@polyxd/mcp

@polyxd/mcp

An MCP server for Polyxd. The host's model is the generator: the server gives it the spec as instructions, checks the UI documents it writes, and shows them to the user as an MCP App, drawn by the Polyxd renderer in any design-system pack.

npx -y @polyxd/mcp        # stdio

Or connect by URL to the hosted server, which needs no account and no sign-in:

https://mcp.polyxd.com/mcp

Connect by URL

• Claude (claude.ai, and Claude Desktop through your account): Customize > Connectors > Add custom connector, paste the URL, No sign-in.
• Claude Code: claude mcp add --transport http polyxd https://mcp.polyxd.com/mcp
• ChatGPT: turn on developer mode (Settings > Security and login), then add it at chatgpt.com/plugins with the URL and no authentication.
• Cursor: { "mcpServers": { "polyxd": { "url": "https://mcp.polyxd.com/mcp" } } } in ~/.cursor/mcp.json.
• VS Code: { "servers": { "polyxd": { "type": "http", "url": "https://mcp.polyxd.com/mcp" } } } in .vscode/mcp.json.

Add it to a host over stdio

Claude Desktop (claude_desktop_config.json) and most other MCP clients:

{
  "mcpServers": {
    "polyxd": { "command": "npx", "args": ["-y", "@polyxd/mcp"] }
  }
}

Claude Code:

claude mcp add polyxd -- npx -y @polyxd/mcp

From a clone of this repository: npm install && npm run build -w @polyxd/mcp, then use node <repo>/packages/mcp/dist/bin.js as the command.

Tools

Tool	What it does	structuredContent
polyxd_guide	The spec as instructions: how to use these tools, then the generator prompt the demos use (document shape, rules for generated screens, bindings, actions, every component and its props, the patterns).	{ specVersion, guide }, the guide being the same text as the content
polyxd_validate	Validates a document. Every issue comes with its JSON Pointer, the component it is in, and a hint saying what to change.	{ valid, errors, warnings, issues[{ severity, pointer, component?, message, hint? }] }
polyxd_verify	The verifier's document checks (staticAudit from @polyxd/verifier/static, so installing the server installs no Playwright), optionally against a Design Direction (an object, or calm-finance / playful-personal) and a capability registry. A compact report.	{ errors, warnings, findings[{ severity, check, message, hint? }], direction?, rules }
polyxd_show	Validates, then returns the document for display. pack picks the design-system pack (default material3), mode picks light or dark (default: the host's theme). Shows nothing when there are errors.	{ shown: true, document, pack, packName, mode?, specVersion }, or { shown: false, issues } with isError
polyxd_packs	The packs: published design systems and original templates.	{ default, packs[{ name, displayName, template, package, description }] }
polyxd_components	The components with a one-line summary each, or one component's full definition by name.	{ components[{ name, category, summary, shell }] }, or { component } with the full definition

Every tool is read-only. data can be passed to polyxd_validate, polyxd_verify and polyxd_show separately from the document; it replaces the document's own data.

Every tool declares an outputSchema (JSON Schema 2020-12, in src/output-schemas.ts, each property described for the model), and every successful result carries structuredContent that conforms to it; the SDK checks each one before it is sent. Error results (isError: true) are not held to the schema: polyxd_show's not-shown result still conforms, the others carry text only.

Resources

• ui://polyxd/surface.html (text/html;profile=mcp-app): the MCP App.
• polyxd://examples/<name>.json: the spec's example documents.
• polyxd://directions/<name>.json: the spec's example Design Directions.

The MCP App

polyxd_show declares _meta.ui.resourceUri: "ui://polyxd/surface.html", per the MCP Apps extension (SEP-1865, 2026-01-26), and ChatGPT's alias openai/outputTemplate. A host that supports MCP Apps reads that resource and renders it in a sandboxed iframe. The page is self-contained: the @polyxd/web renderer, the renderer's stylesheet and every pack's theme, inlined, with no external origins, so the host's default Content Security Policy is enough.

The resource's _meta says so for each host (appResourceMeta()):

• ui.csp: { connectDomains: [], resourceDomains: [] }. The page loads nothing.
• ui.prefersBorder: true.
• ChatGPT's own keys: openai/widgetCSP (the same empty lists), openai/widgetPrefersBorder, openai/widgetDescription, and openai/widgetDomain: "https://mcp.polyxd.com", the origin ChatGPT gives the app's sandbox.
• No ui.domain. Claude wants a hash of the connector URL there and ChatGPT wants an origin, and Claude refuses to render an app whose ui.domain is not its own format. The page needs no stable origin, so it sets none, and ChatGPT reads openai/widgetDomain instead.

The page speaks the MCP Apps protocol over postMessage: ui/initialize, then ui/notifications/initialized. It renders the document from the ui/notifications/tool-result notification's structuredContent, in the chosen pack, light or dark from the host's theme unless mode was given. It reports its height with ui/notifications/size-changed.

When the user presses an action, the page sends ui/message with a user message naming the action, the button's label and the action's context (the values bound into it, including what the user typed):

[Polyxd] In the "New task" screen I pressed "Add task" (action task.save).
Context: {"title":"Buy milk","due":"2026-10-01"}

The model receives that as the user's next message and acts on it. Closing the screen (ui.dismiss) arrives the same way. A host without MCP Apps support gets polyxd_show's text summary instead.

The prompt

The generator prompt comes from @polyxd/runtime, which builds it from the spec. src/packs.generated.ts lists the packs, and src/spec-files.generated.ts imports the spec's component definitions and examples as JSON modules, so the server reads no files. npm run sync -w @polyxd/mcp rewrites both, and the tests fail if either is out of date.

Over HTTP

@polyxd/mcp/http serves the same server over MCP's Streamable HTTP transport, as a web-standard (Request) => Promise<Response> handler. It uses nothing from Node, so it runs in a Cloudflare Worker, Deno or Bun.

import { createHttpHandler, rateLimiter } from "@polyxd/mcp/http";
import viewHtml from "@polyxd/mcp/view.html"; // bundled as text

const mcp = createHttpHandler({ viewHtml: () => viewHtml, rateLimit: rateLimiter({ limit: 600, windowMs: 60_000 }) });
export default { fetch: (request: Request) => (new URL(request.url).pathname === "/mcp" ? mcp(request) : new Response(null, { status: 404 })) };

The transport is the SDK's createMcpHandler, stateless: a fresh server per request, no sessions. For 2025-era clients GET and DELETE answer 405, as the spec asks of a server with no standalone stream and no sessions. Clients on the 2026-07-28 protocol, which send the version with every request, work too. Around it:

• Body limit: maxBodyBytes, default 1 MiB, answered 413. It is checked on Content-Length first and again as the body is read.
• Timeout: timeoutMs, default 15 seconds, for the whole answer. Late answers get a JSON-RPC error: a 504, or on an SSE stream an error for the request's id.
• Rate limit: rateLimit(request) returns false for 429 with Retry-After. rateLimiter() is a fixed window in memory, keyed on CF-Connecting-IP. It counts per process or per Worker isolate, so it is best effort.
• CORS: any origin, since the server holds no credentials and no one's data. Preflights allow the headers they ask for, and responses expose Mcp-Session-Id and Mcp-Protocol-Version.
• Logs: one line per request with the method, path, status and duration. Nothing else: not a body, a header, a query string, an address or an error message.

The hosted server

https://mcp.polyxd.com/mcp is apps/mcp, a Cloudflare Worker: /mcp is the handler above, /health answers JSON, and / redirects to the docs. The view is bundled into the Worker as text (about 2.4 MB in all, 385 KB gzipped). Its rate limit is Cloudflare's rate limiting binding, 600 requests a minute per IP address. That binding counts per Cloudflare location, so it guards against floods rather than enforcing an exact quota. Claude and ChatGPT call from their own servers, which is why the limit is generous. Cloudflare's invocation logs, traces and Issues are off in wrangler.jsonc, so the Worker's own lines are all that is kept. With a POSTHOG_KEY secret set, the hosted Worker also counts initializes and tool calls anonymously (apps/mcp/src/analytics.ts): names and counts only, never a document, its data or an address. This package, and npx @polyxd/mcp, send nothing anywhere.

npm run dev -w @polyxd/mcp-remote                        # wrangler dev on http://127.0.0.1:8787/mcp
node apps/mcp/scripts/smoke.ts http://127.0.0.1:8787/mcp  # every tool, in both protocol eras, over real HTTP
npm run deploy -w @polyxd/mcp-remote                     # builds @polyxd/mcp, then wrangler deploy

Deploying creates the mcp.polyxd.com custom domain on the polyxd.com zone. workers.dev and preview URLs are off.

MCP Registry

server.json describes the hosted server for the MCP Registry as com.polyxd/mcp. A name under com.polyxd needs proof that you control polyxd.com:

openssl genpkey -algorithm Ed25519 -out key.pem
echo "polyxd.com. IN TXT \"v=MCPv1; k=ed25519; p=$(openssl pkey -in key.pem -pubout -outform DER | tail -c 32 | base64)\""
mcp-publisher login dns --domain polyxd.com --private-key "$(openssl pkey -in key.pem -noout -text | grep -A3 'priv:' | tail -n +2 | tr -d ' :\n')"
mcp-publisher publish      # from packages/mcp

This needs OpenSSL 3 (macOS's LibreSSL has no Ed25519). Keep key.pem private and out of the repository. The TXT record goes on the apex, polyxd.com. HTTP proof works instead: serve the same v=MCPv1; k=ed25519; p=... line at https://polyxd.com/.well-known/mcp-registry-auth and use mcp-publisher login http. It has to be polyxd.com, not mcp.polyxd.com, whose proof would only cover names under com.polyxd.mcp.

Once @polyxd/mcp is on npm, add it to server.json so hosts can offer the stdio server too. package.json already carries the mcpName the registry checks:

"packages": [{ "registryType": "npm", "identifier": "@polyxd/mcp", "version": "0.3.0", "transport": { "type": "stdio" } }]

Keep version in server.json in step with package.json; a test checks it.

Build and test

npm run build -w @polyxd/mcp     # tsc, then scripts/build-view.ts writes dist/view.html
npm test -w @polyxd/mcp
npm test -w @polyxd/mcp-remote   # the Worker's routes

The tests connect a client to the server in memory and call every tool on every spec example and on broken documents; start the built bin and speak JSON-RPC to it over stdio; serve it over Streamable HTTP to the SDK client in both protocol eras, and send the raw requests a client library never would (bad JSON, oversize bodies, too many requests, preflights, slow answers); bundle the HTTP entry for a Worker and check it needs nothing from Node; and load the MCP App in headless Chromium with the test playing the host, from the handshake to an action arriving as ui/message. The browser tests skip when Playwright has no Chromium (npx playwright install chromium).