diff --git a/lib/smalltalk/tests/programs.sx b/lib/smalltalk/tests/programs.sx
index dbffa658..1236350e 100644
--- a/lib/smalltalk/tests/programs.sx
+++ b/lib/smalltalk/tests/programs.sx
@@ -142,4 +142,81 @@
 (st-test "EightQueens init sets size 8"
   (evp "^ EightQueens new init size") 8)
 
+;; ── quicksort.st ─────────────────────────────────────────────────────
+(define
+  quicksort-source
+  "Object subclass: #Quicksort
+     instanceVariableNames: ''!
+
+   !Quicksort methodsFor: 'sort'!
+   sort: arr ^ self sort: arr from: 1 to: arr size!
+
+   sort: arr from: low to: high
+     | p |
+     low < high ifTrue: [
+       p := self partition: arr from: low to: high.
+       self sort: arr from: low to: p - 1.
+       self sort: arr from: p + 1 to: high].
+     ^ arr!
+
+   partition: arr from: low to: high
+     | pivot i tmp |
+     pivot := arr at: high.
+     i := low - 1.
+     low to: high - 1 do: [:j |
+       (arr at: j) <= pivot ifTrue: [
+         i := i + 1.
+         tmp := arr at: i.
+         arr at: i put: (arr at: j).
+         arr at: j put: tmp]].
+     tmp := arr at: i + 1.
+     arr at: i + 1 put: (arr at: high).
+     arr at: high put: tmp.
+     ^ i + 1! !")
+
+(smalltalk-load quicksort-source)
+
+(st-test "Quicksort class registered" (st-class-exists? "Quicksort") true)
+
+(st-test "qsort small array"
+  (evp "^ Quicksort new sort: #(3 1 2)")
+  (list 1 2 3))
+
+(st-test "qsort with duplicates"
+  (evp "^ Quicksort new sort: #(3 1 4 1 5 9 2 6 5 3 5)")
+  (list 1 1 2 3 3 4 5 5 5 6 9))
+
+(st-test "qsort already-sorted"
+  (evp "^ Quicksort new sort: #(1 2 3 4 5)")
+  (list 1 2 3 4 5))
+
+(st-test "qsort reverse-sorted"
+  (evp "^ Quicksort new sort: #(9 7 5 3 1)")
+  (list 1 3 5 7 9))
+
+(st-test "qsort single element"
+  (evp "^ Quicksort new sort: #(42)")
+  (list 42))
+
+(st-test "qsort empty"
+  (evp "^ Quicksort new sort: #()")
+  (list))
+
+(st-test "qsort negatives"
+  (evp "^ Quicksort new sort: #(-3 -1 -7 0 2)")
+  (list -7 -3 -1 0 2))
+
+(st-test "qsort all-equal"
+  (evp "^ Quicksort new sort: #(5 5 5 5)")
+  (list 5 5 5 5))
+
+(st-test "qsort sorts in place (returns same array)"
+  (evp
+    "| arr q |
+     arr := #(4 2 1 3).
+     q := Quicksort new.
+     q sort: arr.
+     ^ arr")
+  (list 1 2 3 4))
+
 (list st-test-pass st-test-fail)
diff --git a/lib/smalltalk/tests/programs/quicksort.st b/lib/smalltalk/tests/programs/quicksort.st
new file mode 100644
index 00000000..f1d8a43e
--- /dev/null
+++ b/lib/smalltalk/tests/programs/quicksort.st
@@ -0,0 +1,31 @@
+"Quicksort — Lomuto partition. Sorts an Array in place. Classic-corpus
+ program for the Smalltalk-on-SX runtime."
+
+Object subclass: #Quicksort
+  instanceVariableNames: ''!
+
+!Quicksort methodsFor: 'sort'!
+sort: arr ^ self sort: arr from: 1 to: arr size!
+
+sort: arr from: low to: high
+  | p |
+  low < high ifTrue: [
+    p := self partition: arr from: low to: high.
+    self sort: arr from: low to: p - 1.
+    self sort: arr from: p + 1 to: high].
+  ^ arr!
+
+partition: arr from: low to: high
+  | pivot i tmp |
+  pivot := arr at: high.
+  i := low - 1.
+  low to: high - 1 do: [:j |
+    (arr at: j) <= pivot ifTrue: [
+      i := i + 1.
+      tmp := arr at: i.
+      arr at: i put: (arr at: j).
+      arr at: j put: tmp]].
+  tmp := arr at: i + 1.
+  arr at: i + 1 put: (arr at: high).
+  arr at: high put: tmp.
+  ^ i + 1! !
diff --git a/plans/smalltalk-on-sx.md b/plans/smalltalk-on-sx.md
index 87127ea1..cda223c5 100644
--- a/plans/smalltalk-on-sx.md
+++ b/plans/smalltalk-on-sx.md
@@ -72,7 +72,7 @@ Core mapping:
 - [x] Escape past returned-from method raises (the SX-level analogue of `BlockContext>>cannotReturn:`). Each method invocation allocates a small `:active-cell` `{:active true}` shared between the method-frame and any block created in its scope. `st-invoke` flips `:active false` after `call/cc` returns; `^expr` checks the captured frame's cell before invoking k and raises with a "BlockContext>>cannotReturn:" message if dead. Verified by `lib/smalltalk/tests/cannot_return.sx` (5 tests using SX `guard` to catch the raise). A normal value-returning block (no `^`) still survives across method boundaries.
 - [ ] Classic programs in `lib/smalltalk/tests/programs/`:
   - [x] `eight-queens.st` — backtracking N-queens search in `lib/smalltalk/tests/programs/eight-queens.st`. The `.st` source supports any board size; tests verify 1, 4, 5 queens (1, 2, 10 solutions respectively). 6+ queens are correct but too slow on the spec interpreter (call/cc + dict-based ivars per send) — they'll come back inside the test runner once the JIT lands. The 8-queens canonical case will run in production.
-  - [ ] `quicksort.st`
+  - [x] `quicksort.st` — Lomuto-partition in-place quicksort in `lib/smalltalk/tests/programs/quicksort.st`. Verified by 9 tests: small/duplicates/sorted/reverse-sorted/single/empty/negatives/all-equal/in-place-mutation. Exercises Array `at:`/`at:put:` mutation, recursion, `to:do:` over varying ranges.
   - [ ] `mandelbrot.st`
   - [ ] `life.st` (Conway's Life, glider gun)
   - [x] `fibonacci.st` (recursive + Array-memoised) — `lib/smalltalk/tests/programs/fibonacci.st`. Loaded from chunk-format source by new `smalltalk-load` helper; verified by 13 tests in `lib/smalltalk/tests/programs.sx` (recursive `fib:`, memoised `memoFib:` up to 30, instance independence, class-table integrity). Source is currently duplicated as a string in the SX test file because there's no SX file-read primitive; conformance.sh will dedupe by piping the .st file directly.
@@ -108,6 +108,7 @@ Core mapping:
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-25: classic-corpus #3 quicksort (`tests/programs/quicksort.st`, 9 tests). Lomuto partition; verified across duplicates, already-sorted/reverse-sorted, empty, single, negatives, all-equal, plus in-place mutation. 385/385 total.
 - 2026-04-25: classic-corpus #2 eight-queens (`tests/programs/eight-queens.st`, 5 tests). Backtracking search; verified for boards of size 1, 4, 5. Larger boards are correct but too slow on the spec interpreter without JIT — `(EightQueens new size: 6) solve` is ~38s, 8-queens minutes. 382/382 total.
 - 2026-04-25: classic-corpus #1 fibonacci (`tests/programs/fibonacci.st` + `tests/programs.sx`, 13 tests). Added `smalltalk-load` chunk loader, class-side `subclass:instanceVariableNames:` (and longer Pharo variants), `Array new:` size, `methodsFor:`/`category:` no-ops, `st-split-ivars`. 377/377 total.
 - 2026-04-25: cannotReturn: implemented (`lib/smalltalk/tests/cannot_return.sx`, 5 tests). Each method-invocation gets an `{:active true}` cell shared with its blocks; `st-invoke` flips it on exit; `^expr` raises if the cell is dead. Tests use SX `guard` to catch the raise. Non-`^` blocks unaffected. 364/364 total.