Add McpMetaAttribute and integrate with tools, prompts, and resources

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

Author: Copilot

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerPrompt.cs

@@ -138,6 +138,12 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Icons = options?.Icons,
         };
 
+        // Populate Meta from McpMetaAttribute instances if a MethodInfo is in the metadata
+        if (options?.Metadata?.FirstOrDefault(m => m is MethodInfo) is MethodInfo method)
+        {
+            prompt.Meta = AIFunctionMcpServerTool.CreateMetaFromAttributes(method);
+        }
+
         return new AIFunctionMcpServerPrompt(function, prompt, options?.Metadata ?? []);
     }
 

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerResource.cs

@@ -211,6 +211,13 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
 
         string name = options?.Name ?? function.Name;
 
+        // Populate Meta from McpMetaAttribute instances if a MethodInfo is in the metadata
+        JsonObject? meta = null;
+        if (options?.Metadata?.FirstOrDefault(m => m is MethodInfo) is MethodInfo method)
+        {
+            meta = AIFunctionMcpServerTool.CreateMetaFromAttributes(method);
+        }
+
         ResourceTemplate resource = new()
         {
             UriTemplate = options?.UriTemplate ?? DeriveUriTemplate(name, function),
@@ -219,6 +226,7 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Description = options?.Description,
             MimeType = options?.MimeType ?? "application/octet-stream",
             Icons = options?.Icons,
+            Meta = meta,
         };
 
         return new AIFunctionMcpServerResource(function, resource, options?.Metadata ?? []);

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -143,6 +143,12 @@ options.OpenWorld is not null ||
                     ReadOnlyHint = options.ReadOnly,
                 };
             }
+
+            // Populate Meta from McpMetaAttribute instances if a MethodInfo is in the metadata
+            if (options.Metadata?.FirstOrDefault(m => m is MethodInfo) is MethodInfo method)
+            {
+                tool.Meta = CreateMetaFromAttributes(method);
+            }
         }
 
         return new AIFunctionMcpServerTool(function, tool, options?.Services, structuredOutputRequiresWrapping, options?.Metadata ?? []);
@@ -351,6 +357,22 @@ internal static IReadOnlyList<object> CreateMetadata(MethodInfo method)
         return metadata.AsReadOnly();
     }
 
+    /// <summary>Creates a Meta JsonObject from McpMetaAttribute instances on the specified method.</summary>
+    internal static JsonObject? CreateMetaFromAttributes(MethodInfo method)
+    {
+        // Get all McpMetaAttribute instances from the method
+        var metaAttributes = method.GetCustomAttributes<McpMetaAttribute>();
+        
+        JsonObject? meta = null;
+        foreach (var attr in metaAttributes)
+        {
+            meta ??= new JsonObject();
+            meta[attr.Name] = attr.Value;
+        }
+
+        return meta;
+    }
+
     /// <summary>Regex that flags runs of characters other than ASCII digits or letters.</summary>
 #if NET
     [GeneratedRegex("[^0-9A-Za-z]+")]

src/ModelContextProtocol.Core/Server/McpMetaAttribute.cs

@@ -0,0 +1,57 @@
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Used to specify metadata for an MCP server primitive (tool, prompt, or resource).
+/// </summary>
+/// <remarks>
+/// <para>
+/// This attribute can be applied multiple times to a method to specify multiple key/value pairs
+/// of metadata. The metadata is used to populate the <see cref="Tool.Meta"/>, <see cref="Prompt.Meta"/>,
+/// or <see cref="Resource.Meta"/> property of the corresponding primitive.
+/// </para>
+/// <para>
+/// Metadata can be used to attach additional information to primitives, such as model preferences,
+/// version information, or other custom data that should be communicated to MCP clients.
+/// </para>
+/// <example>
+/// <code>
+/// [McpServerTool]
+/// [McpMeta(Name = "model", Value = "gpt-4o")]
+/// [McpMeta(Name = "version", Value = "1.0")]
+/// public string MyTool(string input)
+/// {
+///     return $"Processed: {input}";
+/// }
+/// </code>
+/// </example>
+/// </remarks>
+[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
+public sealed class McpMetaAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpMetaAttribute"/> class.
+    /// </summary>
+    public McpMetaAttribute()
+    {
+    }
+
+    /// <summary>
+    /// Gets or sets the name (key) of the metadata entry.
+    /// </summary>
+    /// <remarks>
+    /// This value is used as the key in the metadata object. It should be a unique identifier
+    /// for this piece of metadata within the context of the primitive.
+    /// </remarks>
+    public required string Name { get; set; }
+
+    /// <summary>
+    /// Gets or sets the value of the metadata entry.
+    /// </summary>
+    /// <remarks>
+    /// This value is stored as a string in the metadata object. Complex values should be
+    /// serialized to JSON strings if needed.
+    /// </remarks>
+    public required string Value { get; set; }
+}