diff --git a/src/lib/api.tsx b/src/lib/api.tsx
index eed9fd3..d3b2976 100644
--- a/src/lib/api.tsx
+++ b/src/lib/api.tsx
@@ -98,6 +98,11 @@ export function clearCache() {
   cache = {};
 }
 
+/* drop specific endpoints from the cache so the next load() refetches */
+export function invalidate(...keys: EpKey[]) {
+  for (const k of keys) delete cache[k];
+}
+
 /* ---- auth context ---- */
 
 interface AuthCtx {
diff --git a/src/lib/fitbit.ts b/src/lib/fitbit.ts
index d50d187..86cd20f 100644
--- a/src/lib/fitbit.ts
+++ b/src/lib/fitbit.ts
@@ -2,7 +2,7 @@
    Android-only: react-native-health-connect is loaded lazily so the module's
    absence (iOS, Expo Go) never crashes the app. */
 import { Platform } from 'react-native';
-import { FITBIT_SYNC_URL, load, postJson } from './api';
+import { FITBIT_SYNC_URL, invalidate, load, postJson } from './api';
 
 export interface SyncResult {
   ok: boolean;
@@ -90,6 +90,7 @@ export async function syncFitbit(onProgress?: (msg: string) => void): Promise<Sy
   await requestPermission(READ_PERMISSIONS.map((r) => ({ accessType: 'read', recordType: r })));
 
   say('Checking server watermark…');
+  invalidate('fitbitLatest'); // watermark must be fresh, never the session-cached copy
   const latest = await load<{
     samples: Record<string, string | null>;
     daily: Record<string, string | null>;
@@ -182,6 +183,9 @@ export async function syncFitbit(onProgress?: (msg: string) => void): Promise<Sy
     ns += res.samples; nd += res.daily; nl += res.sleep;
   }
 
+  // new rows exist server-side — cached copies of these are now stale
+  invalidate('fitbitSummary', 'fitbitLatest');
+
   return {
     ok: true, samples: ns, daily: nd, sleep: nl,
     message: `Synced ${ns} samples, ${nd} daily rows, ${nl} sleep sessions`,