Claude MCP App renders blank (works in Inspector) #615

Summary

A remote MCP App renders correctly in MCP Inspector but appears blank in claude.ai, despite successful tool execution and resource retrieval.

Expected Behavior

When invoking an MCP tool with a ui:// resource, Claude should render the returned HTML app inside the inline app container.

Actual Behavior

In claude.ai:

• Tool executes successfully
• resources/read is called successfully
• Claude shows UI indicating the app is opened
• App container remains blank (no HTML rendered)

In MCP Inspector:

• Same MCP server
• Same ui:// resource
• App renders correctly

Reproduction Steps

1. Start MCP server exposed via HTTPS (ngrok)

2. Connect to claude.ai

3. Invoke tool:

   open_visual_search_app
   

4. Observe:

   • Claude shows tool response
   • “Open Visual Search App” appears
   • App container opens
   • Container is blank

5. Open MCP Inspector:

   npx @modelcontextprotocol/inspector

6. Connect to same MCP endpoint

7. Navigate to Apps tab

8. Open open_visual_search_app

9. Observe:

   • App renders correctly

MCP Configuration

• Transport: Streamable HTTP

• Server: Remote (ngrok HTTPS)

• Tool name: open_visual_search_app

• Resource URI:

  ui://visual-search/view.html
  

resources/read Response

• MIME type:

  text/html;profile=mcp-app
  

• Includes _meta.ui:

  • CSP (connectDomains, resourceDomains)
  • Claude sandbox domain (*.claudemcpcontent.com)

• HTML:

  • Fully inline
  • No external assets required
  • Valid static content renders in Inspector

Minimal Repro

Even with minimal HTML:

<!DOCTYPE html>
<html>
  <body>
    <h1>HELLO MCP</h1>
  </body>
</html>

• Inspector: renders
• Claude: blank

Key Observation

The exact same ui:// resource renders correctly in MCP Inspector but not in claude.ai.

This suggests:

• MCP server implementation is correct
• Resource format is valid
• Issue is specific to Claude’s MCP app rendering/runtime

Debugging Performed

• Verified /mcp endpoint connectivity

• Verified tools/call works

• Verified resources/read returns correct content

• Verified MIME type is text/html;profile=mcp-app

• Tested with:

  • full app
  • minimal static HTML

• Confirmed rendering works in MCP Inspector

Screenshots

Claude (blank app container):

MCP Inspector (correct rendering):

Impact

Blocks use of MCP Apps UI in claude.ai despite valid implementation.
Forces fallback to non-UI tool flows.

Question

• Are remote MCP Apps fully supported in claude.ai?
• Are there additional constraints required for rendering ui:// resources?
• Is this a known issue with the Claude MCP app host?

Additional Context

This appears similar to cases where:

resources/read succeeds but the app iframe/container fails to mount or render the returned HTML.