diff --git a/frontend/src/lib/api/__tests__/client.test.ts b/frontend/src/lib/api/__tests__/client.test.ts
index 5dcf3d6..a00bf1d 100644
--- a/frontend/src/lib/api/__tests__/client.test.ts
+++ b/frontend/src/lib/api/__tests__/client.test.ts
@@ -549,6 +549,84 @@ describe('API Client', () => {
       const second = await api.getFeaturedMarkets();
       expect(second).toEqual([{ id: 1, title: 'Resolved Market' }]);
     });
+
+    it('does not invalidate cache when mutation returns success:false (#1162)', async () => {
+      // Prime a statistics cache entry.
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify({ total_markets: 10 }),
+      });
+      await api.getStatistics();
+      expect(global.fetch).toHaveBeenCalledTimes(1);
+
+      // newsletterSubscribe returns 200 with success:false (business-logic failure).
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify({ success: false, message: 'Email already subscribed' }),
+      });
+      await api.newsletterSubscribe({ email: 'existing@example.com' });
+      expect(global.fetch).toHaveBeenCalledTimes(2);
+
+      // Cache must NOT be invalidated — next getStatistics should return cached value.
+      const cachedStats = await api.getStatistics();
+      expect(cachedStats).toEqual({ total_markets: 10 });
+      // fetch should not have been called again (still 2 calls total).
+      expect(global.fetch).toHaveBeenCalledTimes(2);
+    });
+
+    it('invalidates cache when mutation returns success:true (#1162)', async () => {
+      // Prime a statistics cache entry.
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify({ total_markets: 10 }),
+      });
+      await api.getStatistics();
+      expect(global.fetch).toHaveBeenCalledTimes(1);
+
+      // newsletterSubscribe succeeds.
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify({ success: true, message: 'Subscribed!' }),
+      });
+      await api.newsletterSubscribe({ email: 'new@example.com' });
+      expect(global.fetch).toHaveBeenCalledTimes(2);
+
+      // Cache MUST be invalidated — next getStatistics should hit the network.
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify({ total_markets: 11 }),
+      });
+      const freshStats = await api.getStatistics();
+      expect(freshStats).toEqual({ total_markets: 11 });
+      expect(global.fetch).toHaveBeenCalledTimes(3);
+    });
+
+    it('invalidates cache for mutations that do not use success envelope (#1162)', async () => {
+      // Prime a markets cache entry.
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify([{ id: 1, title: 'Market' }]),
+      });
+      await api.getFeaturedMarkets();
+      expect(global.fetch).toHaveBeenCalledTimes(1);
+
+      // resolveMarket returns { invalidated_keys: N } (no success field).
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify({ invalidated_keys: 2 }),
+      });
+      await api.resolveMarket(1);
+      expect(global.fetch).toHaveBeenCalledTimes(2);
+
+      // Cache MUST be invalidated — next getFeaturedMarkets hits the network.
+      (global.fetch as jest.Mock).mockResolvedValueOnce({
+        ok: true,
+        text: async () => JSON.stringify([{ id: 1, title: 'Resolved' }]),
+      });
+      const freshMarkets = await api.getFeaturedMarkets();
+      expect(freshMarkets).toEqual([{ id: 1, title: 'Resolved' }]);
+      expect(global.fetch).toHaveBeenCalledTimes(3);
+    });
   });
 
   describe('DELETE requests', () => {
diff --git a/frontend/src/lib/api/client.ts b/frontend/src/lib/api/client.ts
index 7c904c5..adc7321 100644
--- a/frontend/src/lib/api/client.ts
+++ b/frontend/src/lib/api/client.ts
@@ -307,11 +307,21 @@ async function request<T>(
 
       // On mutations, invalidate only the affected resource tags instead of
       // the entire cache. Fall back to a full clear for untagged mutations.
+      //
+      // Guard: some endpoints return { success: false, message: '...' } with a
+      // 200 status to signal business-logic failure (e.g. newsletterSubscribe).
+      // Only bust the cache when the mutation actually succeeded — i.e. when the
+      // response has no `success` field (non-envelope endpoints) or when
+      // `success` is explicitly `true`.
       if (method === "POST" || method === "DELETE") {
-        if (options.cacheTags?.length) {
-          apiCache.invalidateByTags(options.cacheTags);
-        } else {
-          apiCache.invalidateByPattern('.*');
+        const bodyObj = (typeof data === 'object' && data !== null) ? data as Record<string, unknown> : null;
+        const succeeded = bodyObj === null || !('success' in bodyObj) || bodyObj['success'] === true;
+        if (succeeded) {
+          if (options.cacheTags?.length) {
+            apiCache.invalidateByTags(options.cacheTags);
+          } else {
+            apiCache.invalidateByPattern('.*');
+          }
         }
       }