I spent the last while building the HMRC integration for TapTax , a Making Tax Digital (MTD) app for UK sole traders. MTD is the UK government's programme that pushes tax filing out of paper and spreadsheets and into software talking directly to HMRC's APIs. I have integrated with a fair few third-party APIs. Stripe, Plaid-style banking, the usual. HMRC is its own animal. Some of it is genuinely well designed, some of it caught me completely off guard, and a couple of things cost me a full d...