Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add HK-hub/AgentSkills --skill build-mcp-appgit clone --depth 1 https://github.com/HK-hub/AgentSkillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/hk-hub/agentskills/build-mcp-app)<a href="https://agentmods.dev/skills/hk-hub/agentskills/build-mcp-app"><img src="https://agentmods.dev/badge/skills/hk-hub/agentskills/build-mcp-app/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/hk-hub/agentskills/build-mcp-app"><img src="https://agentmods.dev/badge/skills/hk-hub/agentskills/build-mcp-app.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00111 | $0.04017 |
| Opus 5.5 | $0.00044 | $0.01607 |
| Sonnet 5.5 | $0.00022 | $0.00803 |
| Haiku 4.5 | $0.00011 | $0.00402 |
Grade A, and why
build-mcp-app scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
This is a copy
100% identical to build-mcp-app — 8 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 353 lines — stays where its author put it; the contents beside it link to each section on GitHub.
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.
When a widget beats plain text
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.
Widgets vs Elicitation — route correctly
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.
Architecture: two deployment shapes
Remote MCP app (most common)
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.
What ships with it
3 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2mo ago First seen · 353 lines · 111 tokens per session scan A e30e4a109048
build-mcp-app is a skill published in the GitHub repository HK-hub/AgentSkills (6 stars, last pushed 2mo ago), licensed MIT. It adds 111 tokens to every session and 4,017 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 0 findings. It is 100% identical to build-mcp-app, differing in 8 lines, and is treated as a copy.
Other skills, from other repositories
spec-brainstorm
Conversational design workshop for substantial work. Interviews the human one question at a time, explores 2-3 approaches with trade-offs, and presents the design section by section for approval before writing only design.md, then stops. Combines requirements discovery with codebase research and architecture design.…
spec-plan
Write approved implementation plans in one of two modes. Explicit Inline mode creates a conversational plan for bounded work. Spec-backed Plan converts an approved design.md into plan.json. Both modes stop after producing their plan output. Trigger after atelier-orchestrator selects a mode, when the user asks to plan…
spec-implement
Continue an approved Spec-backed workflow when the user says "implement", "go", "start", or "do it". After Codex Plan Mode, persist the next missing design.md or plan.json artifact and stop. When both artifacts exist, execute plan.json with TDD and report between batches.
atelier-orchestrator
Skill routing and workflow orchestration. Selects Inline Plan or Spec-backed Plan, routes to the correct workflow skill, and manages transitions between phases. Use when starting any conversation or task to determine which planning mode and skill apply.
oracle-codebase-design
Shared vocabulary and principles for designing deep modules. Use when designing or changing a module's interface, deciding where a seam goes, handling untrusted input at the edges, choosing the level to test at, naming modules, or making code more testable. Also use when another skill needs the deep-module vocabulary.
code-commit
Generate and validate conventional commit messages following the conventionalcommits.org spec. Use whenever the user wants to commit code, mentions commit messages, git commit, or asks to create a commit. Triggers on "commit", "git commit", "conventional", or when reviewing commit message format.