Why does the OAuth redirect hang or return missing_scopes?

Two separate things are happening here.

Two separate things are happening here.

The redirect hang: Terra sends the authentication webhook to your server synchronously. If your server takes longer than 30 seconds to respond, the client-side redirect hangs or errors even though the connection succeeded, so make your webhook endpoint respond quickly.

The missing history: webhooks only deliver data generated after the user connects. For earlier data, call the historical endpoints (e.g. /v2/activity, /v2/daily) with a start_date range and to_webhook=true.

Most providers allow full history. Some impose limits: Garmin allows up to 5 years back (subject to Garmin's backfill limits), Coros allows up to 3 months back from now or 24 hours before the user authenticated (whichever is hit first), and Polar allows up to 30 days back but not before the user granted permission.