From 8e440629ee96e5f6bd0910b5cec699f3221e24ba Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 31 Dec 2025 19:58:58 +0000
Subject: [PATCH 1/2] Initial plan


From 5372d240d0ab6dc76e4771a1a5767ac061d98247 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 31 Dec 2025 20:10:33 +0000
Subject: [PATCH 2/2] Fix sampling handler to use ChatRole.Tool for tool result
 messages

When converting MCP SamplingMessages containing ToolResultContentBlock to
ChatMessages, the handler now correctly uses ChatRole.Tool instead of
ChatRole.User. This fixes compatibility with OpenAI's API which expects
tool results to come with the Tool role.

Fixes #1023

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
---
 .../AIContentExtensions.cs                    |  5 +-
 .../Client/McpClientTests.cs                  | 78 +++++++++++++++++++
 2 files changed, 82 insertions(+), 1 deletion(-)

diff --git a/src/ModelContextProtocol.Core/AIContentExtensions.cs b/src/ModelContextProtocol.Core/AIContentExtensions.cs
index b1ba32bf4..cffc554b3 100644
--- a/src/ModelContextProtocol.Core/AIContentExtensions.cs
+++ b/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));
                     }
                 }
 
diff --git a/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs b/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs
index 86cefcf10..33470f989 100644
--- a/tests/ModelContextProtocol.Tests/Client/McpClientTests.cs
+++ b/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()
     {