build-mcp-app

# Build an MCP App (Interactive UI Widgets) An MCP app is a standard MCP server that **also serves UI resources** — interactive components rendered inline in the chat surface. Build once, runs in Claude *and* ChatGPT and any other host that implements the apps surface. The UI layer is **additive**. Under the hood it's still tools, resources, and the same wire protocol. If you haven't built a plain MCP server before, the `build-mcp-server` skill covers the base layer. This skill adds widgets on top. > **Testing in Claude:** Add the server as a custom connector in claude.ai (via a Cloudflare tunnel for local dev) — this exercises the real iframe sandbox and `hostContext`. See https://claude.com/docs/connectors/building/testing. ## Claude host specifics | `_meta.ui.*` key | Where | Effect | |---|---|---| | `resourceUri` | tool | Which `ui://` resource the host renders for this tool's results. | | `visibility: ["app"]` | tool | Hide a widget-only helper tool (e.g. geometry/image fetcher called via `callServerTool`) from Claude's tool list. | | `prefersBorder: false` | resource | Drop the host's outer card border (mobile). | | `csp.{connectDomains, resourceDomains, baseUriDomains}` | resource | Declare external origins; default is block-all. `frameDomains` is currently restricted in Claude. | - `hostContext.safeAreaInsets: {top, right, bottom, left}` (px) — honor these for notches and the composer overlay. - Directory submission requires OAuth or **authless** (`none`) — stat...