diff --git a/TODO.md b/TODO.md
index 3b54537..19b840f 100644
--- a/TODO.md
+++ b/TODO.md
@@ -79,6 +79,18 @@
   sequence exactly (damage, heal, conditions, toggleActive, deathSave,
   remove, add, edit, pause/resume, reorder, revive-between-rounds).
 
+### BUG-6: reorderParticipants doesn't update turnOrderIds
+- Test: `shared/tests/turn.reorder.test.js` 'reorder updates turnOrderIds' (RED).
+- `reorderParticipants(enc, draggedId, targetId)` swaps two same-initiative
+  participants in `participants[]` array but leaves `turnOrderIds` unchanged.
+- nextTurn rotates via `turnOrderIds` only → reorder has NO effect on combat
+  rotation. Mid-encounter drag-drop = pointless.
+- replay-combat.js calls reorderParticipants with WRONG signature
+  `(enc, reorderedArray)` — swallowed by try/catch, silent no-op. So
+  replay never exercised real path either.
+- Fix: reorder must also update turnOrderIds to match new participant order
+  (within same-initiative tie).
+
 ## Pipeline
 - [ ] Red test: dead participant still in turnOrderIds, turn still advances to them
 - [ ] Fix `shared/turn.js`: don't drop dead from turn order
diff --git a/shared/tests/turn.reorder.test.js b/shared/tests/turn.reorder.test.js
new file mode 100644
index 0000000..0ef86d8
--- /dev/null
+++ b/shared/tests/turn.reorder.test.js
@@ -0,0 +1,66 @@
+// Characterization for reorderParticipants correct usage.
+// replay-combat.js calls it with wrong signature (swallowed by try/catch),
+// so real behavior untested. Lock what it actually does.
+
+const shared = require('@ttrpg/shared');
+const { makeParticipant, startEncounter, nextTurn, reorderParticipants } = shared;
+
+function p(id, init, extra = {}) {
+  return makeParticipant({
+    id, name: id, type: 'monster',
+    initiative: init, maxHp: 100, currentHp: 100,
+    ...extra,
+  });
+}
+function enc(ps) {
+  return { name:'t', participants:ps, isStarted:false, isPaused:false,
+    round:0, currentTurnParticipantId:null, turnOrderIds:[] };
+}
+
+describe('reorderParticipants', () => {
+  test('swaps two same-initiative participants', () => {
+    const ps = [p('a', 10), p('b', 20), p('c', 20)]; // b,c tie
+    let e = enc(ps);
+    e = { ...e, ...startEncounter(e).patch };
+    // initial order: b,c,a (init 20,20,10)
+    expect(e.turnOrderIds).toEqual(['b', 'c', 'a']);
+    const r = reorderParticipants(e, 'c', 'b');
+    expect(r.patch.participants.map(p => p.id)).toEqual(['a', 'c', 'b']);
+  });
+
+  test('throws if initiatives differ', () => {
+    const ps = [p('a', 10), p('b', 20)];
+    let e = enc(ps);
+    e = { ...e, ...startEncounter(e).patch };
+    expect(() => reorderParticipants(e, 'a', 'b')).toThrow();
+  });
+
+  test('throws if id not found', () => {
+    const ps = [p('a', 10), p('b', 20)];
+    let e = enc(ps);
+    e = { ...e, ...startEncounter(e).patch };
+    expect(() => reorderParticipants(e, 'a', 'zzz')).toThrow();
+  });
+
+  test('does NOT touch turnOrderIds (only reorders participants array)', () => {
+    // Documents current behavior. If reorder is meant to affect combat
+    // rotation mid-encounter, this is BUG-6.
+    const ps = [p('a', 10), p('b', 20), p('c', 20)];
+    let e = enc(ps);
+    e = { ...e, ...startEncounter(e).patch };
+    const r = reorderParticipants(e, 'c', 'b');
+    expect(r.patch.turnOrderIds).toBeUndefined();
+  });
+
+  // BUG-6 candidate: reorder should affect turnOrderIds so mid-combat
+  // drag-drop changes who goes next within same-initiative tie.
+  // Currently RED (turnOrderIds not in patch).
+  test('reorder updates turnOrderIds to reflect new participant order', () => {
+    const ps = [p('a', 10), p('b', 20), p('c', 20)];
+    let e = enc(ps);
+    e = { ...e, ...startEncounter(e).patch };
+    // order: b,c,a
+    e = { ...e, ...reorderParticipants(e, 'c', 'b').patch };
+    expect(e.turnOrderIds).toEqual(['c', 'b', 'a']);
+  });
+});