Clear cookies/site data for railway.app and re-try the connect flow, this error usually shows up when there's a stale OAuth session or cached client registration getting in the way. If that doesn't fix it, try in an incognito window or a different browser to rule out an extension (ad blockers / privacy extensions sometimes block the dynamic client registration request).

Also double check you're not behind a VPN or corporate proxy that could be blocking the request to Railway's auth service, that's a common cause of "couldn't register" specifically since it's failing before you even get to the login prompt.

Railway now documents an official Claude connector flow, so I would avoid manually registering the hosted MCP endpoint as a custom connector. Open Claude’s Railway connector listing, click Connect/Install, then complete the Railway OAuth flow. Railway’s documentation says the official connector uses Railway OAuth against Railway’s hosted MCP server and does not require the Railway CLI or a manually supplied client ID/secret.

If you previously added Railway manually as a custom MCP server, remove/disconnect that entry first, then install the official Railway connector from Claude’s connector directory. After connecting, it should appear under Claude → Customize → Connectors.

If the official connector itself still returns “Couldn’t register with Railway’s sign-in service,” test once in a clean/incognito browser session with popup blocking disabled. If it still fails there, capture the failed OAuth/network request’s HTTP status and timestamp — with tokens/cookies redacted — because at that point the documented client flow is correct and the failure is likely in the OAuth registration/exchange rather than your Railway project configuration.

Official Railway docs: the Claude connector is installed from Claude’s connector directory, authenticates through Railway OAuth, and uses Railway’s hosted MCP server.