diff --git a/shared/tests/turn.invariant.test.js b/shared/tests/turn.invariant.test.js
new file mode 100644
index 0000000..666db9e
--- /dev/null
+++ b/shared/tests/turn.invariant.test.js
@@ -0,0 +1,127 @@
+// INVARIANT test: three order lists must always match.
+//   1. display order   = sortParticipantsByInitiative(participants).map(id)
+//   2. turnOrderIds    = frozen rotation array
+//   3. nextTurn order  = walking active rotation via repeated nextTurn
+//
+// Divergence = bug. BUG-6 (reorder), BUG-5 (add/remove) manifest here.
+// RED expected on reorder + others. Locks drift before refactor.
+
+'use strict';
+
+const shared = require('@ttrpg/shared');
+const {
+  makeParticipant, buildCharacterParticipant, buildMonsterParticipant,
+  sortParticipantsByInitiative,
+  startEncounter, nextTurn, addParticipant, removeParticipant,
+  toggleParticipantActive, togglePause, applyHpChange,
+  reorderParticipants, endEncounter,
+} = shared;
+
+function p(id, init, extra = {}) {
+  return makeParticipant({ id, name: id, type: 'monster',
+    initiative: init, maxHp: 100, currentHp: 100, ...extra });
+}
+function enc(ps, extra = {}) {
+  return { name:'t', participants:ps, isStarted:false, isPaused:false,
+    round:0, currentTurnParticipantId:null, turnOrderIds:[], ...extra };
+}
+const apply = (e, r) => r && r.patch ? { ...e, ...r.patch } : e;
+
+// snapshot the 3 lists
+function lists(e) {
+  const display = sortParticipantsByInitiative(e.participants, e.participants).map(x => x.id);
+  const frozen = [...(e.turnOrderIds || [])];
+  // walk full rotation: from current, nextTurn until back to start, collect ids in order
+  const rotation = [];
+  if (e.isStarted && !e.isPaused && e.currentTurnParticipantId) {
+    let cur = e;
+    const start = cur.currentTurnParticipantId;
+    const seen = [];
+    for (let i = 0; i < (cur.turnOrderIds || []).length + 1; i++) {
+      seen.push(cur.currentTurnParticipantId);
+      const nxt = nextTurn(cur);
+      cur = apply(cur, nxt);
+      if (cur.currentTurnParticipantId === start) break;
+    }
+    // rotation = [start, next1, ...] cyclic. Normalize to start at frozen[0]
+    // so raw-array compare matches (same cycle, canonical start).
+    const head = frozen[0];
+    const offset = seen.indexOf(head);
+    if (offset >= 0) {
+      rotation.push(...seen.slice(offset), ...seen.slice(0, offset));
+    } else {
+      rotation.push(...seen); // head not in rotation (inactive?) — leave as-is
+    }
+  }
+  return { display, frozen, rotation };
+}
+
+describe('3-list invariant: display === turnOrderIds === nextTurn rotation', () => {
+  test('startEncounter: all three match', () => {
+    let e = enc([p('a',10),p('b',7),p('c',3)]);
+    e = apply(e, startEncounter(e));
+    const { display, frozen, rotation } = lists(e);
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+
+  test('tie: drag order in participants[] preserved across all 3', () => {
+    // a,b both init=10. participants[] order [a,b] = display tiebreak.
+    let e = enc([p('a',10),p('b',10),p('c',3)]);
+    e = apply(e, startEncounter(e));
+    const { display, frozen, rotation } = lists(e);
+    expect(display.indexOf('a')).toBeLessThan(display.indexOf('b')); // a before b
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+
+  test('reorder via drag: all 3 reflect new order (BUG-6 RED)', () => {
+    // a,b,c init 10,10,3. drag b before a.
+    let e = enc([p('a',10),p('b',10),p('c',3)]);
+    e = apply(e, startEncounter(e));
+    e = apply(e, reorderParticipants(e, 'b', 'a')); // b dragged before a
+    const { display, frozen, rotation } = lists(e);
+    expect(display.indexOf('b')).toBeLessThan(display.indexOf('a'));
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+
+  test('add mid-encounter: all 3 match (BUG-5 RED)', () => {
+    let e = enc([p('a',10),p('b',5)]);
+    e = apply(e, startEncounter(e));
+    e = apply(e, addParticipant(e, p('c',7)));
+    const { display, frozen, rotation } = lists(e);
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+
+  test('remove mid-encounter: all 3 match', () => {
+    let e = enc([p('a',10),p('b',7),p('c',3)]);
+    e = apply(e, startEncounter(e));
+    e = apply(e, removeParticipant(e, 'b'));
+    const { display, frozen, rotation } = lists(e);
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+
+  test('toggle active off+on: all 3 match', () => {
+    let e = enc([p('a',10),p('b',7),p('c',3)]);
+    e = apply(e, startEncounter(e));
+    e = apply(e, nextTurn(e)); // b current
+    e = apply(e, toggleParticipantActive(e, 'a')); // a off
+    e = apply(e, toggleParticipantActive(e, 'a')); // a on
+    const { display, frozen, rotation } = lists(e);
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+
+  test('hp death + revive: all 3 match', () => {
+    let e = enc([p('a',10),p('b',7),p('c',3)]);
+    e = apply(e, startEncounter(e));
+    e = apply(e, applyHpChange(e, 'b', 'damage', 100)); // b dies
+    e = apply(e, applyHpChange(e, 'b', 'heal', 50)); // b revive
+    const { display, frozen, rotation } = lists(e);
+    expect(frozen).toEqual(display);
+    expect(rotation).toEqual(display);
+  });
+});