I saw a post from 2012 asking for this, in fact: FreeAgent V2 Unsupported Operations - #4 by Chris_Page
It’s quite frustrating, getting on for 8 years later, to still not have any efficient way of retrieving an invoice by reference. For the end users (including customers), the reference will be the most common and natural way they refer to the invoice - so not having an easy way to use that to do a lookup makes development much less straightforward.
It also can’t be good for FreeAgent server load - to have to repeatedly respond to requests for the list of all invoices, just to track down an invoice with a matching reference, is surely a waste of resources at the API end.
FreeAgent team - could this please be added to the feature request list? Something like the following would be ideal: