5 things that surprised me building on HMRC's Making Tax Digital API

I spent the last while building the HMRC integration for . It tells you exactly which header is missing or malformed. Build against it from day one.

3. Quarterly updates are cumulative, not "this quarter"

You file four times a year, so the obvious model is "four quarters, four payloads, each holding that quarter's numbers." That is wrong, and it is the kind of wrong that passes your first test and then quietly corrupts everything.

Each quarterly update is cumulative: year-to-date from 6 April, not the delta for the quarter that just closed (