I just had trouble diagnosing a LanguageClient which did not resolve or reject the onReady promise, and I could not find an exception noted anywhere.

Similar to this issue - there was an exception in initializeFeatures https://github.com/Microsoft/vscode-languageserver-node/blob/020764e3eacc76138ffbf63f09489adbfb9e1be8/client/src/client.ts#L2614

Is this as simple as moving then(..., ...) to then(...).catch(...) ?