diff --git a/.beads/issues.jsonl b/.beads/issues.jsonl
index 5647bb2..5141163 100644
--- a/.beads/issues.jsonl
+++ b/.beads/issues.jsonl
@@ -5,6 +5,7 @@
 {"_type":"issue","id":"jolt-x8s","title":"CRITICAL: collections not callable as IFn (vector/map/set)","status":"closed","priority":1,"issue_type":"bug","owner":"yogthos@gmail.com","created_at":"2026-06-04T17:46:14Z","created_by":"Yogthos","updated_at":"2026-06-04T17:50:54Z","closed_at":"2026-06-04T17:50:54Z","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"jolt-alz","title":"CRITICAL: self-referential lazy-seq/lazy-cat yields only literal prefix","status":"closed","priority":1,"issue_type":"bug","owner":"yogthos@gmail.com","created_at":"2026-06-04T17:46:13Z","created_by":"Yogthos","updated_at":"2026-06-04T18:09:15Z","closed_at":"2026-06-04T18:09:15Z","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"jolt-wec","title":"CRITICAL: multi-collection map returns unrealized thunk","status":"closed","priority":1,"issue_type":"bug","owner":"yogthos@gmail.com","created_at":"2026-06-04T17:46:12Z","created_by":"Yogthos","updated_at":"2026-06-04T18:06:02Z","closed_at":"2026-06-04T18:06:02Z","dependency_count":0,"dependent_count":0,"comment_count":0}
+{"_type":"issue","id":"jolt-ns4","title":"Persistent collections + build-time mutable/immutable toggle","description":"Add JOLT_MUTABLE build flag (config.janet). Fix list-mutation bug (conj mutated original). Round 2: structural-sharing persistent vector (32-way trie) hooked into all core ops. Round 3: persistent lists / HAMT maps.","status":"open","priority":2,"issue_type":"feature","owner":"yogthos@gmail.com","created_at":"2026-06-04T22:15:39Z","created_by":"Yogthos","updated_at":"2026-06-04T22:15:39Z","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"jolt-gxr","title":"HIGH: aliased namespace calls (require :as) don't resolve (s/join)","status":"closed","priority":2,"issue_type":"bug","owner":"yogthos@gmail.com","created_at":"2026-06-04T18:09:58Z","created_by":"Yogthos","updated_at":"2026-06-04T18:45:52Z","closed_at":"2026-06-04T18:45:52Z","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"jolt-4z9","title":"HIGH: dispatch — multimethod :default, protocol-on-record, extend-protocol","status":"closed","priority":2,"issue_type":"bug","owner":"yogthos@gmail.com","created_at":"2026-06-04T18:09:57Z","created_by":"Yogthos","updated_at":"2026-06-04T18:40:43Z","closed_at":"2026-06-04T18:40:43Z","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"_type":"issue","id":"jolt-40n","title":"HIGH: update/update-in/assoc-in fail on map literals (struct)","status":"closed","priority":2,"issue_type":"bug","owner":"yogthos@gmail.com","created_at":"2026-06-04T18:09:56Z","created_by":"Yogthos","updated_at":"2026-06-04T18:40:42Z","closed_at":"2026-06-04T18:40:42Z","dependency_count":0,"dependent_count":0,"comment_count":0}
diff --git a/src/jolt/config.janet b/src/jolt/config.janet
new file mode 100644
index 0000000..f4e6a11
--- /dev/null
+++ b/src/jolt/config.janet
@@ -0,0 +1,15 @@
+# Build-time collection mode.
+#
+# Jolt can be built with either immutable (persistent) collections — proper
+# Clojure value semantics — or fast Janet-native mutable collections.
+#
+#   jpm build                 # immutable (default)
+#   JOLT_MUTABLE=1 jpm build  # mutable
+#
+# This reads the environment at module-load time, so for a jpm-compiled
+# executable the value is fixed when the binary is built (a true compile flag).
+# `mutable?` is a constant, so the type-mode branches throughout core fold away.
+(def mutable? (= "1" (os/getenv "JOLT_MUTABLE")))
+
+# Convenience: immutable? is the default.
+(def immutable? (not mutable?))
diff --git a/src/jolt/core.janet b/src/jolt/core.janet
index 4645ea3..7d7bf66 100644
--- a/src/jolt/core.janet
+++ b/src/jolt/core.janet
@@ -4,6 +4,7 @@
 (use ./types)
 (use ./phm)
 (use ./regex)
+(use ./config)
 
 # ============================================================
 # Predicates
@@ -195,7 +196,8 @@
     (tuple/slice (tuple ;(array/concat (array/slice coll) xs)))
     (if (array? coll)
       (do
-        (var result coll)
+        # lists prepend; copy first unless built in mutable mode
+        (var result (if mutable? coll (array/slice coll)))
         (var i 0)
         (while (< i (length xs))
           (set result (array/insert result 0 (xs i)))