diff --git a/lib/haskell/eval.sx b/lib/haskell/eval.sx
index 6448e4c4..13272701 100644
--- a/lib/haskell/eval.sx
+++ b/lib/haskell/eval.sx
@@ -87,7 +87,17 @@
   hk-mk-builtin
   (fn
     (name fn arity)
-    {:type "builtin" :name name :fn fn :arity arity :collected (list)}))
+    {:type "builtin" :name name :fn fn :arity arity :lazy false :collected (list)}))
+
+;; A lazy built-in receives its collected args as raw thunks (or
+;; values, if those happened to be eager) — the implementation is
+;; responsible for forcing exactly what it needs. Used for `seq`
+;; and `deepseq`, which are non-strict in their second argument.
+(define
+  hk-mk-lazy-builtin
+  (fn
+    (name fn arity)
+    {:type "builtin" :name name :fn fn :arity arity :lazy true :collected (list)}))
 
 ;; ── Apply a function value to one argument ──────────────────
 (define
@@ -199,11 +209,15 @@
         ((< (len collected) arity)
           (assoc b "collected" collected))
         (:else
-          ;; Built-ins are strict in all their arguments. Force each
-          ;; collected thunk before invoking the underlying SX fn.
-          (apply
-            (get b "fn")
-            (map hk-force collected)))))))
+          ;; Strict built-ins force every collected arg before
+          ;; calling. Lazy ones (`seq`, `deepseq`) receive the raw
+          ;; thunks so they can choose what to force.
+          (cond
+            ((get b "lazy") (apply (get b "fn") collected))
+            (:else
+              (apply
+                (get b "fn")
+                (map hk-force collected)))))))))
 
 ;; ── Bool helpers (Bool values are tagged conses) ────────────
 (define
@@ -583,6 +597,23 @@ plus a b = a + b
         env
         "id"
         (hk-mk-builtin "id" (fn (x) x) 1))
+      ;; `seq a b` — strict in `a`, lazy in `b`. Forces `a` to WHNF
+      ;; and returns `b` unchanged (still a thunk if it was one).
+      (dict-set!
+        env
+        "seq"
+        (hk-mk-lazy-builtin
+          "seq"
+          (fn (a b) (do (hk-force a) b))
+          2))
+      ;; `deepseq a b` — like seq but forces `a` to normal form.
+      (dict-set!
+        env
+        "deepseq"
+        (hk-mk-lazy-builtin
+          "deepseq"
+          (fn (a b) (do (hk-deep-force a) b))
+          2))
       ;; Operators as first-class values
       (dict-set! env "+" (hk-make-binop-builtin "+" "+"))
       (dict-set! env "-" (hk-make-binop-builtin "-" "-"))
diff --git a/lib/haskell/tests/seq.sx b/lib/haskell/tests/seq.sx
new file mode 100644
index 00000000..c46ecab3
--- /dev/null
+++ b/lib/haskell/tests/seq.sx
@@ -0,0 +1,85 @@
+;; seq / deepseq tests. seq is strict in its first arg (forces to
+;; WHNF) and returns the second arg unchanged. deepseq additionally
+;; forces the first arg to normal form.
+
+(define
+  hk-prog-val
+  (fn
+    (src name)
+    (hk-deep-force (get (hk-eval-program (hk-core src)) name))))
+
+(define hk-as-list
+  (fn (xs)
+    (cond
+      ((and (list? xs) (= (first xs) "[]")) (list))
+      ((and (list? xs) (= (first xs) ":"))
+        (cons (nth xs 1) (hk-as-list (nth xs 2))))
+      (:else xs))))
+
+(define
+  hk-eval-list
+  (fn (src) (hk-as-list (hk-eval-expr-source src))))
+
+;; ── seq returns its second arg ──
+(hk-test
+  "seq with primitive first arg"
+  (hk-eval-expr-source "seq 1 99")
+  99)
+
+(hk-test
+  "seq forces first arg via let"
+  (hk-eval-expr-source "let x = 1 + 2 in seq x x")
+  3)
+
+(hk-test
+  "seq second arg is whatever shape"
+  (hk-eval-expr-source "seq 0 \"hello\"")
+  "hello")
+
+;; ── seq enables previously-lazy bottom to be forced ──
+;; Without seq the let-binding `x = error …` is never forced;
+;; with seq it must be forced because seq is strict in its first
+;; argument. We don't run that error case here (it would terminate
+;; the test), but we do verify the negative — that without seq,
+;; the bottom bound is never demanded.
+(hk-test
+  "lazy let — bottom never forced when unused"
+  (hk-eval-expr-source "let x = error \"never\" in 42")
+  42)
+
+;; ── deepseq forces nested structure ──
+(hk-test
+  "deepseq with finite list"
+  (hk-eval-expr-source "deepseq [1, 2, 3] 7")
+  7)
+
+(hk-test
+  "deepseq with constructor value"
+  (hk-eval-expr-source "deepseq (Just 5) 11")
+  11)
+
+(hk-test
+  "deepseq with tuple"
+  (hk-eval-expr-source "deepseq (1, 2) 13")
+  13)
+
+;; ── seq + arithmetic ──
+(hk-test
+  "seq used inside arithmetic doesn't poison the result"
+  (hk-eval-expr-source "(seq 1 5) + (seq 2 7)")
+  12)
+
+;; ── seq in user code ──
+(hk-test
+  "seq via fun-clause"
+  (hk-prog-val
+    "f x = seq x (x + 1)\nresult = f 10"
+    "result")
+  11)
+
+(hk-test
+  "seq sequences list construction"
+  (hk-eval-list "[seq 1 10, seq 2 20]")
+  (list 10 20))
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}
diff --git a/plans/haskell-on-sx.md b/plans/haskell-on-sx.md
index adc2c8d7..3c46b2c2 100644
--- a/plans/haskell-on-sx.md
+++ b/plans/haskell-on-sx.md
@@ -79,7 +79,7 @@ Key mappings:
 - [x] `force` = SX eval-thunk-to-WHNF primitive
 - [x] Pattern match forces scrutinee before matching
 - [x] Infinite structures: `repeat x`, `iterate f x`, `[1..]`, Fibonacci stream (sieve deferred — needs lazy `++` and is exercised under `Classic programs`)
-- [ ] `seq`, `deepseq` from Prelude
+- [x] `seq`, `deepseq` from Prelude
 - [ ] Do-notation for a stub `IO` monad (just threading, no real side effects yet)
 - [ ] Classic programs in `lib/haskell/tests/programs/`:
   - [ ] `fib.hs` — infinite Fibonacci stream
@@ -114,6 +114,22 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-25** — Phase 3 `seq` + `deepseq`. Built-ins were strict
+  in all args by default (every collected thunk forced before
+  invoking the underlying SX fn) — that defeats `seq`'s purpose,
+  which is strict in its first argument and lazy in its second.
+  Added a tiny `lazy` flag on the builtin record (set by a new
+  `hk-mk-lazy-builtin` constructor) and routed `hk-apply-builtin`
+  to skip the auto-force when the flag is true. `seq a b` calls
+  `hk-force a` then returns `b` unchanged so its laziness is
+  preserved; `deepseq` does the same with `hk-deep-force`. 9 new
+  tests in `lib/haskell/tests/seq.sx` cover primitive, computed,
+  and let-bound first args, deepseq on a list / `Just` /
+  tuple, seq inside arithmetic, seq via a fun-clause, and
+  `[seq 1 10, seq 2 20]` to confirm seq composes inside list
+  literals. The lazy-when-unused negative case is also tested:
+  `let x = error "never" in 42 == 42`. 368/368 green.
+
 - **2026-04-24** — Phase 3 infinite structures + Prelude. Two
   evaluator changes turn the lazy primitives into a working
   language: