Revert to ICallToolResultTyped with ToCallToolResult conversion method

Restore the previous interface design where CallToolResult<T> implements
ICallToolResultTyped.ToCallToolResult(JsonSerializerOptions) with the
serialization logic inside the generic type. Remove the data-only
ICallToolResultTypedContent interface and ConvertCallToolResultOfT helper.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/CallToolResultOfT.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using System.Text.Json.Nodes;
 
 namespace ModelContextProtocol.Protocol;
@@ -22,7 +23,7 @@ namespace ModelContextProtocol.Protocol;
 /// the SDK to handle serialization of a strongly-typed result.
 /// </para>
 /// </remarks>
-public sealed class CallToolResult<T> : ICallToolResultTypedContent
+public sealed class CallToolResult<T> : ICallToolResultTyped
 {
     /// <summary>
     /// Gets or sets the typed content returned by the tool.
@@ -57,15 +58,30 @@ public sealed class CallToolResult<T> : ICallToolResultTypedContent
     public JsonObject? Meta { get; set; }
 
     /// <inheritdoc/>
-    (object? Content, Type ContentType, bool? IsError, JsonObject? Meta) ICallToolResultTypedContent.GetContent() =>
-        (Content, typeof(T), IsError, Meta);
+    CallToolResult ICallToolResultTyped.ToCallToolResult(JsonSerializerOptions serializerOptions)
+    {
+        var typeInfo = serializerOptions.GetTypeInfo(typeof(T));
+
+        string json = JsonSerializer.Serialize(Content, typeInfo);
+        JsonNode? structuredContent = JsonSerializer.SerializeToNode(Content, typeInfo);
+
+        return new()
+        {
+            Content = [new TextContentBlock { Text = json }],
+            StructuredContent = structuredContent,
+            IsError = IsError,
+            Meta = Meta,
+        };
+    }
 }
 
 /// <summary>
-/// Internal interface for accessing the content of a <see cref="CallToolResult{T}"/> without reflection.
+/// Internal interface for converting strongly-typed tool results to <see cref="CallToolResult"/>.
 /// </summary>
-internal interface ICallToolResultTypedContent
+internal interface ICallToolResultTyped
 {
-    /// <summary>Gets the content, its type, the error flag, and metadata.</summary>
-    (object? Content, Type ContentType, bool? IsError, JsonObject? Meta) GetContent();
+    /// <summary>
+    /// Converts the strongly-typed result to a <see cref="CallToolResult"/>.
+    /// </summary>
+    CallToolResult ToCallToolResult(JsonSerializerOptions serializerOptions);
 }

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -319,7 +319,7 @@ public override async ValueTask<CallToolResult> InvokeAsync(
 
             CallToolResult callToolResponse => callToolResponse,
 
-            _ when ConvertCallToolResultOfT(result, AIFunction.JsonSerializerOptions) is { } converted => converted,
+            _ when result is ICallToolResultTyped typed => typed.ToCallToolResult(AIFunction.JsonSerializerOptions),
 
             _ => new()
             {
@@ -626,33 +626,4 @@ private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumer
             IsError = allErrorContent && hasAny
         };
     }
-
-    /// <summary>
-    /// If <paramref name="result"/> is a <see cref="CallToolResult{T}"/>, converts it to a <see cref="CallToolResult"/>.
-    /// </summary>
-    private static CallToolResult? ConvertCallToolResultOfT(object result, JsonSerializerOptions serializerOptions)
-    {
-        Type type = result.GetType();
-        if (!type.IsGenericType || type.GetGenericTypeDefinition() != typeof(CallToolResult<>))
-        {
-            return null;
-        }
-
-        // Use the internal accessor to get the untyped content, IsError, and Meta
-        // without reflection, avoiding trimming warnings.
-        var (content, contentType, isError, meta) = ((ICallToolResultTypedContent)result).GetContent();
-
-        var typeInfo = serializerOptions.GetTypeInfo(contentType);
-
-        string json = JsonSerializer.Serialize(content, typeInfo);
-        JsonNode? structuredContent = JsonSerializer.SerializeToNode(content, typeInfo);
-
-        return new()
-        {
-            Content = [new TextContentBlock { Text = json }],
-            StructuredContent = structuredContent,
-            IsError = isError,
-            Meta = meta,
-        };
-    }
 }