Add MCP Apps extension support (typed metadata, attribute, and helpers)

[Copilot]

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.

[gabrielwhitehair]

Any idea when this will be merged? Would love to see this feature in the SDK.

[VinKamat]

This is a key capability for "enterprises", emerging and especially those who are already on the MCP train wanting to deliver UI Apps within convo experiences.
Can't wait to see this merged, let's go please.

[FelixOhlhof]

Can you please merge this 🙏

[bharathkreddy]

👀 Tracking this closely - and will adopt this surface as soon as it ships. The tool-side [McpAppUi] and McpServerToolCreateOptions.AppUi look great.

One observation that may be worth addressing in this PR or a fast follow-up: the typed McpUiResourceMeta / McpUiResourceCsp models added to Core aren't reachable via resource registration — only via raw JsonObject on TextResourceContents.Meta. Since per-resource CSP is mandatory for any production MCP App, this leaves the most failure-prone part of the API in raw-JSON territory. Would be great to get a typed path for resources too, mirroring the tool-side ergonomics. Filed as a comment on #1431 with more detail.

[mikekistler]

I think this name is too generic. The MCP Apps spec says "The initial specification focuses on HTML resources (text/html;profile=mcp-app) with a clear path for future extensions." To avoid confusion with potential future resource types, I think we should call this "HtmlMimeType".