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 cyanheads/openfoodfacts-mcp-server --skill add-app-toolgit clone --depth 1 https://github.com/cyanheads/openfoodfacts-mcp-serverWrote 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/cyanheads/openfoodfacts-mcp-server/add-app-tool)<a href="https://agentmods.dev/skills/cyanheads/openfoodfacts-mcp-server/add-app-tool"><img src="https://agentmods.dev/badge/skills/cyanheads/openfoodfacts-mcp-server/add-app-tool/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/cyanheads/openfoodfacts-mcp-server/add-app-tool"><img src="https://agentmods.dev/badge/skills/cyanheads/openfoodfacts-mcp-server/add-app-tool.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 | $0.00041 | $0.03169 |
| Opus 5.5 | $0.00016 | $0.01268 |
| Sonnet 5.5 | $0.00008 | $0.00634 |
| Haiku 4.5 | $0.00004 | $0.00317 |
Grade A, and why
add-app-tool 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 21d 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 add-app-tool — 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 — 240 lines — stays where its author put it; the contents beside it link to each section on GitHub.
When to Use
App tools are rarely the right choice. Reach for one only when all of the following hold:
- A human will actively interact with the result in real time — not just an LLM consuming text.
- The target deployment runs in a client that supports MCP Apps. Many clients (Claude Code, Cursor, most chat UIs) are tool-only and will only ever see the
format()text fallback you have to maintain anyway. - The interaction the UI enables — scrubbing a dense table, approving a multi-step plan, filling a structured form — is core to the workflow, not nice-to-have rendering.
App tools cost more than standard tools: an iframe + CSP setup, app.ontoolresult / callServerTool plumbing, host-context wiring (theme, fonts, styles), and a format() text path that has to be content-complete because most clients see only that. Two surfaces to keep in sync, two failure modes per change.
Default to add-tool. This skill is the how-to once that bar is cleared — the "whether to" decision belongs in design-mcp-server.
Context
MCP Apps extend the standard tool pattern with an interactive HTML UI rendered in a sandboxed iframe by the host. Each MCP App consists of two definitions:
- App tool (
.app-tool.ts) — usesappTool()builder, declaresresourceUripointing to the UI resource - App resource (
.app-resource.ts) — usesappResource()builder, serves the bundled HTML
Both builders are exported from @cyanheads/mcp-ts-core. They handle _meta.ui.resourceUri, the compat key (ui/resourceUri), and the correct MIME type (text/html;profile=mcp-app) automatically.
For the full API, Context interface, and error codes, read the framework's CLAUDE.md/AGENTS.md (loaded at session start).
Steps
- Confirm the three conditions in "When to Use" apply — if any is uncertain, default to
add-toolinstead. Then gather the tool's name, purpose, input/output shape, and what the UI should display from the user's request — ask only if genuinely absent - Choose a URI — convention:
ui://{{tool-name}}/app.html - Create the app tool at
src/mcp-server/tools/definitions/{{tool-name}}.app-tool.ts - Create the app resource at
src/mcp-server/resources/definitions/{{tool-name}}-ui.app-resource.ts - Register both in the project's existing
createApp()arrays (directly insrc/index.tsfor fresh scaffolds, or via barrels if the repo already has them) - Run
bun run devcheck— the linter validates_meta.uiand cross-checks tool/resource pairing - Smoke-test with
bun run rebuild && bun run start:stdio < /dev/null(orstart:http) — theCore services constructedlog record must list the tool in itstoolsfield and its UI resource inresources(the message text shows only counts)
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.
- 21d ago Changed · -5 lines 6ad5aead35ab
- 1mo ago Changed db57241d6a6b
- 2mo ago First seen · 245 lines · 41 tokens per session scan A 96f3fdedf0a4
add-app-tool is a skill published in the GitHub repository cyanheads/openfoodfacts-mcp-server (1 stars, last pushed 2d ago), licensed Apache-2.0. It adds 41 tokens to every session and 3,169 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. It is 100% identical to add-app-tool, differing in 0 lines, and is treated as a copy.
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