MCP App CSP Explained: Why Your Widget Won't Render

You built an MCP App. The tool works. The server returns data. But the widget renders as a blank iframe.

You've hit the #1 problem in MCP App development: Content Security Policy.

This post explains exactly how CSP works in MCP Apps, what the three domain arrays do, the mistakes that cause silent failures, and how to debug them. By the end, you'll never stare at a blank widget again.

The sandbox model

Every MCP App widget runs inside a sandboxed iframe. On ChatGPT, that iframe lives at a domain like yourapp.web-sandbox.oaiusercontent.com. On Claude, it's computed from a hash of your server URL. On VS Code, it's host-controlled.

The sandbox blocks everything by default. No external API calls. No CDN images. No Google Fonts. No WebSocket connections. Nothing leaves the iframe unless you explicitly declare it.

You declare allowed domains in _meta.ui.csp on your MCP resource. The host reads this and sets the iframe's Content Security Policy. If a domain isn't declared, the browser blocks the request before it even happens.

Here's what a declaration looks like:

_meta: {

  ui: {

    csp: {

      connectDomains: ["https://api.example.com"],

      resourceDomains: ["https://cdn.example.com"],

    }

  }

}

Simple enough. But the devil is in knowing which array to put each domain in.

The three domain arrays

connectDomains — runtime connections

Controls: fetch(), XMLHttpRequest, WebSocket, EventSource, navigator.sendBeacon()

Maps to the CSP connect-src directive.

Use when: your widget calls an API at runtime.

// This fetch will be BLOCKED unless api.stripe.com is in connectDomains

const charges = await fetch("https://api.stripe.com/v1/charges");



// Same for WebSockets

const ws = new WebSocket("wss://realtime.example.com/feed");

resourceDomains — static assets

Controls: <script src>, <link rel="stylesheet">, <img src>, <video>, <audio>, @font-face, CSS url(), @import

Maps to CSP script-src, style-src, img-src, font-src, media-src.

Use when: your widget loads assets from external CDNs.

<!-- These will be BLOCKED unless the domains are in resourceDomains -->

<script src="https://cdn.example.com/chart.js"></script>

<link href="https://fonts.googleapis.com/css2?family=Inter" rel="stylesheet">

<img src="https://res.cloudinary.com/demo/image/upload/sample.jpg">

frameDomains — nested iframes

Controls: <iframe src>

Maps to CSP frame-src.

Use when: your widget embeds third-party content like YouTube videos, Google Maps, or Spotify players.

<!-- BLOCKED unless youtube.com is in frameDomains -->

<iframe src="https://www.youtube.com/embed/abc123"></iframe>

Without frameDomains, nested iframes are blocked entirely. Note that ChatGPT reviews apps with frameDomains more strictly — only use it when you actually embed iframes.

The five mistakes that break your widget

1. Using resourceDomains for API calls

This is the most common mistake. Your widget calls fetch() to an API, and you put the domain in resourceDomains because "it's a resource." It isn't — fetch() is a runtime connection.

// Wrong: API domain in resourceDomains

csp: {

  resourceDomains: ["https://api.example.com"]

}



// Correct: API domain in connectDomains

csp: {

  connectDomains: ["https://api.example.com"]

}

The rule: if your JavaScript code calls it at runtime, it goes in connectDomains. If an HTML tag loads it as a static asset, it goes in resourceDomains.

2. Forgetting the font file domain

Google Fonts is a two-domain system. The CSS is served from fonts.googleapis.com, but the actual font files (.woff2) come from fonts.gstatic.com. If you only declare the first, the CSS loads but the fonts don't.

// Wrong: CSS loads, fonts don't

csp: {

  resourceDomains: ["https://fonts.googleapis.com"]

}



// Correct: both domains declared

csp: {

  resourceDomains: [

    "https://fonts.googleapis.com",

    "https://fonts.gstatic.com"

  ]

}

Your widget will render with fallback system fonts — a subtle visual bug that's easy to miss during development but obvious to users.

3. Missing the WebSocket protocol

WebSocket connections use wss://, not https://. If you declare the HTTPS version, the WebSocket connection still fails.

// Wrong: wss:// connections are still blocked

csp: {

  connectDomains: ["https://realtime.example.com"]

}



// Correct: use the wss:// scheme

csp: {

  connectDomains: ["wss://realtime.example.com"]

}



// Also correct: declare both if you use both

csp: {

  connectDomains: [

    "https://api.example.com",

    "wss://realtime.example.com"

  ]

}

4. Services that need both arrays

Some services serve both static assets AND API responses from the same or related domains. Mapbox is a classic example — it serves API responses (tile coordinates) and image tiles (actual map pictures) from the same origins.

// Wrong: only connect, map tiles don't render

csp: {

  connectDomains: ["https://api.mapbox.com"]

}



// Correct: both connect and resource

csp: {

  connectDomains: ["https://api.mapbox.com"],

  resourceDomains: ["https://api.mapbox.com"]

}

Other services that commonly need both: Cloudinary (API + image CDN), Firebase (API + hosting), Supabase (API + storage).

5. Works in dev, breaks when published

ChatGPT has a more relaxed CSP in developer mode. When you publish your app, stricter rules apply. Two things that catch people:

Missing _meta.ui.domain. Developer mode works without it. Published mode requires it — this is the domain ChatGPT uses to scope your widget's sandbox origin.

_meta: {

  ui: {

    domain: "https://myapp.example.com",  // required for published apps

    csp: { /* ... */ }

  }

}

Missing openai/widgetCSP. Some published apps need the ChatGPT-specific CSP format alongside the standard _meta.ui.csp:

_meta: {

  ui: {

    csp: {

      connectDomains: ["https://api.example.com"],

      resourceDomains: ["https://cdn.example.com"]

    }

  },

  // ChatGPT compatibility layer

  "openai/widgetCSP": {

    connect_domains: ["https://api.example.com"],

    resource_domains: ["https://cdn.example.com"]

  }

}

Note the naming difference: connectDomains (camelCase) in the standard spec vs connect_domains (snake_case) in the ChatGPT extension.

How to debug CSP violations

When CSP blocks a request, the browser logs it to the console. Here's how to find it:

1. Open DevTools (F12 or Cmd+Opt+I)


2. Go to the Console tab


3. Look for red errors starting with Refused to
   

The error message tells you exactly what was blocked:

Refused to connect to 'https://api.example.com/data'

because it violates the following Content Security Policy directive:

"connect-src 'self'"

This tells you:

• 
  What was blocked: https://api.example.com/data
  


• 
  Which directive: connect-src — you need connectDomains
  


• 
  Current policy: only 'self' is allowed — the domain isn't declared

For font issues:

Refused to load the font 'https://fonts.gstatic.com/s/inter/...'

because it violates the following Content Security Policy directive:

"font-src 'self'"

This means fonts.gstatic.com needs to be in resourceDomains.

Debugging checklist

When your widget is blank or partially broken:

1. Open DevTools Console — look for Refused to errors


2. For each error, identify the directive (connect-src, font-src, script-src, etc.)


3. Map the directive to the right array:
   
   
   
   
   • 
     connect-src → connectDomains
     
   
   
   • 
     script-src, style-src, img-src, font-src, media-src → resourceDomains
     
   
   
   • 
     frame-src → frameDomains
     
   
   
   
   


4. Add the blocked domain to the correct array


5. Restart your MCP server and test again

Copy-paste patterns

Here are CSP declarations for common use cases:

API calls only:

csp: {

  connectDomains: ["https://api.yourbackend.com"]

}

CDN images:

csp: {

  resourceDomains: ["https://cdn.yourbackend.com"]

}

Google Fonts:

csp: {

  resourceDomains: [

    "https://fonts.googleapis.com",

    "https://fonts.gstatic.com"

  ]

}

Full stack — API + CDN + Fonts:

csp: {

  connectDomains: ["https://api.yourbackend.com"],

  resourceDomains: [

    "https://cdn.yourbackend.com",

    "https://fonts.googleapis.com",

    "https://fonts.gstatic.com"

  ]

}

Mapbox maps:

csp: {

  connectDomains: [

    "https://api.mapbox.com",

    "https://events.mapbox.com"

  ],

  resourceDomains: [

    "https://api.mapbox.com",

    "https://cdn.mapbox.com"

  ]

}

Embedded YouTube:

csp: {

  frameDomains: ["https://www.youtube.com"]

}

Other sandbox restrictions

CSP isn't the only thing the sandbox blocks. These browser APIs are also restricted inside MCP App iframes:

• 
  localStorage / sessionStorage — may throw SecurityError. Use in-memory state instead.


• 
  eval() / new Function() — blocked by default. Some charting libraries use eval() internally — check before picking a dependency.


• 
  window.open() — blocked. Use the MCP Apps bridge for navigation.


• 
  document.cookie — no cookies in sandboxed iframes.


• 
  navigator.clipboard — blocked.


• 
  alert() / confirm() / prompt() — blocked.

If your widget depends on any of these, it will fail silently even if your CSP is perfect.

Platform differences

The MCP Apps spec is standard, but each host implements it differently:

If you're building for multiple platforms, test on each. A widget that works on ChatGPT might fail on Claude or VS Code due to these subtle differences.

Skip the debugging entirely

Getting CSP right by hand is tedious. Every time you add a new external dependency — a font, an analytics script, an API endpoint — you need to update _meta.ui.csp and hope you picked the right array.

MCPR is an open-source MCP proxy that handles this for you. It sits between the AI client and your MCP server, reads your _meta.ui.csp declarations, and injects the correct CSP headers automatically — so you declare once and it works across ChatGPT, Claude, and VS Code.

cargo install mcpr

mcpr --config mcpr.toml

If you don't want to self-host, MCPR Cloud gives you a managed tunnel with a free subdomain. Claim yours at cloud.mcpr.app and start proxying in minutes — CSP handled, auth included, every tool call observable.

• Star us on GitHub


• Get started with MCPR

TL;DR:

• 
  connectDomains = fetch(), WebSocket, XHR (runtime connections)


• 
  resourceDomains = images, fonts, scripts, stylesheets (static assets)


• 
  frameDomains = nested iframes (use sparingly)


• Debug with DevTools Console — look for "Refused to" errors


• Google Fonts needs both fonts.googleapis.com AND fonts.gstatic.com
  


• Test on every platform you ship to — they're all slightly different