diff --git a/src/services/splitApi.ts b/src/services/splitApi.ts
index b7163b93..6860b022 100644
--- a/src/services/splitApi.ts
+++ b/src/services/splitApi.ts
@@ -29,7 +29,6 @@ export function splitApiFactory(
   const urls = settings.urls;
   const filterQueryString = settings.sync.__splitFiltersValidation && settings.sync.__splitFiltersValidation.queryString;
   const SplitSDKImpressionsMode = settings.sync.impressionsMode;
-  const flagSpecVersion = settings.sync.flagSpecVersion;
   const splitHttpClient = splitHttpClientFactory(settings, platform);
 
   return {
@@ -45,7 +44,7 @@ export function splitApiFactory(
     },
 
     fetchAuth(userMatchingKeys?: string[]) {
-      let url = `${urls.auth}/v2/auth?s=${flagSpecVersion}`;
+      let url = `${urls.auth}/v2/auth?s=${settings.sync.flagSpecVersion}`;
       if (userMatchingKeys) { // `userMatchingKeys` is undefined in server-side
         const queryParams = userMatchingKeys.map(userKeyToQueryParam).join('&');
         if (queryParams) url += '&' + queryParams;
@@ -54,7 +53,7 @@ export function splitApiFactory(
     },
 
     fetchSplitChanges(since: number, noCache?: boolean, till?: number, rbSince?: number) {
-      const url = `${urls.sdk}/splitChanges?s=${flagSpecVersion}&since=${since}${rbSince ? '&rbSince=' + rbSince : ''}${filterQueryString || ''}${till ? '&till=' + till : ''}`;
+      const url = `${urls.sdk}/splitChanges?s=${settings.sync.flagSpecVersion}&since=${since}${rbSince ? '&rbSince=' + rbSince : ''}${filterQueryString || ''}${till ? '&till=' + till : ''}`;
       return splitHttpClient(url, noCache ? noCacheHeaderOptions : undefined, telemetryTracker.trackHttp(SPLITS))
         .catch((err) => {
           if (err.statusCode === 414) settings.log.error(ERROR_TOO_MANY_SETS);
diff --git a/src/sync/polling/fetchers/splitChangesFetcher.ts b/src/sync/polling/fetchers/splitChangesFetcher.ts
index d134601b..58f87e9a 100644
--- a/src/sync/polling/fetchers/splitChangesFetcher.ts
+++ b/src/sync/polling/fetchers/splitChangesFetcher.ts
@@ -1,11 +1,29 @@
+import { ISettings } from '../../../types';
+import { ISplitChangesResponse } from '../../../dtos/types';
 import { IFetchSplitChanges, IResponse } from '../../../services/types';
+import { IStorageBase } from '../../../storages/types';
+import { FLAG_SPEC_VERSION } from '../../../utils/constants';
+import { base } from '../../../utils/settingsValidation';
 import { ISplitChangesFetcher } from './types';
+import { LOG_PREFIX_SYNC_SPLITS } from '../../../logger/constants';
+
+const PROXY_CHECK_INTERVAL_MILLIS_CS = 60 * 60 * 1000; // 1 hour in Client Side
+const PROXY_CHECK_INTERVAL_MILLIS_SS = 24 * PROXY_CHECK_INTERVAL_MILLIS_CS; // 24 hours in Server Side
+
+function sdkEndpointOverriden(settings: ISettings) {
+  return settings.urls.sdk !== base.urls.sdk;
+}
 
 /**
  * Factory of SplitChanges fetcher.
  * SplitChanges fetcher is a wrapper around `splitChanges` API service that parses the response and handle errors.
  */
-export function splitChangesFetcherFactory(fetchSplitChanges: IFetchSplitChanges): ISplitChangesFetcher {
+// @TODO breaking: drop support for Split Proxy below v5.10.0 and simplify the implementation
+export function splitChangesFetcherFactory(fetchSplitChanges: IFetchSplitChanges, settings: ISettings, storage: Pick<IStorageBase, 'splits' | 'rbSegments'>): ISplitChangesFetcher {
+
+  const log = settings.log;
+  const PROXY_CHECK_INTERVAL_MILLIS = settings.core.key !== undefined ? PROXY_CHECK_INTERVAL_MILLIS_CS : PROXY_CHECK_INTERVAL_MILLIS_SS;
+  let lastProxyCheckTimestamp: number | undefined;
 
   return function splitChangesFetcher(
     since: number,
@@ -14,12 +32,51 @@ export function splitChangesFetcherFactory(fetchSplitChanges: IFetchSplitChanges
     rbSince?: number,
     // Optional decorator for `fetchSplitChanges` promise, such as timeout or time tracker
     decorator?: (promise: Promise<IResponse>) => Promise<IResponse>
-  ) {
+  ): Promise<ISplitChangesResponse> {
+
+    // Recheck proxy
+    if (lastProxyCheckTimestamp && (Date.now() - lastProxyCheckTimestamp) > PROXY_CHECK_INTERVAL_MILLIS) {
+      settings.sync.flagSpecVersion = FLAG_SPEC_VERSION;
+    }
+
+    let splitsPromise = fetchSplitChanges(since, noCache, till, settings.sync.flagSpecVersion === FLAG_SPEC_VERSION ? rbSince : undefined)
+      // Handle proxy error with spec 1.3
+      .catch((err) => {
+        if (err.statusCode === 400 && sdkEndpointOverriden(settings) && settings.sync.flagSpecVersion === FLAG_SPEC_VERSION) {
+          log.error(LOG_PREFIX_SYNC_SPLITS + 'Proxy error detected. If you are using Split Proxy, please upgrade to latest version');
+          lastProxyCheckTimestamp = Date.now();
+          settings.sync.flagSpecVersion = '1.2'; // fallback to 1.2 spec
+          return fetchSplitChanges(since, noCache, till); // retry request without rbSince
+        }
+        throw err;
+      });
 
-    let splitsPromise = fetchSplitChanges(since, noCache, till, rbSince);
     if (decorator) splitsPromise = decorator(splitsPromise);
 
-    return splitsPromise.then(resp => resp.json());
+    return splitsPromise
+      .then(resp => resp.json())
+      .then(data => {
+        // Using flag spec version 1.2
+        if (data.splits) {
+          return {
+            ff: {
+              d: data.splits,
+              s: data.since,
+              t: data.till
+            }
+          };
+        }
+
+        // Proxy recovery
+        if (lastProxyCheckTimestamp) {
+          log.info(LOG_PREFIX_SYNC_SPLITS + 'Proxy error recovered');
+          lastProxyCheckTimestamp = undefined;
+          return Promise.all([storage.splits.clear(), storage.rbSegments.clear()])
+            .then(() => splitChangesFetcher(storage.splits.getChangeNumber() as number, undefined, undefined, storage.rbSegments.getChangeNumber() as number));
+        }
+
+        return data;
+      });
   };
 
 }
diff --git a/src/sync/polling/syncTasks/splitsSyncTask.ts b/src/sync/polling/syncTasks/splitsSyncTask.ts
index d6fed5a2..d385bf77 100644
--- a/src/sync/polling/syncTasks/splitsSyncTask.ts
+++ b/src/sync/polling/syncTasks/splitsSyncTask.ts
@@ -21,7 +21,7 @@ export function splitsSyncTaskFactory(
     settings.log,
     splitChangesUpdaterFactory(
       settings.log,
-      splitChangesFetcherFactory(fetchSplitChanges),
+      splitChangesFetcherFactory(fetchSplitChanges, settings, storage),
       storage,
       settings.sync.__splitFiltersValidation,
       readiness.splits,
diff --git a/src/sync/polling/updaters/__tests__/splitChangesUpdater.spec.ts b/src/sync/polling/updaters/__tests__/splitChangesUpdater.spec.ts
index b1bc79d8..750f1c0d 100644
--- a/src/sync/polling/updaters/__tests__/splitChangesUpdater.spec.ts
+++ b/src/sync/polling/updaters/__tests__/splitChangesUpdater.spec.ts
@@ -170,7 +170,7 @@ describe('splitChangesUpdater', () => {
   fetchMock.once('*', { status: 200, body: splitChangesMock1 }); // @ts-ignore
   const splitApi = splitApiFactory(settingsSplitApi, { getFetch: () => fetchMock }, telemetryTrackerFactory());
   const fetchSplitChanges = jest.spyOn(splitApi, 'fetchSplitChanges');
-  const splitChangesFetcher = splitChangesFetcherFactory(splitApi.fetchSplitChanges);
+  const splitChangesFetcher = splitChangesFetcherFactory(splitApi.fetchSplitChanges, fullSettings, storage);
 
   const readinessManager = readinessManagerFactory(EventEmitter, fullSettings);
   const splitsEmitSpy = jest.spyOn(readinessManager.splits, 'emit');