diff --git a/lib/haskell/testlib.sx b/lib/haskell/testlib.sx
index 5803b741..1c814fd8 100644
--- a/lib/haskell/testlib.sx
+++ b/lib/haskell/testlib.sx
@@ -56,3 +56,21 @@
         (append!
           hk-test-fails
           {:actual actual :expected expected :name name})))))
+
+(define
+  hk-test-error
+  (fn
+    (name thunk expected-substring)
+    (let
+      ((caught (guard (e (true (if (string? e) e (str e)))) (begin (thunk) nil))))
+      (cond
+        ((nil? caught)
+          (do
+            (set! hk-test-fail (+ hk-test-fail 1))
+            (append! hk-test-fails {:actual "no error raised" :expected (str "error containing: " expected-substring) :name name})))
+        ((>= (index-of caught expected-substring) 0)
+          (set! hk-test-pass (+ hk-test-pass 1)))
+        (:else
+          (do
+            (set! hk-test-fail (+ hk-test-fail 1))
+            (append! hk-test-fails {:actual caught :expected (str "error containing: " expected-substring) :name name})))))))
diff --git a/lib/haskell/tests/eval.sx b/lib/haskell/tests/eval.sx
index 7bbeec29..f9ced488 100644
--- a/lib/haskell/tests/eval.sx
+++ b/lib/haskell/tests/eval.sx
@@ -289,6 +289,16 @@
   (hk-deep-force (hk-run "main = head [42]"))
   42)
 
+(hk-test-error
+  "hk-test-error helper: catches matching error"
+  (fn () (hk-deep-force (hk-run "main = error \"boom\"")))
+  "hk-error: boom")
+
+(hk-test-error
+  "hk-test-error helper: catches head [] error"
+  (fn () (hk-deep-force (hk-run "main = head []")))
+  "Prelude.head: empty list")
+
 (hk-test
   "second arg never forced"
   (hk-eval-expr-source "(\\x y -> x) 1 (error \"never\")")
diff --git a/plans/haskell-completeness.md b/plans/haskell-completeness.md
index ae5be084..b0f972ee 100644
--- a/plans/haskell-completeness.md
+++ b/plans/haskell-completeness.md
@@ -133,7 +133,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `fromJust Nothing` → `"Maybe.fromJust: Nothing"`.
 - [x] Top-level `hk-run-io` catches `hk-error` tag and returns it as a tagged
   error result so test suites can inspect it without crashing.
-- [ ] `hk-test-error` helper in `testlib.sx`:
+- [x] `hk-test-error` helper in `testlib.sx`:
   `(hk-test-error "desc" thunk expected-substring)` — asserts the thunk raises
   an `hk-error` whose message contains the given substring.
 - [ ] Tests in `lib/haskell/tests/errors.sx` (≥ 10 tests: error message
@@ -293,6 +293,13 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 9 `hk-test-error` helper in testlib.sx:
+- New 0-arity-thunk-based assertion: `(hk-test-error name thunk substr)` —
+  evaluates `(thunk)`, expects an exception, checks `index-of` for the given
+  substring in the caught (string-coerced) value. Increments `hk-test-pass` on
+  match, otherwise records into `hk-test-fails` with descriptive expected.
+- Added 2 quick uses to `tests/eval.sx` (error and head []). Suite now 66/66.
+
 **2026-05-07** — Phase 9 `hk-run-io` catches errors, appends to io-lines:
 - Wrapped both `hk-run-io` and `hk-run-io-with-input` in `(guard (e (true …)))`
   that appends the caught exception to `hk-io-lines`. Also added `hk-deep-force`