Why is Garmin historical backfill returning empty?

Garmin historical retrieval requires the historical-data sharing option to be granted on Garmin's consent screen, which can default to OFF (and was defaulted…

Garmin historical retrieval requires the historical-data sharing option to be granted on Garmin's consent screen, which can default to OFF (and was defaulted off for some users who authenticated before a recent provider change). Without it you only get live data going forward.

Garmin also fires its historical backfill once per OAuth subscription, so a reauth that reuses the same credential will not re-deliver history.

The fix: have affected users disconnect and reconnect, ticking the historical-data sharing option during reauthorization.

To pull older Garmin data, call the historical data-retrieval endpoints (e.g. /v2/activity, /v2/menstruation) with a start_date and to_webhook=true. This triggers Garmin's backfill, which Terra then pushes to your destination asynchronously. Garmin allows up to 5 years into the past, subject to Garmin's own backfill limits, so some older activities may not be returned.