Structured output

Author: BenjaminMichaelis

EssentialCSharp.Chat.Shared/Services/AIChatService.cs

@@ -239,7 +239,7 @@ private async Task<string> EnrichPromptWithContext(string prompt, bool enableCon
         var inputItems = new List<ResponseItem>
         {
             functionCallItem, // The original function call
-            new FunctionCallOutputResponseItem(functionCallItem.CallId, string.Join("", toolResult.Content.Where(x => x.Type == "text").OfType<TextContentBlock>().Select(x => x.Text)))
+            new FunctionCallOutputResponseItem(functionCallItem.CallId, McpToolResultFormatter.GetModelInput(toolResult))
         };
 #pragma warning restore OPENAI001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
 
@@ -375,7 +375,7 @@ private static async Task<ResponseCreationOptions> CreateResponseOptionsAsync(
                     responseItems.Add(functionCallItem);
                     responseItems.Add(new FunctionCallOutputResponseItem(
                         functionCallItem.CallId,
-                        string.Join("", toolResult.Content.Where(x => x.Type == "text").OfType<TextContentBlock>().Select(x => x.Text))));
+                        McpToolResultFormatter.GetModelInput(toolResult)));
                 }
                 continue;
             }

EssentialCSharp.Chat.Shared/Services/McpToolResultFormatter.cs

@@ -0,0 +1,24 @@
+using ModelContextProtocol.Protocol;
+
+namespace EssentialCSharp.Chat.Common.Services;
+
+public static class McpToolResultFormatter
+{
+    public static string GetModelInput(CallToolResult toolResult)
+    {
+        if (toolResult.StructuredContent is { } structuredContent)
+        {
+            return structuredContent.GetRawText();
+        }
+
+        return GetPrimaryTextContent(toolResult.Content);
+    }
+
+    public static string GetPrimaryTextContent(IEnumerable<ContentBlock> contentBlocks) =>
+        contentBlocks
+            .Where(x => x.Type == "text")
+            .OfType<TextContentBlock>()
+            .Select(x => x.Text)
+            .FirstOrDefault(text => !string.IsNullOrWhiteSpace(text))
+        ?? string.Empty;
+}

EssentialCSharp.Chat.Tests/McpToolResultFormatterTests.cs

@@ -0,0 +1,49 @@
+using System.Text.Json;
+using EssentialCSharp.Chat.Common.Services;
+using ModelContextProtocol.Protocol;
+
+namespace EssentialCSharp.Chat.Tests;
+
+public class McpToolResultFormatterTests
+{
+    [Test]
+    public async Task GetModelInput_PrefersStructuredContent_WhenAvailable()
+    {
+        JsonElement structuredContent = JsonSerializer.SerializeToElement(new
+        {
+            diagnostic = "CS8600",
+            relevantSections = Array.Empty<object>()
+        });
+
+        CallToolResult toolResult = new()
+        {
+            Content =
+            [
+                new TextContentBlock { Text = "# Book Help for: CS8600" },
+                new TextContentBlock { Text = "{\"diagnostic\":\"CS8600\"}" }
+            ],
+            StructuredContent = structuredContent
+        };
+
+        string modelInput = McpToolResultFormatter.GetModelInput(toolResult);
+
+        await Assert.That(modelInput).IsEqualTo(structuredContent.GetRawText());
+    }
+
+    [Test]
+    public async Task GetModelInput_FallsBackToFirstTextBlock_WhenStructuredContentIsMissing()
+    {
+        CallToolResult toolResult = new()
+        {
+            Content =
+            [
+                new TextContentBlock { Text = "# Readable content" },
+                new TextContentBlock { Text = "{\"json\":\"fallback\"}" }
+            ]
+        };
+
+        string modelInput = McpToolResultFormatter.GetModelInput(toolResult);
+
+        await Assert.That(modelInput).IsEqualTo("# Readable content");
+    }
+}