> For the complete documentation index, see [llms.txt](https://adidas.gitbook.io/api-guidelines/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://adidas.gitbook.io/api-guidelines/general-guidelines/api-first.md).

# API First

Everyone **MUST** follow the **API First** principle.

The API first principle is an extension of design-first principle. Therefore, a development of an API **MUST** always start with API design without any upfront coding activities.

**API design** (e.g., description, schema) **is the master of truth, not the API implementation.**

API implementation **MUST** always be compliant to a particular API design which represents the [contract](/api-guidelines/general-guidelines/contract.md) between API, and it's consumer.