Restore UseStructuredContent on McpServerToolCreateOptions

Keep UseStructuredContent as a non-breaking public property alongside the new
OutputSchema property. UseStructuredContent on options (or the attribute) triggers
output schema inference from the return type; OutputSchema allows providing an
explicit schema directly. Also fix JsonNode-to-JsonElement conversion in
CallToolResult<T>.ToCallToolResult after StructuredContent type change on main.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

src/ModelContextProtocol.Core/Protocol/CallToolResultOfT.cs

@@ -57,7 +57,9 @@ CallToolResult ICallToolResultTyped.ToCallToolResult(JsonSerializerOptions seria
         return new()
         {
             Content = [new TextContentBlock { Text = structuredContent?.ToString() ?? "null" }],
-            StructuredContent = structuredContent,
+            StructuredContent = structuredContent is not null
+                ? JsonSerializer.Deserialize<JsonElement>(structuredContent, McpJsonUtilities.JsonContext.Default.JsonElement)
+                : null,
             IsError = IsError,
             Meta = Meta,
         };

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -174,7 +174,6 @@ private static McpServerToolCreateOptions DeriveOptions(MethodInfo method, McpSe
     {
         McpServerToolCreateOptions newOptions = options?.Clone() ?? new();
 
-        bool useStructuredContent = false;
         if (method.GetCustomAttribute<McpServerToolAttribute>() is { } toolAttr)
         {
             newOptions.Name ??= toolAttr.Name;
@@ -211,7 +210,10 @@ private static McpServerToolCreateOptions DeriveOptions(MethodInfo method, McpSe
                 newOptions.Execution.TaskSupport ??= taskSupport;
             }
 
-            useStructuredContent = toolAttr.UseStructuredContent;
+            if (toolAttr.UseStructuredContent)
+            {
+                newOptions.UseStructuredContent = true;
+            }
         }
 
         if (method.GetCustomAttribute<DescriptionAttribute>() is { } descAttr)
@@ -223,10 +225,10 @@ private static McpServerToolCreateOptions DeriveOptions(MethodInfo method, McpSe
         newOptions.Metadata ??= CreateMetadata(method);
 
         // Generate the output schema from the return type if needed.
-        // UseStructuredContent on the attribute uses T from CallToolResult<T> or the return type directly.
+        // UseStructuredContent on the attribute or options uses T from CallToolResult<T> or the return type directly.
         // CallToolResult<T> return types automatically infer the schema from T even without UseStructuredContent.
         Type? outputSchemaType = GetCallToolResultContentType(method.ReturnType);
-        if (outputSchemaType is null && useStructuredContent)
+        if (outputSchemaType is null && newOptions.UseStructuredContent)
         {
             outputSchemaType = method.ReturnType;
         }

src/ModelContextProtocol.Core/Server/McpServerToolCreateOptions.cs

@@ -117,6 +117,18 @@ public sealed class McpServerToolCreateOptions
     /// </remarks>
     public bool? ReadOnly { get; set; }
 
+    /// <summary>
+    /// Gets or sets a value that indicates whether the tool should report an output schema for structured content.
+    /// </summary>
+    /// <value>
+    /// The default is <see langword="false"/>.
+    /// </value>
+    /// <remarks>
+    /// When enabled, the tool will attempt to populate the <see cref="Tool.OutputSchema"/>
+    /// and provide structured content in the <see cref="CallToolResult.StructuredContent"/> property.
+    /// </remarks>
+    public bool UseStructuredContent { get; set; }
+
     /// <summary>
     /// Gets or sets a JSON Schema object to use as the tool's output schema.
     /// </summary>
@@ -218,6 +230,7 @@ internal McpServerToolCreateOptions Clone() =>
             Idempotent = Idempotent,
             OpenWorld = OpenWorld,
             ReadOnly = ReadOnly,
+            UseStructuredContent = UseStructuredContent,
             OutputSchema = OutputSchema,
             SerializerOptions = SerializerOptions,
             SchemaCreateOptions = SchemaCreateOptions,