> 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/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 is the source of truth for field…

**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.