MCP App UI renders text only in Claude Desktop on Windows — reproduces via local MCPB and remote custom connector

### What happened?

Title: MCP App UI renders text only in Claude Desktop on Windows — reproduces via local MCPB and remote custom connector

I built a custom MCP App that registers:

- UI resource: ui://studymeta/learner-card
- MIME type: text/html;profile=mcp-app
- Tool metadata: _meta.ui.resourceUri = ui://studymeta/learner-card

The tool call succeeds and returns valid learner-card data. However, Claude Desktop shows only the text tool result; no interactive iframe/card is rendered.

This reproduces through both:
1. A local MCPB installation
2. A public remote Streamable HTTP custom connector with OAuth

The remote connector authenticates successfully and the tool is invoked successfully. The issue is limited to MCP Apps UI rendering.

Environment:
- Windows 11
- Claude Desktop
- Custom StudyMeta MCP connector
- MCP Apps resource is self-contained HTML with no external CDN assets

Expected:
The learner card should render inline and display interactive buttons.

Actual:
Only the text fallback is shown. Claude may say that the widget/card was rendered, but no visible card or iframe appears.

Could you confirm whether MCP Apps UI rendering is currently supported for custom connectors in Claude Desktop on Windows, and whether there is any account, feature flag, or connector requirement needed to enable it?
Claude Desktop version: Claude 1.46388.4 (50e62f) 2026-09-05T01:57:47.000Z
Windows version: Windows 11
Remote MCP endpoint: https://metastudy-git-codex-fki-ir-deck-drpaiks-projects.vercel.app/api/mcp

### What did you expect to happen?

After Claude invokes render_my_learner_card, Claude Desktop should render the StudyMeta learner card inline in the conversation.

The card should display the Korean learner-state rows and the interactive buttons:
- 혼자 풀어보기
- 힌트 받기
- 왜 이렇게 판단했나요?

The UI resource is registered as ui://studymeta/learner-card with MIME type text/html;profile=mcp-app, and the tool includes _meta.ui.resourceUri pointing to that resource.

### Steps to reproduce

1. On Windows 11, connect the StudyMeta MCP server to Claude Desktop.
2. This was tested through both:
- a locally installed MCPB package
- a public remote Streamable HTTP custom connector with OAuth
3. Start a new Claude conversation.
4. Ask Claude to invoke render_my_learner_card with:

{
"domain": "calculus",
"skill_id": "chain_rule",
"demo_mode": true,
"learner_profile_type": "synthetic_demo",
"include_experimental_states": true
}

5. Observe the tool result in the conversation.

Expected: an inline interactive learner card is visible.
Actual: only the text result appears; no card or iframe is visible.

The remote connector authenticates successfully, the tool call succeeds, and learner data is returned. The same behavior occurs with both the local MCPB and remote connector.

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

custom server

### Error messages or logs

```shell
There is no explicit error message in Claude Desktop.

Claude displays only the text fallback, for example:

"StudyMeta learner context is available. Hosts without MCP Apps UI support can use the structured result and existing text tools."

or a model-generated summary stating that the learner card was rendered or is available above.

However, no visible interactive card or iframe appears in the conversation.

Server-side verification:
- resources/list succeeds
- resources/read for ui://studymeta/learner-card succeeds
- tools/list includes _meta.ui.resourceUri
- render_my_learner_card succeeds and returns learner-card data
- OAuth authentication succeeds for the remote connector

This reproduces via both local MCPB and a remote Streamable HTTP custom connector, which suggests the issue is in Claude's MCP Apps UI rendering path rather than the MCP server, OAuth, or data retrieval.
```

### Additional context

A screenshot is attached showing the successful tool call with text-only output and no rendered learner-card UI.