Add Description property to Implementation type per MCP spec (#966)

* Initial plan

* Add Description property to Implementation type

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

* Add test to validate client Description from server perspective

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/Implementation.cs

@@ -37,6 +37,22 @@ public sealed class Implementation : IBaseMetadata
     [JsonPropertyName("version")]
     public required string Version { get; set; }
 
+    /// <summary>
+    /// Gets or sets an optional description of the implementation.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This description helps users and developers understand what the implementation provides
+    /// and its purpose. It should clearly explain the functionality and capabilities offered.
+    /// </para>
+    /// <para>
+    /// The description is typically used in documentation, UI displays, and for providing context
+    /// to users about the server or client they are interacting with.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("description")]
+    public string? Description { get; set; }
+
     /// <summary>
     /// Gets or sets an optional list of icons for this implementation.
     /// </summary>

tests/ModelContextProtocol.Tests/Client/McpClientTests.cs

@@ -34,6 +34,7 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
             {
                 Name = "test-server",
                 Version = "1.0.0",
+                Description = "A test server for unit testing",
                 WebsiteUrl = "https://example.com",
                 Icons =
                 [
@@ -52,6 +53,7 @@ public async Task CanReadServerInfo()
         var serverInfo = client.ServerInfo;
         Assert.Equal("test-server", serverInfo.Name);
         Assert.Equal("1.0.0", serverInfo.Version);
+        Assert.Equal("A test server for unit testing", serverInfo.Description);
         Assert.Equal("https://example.com", serverInfo.WebsiteUrl);
         Assert.NotNull(serverInfo.Icons);
         Assert.Equal(2, serverInfo.Icons.Count);
@@ -69,6 +71,29 @@ public async Task CanReadServerInfo()
         Assert.Equal("dark", icon1.Theme);
     }
 
+    [Fact]
+    public async Task ServerCanReadClientInfo()
+    {
+        var clientOptions = new McpClientOptions
+        {
+            ClientInfo = new Implementation
+            {
+                Name = "test-client",
+                Version = "2.0.0",
+                Description = "A test client for validating client-server communication"
+            }
+        };
+
+        await using McpClient client = await CreateMcpClientForServer(clientOptions);
+
+        // Verify the server received the client info with description
+        var clientInfo = Server.ClientInfo;
+        Assert.NotNull(clientInfo);
+        Assert.Equal("test-client", clientInfo.Name);
+        Assert.Equal("2.0.0", clientInfo.Version);
+        Assert.Equal("A test client for validating client-server communication", clientInfo.Description);
+    }
+
     [Theory]
     [InlineData(null, 10)]
     [InlineData(0.7f, 50)]

tests/ModelContextProtocol.Tests/Protocol/ImplementationTests.cs

@@ -14,6 +14,7 @@ public static void Implementation_SerializationRoundTrip_PreservesAllProperties(
             Name = "test-server",
             Title = "Test MCP Server",
             Version = "1.0.0",
+            Description = "A test MCP server implementation for demonstration purposes",
             Icons =
             [
                 new() { Source = "https://example.com/icon.png", MimeType = "image/png", Sizes = ["48x48"] },
@@ -33,6 +34,7 @@ public static void Implementation_SerializationRoundTrip_PreservesAllProperties(
         Assert.Equal(original.Name, deserialized.Name);
         Assert.Equal(original.Title, deserialized.Title);
         Assert.Equal(original.Version, deserialized.Version);
+        Assert.Equal(original.Description, deserialized.Description);
         Assert.Equal(original.WebsiteUrl, deserialized.WebsiteUrl);
         Assert.NotNull(deserialized.Icons);
         Assert.Equal(original.Icons.Count, deserialized.Icons.Count);
@@ -66,6 +68,7 @@ public static void Implementation_SerializationRoundTrip_WithoutOptionalProperti
         Assert.Equal(original.Name, deserialized.Name);
         Assert.Equal(original.Title, deserialized.Title);
         Assert.Equal(original.Version, deserialized.Version);
+        Assert.Equal(original.Description, deserialized.Description);
         Assert.Equal(original.Icons, deserialized.Icons);
         Assert.Equal(original.WebsiteUrl, deserialized.WebsiteUrl);
     }
@@ -78,6 +81,7 @@ public static void Implementation_HasCorrectJsonPropertyNames()
             Name = "test-server",
             Title = "Test Server",
             Version = "1.0.0",
+            Description = "Test description",
             Icons = [new() { Source = "https://example.com/icon.png" }],
             WebsiteUrl = "https://example.com"
         };
@@ -87,6 +91,7 @@ public static void Implementation_HasCorrectJsonPropertyNames()
         Assert.Contains("\"name\":", json);
         Assert.Contains("\"title\":", json);
         Assert.Contains("\"version\":", json);
+        Assert.Contains("\"description\":", json);
         Assert.Contains("\"icons\":", json);
         Assert.Contains("\"websiteUrl\":", json);
     }