If you've built an MCP App — the HTML widget an MCP server hands a host to render inline — you may have hit this: the tool call succeeds, structuredContent comes back fine, the model announces that a widget rendered, and the user sees nothing. No error. No console output.
Source: [Dev.to](https://dev.to/booyaka101/debugging-a-black-box-36-renders-against-claude-and-the-part-where-my-own-data-was-wrong-43ag)