In previous versions, FusionInvoice had a companion download, the FusionInvoice API. It included a few samples that could be the basis for creating your own endpoints allowing for basic operations to be called from external (non-FusionInvoice) systems, like creating new client records, adding invoices or quotes, etc. The implementation of the API made it easy to work with for external systems were PHP based, but more difficult when they were not.
This release includes an entirely rewritten API, which is much more accessible from other applications and can easily be tested with common API testing tools like Postman, cURL, Fiddler, Insomnia, Swagger UI. etc.
We will be providing detailed information about using the new API in upcoming documentation, but in the mean time, if you are using and relying on the previously provided FusionInvoice API, this update will be a breaking change. Please do not update your live FusionInvoice installation until you have the new documentation and have recreated your endpoints using the new API.