Normalize CallToolResult.StructuredContent to JsonElement? and CreateMessageRequestParams.Metadata to JsonObject?

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

Author: Copilot

src/ModelContextProtocol.Core/Protocol/CallToolResult.cs

@@ -1,5 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
-using System.Text.Json.Nodes;
+using System.Text.Json;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
@@ -41,7 +41,7 @@ public sealed class CallToolResult : Result
     /// Gets or sets an optional JSON object representing the structured result of the tool call.
     /// </summary>
     [JsonPropertyName("structuredContent")]
-    public JsonNode? StructuredContent { get; set; }
+    public JsonElement? StructuredContent { get; set; }
 
     /// <summary>
     /// Gets or sets a value that indicates whether the tool call was unsuccessful.

src/ModelContextProtocol.Core/Protocol/CreateMessageRequestParams.cs

@@ -1,5 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
-using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
@@ -68,7 +68,7 @@ public sealed class CreateMessageRequestParams : RequestParams
     /// </para>
     /// </remarks>
     [JsonPropertyName("metadata")]
-    public JsonElement? Metadata { get; set; }
+    public JsonObject? Metadata { get; set; }
 
     /// <summary>
     /// Gets or sets the server's preferences for which model to select.

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -266,7 +266,7 @@ public override async ValueTask<CallToolResult> InvokeAsync(
         object? result;
         result = await AIFunction.InvokeAsync(arguments, cancellationToken).ConfigureAwait(false);
 
-        JsonNode? structuredContent = CreateStructuredResponse(result);
+        JsonElement? structuredContent = CreateStructuredResponse(result);
         return result switch
         {
             AIContent aiContent => new()
@@ -529,33 +529,37 @@ typeProperty.ValueKind is not JsonValueKind.String ||
         return outputSchema;
     }
 
-    private JsonNode? CreateStructuredResponse(object? aiFunctionResult)
+    private JsonElement? CreateStructuredResponse(object? aiFunctionResult)
     {
         if (ProtocolTool.OutputSchema is null)
         {
             // Only provide structured responses if the tool has an output schema defined.
             return null;
         }
 
-        JsonNode? nodeResult = aiFunctionResult switch
+        JsonElement? elementResult = aiFunctionResult switch
         {
-            JsonNode node => node,
-            JsonElement jsonElement => JsonSerializer.SerializeToNode(jsonElement, McpJsonUtilities.JsonContext.Default.JsonElement),
-            _ => JsonSerializer.SerializeToNode(aiFunctionResult, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object))),
+            JsonElement jsonElement => jsonElement,
+            JsonNode node => JsonSerializer.SerializeToElement(node, McpJsonUtilities.JsonContext.Default.JsonNode),
+            null => null,
+            _ => JsonSerializer.SerializeToElement(aiFunctionResult, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object))),
         };
 
         if (_structuredOutputRequiresWrapping)
         {
-            return new JsonObject
+            JsonNode? resultNode = elementResult is { } je
+                ? JsonSerializer.SerializeToNode(je, McpJsonUtilities.JsonContext.Default.JsonElement)
+                : null;
+            return JsonSerializer.SerializeToElement(new JsonObject
             {
-                ["result"] = nodeResult
-            };
+                ["result"] = resultNode
+            }, McpJsonUtilities.JsonContext.Default.JsonObject);
         }
 
-        return nodeResult;
+        return elementResult;
     }
 
-    private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumerable<AIContent> contentItems, JsonNode? structuredContent)
+    private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumerable<AIContent> contentItems, JsonElement? structuredContent)
     {
         List<ContentBlock> contentList = [];
         bool allErrorContent = true;