2 things appear to have fixed it: The externally_connectable permission specified in the manifest.json file should not be wrapped in curly braces, unlike what the WWDC documentation displays "externally_connectable": { "matches": [ "*://mywebsite.com/*", "*://localhost:3000/*" ] } instead of { "externally_connectable": { "matches": [ "*://mywebsite.com/*", "*://localhost:3000/*" ] } } Don't include port numbers in the URL, even if you're on localhost "matches": ["*://localhost/*"]