diff --git a/src/jolt/core.janet b/src/jolt/core.janet
index 4fcfb0d..3f7be35 100644
--- a/src/jolt/core.janet
+++ b/src/jolt/core.janet
@@ -448,7 +448,9 @@
         (if (and (indexed? r) (= 2 (length r)) (function? (in r 1)))
           r
           (coll->cells r)))
-      (if (lazy-seq? c) (realize-ls c)
+      (if (lazy-seq? c)
+        (let [cell (realize-ls c)]
+          (if (= :jolt/pending cell) nil cell))
         (if (indexed? c)
           (if (= 0 (length c)) nil
             (if (and (= 2 (length c)) (function? (in c 1)))
@@ -461,34 +463,21 @@
           nil)))))
 
 (defn core-concat [& colls]
-  "Lazy concatenation — returns a lazy-seq that yields elements one at a time.
-  Fully functional: each rest thunk captures its own state, so multiple access
-  paths (e.g. fib-seq used simultaneously in (rest fib-seq) and fib-seq) work
-  without shared-mutable-state corruption."
-  (defn step [cs]
-    "Return a thunk that produces nil (end) or @[value, rest-thunk]."
-    (if (= 0 (length cs))
-      (fn [] nil)
-      (let [c (in cs 0)
-            remaining (array/slice cs 1)]
+  "Lazy concatenation — returns a lazy-seq that yields elements one at a time."
+  (var cs (if (tuple? colls) (array/slice colls) colls))
+  (var cur nil)
+  (defn next-cell []
+    (var cell (coll->cells cur))
+    (while (and (nil? cell) (not (nil? cs)) (> (length cs) 0))
+      (set cur (in cs 0))
+      (set cs (array/slice cs 1))
+      (set cell (coll->cells cur)))
+    (if (nil? cell) nil
+      @[(in cell 0)
         (fn []
-          (let [cell (coll->cells c)]
-            (if (nil? cell)
-              ((step remaining))
-              (let [value (in cell 0)
-                    rest-fn (in cell 1)]
-                @[value
-                  (fn []
-                    (if (nil? rest-fn)
-                      ((step remaining))
-                      (let [new-cs (array/new-filled (+ 1 (length remaining)) nil)]
-                        (put new-cs 0 rest-fn)
-                        (var i 0)
-                        (while (< i (length remaining))
-                          (put new-cs (+ i 1) (in remaining i))
-                          (++ i))
-                        ((step new-cs)))))])))))))
-  (make-lazy-seq (step (if (tuple? colls) (array/slice colls) colls))))
+          (set cur (in cell 1))
+          (next-cell))]))
+  (make-lazy-seq next-cell))
 
 (defn core-reverse [coll]
   (if (nil? coll) @[]
diff --git a/src/jolt/phm.janet b/src/jolt/phm.janet
index 8a802d9..bc4e4a2 100644
--- a/src/jolt/phm.janet
+++ b/src/jolt/phm.janet
@@ -131,11 +131,12 @@
 (defn realize-ls
   "Force a LazySeq cell. Returns nil (empty) or [first-val, rest-thunk].
   If the thunk returns another lazy-seq, recursively realize it.
-  Detects self-referencing cycles by setting :realized before calling thunk."
+  Uses :jolt/pending sentinel to detect self-referencing cycles."
   [ls]
   (if (get ls :realized)
     (ls :val)
     (do
+      (put ls :val :jolt/pending)
       (put ls :realized true)
       (let [raw ((ls :fn))
             v (if (lazy-seq? raw) (realize-ls raw) raw)]
@@ -144,7 +145,7 @@
 
 (defn ls-first [ls]
   (let [cell (realize-ls ls)]
-    (if (or (nil? cell) (= 0 (length cell))) nil (in cell 0))))
+    (if (or (nil? cell) (= :jolt/pending cell) (= 0 (length cell))) nil (in cell 0))))
 
 (defn ls-rest [ls]
   (let [cell (realize-ls ls)]