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

[linxule]

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.