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This skill should be used when the user wants to build an "MCP app", add "interactive UI" or "widgets" to an MCP server, "render components in chat", build "MCP UI resources", make a tool that shows a "form", "picker", "dashboard" or "confirmation dialog" inline in the conversation, or mentions "apps SDK" in the context of MCP. Use AFTER the build-mcp-server skill has settled the deployment model, or when the user already knows they want UI widgets.
Copy the install command and let the AI configure it · recommended for beginners
Please install the "build-mcp-app" skill from askskill: 1. Download https://raw.githubusercontent.com/anthropics/claude-plugins-official/main/plugins/mcp-server-dev/skills/build-mcp-app/SKILL.md 2. Save it as ~/.claude/skills/build-mcp-app/SKILL.md 3. Reload skills and tell me it's ready
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.
_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.none) — static bearer is private-deploy only and blocks listing — plus tool annotations and 3–5 PNG screenshots; see references/directory-checklist.md.Don't add UI for its own sake — most tools are fine returning text or JSON. Add a widget when one of these is true:
| Signal | Widget type |
|---|---|
| Tool needs structured input Claude can't reliably infer | Form |
| User must pick from a list Claude can't rank (files, contacts, records) | Picker / table |
| Destructive or billable action needs explicit confirmation | Confirm dialog |
| Output is spatial or visual (charts, maps, diffs, previews) | Display widget |
| Long-running job the user wants to watch | Progress / live status |
If none apply, skip the widget. Text is faster to build and faster for the user.
Before building a widget, check if elicitation covers it. Elicitation is spec-native, zero UI code, works in any compliant host.
| Need | Elicitation | Widget |
|---|---|---|
| Confirm yes/no | ✅ | overkill |
| Pick from short enum | ✅ | overkill |
| Fill a flat form (name, email, date) | ✅ | overkill |
| Pick from a large/searchable list | ❌ (no scroll/search) | ✅ |
| Visual preview before choosing | ❌ | ✅ |
| Chart / map / diff view | ❌ | ✅ |
| Live-updating progress | ❌ | ✅ |
If elicitation covers it, use it. See ../build-mcp-server/references/elicitation.md.
Hosted streamable-HTTP server. Widget templates are served as resources; tool results reference them. The host fetches the resource, renders it in an iframe sandbox, and brokers messages between the widget and Claude.
┌──────────┐ tools/call ┌────────────┐
│ Claude │─────────────> │ MCP server │
│ host │<── result ────│ (remote) │
│ │ + widget ref │ │
│ │ │ │
│ │ resources/read│ │
│ │─────────────> │ widget │
│ ┌──────┐ │<── template ──│ HTML/JS │
│ │iframe│ │ └────────────┘
│ │widget│ │
│ └──────┘ │
└──────────┘
Same widget mechanism, but the server runs locally inside an MCPB bundle. Use this when the widget needs to drive a local application — e.g., a file picker that browses the actual local disk, a dialog that controls a desktop app.
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Manage Discord channel access — approve pairings, edit allowlists, set DM/group policy. Use when the user asks to pair, approve someone, check who's allowed, or change policy for the Discord channel.
Set up the Discord channel — save the bot token and review access policy. Use when the user pastes a Discord bot token, asks to configure Discord, asks "how do I set this up" or "who can reach me," or wants to check channel status.
Analyze a codebase and recommend Claude Code automations (hooks, subagents, skills, plugins, MCP servers). Use when user asks for automation recommendations, wants to optimize their Claude Code setup, mentions improving Claude Code workflows, asks how to first set up Claude Code for a project, or wants to know what Claude Code features they should use.
Iterate on the Cardputer-Adv MicroPython app bundle (Claude Buddy, Snake, Hello) after the device is already provisioned via m5-onboard. Use when the user wants to add a new app, push a single changed .py without re-flashing, watch device serial logs, or run a one-shot REPL command. Trigger on "add an app", "push to the cardputer", "tail the device", "run on the device", or follow-up work after /maker-setup.
End-to-end onboarding for a freshly-plugged-in M5Stack ESP32 device (Cardputer, Cardputer-Adv, Core, CoreS3, Stick) — detect on USB, flash UIFlow 2.0 firmware, and install the Claude Buddy MicroPython app bundle. Use whenever the user plugs in or wants to flash/provision/reset an M5Stack or ESP32 board, or says "m5-onboard go".
Generate an explorable HTML report of Claude Code session usage (tokens, cache, subagents, skills, expensive prompts) from ~/.claude/projects transcripts.