> For the complete documentation index, see [llms.txt](https://docs.tryterra.co/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tryterra.co/help-center/help-topics/data-api-sdk/data-model-fields-and-units/null-fields-are-provider-dependent.md).

# Why are so many data fields null?

Null or empty fields almost always mean the provider's API does not expose that metric (even if it appears in their consumer app), not a Terra error.

Null or empty fields almost always mean the **provider's API does not expose that metric** (even if it appears in their consumer app), not a Terra error. Terra cannot request extra metrics from a provider: new fields appear only once the provider adds them to their API, after which Terra standardises and surfaces them.

Common examples:

* Scores and strain are provider-dependent (Apple does not expose them).
* Oura records HRV only during sleep, so daily `avg_hrv_sdnn` is never populated (`rmssd` appears in sleep payloads).
* WHOOP does not expose detailed HR samples, so `total_stress_score` is null.
* `duration_asleep_state_seconds` can be null even with stage breakdowns, in which case asleep duration is effectively the sum of the stages.
* `device_data.serial_number` is usually empty because manufacturers rarely expose it.
* Sleep stages are supported via `hypnogram_samples`, but extra biometrics depend on what the integration pulls.

To see exactly which fields a provider populates, **use the Payload Simulator** on the [dashboard](https://github.com/tryterra/api-faqs/blob/main/dashboard/README.md) or check the [provider data-availability documentation](https://docs.tryterra.co/reference/health-and-fitness-api/supported-integrations). Where a received payload differs from the docs, the payload is the source of truth. Unsupported fields can be logged as feature requests for the integration team.