“Refused to load the script”

The host built the iframe's CSP from the view's _meta.ui.csp, and the view asked for something from an origin that list does not allow; the browser blocked it. The three common causes: (1) The origin is not declared: resourceDomains for scripts, styles, images and fonts; connectDomains for fetch and WebSocket; frameDomains for iframes. With no _meta.ui.csp at all the host applies a default that allows nothing external. To tell: The doctor fails at resource-csp-undeclared (or warns at resource-csp-connect) and names the origin and the list it belongs in. (2) The CSP is declared where this host does not read it: only under _meta["openai/widgetCSP"] (ChatGPT's legacy key), or only on the resources/list entry and not on the resources/read content. To tell: The doctor warns at resource-legacy-csp or resource-meta-location. (3) The host does not apply a list you did declare: claude.ai's sandbox was seen dropping resourceDomains, so declared external images showed broken. To tell: The doctor passes, the origin is declared, and the block happens in one host only. One command shows which: npx --allow-git=root github:agentwares/mcp-apps-doctor https://your-server.example/mcp.

Check your server now

npx --allow-git=root github:agentwares/mcp-apps-doctor https://your-server.example/mcp

Discovery only: no credential, no tool called. It prints the first broken step, whose it is, and the fix with its source, and shows the view in a sandboxed preview.

Who prints it

The browser, in the view's iframe console: `Refused to load the script '…' because it violates the following Content Security Policy directive: "script-src …"`.

The three causes, and how to tell them apart

1. Cause 1. The origin is not declared: resourceDomains for scripts, styles, images and fonts; connectDomains for fetch and WebSocket; frameDomains for iframes. With no _meta.ui.csp at all the host applies a default that allows nothing external.
   The check flags it at resource-csp-undeclared. The doctor fails at resource-csp-undeclared (or warns at resource-csp-connect) and names the origin and the list it belongs in.
2. Cause 2. The CSP is declared where this host does not read it: only under _meta["openai/widgetCSP"] (ChatGPT's legacy key), or only on the resources/list entry and not on the resources/read content.
   The check flags it at resource-legacy-csp. The doctor warns at resource-legacy-csp or resource-meta-location.
3. Cause 3. The host does not apply a list you did declare: claude.ai's sandbox was seen dropping resourceDomains, so declared external images showed broken.
   The check cannot see this one from outside: it happens inside the host or the portal. The doctor passes, the origin is declared, and the block happens in one host only.

Sources and public reports

• hosts MUST construct the CSP from the declared domains, and MUST NOT allow undeclared ones [source: MCP Apps spec 2026-01-26, UI Resource Format, read 8 October 2026]
• by default the frame only runs scripts that are inline or from its own origin [source: Claude, Add an interactive UI to your MCP server, read 8 October 2026]
• claude.ai's sandbox proxy did not apply resourceDomains [observed in modelcontextprotocol/ext-apps#761, opened 28 August 2026]

Keep checking

agentcheck's free watch checks this server every hour and emails when it stops answering or its tool list changes; no account. It does not re-read the ui:// views.

mcpcheck Server Pro diffs the server's tool catalog every night — removed tools, tightened schemas, changed descriptions — with the client-compat matrix, OAuth conformance and 90 days of history; $49 a server a month, first run free. It does not re-run these MCP Apps checks.

Other messages

• “MCP App renders blank”
• “Resource not found: ui://”
• “Tool has no UI”
• “Invalid ui.domain format”
• “ui.domain mismatch”
• “annotations_required”
• “domain_verification_required”
• “frame_domain_explanation_required”
• “screenshots_not_allowed”
• “Failed to fetch template”

Written 8 October 2026 from the sources above. The MCP Apps check · MCP Liveness · Terms · Privacy