`downloadFile` not implemented on Claude iOS (error -32601)

app.downloadFile() returns JSON-RPC error -32601 (method not found) on Claude iOS. The capability is absent from the host's advertised capabilities.

Environment

• Host: claude-ios/1.260323.4
• SDK: @modelcontextprotocol/ext-apps 1.2.2
• Desktop/web: works as expected

Host capabilities reported on iOS

{
  "openLinks": {},
  "serverTools": { "listChanged": true },
  "serverResources": { "listChanged": true },
  "logging": {},
  "updateModelContext": { "text": {}, "image": {} },
  "message": { "text": {}, "image": {} }
}

No downloadFile entry.

Reproduction

const app = new App({ name: "test", version: "1.0.0" });
await app.connect();

await app.downloadFile({
  contents: [{
    type: "resource",
    resource: {
      uri: "file:///test.txt",
      mimeType: "text/plain",
      text: "hello",
    },
  }],
});
// → MCP error -32601: ui/download-file

This is a minimal text payload — the issue is the missing host handler, not payload size or format.

Context

Discovered while building mcp-music-studio, which exports audio files via downloadFile. Both the EmbeddedResource (blob) and plain text variants fail identically. Android is untested but likely the same.

The runtime capability check (app.getHostCapabilities()?.downloadFile) is a clean detection path — apps can gate on this today and automatically pick up the feature when it ships.

Start with App.downloadFile and app.getHostCapabilities(), then reproduce the minimal text payload on Claude iOS 1.260323.4 and compare its advertised capabilities with desktop/web behavior. Done means the iOS host advertises downloadFile and the provided reproduction succeeds without error -32601.