Two undocumented bugs in MCP Apps I found building a task panel for Claude

Short summary

Author documents two undocumented bugs in MCP Apps discovered while building Wingman: _meta.ui.resourceUri must be set on the CallToolResult itself, not nested in structuredContent; and explicit display:flex CSS rules outrank [hidden] unless marked !important. Post also documents three iframe sandbox constraints that silently fail (confirm(), clipboard.writeText, URL.createObjectURL) and shares debugging lessons on checking nested field placement and CSS specificity before assuming logic is wrong.

• •_meta.ui.resourceUri placement: must be on CallToolResult top-level, not inside structuredContent
• •CSS specificity bug: explicit display rules outrank [hidden] attribute without !important flag
• •Three silent iframe sandbox failures: confirm(), clipboard API, and blob downloads produce no errors

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