.Net: Fix #13262: GeminiRequest to handle single turn requests correctly

dotnet/src/Connectors/Connectors.Google.UnitTests/Core/Gemini/GeminiRequestTests.cs

@@ -694,6 +694,49 @@ public void FromPromptAndExecutionSettingsWithThinkingConfigReturnsInGenerationC
         Assert.Equal(executionSettings.ThinkingConfig.ThinkingBudget, request.Configuration?.ThinkingConfig?.ThinkingBudget);
     }
 
+    [Fact]
+    public void FromChatHistorySingleAssistantMessageSetsRoleToNull()
+    {
+        // Arrange - Single assistant message (issue #13262 scenario)
+        ChatHistory chatHistory = [];
+        chatHistory.AddAssistantMessage("assistant-message");
+        var executionSettings = new GeminiPromptExecutionSettings();
+
+        // Act
+        var request = GeminiRequest.FromChatHistoryAndExecutionSettings(chatHistory, executionSettings);
+
+        // Assert - Role should be null to fix issue #13262 (Gemini requires single-turn requests to end with user role or no role)
+        Assert.Single(request.Contents);
+        Assert.Null(request.Contents[0].Role);
+        Assert.Equal("assistant-message", request.Contents[0].Parts![0].Text);
+    }
+
+    [Fact]
+    public void FromChatHistoryMultiTurnConversationPreservesAllRoles()
+    {
+        // Arrange - Multi-turn conversation should not be affected by the fix
+        ChatHistory chatHistory = [];
+        chatHistory.AddUserMessage("user-message-1");
+        chatHistory.AddAssistantMessage("assistant-message-1");
+        chatHistory.AddUserMessage("user-message-2");
+        chatHistory.AddAssistantMessage("assistant-message-2");
+        var executionSettings = new GeminiPromptExecutionSettings();
+
+        // Act
+        var request = GeminiRequest.FromChatHistoryAndExecutionSettings(chatHistory, executionSettings);
+
+        // Assert - All roles should be preserved in multi-turn conversations
+        Assert.Equal(4, request.Contents.Count);
+        Assert.Equal(AuthorRole.User, request.Contents[0].Role);
+        Assert.Equal(AuthorRole.Assistant, request.Contents[1].Role);
+        Assert.Equal(AuthorRole.User, request.Contents[2].Role);
+        Assert.Equal(AuthorRole.Assistant, request.Contents[3].Role);
+        Assert.Equal("user-message-1", request.Contents[0].Parts![0].Text);
+        Assert.Equal("assistant-message-1", request.Contents[1].Parts![0].Text);
+        Assert.Equal("user-message-2", request.Contents[2].Parts![0].Text);
+        Assert.Equal("assistant-message-2", request.Contents[3].Parts![0].Text);
+    }
+
     private sealed class DummyContent(object? innerContent, string? modelId = null, IReadOnlyDictionary<string, object?>? metadata = null) :
         KernelContent(innerContent, modelId, metadata);
 

dotnet/src/Connectors/Connectors.Google/Core/Gemini/Models/GeminiRequest.cs

@@ -119,11 +119,19 @@ private static GeminiRequest CreateGeminiRequest(string prompt)
 
     private static GeminiRequest CreateGeminiRequest(ChatHistory chatHistory)
     {
+        var contents = chatHistory
+            .Where(message => message.Role != AuthorRole.System)
+            .Select(CreateGeminiContentFromChatMessage).ToList();
+
+        // Gemini specific fix: single turn requests must end with "user" role or no role, prevents issue #13262
+        if (contents.Count == 1 && contents[0].Role == AuthorRole.Assistant)
+        {
+            contents[0].Role = null;
+        }
+
         GeminiRequest obj = new()
         {
-            Contents = chatHistory
-                .Where(message => message.Role != AuthorRole.System)
-                .Select(CreateGeminiContentFromChatMessage).ToList(),
+            Contents = contents,
             SystemInstruction = CreateSystemMessages(chatHistory)
         };
         return obj;