MCP Apps: Pyodide/WASM apps take >2 minutes to load in Claude.ai webview

Summary

MCP Apps that use Pyodide (Python in WebAssembly) take over 2 minutes to load in Claude.ai's MCP App webview. The same app loads in ~5 seconds in VS Code Copilot Chat.

Claude believes the root cause is that Claude.ai's webview does not set Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy headers, so window.crossOriginIsolated is false and Pyodide cannot use SharedArrayBuffer.

Reproduction

1. Start an MCP server that returns an MCP App using Pyodide:

   pip install "panel-live[mcp] @ git+https://github.com/panel-extensions/panel-live.git"
   panel-live mcp --transport streamable-http --port 5002

2. Expose via ngrok: ngrok http 5002
3. In Claude.ai: Settings > Connectors > Add custom connector > paste ngrok URL + /mcp
4. Ask: "Create an interactive sine wave with a frequency slider"
5. Observe: the MCP App webview shows "Loading Python runtime..." for >2 minutes

The same flow in VS Code Copilot Chat (which Claude claims provides COOP/COEP headers) loads in ~5 seconds.

Expected behavior

MCP App webviews should set these HTTP headers on the iframe/webview:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

This enables window.crossOriginIsolated = true, which allows SharedArrayBuffer, which Pyodide uses for efficient memory sharing between the main thread and workers.

Why this matters

Pyodide (Python in WebAssembly) is the runtime behind panel-live MCP tool that render interactive data apps in chat.

Without cross-origin isolation, Pyodide falls back to a slow single-threaded path. The 2+ minute load time makes the experience impractical for users.

Environment

• Claude.ai web (Pro plan)
• MCP server: panel-live with FastMCP 3.0.2, streamable-http transport
• Tunnel: ngrok
• Browser: Chrome (latest)

Gif

This gif is speeded up 10 times. You can see that panel-live can load. But it takes +2 mins.

Workaround

According to Claude VS Code Copilot Chat provides COOP/COEP headers

Panel Live loads in ~5 seconds.

Claude.ai

Reported in 215473300183121.

Latest Updates

Please refer to https://panel-extensions.github.io/panel-live/tutorials/getting-started-mcp/ for the latest updates for installing and using the panel-live MCP server.

Please report issues at https://github.com/panel-extensions/panel-live.