CSP & CORS

Unlike regular web apps, MCP Apps HTML is served as an MCP resource and runs in a sandboxed iframe with no same-origin server. Any app that makes network requests must configure Content Security Policy (CSP) and possibly CORS.

CSP controls what the browser allows. You must declare all origins in _meta.ui.csp (McpUiResourceCsp) — including localhost during development. Declare connectDomains for fetch/XHR/WebSocket requests and resourceDomains for scripts, stylesheets, images, and fonts.

CORS controls what the API server allows. Public APIs that respond with Access-Control-Allow-Origin: * or use API key authentication work without CORS configuration. For APIs that allowlist specific origins, use _meta.ui.domain to give the app a stable origin that the API server can allowlist. The format is host-specific, so check each host's documentation for its supported format.

// Computes a stable origin from an MCP server URL for hosting in Claude.
function computeAppDomainForClaude(mcpServerUrl: string): string {
  const hash = crypto
    .createHash("sha256")
    .update(mcpServerUrl)
    .digest("hex")
    .slice(0, 32);
  return `${hash}.claudemcpcontent.com`;
}

const APP_DOMAIN = computeAppDomainForClaude("https://example.com/mcp");

registerAppResource(
  server,
  "Company Dashboard",
  "ui://dashboard/view.html",
  {
    description: "Internal dashboard with company data",
  },
  async () => ({
    contents: [
      {
        uri: "ui://dashboard/view.html",
        mimeType: RESOURCE_MIME_TYPE,
        text: dashboardHtml,
        _meta: {
          ui: {
            // CSP: tell browser the app is allowed to make requests
            csp: {
              connectDomains: ["https://api.example.com"],
            },
            // CORS: give app a stable origin for the API server to allowlist
            //
            // (Public APIs that use `Access-Control-Allow-Origin: *` or API
            // key auth don't need this.)
            domain: APP_DOMAIN,
          },
        },
      },
    ],
  }),
);
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Note that _meta.ui.csp and _meta.ui.domain are set in the contents[] objects returned by the resource read callback, not in registerAppResource()'s config object.