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import { getUiCapability, EXTENSION_ID, RESOURCE_MIME_TYPE } from "sunpeak/mcp";

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Overview

Not all MCP hosts support Apps yet. Servers should check whether the connecting host supports MCP Apps before registering UI-enabled tools — if it doesn’t, register text-only fallback tools instead. This keeps your server compatible with every host while delivering rich UIs where available.

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getUiCapability

Extracts the MCP Apps capability from the client’s extensions field.

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Signature

function getUiCapability(
  clientCapabilities: ClientCapabilities | null | undefined,
): McpUiClientCapabilities | undefined

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Returns

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return
McpUiClientCapabilities | undefined
The capability object if the host supports MCP Apps, or undefined if not.
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mimeTypes
string[]
Supported MIME types. Must include "text/html;profile=mcp-app" for MCP Apps support.

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Usage

Use server.server.oninitialized to check capabilities after the MCP handshake: 
import {
  getUiCapability,
  registerAppTool,
  RESOURCE_MIME_TYPE,
} from "sunpeak/mcp";

server.server.oninitialized = () => {
  const clientCapabilities = server.server.getClientCapabilities();
  const uiCap = getUiCapability(clientCapabilities);

  if (uiCap?.mimeTypes?.includes(RESOURCE_MIME_TYPE)) {
    // Host supports MCP Apps — register tool with UI
    registerAppTool(
      server,
      "weather",
      {
        description: "Get weather with interactive dashboard",
        _meta: { ui: { resourceUri: "ui://weather/dashboard" } },
      },
      weatherHandler,
    );
  } else {
    // Host does not support MCP Apps — register text-only fallback
    server.registerTool(
      "weather",
      { description: "Get weather information" },
      textWeatherHandler,
    );
  }
};

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EXTENSION_ID

The extension identifier used in the MCP extensions field for capability negotiation. 
const EXTENSION_ID = "io.modelcontextprotocol/ui";
Hosts advertise MCP Apps support by including this key in their extensions capabilities during the MCP initialization handshake. getUiCapability reads from this key internally.