diff --git a/packages/firebase/src/config.ts b/packages/firebase/src/config.ts
index c7b1de1e3..87bfad460 100644
--- a/packages/firebase/src/config.ts
+++ b/packages/firebase/src/config.ts
@@ -199,8 +199,20 @@ const mergeConfig = {
 };
 _config.set(mergeConfig);
 
+export type AntiFraudConfig = false | {
+  checkoutWindowMs?: number;
+  checkoutLimitAddr?: number;
+  checkoutLimitIp?: number;
+  ttlHours?: number;
+};
+
+export const checkoutRateLimitsCollection = 'checkoutRateLimits';
+
 export const config = _config as {
-  get(): BaseConfig & typeof mergeConfig & { metafields: Record<string, any> };
+  get(): BaseConfig & typeof mergeConfig & {
+    metafields: Record<string, any>;
+    checkoutAntiFraud?: AntiFraudConfig;
+  };
   // eslint-disable-next-line
   set(config: any): void;
 };
diff --git a/packages/firebase/src/handlers/clean-checkout-rate-limits.ts b/packages/firebase/src/handlers/clean-checkout-rate-limits.ts
new file mode 100644
index 000000000..4bfa3c628
--- /dev/null
+++ b/packages/firebase/src/handlers/clean-checkout-rate-limits.ts
@@ -0,0 +1,28 @@
+import { getFirestore, Timestamp } from 'firebase-admin/firestore';
+import { logger, checkoutRateLimitsCollection } from '../config';
+
+const BATCH_SIZE = 300;
+const MAX_BATCHES = 50;
+
+export default async () => {
+  const db = getFirestore();
+  const now = Timestamp.now();
+  let totalDeleted = 0;
+  for (let batchNumber = 0; batchNumber < MAX_BATCHES; batchNumber += 1) {
+    // eslint-disable-next-line no-await-in-loop
+    const snapshot = await db.collection(checkoutRateLimitsCollection)
+      .where('expireAt', '<=', now)
+      .limit(BATCH_SIZE)
+      .get();
+    if (snapshot.empty) break;
+    const batch = db.batch();
+    snapshot.docs.forEach((doc) => batch.delete(doc.ref));
+    // eslint-disable-next-line no-await-in-loop
+    await batch.commit();
+    totalDeleted += snapshot.size;
+    if (snapshot.size < BATCH_SIZE) break;
+  }
+  if (totalDeleted) {
+    logger.info(`Cleaned ${totalDeleted} expired checkout rate limit docs`);
+  }
+};
diff --git a/packages/firebase/src/index.ts b/packages/firebase/src/index.ts
index 48a51c54b..553950487 100644
--- a/packages/firebase/src/index.ts
+++ b/packages/firebase/src/index.ts
@@ -3,6 +3,7 @@
 import * as functions from 'firebase-functions/v1';
 import config from './config';
 import checkStoreEvents from './handlers/check-store-events';
+import cleanCheckoutRateLimits from './handlers/clean-checkout-rate-limits';
 
 const { httpsFunctionOptions: { region } } = config.get();
 
@@ -18,3 +19,9 @@ export const cronStoreEvents = functionBuilder.pubsub
   .onRun(() => {
     return checkStoreEvents();
   });
+
+export const cronCleanCheckoutRateLimits = functionBuilder.pubsub
+  .schedule(process.env.CRONTAB_CLEAN_CHECKOUT_RATE_LIMITS || '17 4 * * *')
+  .onRun(() => {
+    return cleanCheckoutRateLimits();
+  });
diff --git a/packages/modules/src/firebase/antifraud-rate-limit.ts b/packages/modules/src/firebase/antifraud-rate-limit.ts
new file mode 100644
index 000000000..9e71ba395
--- /dev/null
+++ b/packages/modules/src/firebase/antifraud-rate-limit.ts
@@ -0,0 +1,92 @@
+import { getFirestore, Timestamp } from 'firebase-admin/firestore';
+import {
+  logger,
+  checkoutRateLimitsCollection as COLLECTION,
+  type AntiFraudConfig,
+} from '@cloudcommerce/firebase/lib/config';
+
+const firstHeader = (
+  headers: Record<string, string | string[] | undefined>,
+  name: string,
+) => {
+  const value = headers[name];
+  return Array.isArray(value) ? value[0] : value;
+};
+
+const checkKey = async (
+  firestore: ReturnType<typeof getFirestore>,
+  key: string,
+  limit: number,
+  now: number,
+  checkoutWindowMs: number,
+  expireAt: Timestamp,
+): Promise<{ blocked: boolean; reason: string }> => {
+  const ref = firestore.collection(COLLECTION).doc(key);
+  let blocked = false;
+
+  await firestore.runTransaction(async (t) => {
+    const doc = await t.get(ref);
+    const data = doc.exists ? doc.data()! : { count: 0, windowStart: now };
+    const withinWindow = (now - data.windowStart) < checkoutWindowMs;
+    const count = withinWindow ? data.count + 1 : 1;
+    const windowStart = withinWindow ? data.windowStart : now;
+    t.set(ref, {
+      count, windowStart, updatedAt: now, key, expireAt,
+    });
+    if (count > limit) blocked = true;
+  });
+
+  return { blocked, reason: key.startsWith('addr') ? 'address' : 'ip' };
+};
+
+export default async function antiFraudRateLimit(
+  req: { body: any; headers: Record<string, string | string[] | undefined>; ip?: string },
+  options: Exclude<AntiFraudConfig, false> = {},
+): Promise<{ blocked: boolean; reason?: string }> {
+  const {
+    checkoutWindowMs = 10 * 60 * 1000,
+    checkoutLimitAddr = 10,
+    checkoutLimitIp = 10,
+    ttlHours = 24,
+  } = options;
+
+  try {
+    const firestore = getFirestore();
+    const now = Date.now();
+    const expireAt = Timestamp.fromMillis(now + ttlHours * 60 * 60 * 1000);
+
+    // Client IP may pass through CDN layers; mirror the SSR header chain,
+    // preferring edge-set headers over the spoofable X-Forwarded-For.
+    const ipsHeader = firstHeader(req.headers, 'x-real-ip')
+      || firstHeader(req.headers, 'x-forwarded-for')
+      || firstHeader(req.headers, 'cf-connecting-ip')
+      || firstHeader(req.headers, 'fastly-client-ip');
+    const realIp = (ipsHeader?.split(',')[0] || req.ip)?.trim() || '';
+
+    const to = req.body?.shipping?.to;
+    const zip = String(to?.zip || '').replace(/\D/g, '');
+    const number = String(to?.number || '');
+    const addrKey = zip && number ? `addr_${zip}_${number}` : null;
+    const ipKey = realIp ? `ip_${realIp.replace(/[.:]/g, '_')}` : null;
+
+    const checks: Array<{ key: string; limit: number }> = [];
+    if (addrKey) checks.push({ key: addrKey, limit: checkoutLimitAddr });
+    if (ipKey) checks.push({ key: ipKey, limit: checkoutLimitIp });
+
+    const results = await Promise.all(
+      checks.map(({ key, limit }) => {
+        return checkKey(firestore, key, limit, now, checkoutWindowMs, expireAt);
+      }),
+    );
+
+    const hit = results.find((r) => r.blocked);
+    if (hit) {
+      logger.warn(`Checkout blocked by rate limit — reason: ${hit.reason}`);
+      return { blocked: true, reason: hit.reason };
+    }
+  } catch (err) {
+    logger.warn('Anti-fraud rate limit check failed, allowing checkout', { err });
+  }
+
+  return { blocked: false };
+}
diff --git a/packages/modules/src/firebase/serve-modules-api.ts b/packages/modules/src/firebase/serve-modules-api.ts
index cc7e7f459..cfddef8e7 100644
--- a/packages/modules/src/firebase/serve-modules-api.ts
+++ b/packages/modules/src/firebase/serve-modules-api.ts
@@ -1,9 +1,11 @@
 import type { Request, Response } from 'firebase-functions/v1';
+import config, { logger } from '@cloudcommerce/firebase/lib/config';
 import { schemas } from '../index';
 import handleModule from './handle-module';
 import checkout from './checkout';
+import antiFraudRateLimit from './antifraud-rate-limit.js';
 
-export default (req: Request, res: Response) => {
+export default async (req: Request, res: Response) => {
   const { method } = req;
   if (method !== 'POST' && method !== 'GET') {
     return res.sendStatus(405);
@@ -38,6 +40,25 @@ export default (req: Request, res: Response) => {
             message: 'GET is acceptable only to JSON schema, at /@checkout/schema',
           });
       }
+      const { checkoutAntiFraud } = config.get();
+      if (checkoutAntiFraud !== false) {
+        let blocked = false;
+        try {
+          ({ blocked } = await antiFraudRateLimit(
+            req,
+            typeof checkoutAntiFraud === 'object' ? checkoutAntiFraud : {},
+          ));
+        } catch (err) {
+          logger.warn('Anti-fraud guard threw, allowing checkout', { err });
+        }
+        if (blocked) {
+          return res.status(429).json({
+            error_code: 'CKT429',
+            message: 'Too many checkout attempts. Try again later.',
+          });
+        }
+      }
+
       return checkout(req, res);
     }
     if (url === '/@checkout/schema') {