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Many Semicola tools open an interactive page (a widget) next to the chat: the campaign receipt, the creative composer, the storefront’s Listing. They follow the MCP Apps convention, so any host that supports MCP Apps renders them.

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Tool-result contract

A widget-bearing tool declares its widget as a ui://semicola/<view>/mcp-app.html resource (the tool catalog lists it as Opens widget). Its result carries:
  • short, model-facing text in content;
  • the widget’s data in structuredContent;
  • host-only data in _meta, which the model doesn’t see.
The host reads the ui:// resource with resources/read, renders it in a sandboxed iframe, and hands the tool result to it over the MCP Apps bridge. resources/list returns the live set of widgets; don’t hard-code a list, because it changes with the product. Widgets call back through the host with the same tools, so every write still goes through the tool’s confirmation rules (see Semi confirmations).

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Widget HTML over REST

For a host that renders widgets without a live MCP client: Both need authentication. Pass the HTML to a sandboxed iframe as srcdoc. The response carries the widget’s content security policy in the x-mcp-app-csp header, which the host should apply, and cache-control: no-store. An unknown view returns 404 (“Unknown widget view.”). Treat the view name as opaque: use the one from the tool’s resource URI. Semicola’s own app renders widgets in an iframe sandboxed to allow-scripts allow-forms allow-downloads, with form-action 'none' so a form never submits anywhere.

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Not available yet

  • Session-bound page capabilities and host-only page aliases (the Teach and Library pages’ private material tools).