STAC API aims to define a core of functionality, with richer capabilities enabled by extensions. The list of STAC API Extensions is maintained at https://stac-api-extensions.github.io.
This document explains the process of creating and maturing an extension.
Anyone is welcome to create an extension (see info on this below), and is encouraged to add the extension to the official list by creating an issue or PR on https://github.com/stac-api-extensions/stac-api-extensions.github.io.
All extensions must include a maturity classification, so that STAC API specification users can easily get a sense of how much they can count on the extension.
Each extension has its own conformance class, which is specified with one or more conformance URIs
that are defined for the extension. These must be listed in the conformsTo JSON of the Landing Page,
as specified by STAC API - Core, to let clients know that they can use the functionality.
A number of extensions define functionality that could be used easily in a number of endpoints, such as additional parameters for search through either STAC API - Item Search or STAC API - Features.
Creating new extensions requires creating an OpenAPI fragment to define it, along with a README markdown file that gives an overview of the functionality. In the README, a conformance URI should be provided, so clients can use it to tell if a service has the indicated functionality. It is also recommended to note the 'extension maturity', as defined above, so others can know how widely it is used.
The new extension can live anywhere online, with a recommendation of using a GitHub repository to be able to track changes. The first step in sharing the extension is to add it to the official list, as described above. If it is of something that the wider community may be interested in, then it may be added to the stac-api-extensions GitHub organization.