Tuleap is a Free and Open Source software developed since 2001 under GPLv2 license. Contributions are welcomed, on Tuleap Core, on plugins, in documentation, etc.
Development of a Tuleap extension, outside of the main development tree is hard and you are likely to shoot yourself in the foot if you do so. The main reason is that Tuleap internal API is not guaranteed, it can change at any moment for any reasons.
REST API & Webhooks, on the other hand are very carefully maintained and we ensure, as much as possible the backward compatibility. We (the core team) strongly recommend to look this way for your developments.
Tuleap integration development
This section covers how you can develop a Tuleap extension without getting your hands dirty with Internal code.
Tuleap core & plugins development
In order to contribute to Tuleap core & plugins, please see dedicated Developer guide.