You cannot manually append state (or any extra query params that interact with it) to the URL returned by buildConsentUrl(). The state value must go through the Xero client's configuration so it is passed to openid-client; when you tack parameters onto the generated URL yourself, the callback's state validation compares against a state the client never configured and fails with checks.state argument is missing. This is deliberate CSRF protection, not a bug. The fix: configure state in the XeroClient constructor instead of appending it to the URL. Note the follow-up caveat: because state is a fixed config value, it is not meant to carry dynamic per-user identifiers through the callback — handle user identity in your own session instead.
Source: https://github.com/XeroAPI/xero-node/issues/514
Source: https://github.com/XeroAPI/xero-node/issues/514