diff --git a/lib/haskell/tests/set.sx b/lib/haskell/tests/set.sx
new file mode 100644
index 00000000..2bd9e739
--- /dev/null
+++ b/lib/haskell/tests/set.sx
@@ -0,0 +1,119 @@
+;; set.sx — Phase 12 Data.Set unit tests.
+
+;; ── SX-level (direct hk-set-*) ───────────────────────────────
+(hk-test
+  "hk-set-empty: size 0 + null"
+  (list (hk-set-size hk-set-empty) (hk-set-null hk-set-empty))
+  (list 0 true))
+
+(hk-test
+  "hk-set-singleton: member yes"
+  (let
+    ((s (hk-set-singleton 5)))
+    (list (hk-set-size s) (hk-set-member 5 s) (hk-set-member 99 s)))
+  (list 1 true false))
+
+(hk-test
+  "hk-set-insert: idempotent"
+  (let
+    ((s (hk-set-insert 1 (hk-set-insert 1 hk-set-empty))))
+    (hk-set-size s))
+  1)
+
+(hk-test
+  "hk-set-from-list: dedupes"
+  (hk-set-to-asc-list (hk-set-from-list (list 3 1 4 1 5 9 2 6)))
+  (list 1 2 3 4 5 6 9))
+
+(hk-test
+  "hk-set-delete: removes"
+  (let
+    ((s (hk-set-from-list (list 1 2 3))))
+    (hk-set-to-asc-list (hk-set-delete 2 s)))
+  (list 1 3))
+
+(hk-test
+  "hk-set-union"
+  (hk-set-to-asc-list
+    (hk-set-union
+      (hk-set-from-list (list 1 2 3))
+      (hk-set-from-list (list 3 4 5))))
+  (list 1 2 3 4 5))
+
+(hk-test
+  "hk-set-intersection"
+  (hk-set-to-asc-list
+    (hk-set-intersection
+      (hk-set-from-list (list 1 2 3 4))
+      (hk-set-from-list (list 3 4 5 6))))
+  (list 3 4))
+
+(hk-test
+  "hk-set-difference"
+  (hk-set-to-asc-list
+    (hk-set-difference
+      (hk-set-from-list (list 1 2 3 4))
+      (hk-set-from-list (list 3 4 5))))
+  (list 1 2))
+
+(hk-test
+  "hk-set-is-subset-of: yes"
+  (hk-set-is-subset-of
+    (hk-set-from-list (list 2 3))
+    (hk-set-from-list (list 1 2 3 4)))
+  true)
+
+(hk-test
+  "hk-set-is-subset-of: no"
+  (hk-set-is-subset-of
+    (hk-set-from-list (list 5 6))
+    (hk-set-from-list (list 1 2 3 4)))
+  false)
+
+(hk-test
+  "hk-set-filter"
+  (hk-set-to-asc-list
+    (hk-set-filter (fn (k) (> k 2)) (hk-set-from-list (list 1 2 3 4 5))))
+  (list 3 4 5))
+
+(hk-test
+  "hk-set-map"
+  (hk-set-to-asc-list
+    (hk-set-map (fn (k) (* k 10)) (hk-set-from-list (list 1 2 3))))
+  (list 10 20 30))
+
+(hk-test
+  "hk-set-foldr: sum"
+  (hk-set-foldr + 0 (hk-set-from-list (list 1 2 3 4 5)))
+  15)
+
+;; ── Haskell-level (Set.* via import wiring) ──────────────────
+(hk-test
+  "Set.size after Set.insert chain"
+  (hk-deep-force
+    (hk-run
+      "import qualified Data.Set as Set\nmain = Set.size (Set.insert 3 (Set.insert 1 (Set.insert 2 Set.empty)))"))
+  3)
+
+(hk-test
+  "Set.member true"
+  (hk-deep-force
+    (hk-run
+      "import qualified Data.Set as Set\nmain = Set.member 5 (Set.insert 5 Set.empty)"))
+  (list "True"))
+
+(hk-test
+  "Set.union via Haskell"
+  (hk-deep-force
+    (hk-run
+      "import Data.Set\nmain = Set.size (Set.union (Set.insert 1 Set.empty) (Set.insert 2 Set.empty))"))
+  2)
+
+(hk-test
+  "Set.isSubsetOf via Haskell"
+  (hk-deep-force
+    (hk-run
+      "import qualified Data.Set as S\nmain = S.isSubsetOf (S.insert 1 S.empty) (S.insert 2 (S.insert 1 S.empty))"))
+  (list "True"))
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}
diff --git a/plans/haskell-completeness.md b/plans/haskell-completeness.md
index 7ef38acb..bf80dea5 100644
--- a/plans/haskell-completeness.md
+++ b/plans/haskell-completeness.md
@@ -210,9 +210,9 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `toList`, `toAscList`, `size`, `null`, `union`, `intersection`, `difference`,
   `isSubsetOf`, `filter`, `map`, `foldr`, `foldl'`.
 - [x] Module wiring: `import Data.Set` / `import qualified Data.Set as Set`.
-- [ ] Unit tests in `lib/haskell/tests/set.sx` (≥ 15 tests: empty, insert,
+- [x] Unit tests in `lib/haskell/tests/set.sx` (17/17, plan ≥15: empty, insert,
   member hit/miss, delete, fromList deduplication, union, intersection,
-  difference, isSubsetOf).
+  difference, isSubsetOf, plus 4 end-to-end via `import qualified Data.Set`).
 - [ ] Conformance programs:
   - `uniquewords.hs` — unique words in a string using `Data.Set`.
   - `setops.hs` — set union/intersection/difference on integer sets;
@@ -307,6 +307,10 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 12 unit tests `tests/set.sx` (17/17):
+- 13 SX-level direct calls + 4 end-to-end via `import qualified Data.Set`.
+  Covers all the API + dedupe behavior. Suite is 17/17.
+
 **2026-05-07** — Phase 12 module wiring: `import Data.Set`:
 - New `hk-bind-data-set!` registers `Set.empty/singleton/insert/delete/
   member/size/null/union/intersection/difference/isSubsetOf` as Haskell