Add McpMetaAttribute for attaching metadata to tools, prompts, and resources (#857)

* Initial plan

* Add McpMetaAttribute and integrate with tools, prompts, and resources

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

* Add tests and sample usage for McpMetaAttribute

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

* Add Meta property to options classes and update logic to merge with attributes

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

* Fix missing using directives and clarify McpMetaAttribute documentation

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

* Make Name and Value constructor parameters in McpMetaAttribute and check AIFunction.Metadata.UnderlyingMethod

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

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

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

* Fix UnderlyingMethod property path, use SerializerOptions, and fix JsonObject cref

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

* Simplify MethodInfo resolution to use function.UnderlyingMethod directly

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

* Fix IL2026 trimming warning by providing type info to JsonSerializer.SerializeToNode

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

* Fix remaining issues

* Revert to string-based JSON values for McpMetaAttribute with StringSyntax annotation

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

* Add StringSyntax attribute to Value property and fix sample code to use escaped quotes

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

* Fix tests and a bit of cleanup

* Change approach and add lots of tests

* Use JsonContext for serialization and fix floating point test comparison

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

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: stephentoub <[email protected]>
Co-authored-by: Stephen Toub <[email protected]>

Author: Copilot

samples/AspNetCoreMcpServer/Tools/WeatherTools.cs

@@ -17,6 +17,8 @@ public WeatherTools(IHttpClientFactory httpClientFactory)
     }
 
     [McpServerTool, Description("Get weather alerts for a US state.")]
+    [McpMeta("category", "weather")]
+    [McpMeta("dataSource", "weather.gov")]
     public async Task<string> GetAlerts(
         [Description("The US state to get alerts for. Use the 2 letter abbreviation for the state (e.g. NY).")] string state)
     {
@@ -46,6 +48,8 @@ public async Task<string> GetAlerts(
     }
 
     [McpServerTool, Description("Get weather forecast for a location.")]
+    [McpMeta("category", "weather")]
+    [McpMeta("recommendedModel", "gpt-4")]
     public async Task<string> GetForecast(
         [Description("Latitude of the location.")] double latitude,
         [Description("Longitude of the location.")] double longitude)

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -47,7 +47,7 @@ public sealed class ClientCapabilities
     /// </para>
     /// <para>
     /// The server can use <see cref="McpServer.RequestRootsAsync"/> to request the list of
-    /// available roots from the client, which will trigger the client's <see cref="ModelContextProtocol.Client.McpClientHandlers.RootsHandler"/>.
+    /// available roots from the client, which will trigger the client's <see cref="McpClientHandlers.RootsHandler"/>.
     /// </para>
     /// </remarks>
     [JsonPropertyName("roots")]

src/ModelContextProtocol.Core/Protocol/ElicitationCapability.cs

@@ -13,7 +13,7 @@ namespace ModelContextProtocol.Protocol;
 /// </para>
 /// <para>
 /// When this capability is enabled, an MCP server can request the client to provide additional information
-/// during interactions. The client must set a <see cref="ModelContextProtocol.Client.McpClientHandlers.ElicitationHandler"/> to process these requests.
+/// during interactions. The client must set a <see cref="McpClientHandlers.ElicitationHandler"/> to process these requests.
 /// </para>
 /// <para>
 /// This class is intentionally empty as the Model Context Protocol specification does not

src/ModelContextProtocol.Core/Protocol/SamplingCapability.cs

@@ -14,7 +14,7 @@ namespace ModelContextProtocol.Protocol;
 /// </para>
 /// <para>
 /// When this capability is enabled, an MCP server can request the client to generate content
-/// using an AI model. The client must set a <see cref="ModelContextProtocol.Client.McpClientHandlers.SamplingHandler"/> to process these requests.
+/// using an AI model. The client must set a <see cref="McpClientHandlers.SamplingHandler"/> to process these requests.
 /// </para>
 /// <para>
 /// This class is intentionally empty as the Model Context Protocol specification does not

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerPrompt.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Reflection;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 
 namespace ModelContextProtocol.Server;
 
@@ -138,6 +139,11 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Icons = options?.Icons,
         };
 
+        // Populate Meta from options and/or McpMetaAttribute instances if a MethodInfo is available
+        prompt.Meta = function.UnderlyingMethod is not null ?
+            AIFunctionMcpServerTool.CreateMetaFromAttributes(function.UnderlyingMethod, options?.Meta, options?.SerializerOptions) :
+            options?.Meta;
+
         return new AIFunctionMcpServerPrompt(function, prompt, options?.Metadata ?? []);
     }
 

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerResource.cs

@@ -9,6 +9,7 @@
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.RegularExpressions;
 
 namespace ModelContextProtocol.Server;
@@ -219,6 +220,9 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Description = options?.Description,
             MimeType = options?.MimeType ?? "application/octet-stream",
             Icons = options?.Icons,
+            Meta = function.UnderlyingMethod is not null ?
+                AIFunctionMcpServerTool.CreateMetaFromAttributes(function.UnderlyingMethod, options?.Meta, options?.SerializerOptions) :
+                options?.Meta,
         };
 
         return new AIFunctionMcpServerResource(function, resource, options?.Metadata ?? []);

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -6,6 +6,7 @@
 using System.Reflection;
 using System.Text.Json;
 using System.Text.Json.Nodes;
+using System.Text.Json.Serialization.Metadata;
 using System.Text.RegularExpressions;
 
 namespace ModelContextProtocol.Server;
@@ -143,6 +144,11 @@ options.OpenWorld is not null ||
                     ReadOnlyHint = options.ReadOnly,
                 };
             }
+
+            // Populate Meta from options and/or McpMetaAttribute instances if a MethodInfo is available
+            tool.Meta = function.UnderlyingMethod is not null ?
+                CreateMetaFromAttributes(function.UnderlyingMethod, options.Meta, options.SerializerOptions) :
+                options.Meta;
         }
 
         return new AIFunctionMcpServerTool(function, tool, options?.Services, structuredOutputRequiresWrapping, options?.Metadata ?? []);
@@ -351,6 +357,25 @@ internal static IReadOnlyList<object> CreateMetadata(MethodInfo method)
         return metadata.AsReadOnly();
     }
 
+    /// <summary>Creates a Meta <see cref="JsonObject"/> from <see cref="McpMetaAttribute"/> instances on the specified method.</summary>
+    /// <param name="method">The method to extract <see cref="McpMetaAttribute"/> instances from.</param>
+    /// <param name="meta">Optional <see cref="JsonObject"/> to seed the Meta with. Properties from this object take precedence over attributes.</param>
+    /// <param name="serializerOptions">Optional <see cref="JsonSerializerOptions"/> to use for serialization. This parameter is ignored when parsing JSON strings from attributes.</param>
+    /// <returns>A <see cref="JsonObject"/> with metadata, or null if no metadata is present.</returns>
+    internal static JsonObject? CreateMetaFromAttributes(MethodInfo method, JsonObject? meta = null, JsonSerializerOptions? serializerOptions = null)
+    {
+        // Transfer all McpMetaAttribute instances to the Meta JsonObject, ignoring any that would overwrite existing properties.
+        foreach (var attr in method.GetCustomAttributes<McpMetaAttribute>())
+        {
+            if (meta?.ContainsKey(attr.Name) is not true)
+            {
+                (meta ??= [])[attr.Name] = JsonNode.Parse(attr.JsonValue);
+            }
+        }
+
+        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 +1,103 @@
+using ModelContextProtocol.Protocol;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Used to specify metadata for an MCP server primitive (tool, prompt, or resource).
+/// </summary>
+/// <remarks>
+/// <para>
+/// 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>
+/// This attribute can be applied multiple times to a method to specify multiple key/value pairs
+/// of metadata. However, the same key should not be used more than once; doing so will result
+/// in undefined behavior.
+/// </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>
+/// </remarks>
+/// <example>
+/// <code>
+/// [McpServerTool]
+/// [McpMeta("model", "gpt-4o")]
+/// [McpMeta("version", "1.0")]
+/// [McpMeta("priority", 5.0)]
+/// [McpMeta("isBeta", true)]
+/// [McpMeta("tags", JsonValue = """["a","b"]""")]
+/// public string MyTool(string input) => $"Processed: {input}";
+/// </code>
+/// </example>
+[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
+public sealed class McpMetaAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpMetaAttribute"/> class with a string value.
+    /// </summary>
+    /// <param name="name">The name (key) of the metadata entry.</param>
+    /// <param name="value">The string value of the metadata entry. If null, the value will be serialized as JSON null.</param>
+    public McpMetaAttribute(string name, string? value = null)
+    {
+        Name = name;
+        JsonValue = value is null ? "null" : JsonSerializer.Serialize(value, McpJsonUtilities.JsonContext.Default.String);
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpMetaAttribute"/> class with a double value.
+    /// </summary>
+    /// <param name="name">The name (key) of the metadata entry.</param>
+    /// <param name="value">The double value of the metadata entry.</param>
+    public McpMetaAttribute(string name, double value)
+    {
+        Name = name;
+        JsonValue = JsonSerializer.Serialize(value, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(double)));
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpMetaAttribute"/> class with a boolean value.
+    /// </summary>
+    /// <param name="name">The name (key) of the metadata entry.</param>
+    /// <param name="value">The boolean value of the metadata entry.</param>
+    public McpMetaAttribute(string name, bool value)
+    {
+        Name = name;
+        JsonValue = JsonSerializer.Serialize(value, McpJsonUtilities.JsonContext.Default.Boolean);
+    }
+
+    /// <summary>
+    /// Gets 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 string Name { get; }
+
+    /// <summary>
+    /// Gets or sets the value of the metadata entry as a JSON string.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This value must be well-formed JSON. It will be parsed and added to the metadata <see cref="JsonObject"/>.
+    /// Simple values can be represented as JSON literals like <c>"\"my-string\""</c>, <c>"123"</c>, 
+    /// <c>"true"</c>, etc. Complex structures can be represented as JSON objects or arrays.
+    /// </para>
+    /// <para>
+    /// Setting this property will override any value provided via the constructor.
+    /// </para>
+    /// <para>
+    /// For programmatic scenarios where you want to construct complex metadata without dealing with
+    /// JSON strings, use the <see cref="McpServerToolCreateOptions.Meta"/>, 
+    /// <see cref="McpServerPromptCreateOptions.Meta"/>, or <see cref="McpServerResourceCreateOptions.Meta"/> 
+    /// property to provide a JsonObject directly.
+    /// </para>
+    /// </remarks>
+    [StringSyntax(StringSyntaxAttribute.Json)]
+    public string JsonValue { get; set; }
+}

src/ModelContextProtocol.Core/Server/McpServerPromptCreateOptions.cs

@@ -2,6 +2,7 @@
 using ModelContextProtocol.Protocol;
 using System.ComponentModel;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 
 namespace ModelContextProtocol.Server;
 
@@ -86,6 +87,21 @@ public sealed class McpServerPromptCreateOptions
     /// </remarks>
     public IList<Icon>? Icons { get; set; }
 
+    /// <summary>
+    /// Gets or sets metadata reserved by MCP for protocol-level metadata.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This <see cref="JsonObject"/> is used to seed the <see cref="Prompt.Meta"/> property. Any metadata from
+    /// <see cref="McpMetaAttribute"/> instances on the method will be added to this object, but
+    /// properties already present in this <see cref="JsonObject"/> will not be overwritten.
+    /// </para>
+    /// <para>
+    /// Implementations must not make assumptions about its contents.
+    /// </para>
+    /// </remarks>
+    public JsonObject? Meta { get; set; }
+
     /// <summary>
     /// Creates a shallow clone of the current <see cref="McpServerPromptCreateOptions"/> instance.
     /// </summary>
@@ -100,5 +116,6 @@ internal McpServerPromptCreateOptions Clone() =>
             SchemaCreateOptions = SchemaCreateOptions,
             Metadata = Metadata,
             Icons = Icons,
+            Meta = Meta,
         };
 }

src/ModelContextProtocol.Core/Server/McpServerResourceCreateOptions.cs

@@ -2,6 +2,7 @@
 using ModelContextProtocol.Protocol;
 using System.ComponentModel;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 
 namespace ModelContextProtocol.Server;
 
@@ -101,6 +102,21 @@ public sealed class McpServerResourceCreateOptions
     /// </remarks>
     public IList<Icon>? Icons { get; set; }
 
+    /// <summary>
+    /// Gets or sets metadata reserved by MCP for protocol-level metadata.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This <see cref="JsonObject"/> is used to seed the <see cref="Resource.Meta"/> property. Any metadata from
+    /// <see cref="McpMetaAttribute"/> instances on the method will be added to this object, but
+    /// properties already present in this <see cref="JsonObject"/> will not be overwritten.
+    /// </para>
+    /// <para>
+    /// Implementations must not make assumptions about its contents.
+    /// </para>
+    /// </remarks>
+    public JsonObject? Meta { get; set; }
+
     /// <summary>
     /// Creates a shallow clone of the current <see cref="McpServerResourceCreateOptions"/> instance.
     /// </summary>
@@ -117,5 +133,6 @@ internal McpServerResourceCreateOptions Clone() =>
             SchemaCreateOptions = SchemaCreateOptions,
             Metadata = Metadata,
             Icons = Icons,
+            Meta = Meta,
         };
 }