Add CallToolResult.ToChatMessage extension method (#575)

To simplify non-AIFunction usage getting a ChatMessage with the appropriate FunctionResultContent.

Author: stephentoub

src/ModelContextProtocol.Core/AIContentExtensions.cs

@@ -39,6 +39,29 @@ public static ChatMessage ToChatMessage(this PromptMessage promptMessage)
         };
     }
 
+    /// <summary>
+    /// Converts a <see cref="CallToolResult"/> to a <see cref="ChatMessage"/> object.
+    /// </summary>
+    /// <param name="result">The tool result to convert.</param>
+    /// <param name="callId">The identifier for the function call request that triggered the tool invocation.</param>
+    /// <returns>A <see cref="ChatMessage"/> object created from the tool result.</returns>
+    /// <remarks>
+    /// This method transforms a protocol-specific <see cref="CallToolResult"/> from the Model Context Protocol
+    /// into a standard <see cref="ChatMessage"/> object that can be used with AI client libraries. It produces a
+    /// <see cref="ChatRole.Tool"/> message containing a <see cref="FunctionResultContent"/> with result as a
+    /// serialized <see cref="JsonElement"/>.
+    /// </remarks>
+    public static ChatMessage ToChatMessage(this CallToolResult result, string callId)
+    {
+        Throw.IfNull(result);
+        Throw.IfNull(callId);
+
+        return new(ChatRole.Tool, [new FunctionResultContent(callId, JsonSerializer.SerializeToElement(result, McpJsonUtilities.JsonContext.Default.CallToolResult))
+        {
+             RawRepresentation = result,
+        }]);
+    }
+
     /// <summary>
     /// Converts a <see cref="GetPromptResult"/> to a list of <see cref="ChatMessage"/> objects.
     /// </summary>

tests/ModelContextProtocol.Tests/AIContentExtensionsTests.cs

@@ -0,0 +1,28 @@
+using Microsoft.Extensions.AI;
+using ModelContextProtocol.Protocol;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests;
+
+public class AIContentExtensionsTests
+{
+    [Fact]
+    public void CallToolResult_ToChatMessage_ProducesExpectedAIContent()
+    {
+        CallToolResult toolResult = new() { Content = [new TextContentBlock { Text = "This is a test message." }] };
+
+        Assert.Throws<ArgumentNullException>(() => AIContentExtensions.ToChatMessage(null!, "call123"));
+        Assert.Throws<ArgumentNullException>(() => AIContentExtensions.ToChatMessage(toolResult, null!));
+
+        ChatMessage message = AIContentExtensions.ToChatMessage(toolResult, "call123");
+        
+        Assert.NotNull(message);
+        Assert.Equal(ChatRole.Tool, message.Role);
+        
+        FunctionResultContent frc = Assert.IsType<FunctionResultContent>(Assert.Single(message.Contents));
+        Assert.Same(toolResult, frc.RawRepresentation);
+        Assert.Equal("call123", frc.CallId);
+        JsonElement result = Assert.IsType<JsonElement>(frc.Result);
+        Assert.Contains("This is a test message.", result.ToString());
+    }
+}