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/openfda-mcp-server --skill add-app-toolgit clone --depth 1 https://github.com/cyanheads/openfda-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/openfda-mcp-server/add-app-tool)<a href="https://agentmods.dev/skills/cyanheads/openfda-mcp-server/add-app-tool"><img src="https://agentmods.dev/badge/skills/cyanheads/openfda-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/openfda-mcp-server/add-app-tool"><img src="https://agentmods.dev/badge/skills/cyanheads/openfda-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 | $0.02891 |
| Opus 5 | $0.00020 | $0.01445 |
| Sonnet 5 | $0.00008 | $0.00578 |
| Haiku 4.5 | $0.00004 | $0.00289 |
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 9d 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 — 245 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(orstart:http)
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.
- 9d ago First seen · 245 lines · 41 tokens per session scan A 96f3fdedf0a4
add-app-tool is a skill published in the GitHub repository cyanheads/openfda-mcp-server (4 stars, last pushed 14d ago), licensed Apache-2.0. It adds 41 tokens to every session and 2,891 once invoked, about $0.0002 per session on Opus 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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