diff --git a/host/chez/atoms.ss b/host/chez/atoms.ss
index f7a7fe9..816d6de 100644
--- a/host/chez/atoms.ss
+++ b/host/chez/atoms.ss
@@ -17,10 +17,13 @@
 ;; (watches/validators are overlay features layered via jolt.host/ref-put!).
 (define (jolt-atom-new v . _opts) (make-jolt-atom v))
 
+;; deref reads an atom; it also unwraps a `reduced` (Clojure @(reduced x) => x,
+;; which the overlay's `unreduced` relies on). The reduced record is in seq.ss.
 (define (jolt-deref x)
-  (if (jolt-atom? x)
-      (jolt-atom-val x)
-      (error #f "deref: unsupported reference type" x)))
+  (cond
+    ((jolt-atom? x) (jolt-atom-val x))
+    ((jolt-reduced? x) (jolt-reduced-val x))
+    (else (error #f "deref: unsupported reference type" x))))
 
 ;; (swap! a f arg*) -> (reset! a (f @a arg*)); f is invoked through jolt-invoke
 ;; (a jolt fn value, keyword, or invokable collection).
diff --git a/host/chez/natives-seq.ss b/host/chez/natives-seq.ss
new file mode 100644
index 0000000..c3b5a01
--- /dev/null
+++ b/host/chez/natives-seq.ss
@@ -0,0 +1,99 @@
+;; seq-native shims (jolt-y6mv) — seed-native seq fns the overlay assumes are
+;; clojure.core natives but which live in the Janet seed (src/jolt/core_coll.janet),
+;; so they have no def-var! in the assembled prelude and resolve to jolt-nil on
+;; Chez. This was the dominant prelude-parity crash bucket ('apply jolt-nil').
+;; Each is a pure fn over the existing seq layer (seq.ss) — collection arities
+;; only; the 1-arg transducer arities are jolt-kxsr. Loaded last (after
+;; converters.ss for jolt-compare and seq.ss for the reduced record).
+
+;; reduced / reduced? — the box itself is the jolt-reduced record from seq.ss
+;; (so the reduce machinery there can see it); these just expose the constructor
+;; and predicate. (deref a-reduced) is handled in atoms.ss.
+(define (jolt-reduced-new x) (make-jolt-reduced x))
+(define (jolt-reduced-pred x) (jolt-reduced? x))
+
+;; mapcat: (mapcat f coll & colls) — map f across the colls (stops at shortest),
+;; then concat the results. Collection arity only.
+(define (jolt-mapcat f . colls)
+  (apply jolt-concat (seq->list (apply jolt-map f colls))))
+
+;; take-while / drop-while over the seq layer.
+(define (take-while-seq pred s)
+  (if (jolt-nil? s) jolt-empty-list
+      (let ((x (seq-first s)))
+        (if (jolt-truthy? (jolt-invoke pred x))
+            (cseq-lazy x (lambda () (take-while-seq pred (jolt-seq (seq-more s)))))
+            jolt-empty-list))))
+(define (jolt-take-while pred coll) (take-while-seq pred (jolt-seq coll)))
+(define (jolt-drop-while pred coll)
+  (let loop ((s (jolt-seq coll)))
+    (if (and (not (jolt-nil? s)) (jolt-truthy? (jolt-invoke pred (seq-first s))))
+        (loop (jolt-seq (seq-more s)))
+        (if (jolt-nil? s) jolt-empty-list s))))
+
+;; partition: (partition n coll), (partition n step coll), or
+;; (partition n step pad coll). Only complete partitions of size n are kept;
+;; with pad, a short final partition is padded from pad (and may be < n if pad
+;; runs out). Each partition is a seq; the whole result is a lazy seq of seqs.
+(define jolt-partition
+  (case-lambda
+    ((n coll) (partition* (->idx n) (->idx n) #f #f coll))
+    ((n step coll) (partition* (->idx n) (->idx step) #f #f coll))
+    ((n step pad coll) (partition* (->idx n) (->idx step) #t pad coll))))
+(define (take-n n s)               ; -> (values list-of-first-n remaining-seq taken-count)
+  (let loop ((n n) (s s) (acc '()))
+    (if (or (fx<=? n 0) (jolt-nil? s))
+        (values (reverse acc) s (length acc))
+        (loop (fx- n 1) (jolt-seq (seq-more s)) (cons (seq-first s) acc)))))
+(define (partition* n step has-pad pad coll)
+  (let loop ((s (jolt-seq coll)))
+    (if (jolt-nil? s) jolt-empty-list
+        (let-values (((part rest taken) (take-n n s)))
+          (cond
+            ;; full partition: emit it, advance `step` from its START
+            ((fx=? taken n)
+             (cseq-lazy (list->cseq part)
+                        (lambda () (loop (jolt-seq (advance-by step s))))))
+            ;; short final partition with pad: top up to n from pad, then stop
+            ((and has-pad (fx>? taken 0))
+             (let ((padded (append part (take-list (- n taken) (jolt-seq pad)))))
+               (cseq-lazy (list->cseq padded) (lambda () jolt-empty-list))))
+            ;; short final partition, no pad: dropped (Clojure keeps only full ones)
+            (else jolt-empty-list))))))
+(define (advance-by step s)        ; drop `step` elements from s (seq), returns a seq
+  (let loop ((step step) (s s))
+    (if (or (fx<=? step 0) (jolt-nil? s)) s
+        (loop (fx- step 1) (jolt-seq (seq-more s))))))
+(define (take-list n s)            ; up to n elements of seq s as a Scheme list
+  (let loop ((n n) (s s) (acc '()))
+    (if (or (fx<=? n 0) (jolt-nil? s)) (reverse acc)
+        (loop (fx- n 1) (jolt-seq (seq-more s)) (cons (seq-first s) acc)))))
+
+;; sort: (sort coll) uses compare; (sort cmp coll) uses cmp, whose result may be
+;; a 3-way number (<0 / 0 / >0) OR a boolean (a Clojure-style less-than pred).
+(define (cmp->less cmp)
+  (lambda (a b)
+    (let ((r (jolt-invoke cmp a b)))
+      (if (number? r) (< r 0) (jolt-truthy? r)))))
+(define jolt-sort
+  (case-lambda
+    ((coll) (jolt-sort* (cmp->less jolt-compare) coll))
+    ((cmp coll) (jolt-sort* (cmp->less cmp) coll))))
+(define (jolt-sort* less? coll)
+  (let ((s (jolt-seq coll)))
+    (if (jolt-nil? s) jolt-empty-list
+        (list->cseq (list-sort less? (seq->list s))))))
+
+;; identical?: jolt reference identity. The seed defines it as (= a b) over its
+;; value model (core_types.janet core-identical?), where interned keywords/small
+;; values compare equal — so jolt= is the faithful port.
+(define (jolt-identical? a b) (jolt= a b))
+
+(def-var! "clojure.core" "reduced" jolt-reduced-new)
+(def-var! "clojure.core" "reduced?" jolt-reduced-pred)
+(def-var! "clojure.core" "mapcat" jolt-mapcat)
+(def-var! "clojure.core" "take-while" jolt-take-while)
+(def-var! "clojure.core" "drop-while" jolt-drop-while)
+(def-var! "clojure.core" "partition" jolt-partition)
+(def-var! "clojure.core" "sort" jolt-sort)
+(def-var! "clojure.core" "identical?" jolt-identical?)
diff --git a/host/chez/rt.ss b/host/chez/rt.ss
index 2d45ac1..277f59b 100644
--- a/host/chez/rt.ss
+++ b/host/chez/rt.ss
@@ -160,3 +160,8 @@
 ;; extends get/count/contains? to see through a transient. After collections.ss
 ;; (the persistent ops it delegates to).
 (load "host/chez/transients.ss")
+
+;; seq-native shims (jolt-y6mv): mapcat/take-while/drop-while/partition/sort +
+;; reduced/reduced?/identical? — seed-native fns the overlay assumes are core
+;; natives. Over the seq layer + jolt-compare, so loaded after converters.ss.
+(load "host/chez/natives-seq.ss")
diff --git a/host/chez/seq.ss b/host/chez/seq.ss
index 557da91..32b513a 100644
--- a/host/chez/seq.ss
+++ b/host/chez/seq.ss
@@ -30,6 +30,11 @@
 (define-record-type empty-list-t (fields) (nongenerative empty-list-v1))
 (define jolt-empty-list (make-empty-list-t))
 
+;; reduced (jolt-y6mv): a box a reducing fn returns to stop reduce early. The
+;; reduce machinery below unwraps it; (deref a-reduced) / unreduced also read it.
+;; reduced?/reduced are def-var!'d into clojure.core in natives-seq.ss.
+(define-record-type jolt-reduced (fields val) (nongenerative jolt-reduced-v1))
+
 ;; ============================================================================
 ;; jolt-seq — coerce a seqable to a non-empty seq, or jolt-nil when empty
 ;; ============================================================================
@@ -132,8 +137,14 @@
 (define (jolt-filter pred coll) (filter-seq pred (jolt-seq coll) #t))
 (define (jolt-remove pred coll) (filter-seq pred (jolt-seq coll) #f))
 
+;; honors `reduced`: a reducing fn that returns (reduced x) stops the fold and
+;; unwraps to x (so does a reduced INIT). Checked at entry, so the value returned
+;; by the last step is unwrapped on the next turn before the seq is consulted.
 (define (reduce-seq f acc s)
-  (if (jolt-nil? s) acc (reduce-seq f (jolt-invoke f acc (seq-first s)) (jolt-seq (seq-more s)))))
+  (cond
+    ((jolt-reduced? acc) (jolt-reduced-val acc))
+    ((jolt-nil? s) acc)
+    (else (reduce-seq f (jolt-invoke f acc (seq-first s)) (jolt-seq (seq-more s))))))
 (define jolt-reduce
   (case-lambda
     ((f coll) (let ((s (jolt-seq coll)))
diff --git a/test/chez/README.md b/test/chez/README.md
index 9324cb4..6273b8c 100644
--- a/test/chez/README.md
+++ b/test/chez/README.md
@@ -124,12 +124,27 @@ class names, eval-order, with-open — all deferred Phase-2 / dynamic-var gaps).
   `inf`/`-inf`/`nan` and `str` renders `Infinity`/`-Infinity`/`NaN` (Clojure). Plus
   variadic `assoc!`. (`str` of inf *inside a collection* still wants the long form —
   the Phase-2 recursive str renderer — so `[inf inside coll]` is allowlisted.)
+- inc 3n `host/chez/natives-seq.ss` (jolt-y6mv): the dominant prelude-parity crash
+  bucket was `apply non-procedure jolt-nil` — core fns calling seed-native seq fns
+  (`src/jolt/core_coll.janet`) that have no Chez shim, so `var-deref` yields
+  jolt-nil. A static scan of the assembled prelude found 52 referenced-but-undefined
+  `clojure.core` names; this increment shims the safe, high-value seq fns: `mapcat`/
+  `take-while`/`drop-while`/`partition` (collection arities — the 1-arg transducer
+  forms are jolt-kxsr), `sort` (compare default; a comparator may return a 3-way
+  number or a boolean less-than), and `reduced`/`reduced?` — a `jolt-reduced` record
+  in `seq.ss` that `reduce` short-circuits on and `deref` unwraps (so `unreduced`
+  works). `identical?` = `jolt=` (the seed's definition). `list?` was deferred: a
+  Chez lazy seq and a list are both `cseq`, so it can't be told apart without a
+  distinct list type (a real divergence risk).
 
-The remaining buckets are the punch-list the next increments chase: ~360 emit-fail
-(genuine host interop — qualified Java/Janet refs, runtime `defmacro`/`eval`, out of
-the analyzer's subset) and ~715 runtime crashes — more host-coupled natives without a
-shim, transducer arities (jolt-kxsr), `cdr`-on-`()` and `\p{}` regex classes
-(jolt-y1zq), and multimethod dispatch (Phase 2 jolt-9ls5).
+The remaining buckets are the punch-list the next increments chase (at 1467/2497):
+~361 emit-fail (genuine host interop — qualified Java/Janet refs, runtime
+`defmacro`/`eval`, out of the analyzer's subset) and ~655 runtime crashes — still
+~590 `apply jolt-nil` (more host-coupled natives without a shim: `meta`/`with-meta`,
+`format`, the `clojure.string` natives, bit ops, `var`/`volatile`/`future`/
+thread-binding ops), the transducer arities (jolt-kxsr — 1-arg `filter`/`map`/`take`/
+`take-while` + 3-arg `into`), `cdr`-on-`()` and `\p{}` regex classes (jolt-y1zq), and
+multimethod dispatch (Phase 2 jolt-9ls5).
 
 Two host shims landed with the prelude. `host/chez/atoms.ss`: atom/deref/swap!/
 reset! (+ compare-and-set!/swap-vals!/reset-vals!) — host-coupled mutable cells the
diff --git a/test/chez/emit-test.janet b/test/chez/emit-test.janet
index c22b095..71413c2 100644
--- a/test/chez/emit-test.janet
+++ b/test/chez/emit-test.janet
@@ -488,6 +488,44 @@
     (ok (string "numedge: " src) (and (= code 0) (= out want))
         (string "chez=" out " janet=" want " | " err))))
 
+# 3s) seq-native shims + reduced (jolt-y6mv): the dominant prelude-parity crash
+#   bucket was 'apply jolt-nil' — core fns calling seed-native seq fns with no Chez
+#   shim. host/chez/natives-seq.ss shims the safe, high-value ones (mapcat/
+#   take-while/drop-while/partition collection arities, sort) over the seq layer,
+#   plus reduced/reduced? (reduce short-circuits on a reduced; deref unwraps it)
+#   and identical?. They lower to var-deref in prelude mode. Asserted as
+#   (= expected (expr)) -> "true" so seq-vs-vector equality (not print form) is the
+#   contract, exactly like the corpus gate.
+(each src [# reduced
+           "(reduced? (reduced 1))" "(reduced? 1)" "(deref (reduced 9))"
+           "(reduce (fn [a x] (if (> a 2) (reduced a) (+ a x))) 0 [1 2 3 4 5])"
+           "(= [1 1 2 2] (mapcat (fn [x] [x x]) [1 2]))"
+           "(= [1 3 2 4] (mapcat vector [1 2] [3 4]))"
+           "(= [1 2 3] (mapcat identity [[1 2] [3]]))"
+           "(= () (mapcat vector [] [1 2]))"
+           "(= [1 2] (take-while (fn [x] (< x 3)) [1 2 3 1]))"
+           "(= [3 1] (drop-while (fn [x] (< x 3)) [1 2 3 1]))"
+           "(= () (take-while pos? []))"
+           "(= [[1 2] [3 4]] (partition 2 [1 2 3 4 5]))"
+           "(= [[1 2] [4 5]] (partition 2 3 [1 2 3 4 5 6]))"
+           "(= [[1 2] [3 :p]] (partition 2 2 [:p] [1 2 3]))"
+           "(= [1 2 3] (sort [3 1 2]))"
+           "(= [3 2 1] (sort > [1 3 2]))"
+           "(= [nil 1 3] (sort compare [3 nil 1]))"
+           "(= () (sort []))"
+           "(identical? :a :a)" "(identical? :a :b)"]
+  (let [[code out err] (run-prelude src) want (cli-oracle src)]
+    (ok (string "seq-native: " src) (and (= code 0) (= out want))
+        (string "chez=" out " janet=" want " | " err))))
+# reduce-kv honors reduced is an OVERLAY fn over the native reduce — exercise it
+# end-to-end through the assembled -e binary.
+(when (os/stat "bin/jolt-chez")
+  (each src ["(= [:a] (reduce-kv (fn [a i v] (if (= i 1) (reduced a) (conj a v))) [] [:a :b :c]))"
+             "(= 9 (unreduced (reduced 9)))" "(= 9 (unreduced 9))"]
+    (let [[code out err] (run-jolt-chez src) want (cli-oracle src)]
+      (ok (string "seq-native -e: " src) (and (= code 0) (= out want))
+          (string "chez=" out " janet=" want " | " err)))))
+
 # 4) perf signal: emitted fib(30) in-Scheme timing (excludes Chez startup), to
 #    track against the spike ceiling (hand-Scheme fib ~5ms). Informational — the
 #    jolt-truthy? wrapper (~3x) and flonum modeling are known Phase-4 levers.
diff --git a/test/chez/run-corpus-prelude.janet b/test/chez/run-corpus-prelude.janet
index 9269436..114aad5 100644
--- a/test/chez/run-corpus-prelude.janet
+++ b/test/chez/run-corpus-prelude.janet
@@ -135,8 +135,9 @@
 # reach-floor and the suite baseline. The gate fails if parity drops below it, or
 # on any NEW (un-allowlisted) divergence — a real Chez correctness regression.
 # Full-corpus baseline: inc 3j 1220/2497; 3k (converters) 1326; 3l (transients)
-# 1382; 3m (numeric-edge emit + variadic assoc!) 1407. Strided runs scale down.
-(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_PRELUDE_FLOOR") "1407")))
+# 1382; 3m (numeric-edge emit + variadic assoc!) 1407; 3n (seq-native shims +
+# reduced) 1467. Strided runs scale down.
+(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_PRELUDE_FLOOR") "1467")))
 (def floor (if (os/getenv "JOLT_CORPUS_LIMIT") 0 base-floor))
 (when (or (> (length diverged) 0) (< pass floor))
   (printf "REGRESSION: pass %d < floor %d or %d new divergence(s)" pass floor (length diverged)))