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Developing a Backend

Soapbox expects backends to implement the Mastodon API.

At the very least:

Soapbox uses feature-detection on the instance to determine which features to show. By default, a minimal featureset is used.

Feature detection

First thing, Soapbox fetches GET /api/v1/instance to identify the backend. The instance should respond with a version string:

  "title": "Soapbox",
  "short_description": "hello world!",
  // ...
  "version": "2.7.2 (compatible; Pleroma 2.4.52+soapbox)"

The version string should match this format:


The Regex used to parse it:

/^([\w+.]*)(?: \(compatible; ([\w]*) (.*)\))?$/
  • COMPAT_VERSION - The highest Mastodon API version this backend is compatible with. If you're not sure, use a lower version like 2.7.2. It MUST follow semver.
  • BACKEND_NAME - Human-readable name of the backend. No spaces!
  • VERSION - The actual version of the backend. It MUST follow semver.

Typically checks are done against BACKEND_NAME and VERSION.

The version string is similar in purpose to a User-Agent string. The format was first invented by Pleroma, but is now widely used, including by Pixelfed, Mitra, and Soapbox BE.

See features.ts for the complete list of features.

Forks of other software

If your software is a fork of another software, the version string should indicate that. Otherwise, Soapbox will use the minimal featureset.

Forks of Mastodon

Mastodon forks do not need the compat section, and can simply append +[NAME] to the version string (eg Glitch Social):


Forks of Pleroma

For Pleroma forks, the fork name should be in the compat section (eg Soapbox BE):

2.7.2 (compatible; Pleroma 2.4.52+soapbox)

Adding support for a new backend

If the backend conforms to the above format, please modify features.ts and submit a merge request to enable features for your backend!