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 bosens-China/tc39-atlas --skill build-mcp-appgit clone --depth 1 https://github.com/bosens-China/tc39-atlasWrote 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/bosens-china/tc39-atlas/build-mcp-app)<a href="https://agentmods.dev/skills/bosens-china/tc39-atlas/build-mcp-app"><img src="https://agentmods.dev/badge/skills/bosens-china/tc39-atlas/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/bosens-china/tc39-atlas/build-mcp-app"><img src="https://agentmods.dev/badge/skills/bosens-china/tc39-atlas/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 | $0.04747 |
| Opus 5.5 | $0.00044 | $0.01899 |
| Sonnet 5.5 | $0.00022 | $0.00949 |
| Haiku 4.5 | $0.00011 | $0.00475 |
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 1mo 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 — 0 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 — 393 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.
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) — static bearer is private-deploy only and blocks listing — plus toolannotationsand 3–5 PNG screenshots; seereferences/directory-checklist.md.
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 |
What ships with it
6 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.
- 1mo ago First seen · 393 lines · 111 tokens per session scan A d2fd94f00965
build-mcp-app is a skill published in the GitHub repository bosens-China/tc39-atlas (0 stars, last pushed yesterday), licensed MIT. It adds 111 tokens to every session and 4,747 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 0 lines, and is treated as a copy.
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