diff --git a/servers/Azure.Mcp.Server/changelog-entries/1780624204134.yaml b/servers/Azure.Mcp.Server/changelog-entries/1780624204134.yaml
new file mode 100644
index 0000000000..0dca6f6ffe
--- /dev/null
+++ b/servers/Azure.Mcp.Server/changelog-entries/1780624204134.yaml
@@ -0,0 +1,3 @@
+changes:
+  - section: "Bugs Fixed"
+    description: "Fixed an issue where the Cosmos DB QueryItems operation silently parsed non-success (e.g. throttled or auth-failure) responses as data. The response is now validated before parsing so failures surface as errors."
diff --git a/tools/Azure.Mcp.Tools.Cosmos/src/Services/CosmosService.cs b/tools/Azure.Mcp.Tools.Cosmos/src/Services/CosmosService.cs
index 98fca9d2ad..42527e796a 100644
--- a/tools/Azure.Mcp.Tools.Cosmos/src/Services/CosmosService.cs
+++ b/tools/Azure.Mcp.Tools.Cosmos/src/Services/CosmosService.cs
@@ -288,6 +288,8 @@ public async Task<List<JsonElement>> QueryItems(
         while (queryIterator.HasMoreResults)
         {
             using ResponseMessage response = await queryIterator.ReadNextAsync(cancellationToken);
+            response.EnsureSuccessStatusCode();
+
             using var document = JsonDocument.Parse(response.Content);
             items.Add(document.RootElement.Clone());
         }
diff --git a/tools/Azure.Mcp.Tools.Cosmos/tests/Azure.Mcp.Tools.Cosmos.Tests/CosmosServiceTests.cs b/tools/Azure.Mcp.Tools.Cosmos/tests/Azure.Mcp.Tools.Cosmos.Tests/CosmosServiceTests.cs
index 32c60b5893..f8982f963e 100644
--- a/tools/Azure.Mcp.Tools.Cosmos/tests/Azure.Mcp.Tools.Cosmos.Tests/CosmosServiceTests.cs
+++ b/tools/Azure.Mcp.Tools.Cosmos/tests/Azure.Mcp.Tools.Cosmos.Tests/CosmosServiceTests.cs
@@ -195,6 +195,31 @@ await _cacheService.DidNotReceive().GetAsync<CosmosClient>(
             Arg.Any<CancellationToken>());
     }
 
+    [Fact]
+    public async Task QueryItems_NonSuccessResponse_Throws()
+    {
+        // Arrange: route the CosmosClient's transport through the mocked IHttpClientFactory so it always returns 403.
+        var handler = new MockHttpHandler(HttpStatusCode.Forbidden);
+        _httpClientFactory.CreateClient(Arg.Any<string>()).Returns(new HttpClient(handler));
+        var clientOptions = new CosmosClientOptions { HttpClientFactory = () => _httpClientFactory.CreateClient() };
+
+        var credential = Substitute.For<TokenCredential>();
+        var token = new AccessToken("fake-token", DateTimeOffset.UtcNow.AddHours(1));
+        credential.GetTokenAsync(Arg.Any<TokenRequestContext>(), Arg.Any<CancellationToken>())
+            .Returns(new ValueTask<AccessToken>(token));
+        credential.GetToken(Arg.Any<TokenRequestContext>(), Arg.Any<CancellationToken>())
+            .Returns(token);
+
+        using var cosmosClient = new CosmosClient("https://myaccount.documents.azure.com", credential, clientOptions);
+
+        _cacheService.GetAsync<CosmosClient>(Arg.Any<string>(), Arg.Any<string>(), Arg.Any<TimeSpan?>(), Arg.Any<CancellationToken>())
+            .Returns(cosmosClient);
+
+        // Act & Assert: a non-success response must throw rather than being returned as a data item
+        await Assert.ThrowsAnyAsync<CosmosException>(() =>
+            _service.QueryItems("myaccount", "mydb", "mycontainer", "SELECT * FROM c", "sub123", AuthMethod.Key, cancellationToken: TestContext.Current.CancellationToken));
+    }
+
     private sealed class MockHttpHandler(HttpStatusCode statusCode) : HttpMessageHandler
     {
         protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)