MCP App Ready

Stable 2026-01-26 extension

Make your MCP App host-ready.

Paste a tools/resources snapshot. Check ui:// linkage, HTML resource content, MCP App MIME types, visibility, sandbox permissions and CSP origins before testing in Claude, ChatGPT, VS Code or another host.

MCP Apps snapshot

Readiness report

Why this validator exists

MCP Apps joins three surfaces that must agree.

MCP Apps is the first official Model Context Protocol extension. A server links a tool to a predeclared HTML resource, a host loads that resource into a sandbox, and the embedded view communicates through JSON-RPC. A single typo across those layers can leave an otherwise valid MCP tool with a blank or blocked interface.

This checker accepts a compact snapshot with tools, resources and contents arrays. It verifies the relationships that a generic JSON formatter cannot see.

Checks included

  • Modern _meta.ui.resourceUri links and deprecated flat metadata.
  • ui:// resource URIs and exact text/html;profile=mcp-app MIME types.
  • Tool visibility values for model and app access.
  • Matching declarations, resource content and tool references.
  • Exactly one HTML text or base64 blob payload.
  • CSP domain arrays, exact secure origins and wildcard subdomains.
  • Stable camera, microphone, geolocation and clipboard-write permission shapes.
  • Complete HTML5 document warnings and duplicate identifiers.

This is an independent static checker, not an official conformance certificate. Always run the server against a real host or the reference conformance tooling after structural validation.

Safe workflow

Declare the smallest sandbox surface.

  1. Link the tool. Put a stable ui:// URI in modern tool metadata.
  2. Declare the resource. Use the MCP App HTML profile MIME type.
  3. Bundle where possible. A single HTML file reduces CSP and cache surprises.
  4. Allow only required origins. Keep connect, resource, frame and base URI domains separate.
  5. Test progressive fallback. Return useful text for hosts without MCP Apps support.

Focused references

Debug each MCP Apps boundary.

MCP Apps validator guide

Build a complete JSON snapshot and understand linkage diagnostics.

Read the guide →

MCP App CSP checker

Separate network, asset, frame and base URI origins correctly.

Review CSP →

ui:// resource errors

Fix MIME types, missing resources, visibility and legacy metadata.

Browse errors →

FAQ

Before host testing.

What JSON should I paste?

Use an object containing any combination of tools, resources and resources/read content items. The sample shows the compact shape.

Does a valid report guarantee compatibility?

No. It proves only this static snapshot passed the listed checks. Host behavior, HTML execution and JSON-RPC messaging still require runtime testing.

Are external URLs allowed?

Only origins declared in the appropriate CSP list may be permitted by the host, and a host may further restrict them.

Why is ui/resourceUri deprecated?

The stable extension prefers nested _meta.ui.resourceUri. The flat key remains a compatibility signal but should be migrated.

Is this an official MCP project?

No. MCP App Ready is an independent developer utility based on the public stable specification.