/integration/mutable-deftype-test.janet
@@ -0,0 +1,57 @@
+# Mutable deftype fields under shapes/direct-link (jolt-c3q).
+#
+# A deftype field tagged ^:unsynchronized-mutable / ^:volatile-mutable is set! at
+# runtime. Immutable records are shape-recs (immutable tuples) where shapes are
+# active (direct-link), so set! can't mutate them. A deftype with ANY mutable
+# field opts out of the shape-rec layout and uses the mutable table form (which
+# set! already mutates and field reads route through), regardless of :shapes?.
+# Immutable deftypes/records keep the fast shape-rec.
+
+(use ../../src/jolt/api)
+
+(var failures 0)
+(defn- check [label got want]
+  (unless (= got want)
+    (++ failures)
+    (printf "FAIL [%s] got %q want %q" label got want)))
+
+# direct-linking? true => :shapes? on (the mode where immutable records are
+# tuples and this used to error "Can't set! field on non-deftype: tuple").
+(let [ctx (init {:compile? true :direct-linking? true})]
+  (eval-string ctx "(deftype Counter [^:unsynchronized-mutable n])")
+  (eval-string ctx "(def c (->Counter 0))")
+  (check "read initial mutable field" (eval-string ctx "(.-n c)") 0)
+  (eval-string ctx "(set! (.-n c) 5)")
+  (check "read after set!" (eval-string ctx "(.-n c)") 5)
+  (eval-string ctx "(set! (.-n c) (inc (.-n c)))")
+  (check "set! using prior value" (eval-string ctx "(.-n c)") 6)
+  # keyword access reads the same mutated field
+  (check "keyword access sees mutation" (eval-string ctx "(:n c)") 6))
+
+# Mixed mutable + immutable fields: a method reads both, set! touches the mutable.
+(let [ctx (init {:compile? true :direct-linking? true})]
+  (eval-string ctx "(deftype Cell [label ^:unsynchronized-mutable v] Object (toString [_] (str label \"=\" v)))")
+  (eval-string ctx "(def cell (->Cell \"x\" 1))")
+  (check "immutable field reads" (eval-string ctx "(.-label cell)") "x")
+  (check "custom toString before" (eval-string ctx "(str cell)") "x=1")
+  (eval-string ctx "(set! (.-v cell) 42)")
+  (check "mutable field after set!" (eval-string ctx "(.-v cell)") 42)
+  (check "custom toString after mutation" (eval-string ctx "(str cell)") "x=42")
+  (check "immutable field unchanged" (eval-string ctx "(.-label cell)") "x"))
+
+# volatile-mutable is treated the same (also a mutable field).
+(let [ctx (init {:compile? true :direct-linking? true})]
+  (eval-string ctx "(deftype Box [^:volatile-mutable x])")
+  (eval-string ctx "(def b (->Box :a))")
+  (eval-string ctx "(set! (.-x b) :z)")
+  (check "volatile-mutable set!" (eval-string ctx "(.-x b)") :z))
+
+# An all-immutable deftype/record is unaffected: still a shape-rec, fast reads.
+(let [ctx (init {:compile? true :direct-linking? true})]
+  (eval-string ctx "(defrecord Pt [x y])")
+  (check "immutable record reads" (eval-string ctx "(:x (->Pt 3 4))") 3)
+  (check "immutable record reads y" (eval-string ctx "(:y (->Pt 3 4))") 4))
+
+(if (pos? failures)
+  (do (printf "mutable-deftype: %d failure(s)" failures) (os/exit 1))
+  (print "mutable-deftype: all cases passed"))