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 jnPiyush/AgentX --skill mcp-apps-developmentgit clone --depth 1 https://github.com/jnPiyush/AgentXWrote 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/jnpiyush/agentx/mcp-apps-development)<a href="https://agentmods.dev/skills/jnpiyush/agentx/mcp-apps-development"><img src="https://agentmods.dev/badge/skills/jnpiyush/agentx/mcp-apps-development/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/jnpiyush/agentx/mcp-apps-development"><img src="https://agentmods.dev/badge/skills/jnpiyush/agentx/mcp-apps-development.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00065 | $0.03038 |
| Opus 5.5 | $0.00026 | $0.01215 |
| Sonnet 5.5 | $0.00013 | $0.00608 |
| Haiku 4.5 | $0.00006 | $0.00304 |
Grade A, and why
mcp-apps-development 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.
How it starts
The opening of the file, as written. The whole thing — 370 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MCP Apps Development
Build MCP Apps that render interactive UI inside conversational AI clients using the
@modelcontextprotocol/ext-appsSDK.
When to Use
- Building MCP tools that display interactive UI (charts, forms, dashboards)
- Creating visual resource viewers inside AI chat interfaces
- Adding rich media output to existing MCP servers (video, maps, 3D, music)
- Migrating OpenAI chat app plugins to the MCP Apps standard
- Building fullscreen interactive experiences within MCP-compatible hosts
When NOT to Use
- Building headless MCP servers (tools, resources, prompts only) -> use
mcp-server-developmentskill - Creating standalone web applications outside AI clients
- Building API integrations without a visual component
Decision Tree
Need interactive UI in an AI chat client?
+- Tool with visual output?
| +- Register with registerAppTool()
| +- Return _meta.ui.resourceUri linking to a resource
| - Resource renders the UI via App class
+- Standalone visual resource?
| +- Register with registerAppResource()
| - Resource renders UI, host embeds as iframe
+- Migrating from OpenAI plugin?
| +- See Migration section below
| - Key: synchronous globals -> async App handlers
- No UI needed?
- Use mcp-server-development skill instead
Architecture Overview
---------- PostMessage ---------- MCP ----------
| View | <================> | Host | <==========> | Server |
| (App) | (iframe) |(AppBridge| (Client) | (MCP SDK)|
| iframe | | proxy) | | |
---------- ---------- ----------
| | |
App class AppBridge class registerAppTool()
PostMessageTransport proxies MCP requests registerAppResource()
React hooks embeds iframe _meta.ui.resourceUri
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 Changed d64df78171e6
- 1mo ago First seen · 370 lines · 65 tokens per session scan A aa30e2896363
mcp-apps-development is a skill published in the GitHub repository jnPiyush/AgentX (16 stars, last pushed 3d ago), licensed Apache-2.0. It adds 65 tokens to every session and 3,038 once invoked, about $0.0003 per session on Opus 5.5. 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 2026-09-03.
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