Change Value type to object and use JsonSerializer.SerializeToNode, fix nullable warnings

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -378,7 +378,7 @@ internal static IReadOnlyList<object> CreateMetadata(MethodInfo method)
             // Only add the attribute property if it doesn't already exist in the seed
             if (!meta.ContainsKey(attr.Name))
             {
-                meta[attr.Name] = attr.Value;
+                meta[attr.Name] = JsonSerializer.SerializeToNode(attr.Value);
             }
         }
 

src/ModelContextProtocol.Core/Server/McpMetaAttribute.cs

@@ -34,8 +34,8 @@ public sealed class McpMetaAttribute : Attribute
     /// Initializes a new instance of the <see cref="McpMetaAttribute"/> class.
     /// </summary>
     /// <param name="name">The name (key) of the metadata entry.</param>
-    /// <param name="value">The value of the metadata entry.</param>
-    public McpMetaAttribute(string name, string value)
+    /// <param name="value">The value of the metadata entry. This can be any value that can be encoded in .NET metadata.</param>
+    public McpMetaAttribute(string name, object? value)
     {
         Name = name;
         Value = value;
@@ -54,9 +54,16 @@ public McpMetaAttribute(string name, string value)
     /// Gets the value of the metadata entry.
     /// </summary>
     /// <remarks>
-    /// This value is stored as a string in the metadata object. For complex values, use the
+    /// <para>
+    /// This value can be any object that can be encoded in .NET metadata (strings, numbers, booleans, etc.).
+    /// The value will be serialized to JSON using <see cref="System.Text.Json.JsonSerializer"/> when
+    /// populating the metadata JsonObject.
+    /// </para>
+    /// <para>
+    /// For complex JSON structures that cannot be represented as .NET metadata, use the
     /// <see cref="McpServerToolCreateOptions.Meta"/>, <see cref="McpServerPromptCreateOptions.Meta"/>, 
     /// or <see cref="McpServerResourceCreateOptions.Meta"/> property to provide a JsonObject directly.
+    /// </para>
     /// </remarks>
-    public string Value { get; }
+    public object? Value { get; }
 }

tests/ModelContextProtocol.Tests/Server/McpMetaAttributeTests.cs

@@ -14,7 +14,7 @@ public void McpMetaAttribute_OnTool_PopulatesMeta()
         var method = typeof(TestToolClass).GetMethod(nameof(TestToolClass.ToolWithMeta))!;
 
         // Act
-        var tool = McpServerTool.Create(method, null);
+        var tool = McpServerTool.Create(method, target: null);
 
         // Assert
         Assert.NotNull(tool.ProtocolTool.Meta);
@@ -29,7 +29,7 @@ public void McpMetaAttribute_OnPrompt_PopulatesMeta()
         var method = typeof(TestPromptClass).GetMethod(nameof(TestPromptClass.PromptWithMeta))!;
 
         // Act
-        var prompt = McpServerPrompt.Create(method, null);
+        var prompt = McpServerPrompt.Create(method, target: null);
 
         // Assert
         Assert.NotNull(prompt.ProtocolPrompt.Meta);
@@ -44,7 +44,7 @@ public void McpMetaAttribute_OnResource_PopulatesMeta()
         var method = typeof(TestResourceClass).GetMethod(nameof(TestResourceClass.ResourceWithMeta))!;
 
         // Act
-        var resource = McpServerResource.Create(method, null);
+        var resource = McpServerResource.Create(method, target: null);
 
         // Assert
         Assert.NotNull(resource.ProtocolResource.Meta);
@@ -59,7 +59,7 @@ public void McpMetaAttribute_WithoutAttributes_ReturnsNull()
         var method = typeof(TestToolClass).GetMethod(nameof(TestToolClass.ToolWithoutMeta))!;
 
         // Act
-        var tool = McpServerTool.Create(method, null);
+        var tool = McpServerTool.Create(method, target: null);
 
         // Assert
         Assert.Null(tool.ProtocolTool.Meta);
@@ -72,7 +72,7 @@ public void McpMetaAttribute_SingleAttribute_PopulatesMeta()
         var method = typeof(TestToolClass).GetMethod(nameof(TestToolClass.ToolWithSingleMeta))!;
 
         // Act
-        var tool = McpServerTool.Create(method, null);
+        var tool = McpServerTool.Create(method, target: null);
 
         // Assert
         Assert.NotNull(tool.ProtocolTool.Meta);
@@ -93,7 +93,7 @@ public void McpMetaAttribute_OptionsMetaTakesPrecedence()
         var options = new McpServerToolCreateOptions { Meta = seedMeta };
 
         // Act
-        var tool = McpServerTool.Create(method, options);
+        var tool = McpServerTool.Create(method, target: null, options: options);
 
         // Assert
         Assert.NotNull(tool.ProtocolTool.Meta);
@@ -117,7 +117,7 @@ public void McpMetaAttribute_OptionsMetaOnly_NoAttributes()
         var options = new McpServerToolCreateOptions { Meta = seedMeta };
 
         // Act
-        var tool = McpServerTool.Create(method, options);
+        var tool = McpServerTool.Create(method, target: null, options: options);
 
         // Assert
         Assert.NotNull(tool.ProtocolTool.Meta);
@@ -138,7 +138,7 @@ public void McpMetaAttribute_PromptOptionsMetaTakesPrecedence()
         var options = new McpServerPromptCreateOptions { Meta = seedMeta };
 
         // Act
-        var prompt = McpServerPrompt.Create(method, options);
+        var prompt = McpServerPrompt.Create(method, target: null, options: options);
 
         // Assert
         Assert.NotNull(prompt.ProtocolPrompt.Meta);
@@ -163,7 +163,7 @@ public void McpMetaAttribute_ResourceOptionsMetaTakesPrecedence()
         var options = new McpServerResourceCreateOptions { Meta = seedMeta };
 
         // Act
-        var resource = McpServerResource.Create(method, options);
+        var resource = McpServerResource.Create(method, target: null, options: options);
 
         // Assert
         Assert.NotNull(resource.ProtocolResource.Meta);