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 agentmods add skills/webhook-co/webhook/build-mcp-appnpx skills add webhook-co/webhook --skill build-mcp-appgit clone --depth 1 https://github.com/webhook-co/webhookWrote 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/webhook-co/webhook/build-mcp-app)<a href="https://agentmods.dev/skills/webhook-co/webhook/build-mcp-app"><img src="https://agentmods.dev/badge/skills/webhook-co/webhook/build-mcp-app.svg" alt="Measured on agentmods" 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 | $0.00056 | $0.00640 |
| Opus 5 | $0.00028 | $0.00320 |
| Sonnet 5 | $0.00011 | $0.00128 |
| Haiku 4.5 | $0.00006 | $0.00064 |
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 3d 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.
How it starts
The opening of the file, as written. The whole thing — 53 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Build MCP app
An MCP app is an interactive widget — a form, picker, confirmation dialog, chart, or live status view — that an MCP host renders in a sandboxed iframe. Use it only when a widget genuinely beats a text exchange (e.g. confirming a destructive replay, picking an endpoint, watching delivery status live). Most tools should stay text-first; reach for a UI when interaction or at-a-glance state is the point.
Human-UI-testing hard stop (read first)
Anything an MCP app renders is user-facing UI. The repo's non-negotiable applies in full: when a change needs human visual/UX verification you can't do yourself — layout, rendering, interaction behavior, or any user-facing copy — STOP and explicitly flag it for human testing. Do not mark the widget done, approve it, or merge it until a human has eyeballed it. Say plainly what to check (states, edge cases, copy).
When a widget is worth it
- Confirmation dialog for irreversible or cross-tenant-sensitive actions (replay, delete) — make the consequence explicit before the user commits.
- Picker / form when free-text would be error-prone (choosing an endpoint, a time window, an event).
- Chart / live status when trends or real-time delivery state are easier to read visually than as text.
If a plain text response answers the question, don't build a widget.
Build guidance
- Treat the iframe as untrusted and sandboxed: no ambient credentials inside the widget; pass only the minimum data needed, and keep all privileged work behind the MCP server (validate → delegate → respond), never in the widget.
- Never render raw webhook payloads or PII/PHI in a widget; reference events by id and show redacted summaries.
- Reuse
shared/types for anything the widget exchanges with the server — don't invent UI-only shapes. - Follow the design-ux conventions (accessibility, design tokens) and the
writing-voicerule for all labels and copy: sentence case, lowercasewebhook.co/wbhk.my, precise and quietly opinionated.
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.
- 3d ago First seen · 53 lines · 56 tokens per session scan A a8a5ccc59f2a
build-mcp-app is a skill published in the GitHub repository webhook-co/webhook (0 stars, last pushed 2d ago), licensed Apache-2.0. It adds 56 tokens to every session and 640 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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