> 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/faq/help-topics/data-api-sdk/data-model-fields-and-units/schema-source-of-truth-extra-fields.md).

# Why is a webhook field not in the documented schema?

**The authoritative schema is the published OpenAPI specification** in the public GitHub repository, and the [REST API reference](https://docs.tryterra.co/reference/health-and-fitness-api/data-models) is the source of truth for field names and units.

On extra fields:

* New fields may be added over time, so **treat additional or unknown keys as non-breaking and ignore them**, rather than validating strictly against a closed schema.
* The REST docs are authoritative. The payload key is `muscle_mass_g` (value in grams). Where the TypeScript client types disagree with the REST docs, **trust the REST docs**.

The [OpenAPI spec](https://docs.tryterra.co/reference/health-and-fitness-api/openapi-spec) is also published in the docs.