diff --git a/src/apps/epic/models.py b/src/apps/epic/models.py
index 71981df..d413cf2 100644
--- a/src/apps/epic/models.py
+++ b/src/apps/epic/models.py
@@ -85,6 +85,52 @@ class GateSlot(models.Model):
     debited_token_type = models.CharField(max_length=8, null=True, blank=True)
     debited_token_expires_at = models.DateTimeField(null=True, blank=True)
 
+    # ── Seat-occupancy / renewal clock (sprint 2026-05-31) ────────────────
+    # A filled seat's token cost is "current" for one renewal span after
+    # `filled_at`, then sits in a renewal-grace span of equal length before
+    # auto-BYE. Uniform across token types (no exceptions) — keyed on
+    # `filled_at` only; the per-token `debit_token` rules are untouched. A
+    # NULL `filled_at` (ORM fixtures / RESERVED slots) reads current /
+    # never-expired so nothing built without a fill timestamp gets evicted.
+    @property
+    def renewal_span(self):
+        return self.room.renewal_period or timedelta(days=7)
+
+    @property
+    def cost_current_until(self):
+        """End of the cost-current window [A, A+S). None if not filled."""
+        if self.filled_at is None:
+            return None
+        return self.filled_at + self.renewal_span
+
+    @property
+    def grace_expires_at(self):
+        """End of the renewal-grace window [A+S, A+2S) — the auto-BYE
+        threshold. None if not filled."""
+        if self.filled_at is None:
+            return None
+        return self.filled_at + 2 * self.renewal_span
+
+    @property
+    def cost_current(self):
+        """True in [A, A+S). NULL filled_at → True (never-filled / fixtures)."""
+        until = self.cost_current_until
+        return until is None or timezone.now() < until
+
+    @property
+    def in_renewal_grace(self):
+        """True in [A+S, A+2S) — cost lapsed but the seat is still held for
+        renewal. False before the span and after grace expires."""
+        if self.filled_at is None:
+            return False
+        return self.cost_current_until <= timezone.now() < self.grace_expires_at
+
+    @property
+    def grace_expired(self):
+        """True at/after A+2S — past renewal grace, eligible for auto-BYE."""
+        exp = self.grace_expires_at
+        return exp is not None and timezone.now() >= exp
+
 
 class RoomInvite(models.Model):
     PENDING = "PENDING"
diff --git a/src/apps/epic/tests/integrated/test_models.py b/src/apps/epic/tests/integrated/test_models.py
index 6e67b60..618e5f2 100644
--- a/src/apps/epic/tests/integrated/test_models.py
+++ b/src/apps/epic/tests/integrated/test_models.py
@@ -80,6 +80,82 @@ class DebitTokenTest(TestCase):
         self.assertEqual(self.room.gate_status, Room.OPEN)
 
 
+class GateSlotCostCurrentTest(TestCase):
+    """Seat-occupancy / renewal clock (sprint 2026-05-31). A filled seat's
+    token cost is "current" for `filled_at + renewal_period` (the cost-current
+    window), then sits in a renewal-grace window of equal length before its
+    occupant is auto-BYE'd. All derived from `filled_at` + `renewal_period`
+    only — uniform across token types, no new fields. A NULL `filled_at`
+    (ORM fixtures / RESERVED slots) reads as current / never-expired."""
+
+    def setUp(self):
+        self.owner = User.objects.create(email="founder@example.com")
+        self.room = Room.objects.create(
+            name="Test Room", owner=self.owner, renewal_period=timedelta(days=7),
+        )
+        self.slot = self.room.gate_slots.get(slot_number=1)
+
+    def _fill(self, filled_at):
+        self.slot.status = GateSlot.FILLED
+        self.slot.gamer = self.owner
+        self.slot.filled_at = filled_at
+        self.slot.save()
+        return self.slot
+
+    def test_cost_current_true_within_span(self):
+        self._fill(timezone.now())
+        self.assertTrue(self.slot.cost_current)
+
+    def test_cost_current_false_after_span(self):
+        self._fill(timezone.now() - timedelta(days=8))
+        self.assertFalse(self.slot.cost_current)
+
+    def test_cost_current_true_when_filled_at_null(self):
+        # RESERVED / ORM-built fixtures never expire (filled_at is None).
+        self.slot.status = GateSlot.FILLED
+        self.slot.filled_at = None
+        self.slot.save()
+        self.assertTrue(self.slot.cost_current)
+
+    def test_cost_current_until_equals_filled_plus_renewal_period(self):
+        self._fill(timezone.now())
+        self.assertEqual(
+            self.slot.cost_current_until, self.slot.filled_at + timedelta(days=7),
+        )
+
+    def test_in_renewal_grace_true_between_S_and_2S(self):
+        self._fill(timezone.now() - timedelta(days=8))   # past S=7d, before 2S=14d
+        self.assertTrue(self.slot.in_renewal_grace)
+
+    def test_in_renewal_grace_false_before_S(self):
+        self._fill(timezone.now())
+        self.assertFalse(self.slot.in_renewal_grace)
+
+    def test_in_renewal_grace_false_after_2S(self):
+        self._fill(timezone.now() - timedelta(days=15))
+        self.assertFalse(self.slot.in_renewal_grace)
+
+    def test_grace_expired_at_2S(self):
+        self._fill(timezone.now() - timedelta(days=15))  # past 2S=14d
+        self.assertTrue(self.slot.grace_expired)
+
+    def test_grace_expired_false_within_grace(self):
+        self._fill(timezone.now() - timedelta(days=8))
+        self.assertFalse(self.slot.grace_expired)
+
+    def test_grace_expired_false_for_null_filled_at(self):
+        self.slot.status = GateSlot.FILLED
+        self.slot.filled_at = None
+        self.slot.save()
+        self.assertFalse(self.slot.grace_expired)
+
+    def test_renewal_span_falls_back_to_7d_when_period_null(self):
+        self.room.renewal_period = None
+        self.room.save(update_fields=["renewal_period"])
+        self.slot.refresh_from_db()
+        self.assertEqual(self.slot.renewal_span, timedelta(days=7))
+
+
 class CoinTokenInUseTest(TestCase):
     def setUp(self):
         self.owner = User.objects.create(email="founder@example.com")