diff --git a/src/modules/keys/holding-capacity.test.ts b/src/modules/keys/holding-capacity.test.ts
new file mode 100644
index 0000000..f06e7a4
--- /dev/null
+++ b/src/modules/keys/holding-capacity.test.ts
@@ -0,0 +1,45 @@
+import { calculateHoldingCapacity } from './keys.routes';
+
+describe('calculateHoldingCapacity', () => {
+   it('allows a quantity that fits within the wallet cap', () => {
+      expect(
+         calculateHoldingCapacity({
+            circulatingSupply: 1_000,
+            holderCapBps: 2_500,
+            currentHolding: 100,
+            quantity: 150,
+         })
+      ).toEqual({
+         allowed: true,
+         current_holding: 100,
+         maximum_holding: 250,
+         remaining_capacity: 150,
+      });
+   });
+
+   it('rejects a quantity above the remaining capacity', () => {
+      expect(
+         calculateHoldingCapacity({
+            circulatingSupply: 1_000,
+            holderCapBps: 2_500,
+            currentHolding: 100,
+            quantity: 150.01,
+         }).allowed
+      ).toBe(false);
+   });
+
+   it('returns no remaining capacity when a legacy holding exceeds the cap', () => {
+      expect(
+         calculateHoldingCapacity({
+            circulatingSupply: 100,
+            holderCapBps: 1_000,
+            currentHolding: 20,
+            quantity: 1,
+         })
+      ).toMatchObject({
+         allowed: false,
+         maximum_holding: 10,
+         remaining_capacity: 0,
+      });
+   });
+});
diff --git a/src/modules/keys/key-holding-capacity.service.test.ts b/src/modules/keys/key-holding-capacity.service.test.ts
index 0a15e74..daa9644 100644
--- a/src/modules/keys/key-holding-capacity.service.test.ts
+++ b/src/modules/keys/key-holding-capacity.service.test.ts
@@ -101,6 +101,18 @@ describe('key-holding-capacity.service', () => {
       });
    });
 
+   it('reports whether a requested buy fits within the remaining capacity', async () => {
+      mockKey();
+      mockPrisma.keyOwnership.findUnique.mockResolvedValue({ balance: '100' });
+
+      await expect(getKeyHoldingCapacity('key-1', WALLET, 150)).resolves.toMatchObject({
+         allowed: true,
+         current_holding: 100,
+         maximum_holding: 250,
+         remaining_capacity: 150,
+      });
+   });
+
    it('treats a wallet with no position as zero holding', async () => {
       mockKey();
       mockPrisma.keyOwnership.findUnique.mockResolvedValue(null);
diff --git a/src/modules/keys/key-holding-capacity.service.ts b/src/modules/keys/key-holding-capacity.service.ts
index 3905b3b..7dba47a 100644
--- a/src/modules/keys/key-holding-capacity.service.ts
+++ b/src/modules/keys/key-holding-capacity.service.ts
@@ -16,6 +16,9 @@ export interface KeyHoldingCapacity {
    cap: number;
    remaining: number;
    cap_percentage: number;
+   allowed?: boolean;
+   remaining_capacity?: number;
+   maximum_holding?: number;
 }
 
 /**
@@ -51,9 +54,12 @@ export function computeCapPercentage(holding: number, cap: number): number {
  */
 export async function getKeyHoldingCapacity(
    keyId: string,
-   walletAddress: string
+   walletAddress: string,
+   quantity?: number
 ): Promise<KeyHoldingCapacity> {
-   const cacheKey = `keys:holding-capacity:${keyId}:${walletAddress}`;
+   const cacheKey = `keys:holding-capacity:${keyId}:${walletAddress}${
+      quantity === undefined ? '' : `:${quantity}`
+   }`;
    const cached = await cacheGetJson<KeyHoldingCapacity>(cacheKey);
    if (cached) {
       return cached;
@@ -98,6 +104,12 @@ export async function getKeyHoldingCapacity(
       cap_percentage: computeCapPercentage(currentHolding, cap),
    };
 
+   if (quantity !== undefined) {
+      capacity.allowed = quantity <= capacity.remaining;
+      capacity.remaining_capacity = capacity.remaining;
+      capacity.maximum_holding = capacity.cap;
+   }
+
    await cacheSetJson(cacheKey, capacity, HOLDING_CAPACITY_CACHE_TTL_SECONDS);
    return capacity;
 }
diff --git a/src/modules/keys/keys.routes.ts b/src/modules/keys/keys.routes.ts
index ffdde93..1eed0e9 100644
--- a/src/modules/keys/keys.routes.ts
+++ b/src/modules/keys/keys.routes.ts
@@ -158,11 +158,47 @@ const walletQuerySchema = z.object({
    wallet: StellarAddressSchema,
 });
 
+const holdingCapacityQuerySchema = walletQuerySchema.extend({
+   quantity: z.coerce.number().finite().positive().optional(),
+});
+
+/**
+ * Calculate whether a requested buy fits within a creator's per-wallet cap.
+ * Kept as a pure helper for callers and regression tests that use the legacy
+ * basis-point cap representation.
+ */
+export function calculateHoldingCapacity(input: {
+   circulatingSupply: number;
+   holderCapBps: number;
+   currentHolding: number;
+   quantity: number;
+}): {
+   allowed: boolean;
+   remaining_capacity: number;
+   current_holding: number;
+   maximum_holding: number;
+} {
+   const maximumHolding = Math.max(
+      0,
+      input.circulatingSupply * (input.holderCapBps / 10_000)
+   );
+   const remainingCapacity = Math.max(
+      0,
+      maximumHolding - input.currentHolding
+   );
+
+   return {
+      allowed: input.quantity <= remainingCapacity,
+      remaining_capacity: remainingCapacity,
+      current_holding: input.currentHolding,
+      maximum_holding: maximumHolding,
+   };
+}
+
 const lpHistoryQuerySchema = z.object({
    limit: z.coerce.number().int().positive().max(100).optional().default(20),
    cursor: z.string().min(1).optional(),
 });
-
 const priceImpactQuerySchema = z.object({
    quantity: z.string().transform(v => {
       const num = parseInt(v, 10);
@@ -1289,11 +1325,11 @@ router.get('/:keyId/cooldown', async (req, res, next) => {
  */
 
 /**
- * GET /api/v1/keys/:keyId/holding-capacity?wallet=
- * Wallet holding, holder cap, and remaining purchase capacity on a key.
+ * GET /api/v1/keys/:keyId/holding-capacity?wallet=&quantity=
+ * Wallet holding, holder cap, and optional buy pre-check on a key.
  */
 router.get('/:keyId/holding-capacity', async (req, res, next) => {
-   const parsed = walletQuerySchema.safeParse(req.query);
+   const parsed = holdingCapacityQuerySchema.safeParse(req.query);
    if (!parsed.success) {
       sendValidationError(
          res,
@@ -1307,7 +1343,8 @@ router.get('/:keyId/holding-capacity', async (req, res, next) => {
          res,
          await getKeyHoldingCapacity(
             String(req.params.keyId),
-            parsed.data.wallet
+            parsed.data.wallet,
+            parsed.data.quantity
          )
       );
    } catch (error) {