We highly encourage fixes and new features made to the code be submitted to the source code repositories via Github’s pull requests.
SNAP API Documentation
For developers who want to extend or patch SNAP we provide the Javadoc here:
The Sentinel Toolboxes and full source code is open source software, and is distributed freely under the GNU GPL license.
The Developers Wiki contains valuable resources for getting familiar with the software internals, how-tos for making your own modules, Python API, contributing to the software, etc.
There is a dedicated section in the Forum for development oriented topics.