Add icon support to McpServerTool infrastructure and attributes

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

Author: Copilot

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -121,6 +121,7 @@ private static AIFunctionFactoryOptions CreateAIFunctionFactoryOptions(
             Description = options?.Description ?? function.Description,
             InputSchema = function.JsonSchema,
             OutputSchema = CreateOutputSchema(function, options, out bool structuredOutputRequiresWrapping),
+            Icons = options?.Icons,
         };
 
         if (options is not null)
@@ -177,6 +178,15 @@ private static McpServerToolCreateOptions DeriveOptions(MethodInfo method, McpSe
             }
 
             newOptions.UseStructuredContent = toolAttr.UseStructuredContent;
+
+            // Handle icon from attribute if not already specified in options
+            if (newOptions.Icons is null && !string.IsNullOrEmpty(toolAttr.IconSource))
+            {
+                newOptions.Icons = new List<Icon>
+                {
+                    new() { Source = toolAttr.IconSource }
+                };
+            }
         }
 
         if (method.GetCustomAttribute<DescriptionAttribute>() is { } descAttr)

src/ModelContextProtocol.Core/Server/McpServerToolAttribute.cs

@@ -254,4 +254,19 @@ public bool ReadOnly
     /// </para>
     /// </remarks>
     public bool UseStructuredContent { get; set; }
+
+    /// <summary>
+    /// Gets or sets the source URI for the tool's icon.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This can be an HTTP/HTTPS URL pointing to an image file or a data URI with base64-encoded image data.
+    /// When specified, a single icon will be added to the tool.
+    /// </para>
+    /// <para>
+    /// For more advanced icon configuration (multiple icons, MIME type specification, size characteristics),
+    /// use <see cref="McpServerToolCreateOptions.Icons"/> when creating the tool programmatically.
+    /// </para>
+    /// </remarks>
+    public string? IconSource { get; set; }
 }

src/ModelContextProtocol.Core/Server/McpServerToolCreateOptions.cs

@@ -164,6 +164,14 @@ public sealed class McpServerToolCreateOptions
     /// </remarks>
     public IReadOnlyList<object>? Metadata { get; set; }
 
+    /// <summary>
+    /// Gets or sets the icons for this tool.
+    /// </summary>
+    /// <remarks>
+    /// This can be used by clients to display the tool's icon in a user interface.
+    /// </remarks>
+    public IList<Icon>? Icons { get; set; }
+
     /// <summary>
     /// Creates a shallow clone of the current <see cref="McpServerToolCreateOptions"/> instance.
     /// </summary>
@@ -182,5 +190,6 @@ internal McpServerToolCreateOptions Clone() =>
             SerializerOptions = SerializerOptions,
             SchemaCreateOptions = SchemaCreateOptions,
             Metadata = Metadata,
+            Icons = Icons,
         };
 }

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -678,6 +678,87 @@ Instance JSON document does not match the specified schema.
 
     record Person(string Name, int Age);
 
+    [Fact]
+    public void SupportsIconsInCreateOptions()
+    {
+        var icons = new List<Icon>
+        {
+            new() { Source = "https://example.com/icon.png", MimeType = "image/png", Sizes = "48x48" },
+            new() { Source = "https://example.com/icon.svg", MimeType = "image/svg+xml", Sizes = "any" }
+        };
+
+        McpServerTool tool = McpServerTool.Create(() => "test", new McpServerToolCreateOptions
+        {
+            Icons = icons
+        });
+
+        Assert.NotNull(tool.ProtocolTool.Icons);
+        Assert.Equal(2, tool.ProtocolTool.Icons.Count);
+        Assert.Equal("https://example.com/icon.png", tool.ProtocolTool.Icons[0].Source);
+        Assert.Equal("image/png", tool.ProtocolTool.Icons[0].MimeType);
+        Assert.Equal("48x48", tool.ProtocolTool.Icons[0].Sizes);
+        Assert.Equal("https://example.com/icon.svg", tool.ProtocolTool.Icons[1].Source);
+        Assert.Equal("image/svg+xml", tool.ProtocolTool.Icons[1].MimeType);
+        Assert.Equal("any", tool.ProtocolTool.Icons[1].Sizes);
+    }
+
+    [Fact]
+    public void SupportsIconSourceInAttribute()
+    {
+        MethodInfo? method = typeof(IconTestClass).GetMethod(nameof(IconTestClass.ToolWithIconSource));
+        Assert.NotNull(method);
+
+        McpServerTool tool = McpServerTool.Create(method, new IconTestClass());
+
+        Assert.NotNull(tool.ProtocolTool.Icons);
+        Assert.Single(tool.ProtocolTool.Icons);
+        Assert.Equal("https://example.com/tool-icon.png", tool.ProtocolTool.Icons[0].Source);
+        Assert.Null(tool.ProtocolTool.Icons[0].MimeType);
+        Assert.Null(tool.ProtocolTool.Icons[0].Sizes);
+    }
+
+    [Fact]
+    public void CreateOptionsIconsOverrideAttributeIconSource()
+    {
+        MethodInfo? method = typeof(IconTestClass).GetMethod(nameof(IconTestClass.ToolWithIconSource));
+        Assert.NotNull(method);
+
+        var optionsIcons = new List<Icon>
+        {
+            new() { Source = "https://example.com/override-icon.svg", MimeType = "image/svg+xml" }
+        };
+
+        McpServerTool tool = McpServerTool.Create(method, new IconTestClass(), new McpServerToolCreateOptions
+        {
+            Icons = optionsIcons
+        });
+
+        Assert.NotNull(tool.ProtocolTool.Icons);
+        Assert.Single(tool.ProtocolTool.Icons);
+        Assert.Equal("https://example.com/override-icon.svg", tool.ProtocolTool.Icons[0].Source);
+        Assert.Equal("image/svg+xml", tool.ProtocolTool.Icons[0].MimeType);
+    }
+
+    [Fact]
+    public void SupportsToolWithoutIcons()
+    {
+        MethodInfo? method = typeof(IconTestClass).GetMethod(nameof(IconTestClass.ToolWithoutIcon));
+        Assert.NotNull(method);
+
+        McpServerTool tool = McpServerTool.Create(method, new IconTestClass());
+
+        Assert.Null(tool.ProtocolTool.Icons);
+    }
+
+    private class IconTestClass
+    {
+        [McpServerTool(IconSource = "https://example.com/tool-icon.png")]
+        public string ToolWithIconSource() => "result";
+
+        [McpServerTool]
+        public string ToolWithoutIcon() => "result";
+    }
+
     [JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
     [JsonSerializable(typeof(DisposableToolType))]
     [JsonSerializable(typeof(AsyncDisposableToolType))]