.Net: Add ThoughtSignature support for Gemini function calling

dotnet/src/Connectors/Connectors.Google.UnitTests/Core/Gemini/Clients/GeminiChatGenerationFunctionCallingTests.cs

@@ -626,6 +626,83 @@ public async Task IfAutoInvokeShouldAllowFilterToModifyFunctionResultAsync()
         Assert.Contains(ModifiedResult, secondRequestContent);
     }
 
+    [Fact]
+    public async Task FunctionCallWithThoughtSignatureIsCapturedInToolCallAsync()
+    {
+        // Arrange
+        var responseWithThoughtSignature = File.ReadAllText("./TestData/chat_function_with_thought_signature_response.json")
+            .Replace("%nameSeparator%", GeminiFunction.NameSeparator, StringComparison.Ordinal);
+        this._messageHandlerStub.ResponseToReturn.Content = new StringContent(responseWithThoughtSignature);
+
+        var client = this.CreateChatCompletionClient();
+        var chatHistory = CreateSampleChatHistory();
+        var executionSettings = new GeminiPromptExecutionSettings
+        {
+            ToolCallBehavior = GeminiToolCallBehavior.EnableFunctions([this._timePluginNow])
+        };
+
+        // Act
+        var messages = await client.GenerateChatMessageAsync(chatHistory, executionSettings: executionSettings, kernel: this._kernelWithFunctions);
+
+        // Assert
+        Assert.Single(messages);
+        var geminiMessage = messages[0] as GeminiChatMessageContent;
+        Assert.NotNull(geminiMessage);
+        Assert.NotNull(geminiMessage.ToolCalls);
+        Assert.Single(geminiMessage.ToolCalls);
+        Assert.Equal("test-thought-signature-abc123", geminiMessage.ToolCalls[0].ThoughtSignature);
+    }
+
+    [Fact]
+    public async Task TextResponseWithThoughtSignatureIsCapturedInMetadataAsync()
+    {
+        // Arrange
+        var responseWithThoughtSignature = File.ReadAllText("./TestData/chat_text_with_thought_signature_response.json");
+        this._messageHandlerStub.ResponseToReturn.Content = new StringContent(responseWithThoughtSignature);
+
+        var client = this.CreateChatCompletionClient();
+        var chatHistory = CreateSampleChatHistory();
+
+        // Act
+        var messages = await client.GenerateChatMessageAsync(chatHistory);
+
+        // Assert
+        Assert.Single(messages);
+        var geminiMessage = messages[0] as GeminiChatMessageContent;
+        Assert.NotNull(geminiMessage);
+        Assert.NotNull(geminiMessage.Metadata);
+        var metadata = geminiMessage.Metadata as GeminiMetadata;
+        Assert.NotNull(metadata);
+        Assert.Equal("text-response-thought-signature-xyz789", metadata.ThoughtSignature);
+    }
+
+    [Fact]
+    public async Task ThoughtSignatureIsIncludedInSubsequentRequestAsync()
+    {
+        // Arrange - First response has function call with ThoughtSignature
+        var responseWithThoughtSignature = File.ReadAllText("./TestData/chat_function_with_thought_signature_response.json")
+            .Replace("%nameSeparator%", GeminiFunction.NameSeparator, StringComparison.Ordinal);
+        using var handlerStub = new MultipleHttpMessageHandlerStub();
+        handlerStub.AddJsonResponse(responseWithThoughtSignature);
+        handlerStub.AddJsonResponse(this._responseContent); // Second response is text
+
+        using var httpClient = new HttpClient(handlerStub, false);
+        var client = this.CreateChatCompletionClient(httpClient: httpClient);
+        var chatHistory = CreateSampleChatHistory();
+        var executionSettings = new GeminiPromptExecutionSettings
+        {
+            ToolCallBehavior = GeminiToolCallBehavior.AutoInvokeKernelFunctions
+        };
+
+        // Act
+        await client.GenerateChatMessageAsync(chatHistory, executionSettings: executionSettings, kernel: this._kernelWithFunctions);
+
+        // Assert - Check that the second request includes the ThoughtSignature
+        var secondRequestContent = handlerStub.GetRequestContentAsString(1);
+        Assert.NotNull(secondRequestContent);
+        Assert.Contains("test-thought-signature-abc123", secondRequestContent);
+    }
+
     private static ChatHistory CreateSampleChatHistory()
     {
         var chatHistory = new ChatHistory();

dotnet/src/Connectors/Connectors.Google.UnitTests/Core/Gemini/Clients/GeminiChatStreamingFunctionCallingTests.cs

@@ -417,6 +417,63 @@ await client.StreamGenerateChatMessageAsync(chatHistory, executionSettings: exec
             c is GeminiChatMessageContent gm && gm.Role == AuthorRole.Tool && gm.CalledToolResult is not null);
     }
 
+    [Fact]
+    public async Task StreamingTextResponseWithAutoInvokeAndEmptyToolCallsDoesNotEnterToolCallingBranchAsync()
+    {
+        // Arrange - This tests the Phase 6 bug fix: empty ToolCalls list should not trigger tool calling
+        var client = this.CreateChatCompletionClient();
+        var chatHistory = CreateSampleChatHistory();
+        var executionSettings = new GeminiPromptExecutionSettings
+        {
+            ToolCallBehavior = GeminiToolCallBehavior.AutoInvokeKernelFunctions
+        };
+
+        // Response is text-only (no function calls), so ToolCalls will be empty list
+        this._messageHandlerStub.ResponseToReturn.Content = new StringContent(this._responseContent);
+
+        // Act
+        var messages = await client.StreamGenerateChatMessageAsync(
+            chatHistory,
+            executionSettings: executionSettings,
+            kernel: this._kernelWithFunctions).ToListAsync();
+
+        // Assert - Should yield text response without entering tool-calling branch
+        Assert.NotEmpty(messages);
+        Assert.All(messages, m =>
+        {
+            var geminiMessage = m as GeminiStreamingChatMessageContent;
+            Assert.NotNull(geminiMessage);
+            // ToolCalls should be null or empty for text responses
+            Assert.True(geminiMessage.ToolCalls is null || geminiMessage.ToolCalls.Count == 0);
+        });
+    }
+
+    [Fact]
+    public async Task StreamingTextResponseWithAutoInvokeAndNullToolCallsDoesNotEnterToolCallingBranchAsync()
+    {
+        // Arrange - This tests that pattern `is { Count: > 0 }` handles null safely
+        var client = this.CreateChatCompletionClient();
+        var chatHistory = CreateSampleChatHistory();
+        var executionSettings = new GeminiPromptExecutionSettings
+        {
+            ToolCallBehavior = GeminiToolCallBehavior.AutoInvokeKernelFunctions
+        };
+
+        // Response is text-only
+        this._messageHandlerStub.ResponseToReturn.Content = new StringContent(this._responseContent);
+
+        // Act
+        var messages = await client.StreamGenerateChatMessageAsync(
+            chatHistory,
+            executionSettings: executionSettings,
+            kernel: this._kernelWithFunctions).ToListAsync();
+
+        // Assert - Should complete without errors
+        Assert.NotEmpty(messages);
+        // Verify we got text content
+        Assert.Contains(messages, m => !string.IsNullOrEmpty(m.Content));
+    }
+
     private static ChatHistory CreateSampleChatHistory()
     {
         var chatHistory = new ChatHistory();

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

@@ -68,4 +68,71 @@ public void ToStringReturnsCorrectValue()
         // Act & Assert
         Assert.Equal("MyPlugin_MyFunction(location:San Diego, max_price:300)", functionToolCall.ToString());
     }
+
+    [Fact]
+    public void ThoughtSignatureIsNullWhenCreatedFromFunctionCallPart()
+    {
+        // Arrange - Using the FunctionCallPart constructor (no ThoughtSignature)
+        var toolCallPart = new GeminiPart.FunctionCallPart { FunctionName = "MyFunction" };
+        var functionToolCall = new GeminiFunctionToolCall(toolCallPart);
+
+        // Act & Assert
+        Assert.Null(functionToolCall.ThoughtSignature);
+    }
+
+    [Fact]
+    public void ThoughtSignatureIsCapturedWhenCreatedFromGeminiPart()
+    {
+        // Arrange - Using the GeminiPart constructor (with ThoughtSignature)
+        var part = new GeminiPart
+        {
+            FunctionCall = new GeminiPart.FunctionCallPart { FunctionName = "MyFunction" },
+            ThoughtSignature = "test-thought-signature-123"
+        };
+        var functionToolCall = new GeminiFunctionToolCall(part);
+
+        // Act & Assert
+        Assert.Equal("test-thought-signature-123", functionToolCall.ThoughtSignature);
+    }
+
+    [Fact]
+    public void ThoughtSignatureIsNullWhenGeminiPartHasNoSignature()
+    {
+        // Arrange
+        var part = new GeminiPart
+        {
+            FunctionCall = new GeminiPart.FunctionCallPart { FunctionName = "MyFunction" },
+            ThoughtSignature = null
+        };
+        var functionToolCall = new GeminiFunctionToolCall(part);
+
+        // Act & Assert
+        Assert.Null(functionToolCall.ThoughtSignature);
+    }
+
+    [Fact]
+    public void ArgumentsArePreservedWhenCreatedFromGeminiPart()
+    {
+        // Arrange
+        var part = new GeminiPart
+        {
+            FunctionCall = new GeminiPart.FunctionCallPart
+            {
+                FunctionName = "MyPlugin_MyFunction",
+                Arguments = new JsonObject
+                {
+                    { "location", "San Diego" },
+                    { "max_price", 300 }
+                }
+            },
+            ThoughtSignature = "signature-abc"
+        };
+        var functionToolCall = new GeminiFunctionToolCall(part);
+
+        // Act & Assert
+        Assert.NotNull(functionToolCall.Arguments);
+        Assert.Equal(2, functionToolCall.Arguments.Count);
+        Assert.Equal("San Diego", functionToolCall.Arguments["location"]!.ToString());
+        Assert.Equal("signature-abc", functionToolCall.ThoughtSignature);
+    }
 }

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

@@ -0,0 +1,59 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.SemanticKernel.Connectors.Google;
+using Xunit;
+
+namespace SemanticKernel.Connectors.Google.UnitTests.Core.Gemini;
+
+/// <summary>
+/// Unit tests for <see cref="GeminiMetadata"/> class.
+/// </summary>
+public sealed class GeminiMetadataTests
+{
+    [Fact]
+    public void ThoughtSignatureCanBeSetAndRetrieved()
+    {
+        // Arrange & Act
+        var metadata = new GeminiMetadata { ThoughtSignature = "test-signature-123" };
+
+        // Assert
+        Assert.Equal("test-signature-123", metadata.ThoughtSignature);
+    }
+
+    [Fact]
+    public void ThoughtSignatureIsNullByDefault()
+    {
+        // Arrange & Act
+        var metadata = new GeminiMetadata();
+
+        // Assert
+        Assert.Null(metadata.ThoughtSignature);
+    }
+
+    [Fact]
+    public void ThoughtSignatureIsStoredInDictionary()
+    {
+        // Arrange
+        var metadata = new GeminiMetadata { ThoughtSignature = "dict-signature" };
+
+        // Act
+        var hasKey = metadata.TryGetValue("ThoughtSignature", out var value);
+
+        // Assert
+        Assert.True(hasKey);
+        Assert.Equal("dict-signature", value);
+    }
+
+    [Fact]
+    public void ThoughtSignatureCanBeRetrievedFromDictionary()
+    {
+        // Arrange - This simulates deserialized metadata
+        var metadata = new GeminiMetadata { ThoughtSignature = "from-dict" };
+
+        // Act
+        var signature = metadata.ThoughtSignature;
+
+        // Assert
+        Assert.Equal("from-dict", signature);
+    }
+}