Skip to main content
Your app’s view runs in a sandboxed iframe at a real URL, on an origin separate from the Inspector itself. That matters whenever something outside MCPJam checks where a request came from — a referrer-restricted Google Maps key, an OAuth redirect URI, a CORS allowlist.

​
Which origin to allowlist

The view runs on the Inspector’s scheme and port with the hostname swapped between localhost and 127.0.0.1. The swap is what keeps the sandbox on a different origin from the app.Because the swap depends on which one you open the Inspector with, allowlist both:
To read the exact value rather than infer it, open a tool result, expand the CSP Workbench, and look at Sandbox Stack → View origin. It has a copy button.
Cross-origin requests carry the origin, not the full path — MCPJam sends Referrer-Policy: strict-origin-when-cross-origin. So a third party sees http://127.0.0.1:6274/, and an allowlist pattern keyed on the origin matches. A pattern keyed on a path will not.

​
CSP is a separate gate

Allowlisting your origin tells the third party to accept your requests. It does not let your widget make them — MCPJam’s default policy denies everything not declared. Both have to pass. Declare what the API needs in the UI resource’s _meta.ui.csp:
resourceDomains covers scripts, images, styles and fonts; connectDomains covers fetch/XHR/WebSocket. A JavaScript SDK usually needs both. frameDomains is only for APIs you embed as a nested iframe — an embed widget, not a JS SDK. Anything still blocked shows up in the Workbench’s Findings tab with the directive that refused it.

​
_meta.ui.domain

SEP-1865 lets a server request a dedicated origin with _meta.ui.domain. The format is host-specific and each host derives its own — Claude uses sha256(<connector URL>)[:32] + ".claudemcpcontent.com", ChatGPT a per-plugin label — so one declared string cannot match every host. MCPJam derives the origin it serves your view from rather than routing on what you declare; a server-chosen string would otherwise let one server claim another’s origin, and with it that origin’s cookies and storage. If you declare a domain, the Workbench’s Findings tab reports whether it matches what MCPJam serves. A mismatch is informational — it is the normal state for a server already targeting Claude or ChatGPT — and simply means an allowlist keyed on that value will not match requests coming from MCPJam.

​
Rendering mode

MCPJam mounts a view by writing its HTML into a blank document, the same way Claude does. A document written that way without a doctype is parsed in quirks mode, where the box model and percentage heights differ from what you probably tested against. Start your resource HTML with <!DOCTYPE html>. The readiness report flags this as claude.apps.design.doctype.