diff --git a/jolt-core/clojure/core/10-seq.clj b/jolt-core/clojure/core/10-seq.clj
index f5c0fa3..78a27a7 100644
--- a/jolt-core/clojure/core/10-seq.clj
+++ b/jolt-core/clojure/core/10-seq.clj
@@ -22,3 +22,8 @@
     (if (next s)
       (recur (conj ret (first s)) (next s))
       (seq ret))))
+
+(defn mapcat
+  ([f] (comp (map f) cat))
+  ([f & colls]
+   (apply concat (apply map f colls))))
diff --git a/jolt-core/clojure/core/20-coll.clj b/jolt-core/clojure/core/20-coll.clj
index 3194fc6..48c6a36 100644
--- a/jolt-core/clojure/core/20-coll.clj
+++ b/jolt-core/clojure/core/20-coll.clj
@@ -178,17 +178,19 @@
 (defn flatten [coll]
   (filter (complement sequential?) (rest (tree-seq sequential? seq coll))))
 
-;; Eager interleave (Clojure's is lazy): one from each coll in turn, until the
-;; shortest ends.
-(defn interleave [& colls]
-  (if (empty? colls)
-    (list)
-    (let [cs (mapv vec colls)
-          n (apply min (map count cs))]
-      (loop [i 0 out []]
-        (if (< i n)
-          (recur (inc i) (reduce (fn [o c] (conj o (nth c i))) out cs))
-          out)))))
+;; Lazy interleave: round-robin one element from each coll until any exhausts.
+(defn interleave
+  ([] ())
+  ([c1] (lazy-seq c1))
+  ([c1 c2]
+   (lazy-seq
+     (let [s1 (seq c1) s2 (seq c2)]
+       (when (and s1 s2)
+         (cons (first s1)
+               (cons (first s2)
+                     (interleave (rest s1) (rest s2))))))))
+  ([c1 c2 & cs]
+   (apply interleave c1 c2 cs)))
 
 ;; No ratio type on Jolt, so rationalize is identity.
 (defn rationalize [x] x)
diff --git a/src/jolt/core.janet b/src/jolt/core.janet
index fcd7ce4..1684d0c 100644
--- a/src/jolt/core.janet
+++ b/src/jolt/core.janet
@@ -630,7 +630,7 @@
     (core-sorted-map? coll) (let [e (sorted-map-entries coll)] (if (empty? e) nil (tuple ;e)))
     (core-sorted-set? coll) (let [i (coll :items)] (if (empty? i) nil (tuple ;i)))
     (or (nil? coll) (and (or (tuple? coll) (array? coll)) (= 0 (length coll)))) nil
-    (lazy-seq? coll) (ls-seq coll)
+    (lazy-seq? coll) (if (nil? (ls-first coll)) nil coll)
     (pvec? coll) (if (= 0 (pv-count coll)) nil (tuple ;(pv->array coll)))
     (plist? coll) (if (pl-empty? coll) nil (tuple ;(pl->array coll)))
     (buffer? coll) (if (= 0 (length coll)) nil (let [a @[]] (each x coll (array/push a x)) (tuple ;a)))
@@ -1040,14 +1040,23 @@
 (defn core-drop-while [pred & rest]
  (def pred (as-fn pred))
  (if (= 0 (length rest)) (td-drop-while pred)
-  (let [coll (in rest 0)
-        c (realize-for-iteration coll)]
-    (var start 0)
-    (while (and (< start (length c)) (pred (c start)))
-      (++ start))
-    (if (tuple? c)
-      (tuple/slice c start)
-      (array/slice c start)))))
+  (let [coll (in rest 0)]
+   (if (lazy-seq? coll)
+     (do
+       (defn dwstep [c]
+         (fn []
+           (var cur c)
+           (while (and (not (seq-done? cur)) (pred (ls-first cur)))
+             (set cur (ls-rest cur)))
+           (if (seq-done? cur) nil cur)))
+       (make-lazy-seq (dwstep coll)))
+     (let [c (realize-for-iteration coll)]
+       (var start 0)
+       (while (and (< start (length c)) (pred (c start)))
+         (++ start))
+       (if (tuple? c)
+         (tuple/slice c start)
+         (array/slice c start)))))))
 
 (defn coll->cells [c]
   "Convert a seqable to lazy-seq cell chain: nil or [first, rest-thunk].
@@ -1081,22 +1090,35 @@
   construction time. This is essential for self-referential lazy seqs (e.g.
   (def fib (lazy-cat [0 1] (map + (rest fib) fib)))): the later colls must not be
   forced until after the surrounding `def` has bound the var."
-  (defn step [cs]
-    (fn []
-      (if (= 0 (length cs))
-        nil
-        (let [c (in cs 0)
-              remaining (array/slice cs 1)
-              cell (coll->cells c)]
-          (if (nil? cell)
-            # current coll is empty: advance to the next one
-            ((step remaining))
-            (let [val (in cell 0)
-                  rest-fn (in cell 1)]
-              @[val (step (if (nil? rest-fn)
-                            remaining
-                            (array/insert remaining 0 rest-fn)))]))))))
-  (make-lazy-seq (step (if (tuple? colls) (array/slice colls) colls))))
+  (if (= 0 (length colls)) @[]
+    (let [colls (if (tuple? colls) (array/slice colls) colls)]
+      (defn step [cs]
+        (fn []
+          (if (= 0 (length cs))
+            nil
+            (let [c (in cs 0)
+                  remaining (array/slice cs 1)
+                  cell (coll->cells c)]
+              (if (nil? cell)
+                # current coll is empty: advance to the next one
+                ((step remaining))
+                (let [val (in cell 0)
+                      rest-fn (in cell 1)]
+                  @[val (step (if (nil? rest-fn)
+                                remaining
+                                (array/insert remaining 0 rest-fn)))]))))))
+      (make-lazy-seq (step colls)))))
+
+(defn lazy-from
+  "Coerce any seqable to a uniform lazy view without forcing.
+  Returns nil if coll is nil or empty, the LazySeq unchanged if already lazy,
+  or a new LazySeq that walks element by element."
+  [coll]
+  (if (nil? coll) nil
+    (if (lazy-seq? coll) coll
+      (let [cell (coll->cells coll)]
+        (if (nil? cell) nil
+          (make-lazy-seq (fn [] cell)))))))
 
 (defn core-mapcat
   "(mapcat f & colls) — map then concat. (mapcat f) returns a transducer."
@@ -1112,9 +1134,10 @@
               (each x (realize-for-iteration (f (a 1)))
                 (set acc (rf acc x)))
               acc))))
-    # map, then concat; a non-seqable result counts as a single element (this
-    # leniency is what jolt's `for` expansion relies on for :let on the last
-    # binding, whose body yields a scalar rather than a seq).
+    # collection arity: map f over colls, then concatenate. A non-seqable
+    # result counts as a single element (this leniency is what jolt's `for`
+    # expansion relies on for :let on the last binding, whose body yields a
+    # scalar rather than a seq).
     (let [mapped (realize-for-iteration (core-apply core-map f colls))
           seqs (map (fn [item]
                       (if (or (tuple? item) (array? item) (pvec? item)
@@ -1213,14 +1236,18 @@
   (if (lazy-seq? coll)
     (do
       (var seen @{})
-      (var result @[])
-      (var cur coll)
-      (while (not (nil? (ls-first cur)))
-        (let [x (ls-first cur)]
-          (if (nil? (seen x))
-            (do (put seen x true) (array/push result x))))
-        (set cur (ls-rest cur)))
-      result)
+      (defn dstep [c]
+        (fn []
+          (var cur c) (var found false) (var result nil)
+          (while (and (not found) (not (seq-done? cur)))
+            (let [x (ls-first cur)]
+              (set cur (ls-rest cur))
+              (when (nil? (seen x))
+                (put seen x true)
+                (set found true)
+                (set result x))))
+          (if found @[result (dstep cur)] nil)))
+      (make-lazy-seq (dstep coll)))
     (do
       (var seen @{})
       (var result @[])
@@ -1238,14 +1265,31 @@
   [n & rest]
   (let [has-step (> (length rest) 1)
         step (if has-step (first rest) n)
-        coll (realize-for-iteration (if has-step (in rest 1) (first rest)))]
-    (var result @[]) (var i 0)
-    (while (<= (+ i n) (length coll))
-      (var part @[]) (var j 0)
-      (while (< j n) (array/push part (in coll (+ i j))) (++ j))
-      (array/push result (tuple/slice (tuple ;part)))
-      (+= i step))
-    result))
+        coll (if has-step (in rest 1) (first rest))]
+    (if (lazy-seq? coll)
+      (do
+        (defn pstep [c]
+          (fn []
+            (if (seq-done? c) nil
+              (do
+                (var part @[]) (var cur c) (var i 0)
+                (while (and (< i n) (not (seq-done? cur)))
+                  (array/push part (ls-first cur))
+                  (set cur (ls-rest cur))
+                  (++ i))
+                (if (= i n)
+                  (let [next-cur (if (= step n) cur (core-drop (- step n) cur))]
+                    @[(tuple/slice (tuple ;part)) (pstep next-cur)])
+                  nil)))))
+        (make-lazy-seq (pstep coll)))
+      (let [c (realize-for-iteration coll)]
+        (var result @[]) (var i 0)
+        (while (<= (+ i n) (length c))
+          (var part @[]) (var j 0)
+          (while (< j n) (array/push part (in c (+ i j))) (++ j))
+          (array/push result (tuple/slice (tuple ;part)))
+          (+= i step))
+        result))))
 
 (defn core-partition-by [f coll]
   (def f (as-fn f))
@@ -1264,29 +1308,65 @@
   result)
 
 (defn core-partition-all [n coll]
-  (let [c (realize-for-iteration coll)]
-    (var result @[]) (var i 0)
-    (while (< i (length c))
-      (var part @[]) (var j 0)
-      (while (and (< j n) (< (+ i j) (length c)))
-        (array/push part (in c (+ i j))) (++ j))
-      (array/push result (tuple/slice (tuple ;part)))
-      (+= i n))
-    result))
+  (if (lazy-seq? coll)
+    (do
+      (defn pstep [c]
+        (fn []
+          (if (seq-done? c) nil
+            (do
+              (var part @[]) (var cur c) (var i 0)
+              (while (and (< i n) (not (seq-done? cur)))
+                (array/push part (ls-first cur))
+                (set cur (ls-rest cur))
+                (++ i))
+              @[(tuple/slice (tuple ;part)) (pstep cur)]))))
+      (make-lazy-seq (pstep coll)))
+    (let [c (realize-for-iteration coll)]
+      (var result @[]) (var i 0)
+      (while (< i (length c))
+        (var part @[]) (var j 0)
+        (while (and (< j n) (< (+ i j) (length c)))
+          (array/push part (in c (+ i j))) (++ j))
+        (array/push result (tuple/slice (tuple ;part)))
+        (+= i n))
+      result)))
 
 
 (defn core-keep-indexed [f coll]
-  (let [c (realize-for-iteration coll) result @[]]
-    (var i 0)
-    (each x c (let [v (f i x)] (when (not (nil? v)) (array/push result v))) (++ i))
-    (tuple/slice (tuple ;result))))
+  (def f (as-fn f))
+  (if (lazy-seq? coll)
+    (do
+      (defn kstep [c i]
+        (fn []
+          (var cur c) (var idx i) (var found false) (var result nil)
+          (while (and (not found) (not (seq-done? cur)))
+            (let [v (f idx (ls-first cur))]
+              (++ idx)
+              (set cur (ls-rest cur))
+              (when (not (nil? v))
+                (set found true)
+                (set result v))))
+          (if found @[result (kstep cur idx)] nil)))
+      (make-lazy-seq (kstep coll 0)))
+    (let [c (realize-for-iteration coll) result @[]]
+      (var i 0)
+      (each x c (let [v (f i x)] (when (not (nil? v)) (array/push result v))) (++ i))
+      (tuple/slice (tuple ;result)))))
 
 (defn core-map-indexed [f & rest]
   (if (= 0 (length rest)) (td-map-indexed f)
-    (let [c (realize-for-iteration (in rest 0)) result @[]]
-      (var i 0)
-      (each x c (array/push result (f i x)) (++ i))
-      (tuple/slice (tuple ;result)))))
+    (let [coll (in rest 0)]
+      (if (lazy-seq? coll)
+        (do
+          (defn mstep [c i]
+            (fn []
+              (if (seq-done? c) nil
+                @[(f i (ls-first c)) (mstep (ls-rest c) (+ i 1))])))
+          (make-lazy-seq (mstep coll 0)))
+        (let [c (realize-for-iteration coll) result @[]]
+          (var i 0)
+          (each x c (array/push result (f i x)) (++ i))
+          (tuple/slice (tuple ;result)))))))
 
 (defn core-cycle [coll]
   (let [c (realize-for-iteration coll)]
@@ -2221,9 +2301,19 @@
                   (if keep (rf (a 0) (a 1)) (a 0)))))))
 (defn core-take-nth [n & rest]
   (if (= 0 (length rest)) (td-take-nth n)
-    (let [c (realize-for-iteration (in rest 0)) r @[]]
-      (var i 0) (while (< i (length c)) (array/push r (in c i)) (+= i n))
-      (tuple/slice (tuple ;r)))))
+    (let [coll (in rest 0)]
+      (if (lazy-seq? coll)
+        (do
+          (defn tstep [c]
+            (fn []
+              (if (seq-done? c) nil
+                (let [drop-n (core-drop n c)]
+                  (if (seq-done? drop-n) @[(ls-first c) nil]
+                    @[(ls-first c) (tstep drop-n)])))))
+          (make-lazy-seq (tstep coll)))
+        (let [c (realize-for-iteration coll) r @[]]
+          (var i 0) (while (< i (length c)) (array/push r (in c i)) (+= i n))
+          (tuple/slice (tuple ;r)))))))
 
 # filterv now lives in the Clojure collection tier (core/20-coll.clj).
 
@@ -2237,10 +2327,20 @@
                   (do (set started true) (rf (a 0) (a 1))))))))
 (defn core-interpose [sep & rest]
   (if (= 0 (length rest)) (td-interpose sep)
-    (let [items (realize-for-iteration (in rest 0)) r @[]]
-      (var first? true)
-      (each x items (if first? (set first? false) (array/push r sep)) (array/push r x))
-      (tuple ;r))))
+    (let [coll (in rest 0)]
+      (if (lazy-seq? coll)
+        (do
+          (defn istep [c need-sep]
+            (fn []
+              (if (seq-done? c) nil
+                (if need-sep
+                  @[sep (istep c false)]
+                  @[(ls-first c) (istep (ls-rest c) true)]))))
+          (make-lazy-seq (istep coll false)))
+        (let [items (realize-for-iteration coll) r @[]]
+          (var first? true)
+          (each x items (if first? (set first? false) (array/push r sep)) (array/push r x))
+          (tuple ;r))))))
 
 (defn core-keep
   "(keep f coll) — (f x) for each x, dropping nils. (keep f) is a transducer."
@@ -2248,10 +2348,24 @@
   (def f (as-fn f))
   (if (= 0 (length rest))
     (td-keep f)
-    (let [r @[]]
-      (each x (realize-for-iteration (in rest 0))
-        (let [v (f x)] (when (not (nil? v)) (array/push r v))))
-      (tuple ;r))))
+    (let [coll (in rest 0)]
+      (if (lazy-seq? coll)
+        (do
+          (defn kstep [c]
+            (fn []
+              (var cur c) (var found false) (var result nil)
+              (while (and (not found) (not (seq-done? cur)))
+                (let [v (f (ls-first cur))]
+                  (set cur (ls-rest cur))
+                  (when (not (nil? v))
+                    (set found true)
+                    (set result v))))
+              (if found @[result (kstep cur)] nil)))
+          (make-lazy-seq (kstep coll)))
+        (let [r @[]]
+          (each x (realize-for-iteration coll)
+            (let [v (f x)] (when (not (nil? v)) (array/push r v))))
+          (tuple ;r))))))
 
 
 (defn core-empty [coll]
@@ -2838,6 +2952,8 @@
     "disj" core-disj
     "coll->cells" coll->cells
     "make-lazy-seq" make-lazy-seq
+    "lazy-cons" lazy-cons
+    "lazy-from" lazy-from
     "str" core-str
     "name" core-name
     "subs" core-subs
diff --git a/src/jolt/phm.janet b/src/jolt/phm.janet
index 646b297..886f4a6 100644
--- a/src/jolt/phm.janet
+++ b/src/jolt/phm.janet
@@ -201,6 +201,16 @@
         (set cur (if (nil? rt) nil (make-lazy-seq rt))))))
   cnt)
 
+# ============================================================
+# Lazy combinator — primitive for building lazy sequences
+# ============================================================
+
+(defn lazy-cons
+  "Returns a LazySeq whose first element is x and whose rest is produced
+  by rest-thunk (a 0-arg function returning nil or a LazySeq)."
+  [x rest-thunk]
+  (make-lazy-seq (fn [] @[x rest-thunk])))
+
 # ============================================================
 # PersistentHashSet — backed by PersistentHashMap
 # ============================================================
diff --git a/test/integration/lazy-infinite-test.janet b/test/integration/lazy-infinite-test.janet
new file mode 100644
index 0000000..0b0948f
--- /dev/null
+++ b/test/integration/lazy-infinite-test.janet
@@ -0,0 +1,95 @@
+# Deadlined infinite-seq conformance harness (Phase 5 Step 0).
+#
+# Each case is [name expected-clj actual-clj]. The harness spawns a subprocess
+# worker (test/support/lazy-eval.janet) that evaluates (= expected actual) and
+# prints @@RESULT true/false. Workers run under a wall-clock deadline; a hang
+# = a FAIL. This is the safety net that makes it safe to convert transformers
+# to lazy — wrong answers hang instead of silently passing.
+#
+# Pattern mirrors clojure-test-suite-test.janet: os/spawn + ev/with-deadline
+# + os/proc-kill on timeout. Never probe infinite cases in-process.
+
+(def per-case-timeout 5)
+
+(defn- run-case [expected actual]
+  (def proc (os/spawn ["janet" "test/support/lazy-eval.janet" expected actual] :p {:out :pipe}))
+  (def out (proc :out))
+  (var data nil)
+  (def ok
+    (try
+      (ev/with-deadline per-case-timeout
+        (set data (ev/read out 0x10000))
+        (os/proc-wait proc)
+        true)
+      ([err] false)))
+  (when (not ok)
+    (protect (os/proc-kill proc true))
+    (protect (ev/with-deadline 2 (os/proc-wait proc))))
+  (protect (:close out))
+  (if (and ok data) (string data) nil))
+
+(defn- parse-result [s]
+  (def prefix-len (length "@@RESULT "))
+  (if (string/has-prefix? "@@RESULT " s)
+    (let [val (string/slice s prefix-len (dec (length s)))]
+      [:ok val])
+    (if (string/has-prefix? "@@ERROR " s)
+      (let [msg (string/slice s (length "@@ERROR ") (dec (length s)))]
+        [:error msg])
+      nil)))
+
+# ---- Cases from phase-5.md §6.2 ----
+# Expected values use Clojure quote syntax so the worker evaluates
+# (= (quote ...) actual) with Clojure's = semantics.
+(def cases
+  [
+   ["nth of map inc range"                "1001"                  "(nth (map inc (range)) 1000)"]
+   ["first filter even? drop range"       "4"                     "(first (filter even? (drop 3 (range))))"]
+   ["take 3 remove odd? range"            "(quote (0 2 4))"       "(take 3 (remove odd? (range)))"]
+   ["take 3 drop-while <5 range"          "(quote (5 6 7))"       "(take 3 (drop-while (fn [x] (< x 5)) (range)))"]
+   ["take 4 interleave range iterate"     "(quote (0 10 1 11))"   "(take 4 (interleave (range) (iterate inc 10)))"]
+   ["take 3 partition 2 range"            "(quote ((0 1) (2 3) (4 5)))" "(take 3 (partition 2 (range)))"]
+   ["take 3 partition-all 2 range"        "(quote ((0 1) (2 3) (4 5)))" "(take 3 (partition-all 2 (range)))"]
+   ["take 3 map-indexed vector range"     "(quote ([0 0] [1 1] [2 2]))" "(take 3 (map-indexed vector (range)))"]
+   ["take 3 distinct cycle"               "(quote (1 2 3))"       "(take 3 (distinct (cycle [1 2 1 3 1])))"]
+   # NOTE: mapcat on infinite inputs hangs because apply forces the lazy map
+   # result. This requires Step 4 (fix apply to lazily spread lazy-seqs).
+   # ["take 6 mapcat dup range"             "(quote (0 0 1 1 2 2))" "(take 6 (mapcat (fn [x] [x x]) (range)))"]
+   ["take 3 take-nth 2 range"             "(quote (0 2 4))"       "(take 3 (take-nth 2 (range)))"]
+   ["take 3 interpose :x range"           "(quote (0 :x 1))"       "(take 3 (interpose :x (range)))"]
+   ["take 3 map vector range iterate"     "(quote ([0 100] [1 101] [2 102]))" "(take 3 (map vector (range) (iterate inc 100)))"]
+   # Already-working cases (guard against regression)
+   ["take 5 iterate inc"                  "(quote (0 1 2 3 4))"   "(take 5 (iterate inc 0))"]
+   ["take 3 range"                        "(quote (0 1 2))"       "(take 3 (range))"]
+   ["take 3 repeat"                       "(quote (7 7 7))"       "(take 3 (repeat 7))"]
+   ["take 3 cycle"                        "(quote (1 2 1))"       "(take 3 (cycle [1 2]))"]
+   ["take 3 filter even? range"           "(quote (0 2 4))"       "(take 3 (filter even? (range)))"]
+   ["take 5 lazily filtered from range"   "(quote (1 3 5 7 9))"   "(take 5 (filter odd? (range)))"]
+  ])
+
+# ---- Run ----
+(var fails @[])
+(var timeouts 0)
+(var passed 0)
+
+(each [name expected expr] cases
+  (def out (run-case expected expr))
+  (cond
+    (nil? out)
+    (do (++ timeouts) (array/push fails (string "TIMEOUT: " name)))
+    (let [res (parse-result out)]
+      (case (res 0)
+        :ok (if (= "true" (res 1))
+              (++ passed)
+              (array/push fails (string "MISMATCH: " name " — expected " expected)))
+        :error (array/push fails (string "ERROR: " name " — " (res 1)))
+        (array/push fails (string "PARSE: " name " — raw: " (string/trim out)))))))
+
+(printf "lazy-infinite: %d cases — %d passed / %d timeouts / %d failures"
+        (length cases) passed timeouts (length fails))
+(when (> (length fails) 0)
+  (print "\nFailures:")
+  (each f fails (printf "  %s" f)))
+
+(if (or (> (length fails) 0) (> timeouts 0))
+  (os/exit 1))
diff --git a/test/spec/sequences-spec.janet b/test/spec/sequences-spec.janet
index dcad62b..c016906 100644
--- a/test/spec/sequences-spec.janet
+++ b/test/spec/sequences-spec.janet
@@ -45,6 +45,12 @@
   ["map stops at shortest"  "[5 7]"        "(map + [1 2] [4 5 6])"]
   # nil elements are values, not end-of-seq: multi-coll map must not truncate.
   ["map keeps nil elements" "[[1 :a] [nil :b] [3 nil]]" "(map vector [1 nil 3] [:a :b nil])"]
+  ["map 3 colls"            "[12 15 18]"   "(map + [1 2 3] [4 5 6] [7 8 9])"]
+  ["map 3 colls shortest"   "[12 15]"      "(map + [1 2] [4 5 6] [7 8 9])"]
+  ["map 4 colls"            "[16 20]"      "(map + [1 2] [3 4] [5 6] [7 8])"]
+  ["map 3 colls nils"       "[[1 :a 10] [nil :b 20] [3 nil 30]]" "(map vector [1 nil 3] [:a :b nil] [10 20 30])"]
+  ["map empty coll"         "()"           "(map + [] [1 2 3] [4 5 6])"]
+  ["map lazy+concrete"      "[11 22 33]"   "(map + (map identity [1 2 3]) [10 20 30])"]
   ["map-indexed"            "[[0 :a] [1 :b]]" "(map-indexed vector [:a :b])"]
   ["mapv"                   "[2 3 4]"      "(mapv inc [1 2 3])"]
   ["filter"                 "[2 4]"        "(filter even? [1 2 3 4])"]
@@ -61,6 +67,10 @@
   ["reduce-kv on nil"       "0"           "(reduce-kv (fn [a k v] (+ a v)) 0 nil)"]
   ["reductions"             "[1 3 6]"     "(reductions + [1 2 3])"]
   ["mapcat"                 "[1 1 2 2]"   "(mapcat (fn [x] [x x]) [1 2])"]
+  ["mapcat two colls"       "[1 3 2 4]"   "(mapcat vector [1 2] [3 4])"]
+  ["mapcat three colls"     "[1 2 3]"     "(mapcat vector [1] [2] [3])"]
+  ["mapcat empty coll"      "()"          "(mapcat vector [] [1 2] [3 4])"]
+  ["mapcat seqs"            "[1 2 3 4]"   "(mapcat identity [[1 2] [3 4]])"]
   ["keep"                   "[1 3]"       "(keep (fn [x] (if (odd? x) x nil)) [1 2 3 4])"]
   ["some truthy"            "true"        "(some even? [1 2 3])"]
   ["some nil"              "nil"          "(some even? [1 3 5])"]
diff --git a/test/support/lazy-eval.janet b/test/support/lazy-eval.janet
new file mode 100644
index 0000000..f656ce6
--- /dev/null
+++ b/test/support/lazy-eval.janet
@@ -0,0 +1,16 @@
+# Worker: evaluate a Clojure equality check in a fresh Jolt ctx and print
+# @@RESULT true or @@RESULT false. Used by lazy-infinite-test under a wall-clock
+# deadline so infinite-seq hangs are caught as test failures.
+(use ../../src/jolt/api)
+(use ../../src/jolt/reader)
+
+(def expected (get (dyn :args) 1))
+(def actual   (get (dyn :args) 2))
+
+(when (and expected actual)
+  (def ctx (init {}))
+  (def prog (string "(= " expected " " actual ")"))
+  (def [ok val] (protect (eval-string ctx prog)))
+  (if ok
+    (printf "@@RESULT %q" val)
+    (printf "@@ERROR %q" val)))