diff --git a/migrations.py b/migrations.py
index d3bcece..6ede0eb 100644
--- a/migrations.py
+++ b/migrations.py
@@ -845,3 +845,18 @@ async def m011_machine_npub_nullable(db):
         "CREATE UNIQUE INDEX IF NOT EXISTS dca_machines_wallet_id_uq "
         "ON dca_machines (wallet_id)"
     )
+
+
+async def m012_add_max_cash_in_sats(db):
+    """Server-side per-transaction cash-in ceiling (aiolabs/spirekeeper#31).
+
+    The secure `create_withdraw` RPC derives fee/net/attribution server-side,
+    but `principal_sats` is necessarily ATM-attested (only the hardware knows
+    how much cash went in). The bunker ACL / usage caps gate call *rate*, not
+    *sats*, so a single in-rate call could request an arbitrarily large payout.
+    `max_cash_in_sats` bounds that: the handler rejects a cash-in whose
+    principal exceeds it. NULL = no cap.
+    """
+    await db.execute(
+        "ALTER TABLE spirekeeper.super_config ADD COLUMN max_cash_in_sats INTEGER"
+    )
diff --git a/models.py b/models.py
index d779633..99ee552 100644
--- a/models.py
+++ b/models.py
@@ -482,6 +482,10 @@ class SuperConfig(BaseModel):
     super_cash_in_fee_fraction: float = 0.0
     super_cash_out_fee_fraction: float = 0.0
     super_fee_wallet_id: str | None
+    # Per-transaction cash-in ceiling in sats (#31). The bunker ACL gates call
+    # rate, not sats, so this bounds a single ATM-attested principal. NULL = no
+    # cap.
+    max_cash_in_sats: int | None = None
     updated_at: datetime
 
 
@@ -489,6 +493,7 @@ class UpdateSuperConfigData(BaseModel):
     super_cash_in_fee_fraction: float | None = None
     super_cash_out_fee_fraction: float | None = None
     super_fee_wallet_id: str | None = None
+    max_cash_in_sats: int | None = None
 
     @validator(
         "super_cash_in_fee_fraction",
@@ -501,6 +506,14 @@ class UpdateSuperConfigData(BaseModel):
             raise ValueError("super fee fraction must be between 0 and 1")
         return round(float(v), 4)
 
+    @validator("max_cash_in_sats")
+    def _cap_non_negative(cls, v):
+        if v is None:
+            return v
+        if v < 0:
+            raise ValueError("max_cash_in_sats must be >= 0")
+        return int(v)
+
 
 # =============================================================================
 # Operator UX action carriers — partial-tx and balance-settlement features.
diff --git a/static/js/index.js b/static/js/index.js
index c9878d0..053133d 100644
--- a/static/js/index.js
+++ b/static/js/index.js
@@ -103,7 +103,8 @@ window.app = Vue.createApp({
         data: {
           super_cash_in_fee_fraction: 0,
           super_cash_out_fee_fraction: 0,
-          super_fee_wallet_id: ''
+          super_fee_wallet_id: '',
+          max_cash_in_sats: null
         }
       },
 
@@ -576,7 +577,8 @@ window.app = Vue.createApp({
           this.superConfig?.super_cash_in_fee_fraction ?? 0,
         super_cash_out_fee_fraction:
           this.superConfig?.super_cash_out_fee_fraction ?? 0,
-        super_fee_wallet_id: this.superConfig?.super_fee_wallet_id || ''
+        super_fee_wallet_id: this.superConfig?.super_fee_wallet_id || '',
+        max_cash_in_sats: this.superConfig?.max_cash_in_sats ?? null
       }
       this.superFeeDialog.show = true
     },
@@ -604,6 +606,21 @@ window.app = Vue.createApp({
         Number(d.super_cash_in_fee_fraction),
         Number(d.super_cash_out_fee_fraction)
       )) return
+      // Blank cap field -> null (the PUT skips null, so the existing value is
+      // preserved rather than cleared — set 0 to reject every cash-in).
+      const cap =
+        d.max_cash_in_sats === '' ||
+        d.max_cash_in_sats === null ||
+        d.max_cash_in_sats === undefined
+          ? null
+          : Number(d.max_cash_in_sats)
+      if (cap !== null && (!Number.isInteger(cap) || cap < 0)) {
+        Quasar.Notify.create({
+          type: 'negative',
+          message: 'Max cash-in must be a non-negative whole number of sats'
+        })
+        return
+      }
       this.superFeeDialog.saving = true
       try {
         const {data} = await LNbits.api.request(
@@ -611,7 +628,8 @@ window.app = Vue.createApp({
           {
             super_cash_in_fee_fraction: Number(d.super_cash_in_fee_fraction),
             super_cash_out_fee_fraction: Number(d.super_cash_out_fee_fraction),
-            super_fee_wallet_id: (d.super_fee_wallet_id || '').trim() || null
+            super_fee_wallet_id: (d.super_fee_wallet_id || '').trim() || null,
+            max_cash_in_sats: cap
           }
         )
         this.superConfig = data
diff --git a/templates/spirekeeper/index.html b/templates/spirekeeper/index.html
index 53c30d1..01bb973 100644
--- a/templates/spirekeeper/index.html
+++ b/templates/spirekeeper/index.html
@@ -1381,6 +1381,11 @@
             label="Super fee destination wallet_id"
             hint="LNbits wallet that collects the platform fee"
             class="q-mb-md" dense outlined></q-input>
+          <q-input v-model.number="superFeeDialog.data.max_cash_in_sats"
+            label="Max cash-in per transaction (sats — blank = no cap)"
+            hint="Server-side ceiling on a single cash-in's principal. The ATM attests the amount; this bounds a compromised/buggy machine to one capped tx."
+            type="number" step="1" min="0"
+            class="q-mb-md" dense outlined></q-input>
         </q-card-section>
         <q-card-actions align="right">
           <q-btn flat label="Cancel" v-close-popup></q-btn>