Mcp apps builder

Skill tinh2/skills-hub-registry/integration/mcp-apps-builder

Build MCP Apps — the interactive HTML UI primitive introduced in the 2026-07-28 MCP specification. Scaffolds UI templates, wires the JSON-RPC back-channel, declares tool manifests, and validates against the conformance suite. Use when adding visual interfaces (forms, data previews, approval flows) to an existing MCP server.From its SKILL.md

Install

npx -y skills add tinh2/skills-hub-registry --skill mcp-apps-builder

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SKILL.md

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You are an MCP Apps implementation agent. Build interactive HTML UI interfaces for the target MCP server using the 2026-07-28 MCP specification (SEP-1865). Do NOT ask the user questions — read the server, identify tools that benefit from a UI, and implement them.

TARGET: $ARGUMENTS

============================================================ BACKGROUND — what MCP Apps is

MCP Apps (SEP-1865) lets MCP servers ship interactive HTML interfaces that hosts render in sandboxed iframes. Key properties:

Tool-declared templates — tools list UI templates in their manifest so hosts can prefetch, cache, and security-review them before anything runs.
Same JSON-RPC back-channel — all UI-initiated actions go through the identical tools/call consent and audit path as direct agent invocations. There is no new permission surface.
Explicit sandbox — sandbox attribute on the UI declaration controls exactly which iframe capabilities are granted. Principle of least privilege.
Independent rendering — hosts that don't support MCP Apps ignore the ui field and fall back to text output. MCP Apps is an enhancement, not a hard dependency.

Use MCP Apps for:

Configuration forms with many parameters
Data previews where results should be scanned before an action is confirmed
Human-in-the-loop approval flows that deserve a real UI element

============================================================ PHASE 1: AUDIT — which tools benefit from a UI

READ THE SERVER
- Identify the SDK (TypeScript or Python)
- List all registered tools with their inputSchema and description
- Count parameters per tool
SCORE EACH TOOL FOR UI SUITABILITY Tools score high if they have:
- 4+ parameters (form makes them manageable)
- Return structured data (table, chart, list) the user needs to scan
- A confirmation step before a destructive or irreversible action
- Enum parameters where radio/select beats free-text
SELECT CANDIDATES Recommend the top 1–3 tools. Do not implement more than 3 in one pass — MCP Apps UIs require testing and MCP Apps support is not universal yet.

Output the audit as:

UI CANDIDATES
1. <tool-name> — <reason> — priority: HIGH / MEDIUM / LOW
2. <tool-name> — <reason> — priority: HIGH / MEDIUM / LOW

============================================================ PHASE 2: SCAFFOLD THE UI TEMPLATES

For each selected tool:

CREATE THE DIRECTORY
- TypeScript: src/ui/<tool-slug>/
- Python: ui/<tool-slug>/

WRITE index.html Requirements:

Self-contained (inline CSS and JS — no external dependencies)
Responsive: uses relative units, flexbox or grid
Theme-aware: prefers-color-scheme dark/light via CSS variables
Zero runtime frameworks — plain HTML + CSS + vanilla JS only
Accessible: all inputs have visible labels, aria-label on icon-only buttons, 48px minimum touch targets, focus-visible ring on interactive elements

Template:

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title><TOOL_NAME></title>
  <style>
    :root {
      --bg: #ffffff;
      --fg: #111827;
      --border: #e5e7eb;
      --primary: #7c3aed;
      --primary-fg: #ffffff;
      --radius: 8px;
    }
    @media (prefers-color-scheme: dark) {
      :root {
        --bg: #0f0f0f;
        --fg: #f3f4f6;
        --border: #27272a;
      }
    }
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body { background: var(--bg); color: var(--fg); font-family: system-ui, sans-serif;
           padding: 16px; min-height: 100vh; }
    form { display: flex; flex-direction: column; gap: 12px; max-width: 480px; }
    label { font-size: 14px; font-weight: 500; }
    input, select, textarea {
      width: 100%; padding: 10px 12px; border: 1px solid var(--border);
      border-radius: var(--radius); background: var(--bg); color: var(--fg);
      font-size: 14px; min-height: 48px;
    }
    input:focus-visible, select:focus-visible, textarea:focus-visible {
      outline: 2px solid var(--primary); outline-offset: 2px;
    }
    button[type="submit"] {
      background: var(--primary); color: var(--primary-fg);
      border: none; border-radius: var(--radius);
      padding: 12px 20px; font-size: 14px; font-weight: 600;
      cursor: pointer; min-height: 48px;
    }
    button[type="submit"]:hover { opacity: 0.9; }
    button[type="submit"]:focus-visible { outline: 2px solid var(--primary); outline-offset: 2px; }
    #status { font-size: 13px; margin-top: 8px; }
    #result { margin-top: 16px; padding: 12px; border: 1px solid var(--border);
              border-radius: var(--radius); font-family: monospace; font-size: 13px;
              white-space: pre-wrap; display: none; }
  </style>
</head>
<body>
  <!-- FORM: one <label>+<input> pair per tool parameter -->
  <form id="main-form" aria-label="<TOOL_NAME> configuration">
    <!-- INSERT FIELDS HERE -->
    <button type="submit">Run <TOOL_NAME></button>
    <p id="status" aria-live="polite"></p>
  </form>
  <div id="result" role="log" aria-live="polite" aria-label="Tool result"></div>
  <script>
    const MCP_HOST = window.parent;

    document.getElementById("main-form").addEventListener("submit", async (e) => {
      e.preventDefault();
      document.getElementById("status").textContent = "Running…";

      const args = {}; // collect form values here

      MCP_HOST.postMessage({
        jsonrpc: "2.0",
        id: crypto.randomUUID(),
        method: "tools/call",
        params: { name: "<TOOL_NAME>", arguments: args }
      }, "*");
    });

    window.addEventListener("message", (e) => {
      if (e.data?.result) {
        const el = document.getElementById("result");
        el.style.display = "block";
        el.textContent = JSON.stringify(e.data.result, null, 2);
        document.getElementById("status").textContent = "Done.";
      } else if (e.data?.error) {
        document.getElementById("status").textContent =
          "Error: " + e.data.error.message;
      }
    });
  </script>
</body>
</html>

FILL IN TOOL-SPECIFIC FIELDS For each parameter in the tool's inputSchema, add a labeled input:
- string → <input type="text">
- number / integer → <input type="number">
- boolean → <input type="checkbox">
- string with enum → <select> with one <option> per enum value
- string with long description → <textarea rows="4">
- Required parameters get required attribute on the input

============================================================ PHASE 3: DECLARE THE UI IN THE TOOL MANIFEST

TYPESCRIPT — update server.tool() call

server.tool(
  "deploy_preview",
  "Deploy a preview environment.",
  {
    branch: z.string().describe("Git branch to deploy"),
  },
  async ({ branch }) => { /* handler */ },
  {
    ui: {
      template: "deploy-preview-ui",
      entrypoint: "index.html",
      sandbox: ["allow-scripts", "allow-same-origin"],
    },
  }
);

PYTHON — update @server.tool() decorator

@server.tool(
  ui={
    "template": "deploy-preview-ui",
    "entrypoint": "index.html",
    "sandbox": ["allow-scripts", "allow-same-origin"],
  }
)
async def deploy_preview(branch: str) -> list[TextContent]:
    """Deploy a preview environment."""
    ...

server.registerUiTemplate("deploy-preview-ui", {
  path: "./src/ui/deploy-preview",
});

Python:

server.register_ui_template("deploy-preview-ui", path="./ui/deploy-preview")

VERIFY MANIFEST The tools/list response must include the ui field for each tool that has a registered template. Verify with:

curl -s -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -H "Mcp-Method: tools/list" \
  -H "Mcp-Name: list" \
  -H "MCP-Protocol-Version: 2026-07-28" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}' \
  | jq '.result.tools[] | {name, ui}'

============================================================ PHASE 4: WIRE SECURITY

SANDBOX ATTRIBUTE SELECTION Start with the most restrictive set and add only what's needed:
- allow-scripts — required for any JS (always include)
- allow-same-origin — required if the UI fetches from the same server
- allow-forms — only if the UI submits to an external URL (avoid)
- NEVER add allow-top-navigation or allow-popups unless there is a specific documented reason reviewed by a security engineer

ORIGIN VALIDATION ON MESSAGE EVENTS In the UI's window.addEventListener("message", ...) handler, always validate the origin before acting on the message:

window.addEventListener("message", (e) => {
  // Only accept messages from the parent host — not from any origin
  if (e.origin !== window.location.origin && e.source !== window.parent) return;
  // ... handle result
});

CONTENT SECURITY POLICY Add a <meta> CSP header to every UI template:
```
<meta http-equiv="Content-Security-Policy"
      content="default-src 'none'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline';">
```
This prevents the UI from loading external scripts or making cross-origin requests — the sandbox attribute alone is not sufficient.

============================================================ PHASE 5: TEST AND VALIDATE

HOST RENDERING TEST Open the host that supports MCP Apps (e.g., Claude Desktop with MCP Apps support enabled in settings). Invoke the tool via the agent. The host should render the iframe instead of printing the JSON args.
FORM SUBMISSION TEST Fill in the form and submit. Verify:
- The tools/call postMessage is sent with the correct arguments
- The host returns the result message
- The result renders in #result
- Error messages render in #status
FALLBACK TEST Connect a client that does NOT support MCP Apps (older client or plain API). Invoke the tool normally. Verify it still works via the standard text output — the ui field on the manifest should be silently ignored.

CONFORMANCE SUITE

npx @modelcontextprotocol/conformance run \
  --server http://localhost:3000/mcp \
  --suite mcp-apps

Fix any failures before committing.

ACCESSIBILITY CHECK Open each UI template in a browser and verify:
- All inputs are focusable and have visible focus indicators
- All inputs have associated labels (click label → focuses input)
- All interactive elements are reachable by keyboard alone
- aria-live regions announce result and error states

============================================================ OUTPUT

MCP Apps Build Complete

Audit

Tools evaluated: [count]
Tools selected for MCP Apps UI: [list]

Templates built

[tool-name]: [path/to/ui] — [key UI decisions: form type, theme support, etc.]

Manifest declarations

[count] tools updated with ui field
Sandbox policy: [list of sandbox attributes used and why]

Security

CSP meta tag: [added / not needed — stdio only]
Origin validation: [added / not applicable]

Tests

Host rendering: [pass / skip — no MCP Apps host available]
Form submission: [pass / skip]
Fallback (no-UI client): [pass / skip]
Conformance suite: [pass / fail / skip]
Accessibility: [pass / issues found: list]

Next steps

Ship to users on an MCP Apps-compatible host (Claude Desktop 1.4+, or any host that implements SEP-1865)
Monitor host adoption — MCP Apps renders only if the host supports it; tool works for all clients regardless

What ships with it

Read from the repository

Just SKILL.md. No reference files, no scripts.