MCP Apps is the first official MCP extension (io.modelcontextprotocol/ui), enabling servers to deliver interactive UIs inside AI clients. The C# SDK had the foundational primitives (Extensions, _meta) but no typed convenience layer, requiring manual JSON construction that is error-prone and non-discoverable.

New Package: ModelContextProtocol.Extensions.Apps

A new package (ModelContextProtocol.Extensions.Apps) houses all MCP Apps types and helpers. It references ModelContextProtocol.Core for access to server primitives.

New APIs

Constants (McpApps)

• McpApps.ResourceMimeType → "text/html;profile=mcp-app"
• McpApps.ExtensionId → "io.modelcontextprotocol/ui"
• McpApps.SerializerOptions → pre-configured JsonSerializerOptions for MCP Apps types

Typed metadata models

• McpUiToolMeta – ResourceUri, Visibility
• McpUiToolVisibility – Model/App string constants
• McpUiResourceMeta – Csp, Permissions, Domain, PrefersBorder
• McpUiResourceCsp – ConnectDomains, ResourceDomains, FrameDomains, BaseUris
• McpUiResourcePermissions – Allow
• McpUiClientCapabilities – MimeTypes

Client capability helper

McpUiClientCapabilities? caps = McpApps.GetUiCapability(clientCapabilities);
if (caps?.MimeTypes?.Contains(McpApps.ResourceMimeType) is true) { ... }

[McpAppUi] attribute (declarative path)

[McpServerTool]
[McpAppUi(ResourceUri = "ui://weather/view.html")]
[Description("Get current weather for a location")]
public string GetWeather(string location) => ...;

When processed by McpApps.ApplyAppUiAttributes() (or the WithMcpApps() builder extension), this populates the structured _meta.ui object in the tool's metadata.

McpApps.SetAppUi (programmatic path)

McpApps.SetAppUi(tool, new McpUiToolMeta
{
    ResourceUri = "ui://weather/view.html",
    Visibility = [McpUiToolVisibility.Model, McpUiToolVisibility.App]
});

Explicit Meta["ui"] entries (set via McpServerToolCreateOptions.Meta) take precedence over SetAppUi; SetAppUi takes precedence over [McpAppUi] attribute.

WithMcpApps() builder extension

builder.Services
    .AddMcpServer()
    .WithTools<MyToolType>()
    .WithMcpApps();

Automatically processes [McpAppUi] attributes on all registered tools via IPostConfigureOptions<McpServerOptions>.

Notes

• All new APIs are [Experimental(MCPEXP003)] — a dedicated diagnostic ID for MCP Apps.
• Source-generated JSON serialization for Native AOT compatibility via McpAppsJsonContext.
• Documentation added in docs/concepts/apps/apps.md.