Fix CreateSamplingHandler to use ChatRole.Tool for tool result messages

src/ModelContextProtocol.Core/AIContentExtensions.cs

@@ -128,7 +128,10 @@ public static class AIContentExtensions
                 {
                     if (sm.Content?.Select(b => b.ToAIContent()).OfType<AIContent>().ToList() is { Count: > 0 } aiContents)
                     {
-                        messages.Add(new ChatMessage(sm.Role is Role.Assistant ? ChatRole.Assistant : ChatRole.User, aiContents));
+                        ChatRole role = aiContents.All(c => c is FunctionResultContent) ? ChatRole.Tool :
+                            sm.Role is Role.Assistant ? ChatRole.Assistant :
+                            ChatRole.User;
+                        messages.Add(new ChatMessage(role, aiContents));
                     }
                 }
 

tests/ModelContextProtocol.Tests/Client/McpClientTests.cs

@@ -260,6 +260,84 @@ public async Task CreateSamplingHandler_ShouldHandleResourceMessages()
         Assert.Equal("endTurn", result.StopReason);
     }
 
+    [Fact]
+    public async Task CreateSamplingHandler_ShouldUseToolRoleForToolResultMessages()
+    {
+        // Arrange
+        var mockChatClient = new Mock<IChatClient>();
+        var requestParams = new CreateMessageRequestParams
+        {
+            Messages =
+            [
+                new SamplingMessage
+                {
+                    Role = Role.User,
+                    Content = [new TextContentBlock { Text = "What is the weather in Paris?" }]
+                },
+                new SamplingMessage
+                {
+                    Role = Role.Assistant,
+                    Content = [new ToolUseContentBlock
+                    {
+                        Id = "call_weather_123",
+                        Name = "get_weather",
+                        Input = JsonSerializer.SerializeToElement(new { location = "Paris" }, McpJsonUtilities.DefaultOptions)
+                    }]
+                },
+                new SamplingMessage
+                {
+                    Role = Role.User,
+                    Content = [new ToolResultContentBlock
+                    {
+                        ToolUseId = "call_weather_123",
+                        Content = [new TextContentBlock { Text = "Weather: 18°C, sunny" }]
+                    }]
+                }
+            ],
+            MaxTokens = 100
+        };
+
+        IEnumerable<ChatMessage>? capturedMessages = null;
+        var cancellationToken = CancellationToken.None;
+        var expectedResponse = new[] {
+            new ChatResponseUpdate
+            {
+                ModelId = "test-model",
+                FinishReason = ChatFinishReason.Stop,
+                Role = ChatRole.Assistant,
+                Contents = [new TextContent("The weather in Paris is 18°C and sunny.")]
+            }
+        }.ToAsyncEnumerable();
+
+        mockChatClient
+            .Setup(client => client.GetStreamingResponseAsync(It.IsAny<IEnumerable<ChatMessage>>(), It.IsAny<ChatOptions>(), cancellationToken))
+            .Callback<IEnumerable<ChatMessage>, ChatOptions?, CancellationToken>((messages, _, _) => capturedMessages = messages.ToList())
+            .Returns(expectedResponse);
+
+        var handler = mockChatClient.Object.CreateSamplingHandler();
+
+        // Act
+        var result = await handler(requestParams, Mock.Of<IProgress<ProgressNotificationValue>>(), cancellationToken);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.NotNull(capturedMessages);
+        var messagesList = capturedMessages.ToList();
+        Assert.Equal(3, messagesList.Count);
+
+        // First message should be User role (text message)
+        Assert.Equal(ChatRole.User, messagesList[0].Role);
+        Assert.IsType<TextContent>(messagesList[0].Contents.Single());
+
+        // Second message should be Assistant role (tool use)
+        Assert.Equal(ChatRole.Assistant, messagesList[1].Role);
+        Assert.IsType<FunctionCallContent>(messagesList[1].Contents.Single());
+
+        // Third message should be Tool role (tool result) - this is the bug fix
+        Assert.Equal(ChatRole.Tool, messagesList[2].Role);
+        Assert.IsType<FunctionResultContent>(messagesList[2].Contents.Single());
+    }
+
     [Fact]
     public async Task ListToolsAsync_AllToolsReturned()
     {