HMRC rebuilt its losses API in June. Software still on the old version will fail on your 2026-27 losses.

A changelog nobody reads, with real consequences

We run a small monitor that watches HMRC's Making Tax Digital API specification and tells us when it changes. Most weeks it is quiet. On 16 June 2026 it was not: a whole version of the API that records tax losses had moved to production, and the version our first integration was built on had, months earlier, been quietly fenced off from the current tax year.

This is the first in an occasional series where we write those changes down. Not because a landlord should have to care about API versions, but because the gap between “HMRC recognised” and “actually keeping up” is invisible from the outside, and this is a clean example of the difference costing you a failed filing.

What changed, in one paragraph

HMRC released version 7.0 of its Individual Losses (MTD) API to production on 16 June 2026, for tax years 2026-27 onwards. It replaces the two separate resources in version 6.0, brought forward losses and loss claims, with a single group named Losses and Claims and three endpoints: retrieve, create or amend, and delete. Version 6.0 has not been switched off; since 22 January 2026 it simply refuses any tax year after 2025-26.

Put plainly: 6.0 is last year's API. It still works for last year. The moment a landlord has a 2026-27 loss to record, their software has to be speaking 7.0, or the request is rejected before it reaches a human.

Why this matters if you have a property loss

Property losses are ordinary, not exotic. A year of heavy repairs, a long void, a new purchase with early costs, and a property can run at a loss. You carry that loss forward and set it against a later year's profit, which lowers the tax you pay then. Recording it correctly is worth real money.

Under Making Tax Digital, that record goes through this API. If your software tries to lodge a 2026-27 loss using version 6.0, HMRC answers with RULE_TAX_YEAR_FOR_VERSION_NOT_SUPPORTED and does nothing else. There is no partial credit and no silent fallback. The loss is simply not recorded until the software is updated.

So this is not a plumbing detail. It is the difference between a carried-forward loss that reduces your bill and one that never registered because the software was a version behind.

The timeline, dated

Unlike most of HMRC's guidance, this source carries dates. Every entry below is from HMRC's published changelog, read on 21 July 2026.

The pattern is worth noticing. The version cap landed in January, five months before the replacement reached production in June. For anyone reading the changelog, that was five months of warning. For anyone not reading it, the first sign is a rejected filing.

What actually changed in the shape of it

We built our first losses integration against version 6.0, so we can be specific about the size of the change. In 6.0, brought forward losses and loss claims are two separate things, with their own paths and their own operations. Our 6.0 code carried eleven distinct calls to cover them: create, list, retrieve, amend and delete a brought forward loss, and create, list, retrieve, change the type of, delete and reorder a loss claim.

Version 7.0 folds all of that into one resource, Losses and Claims, with three endpoints: retrieve them, create or amend them, and delete them. Eleven calls become three. That HMRC gave the new group a single combined name, where 6.0 had two, is the clearest signal of the merge; we read the collapse from the naming and the endpoint list, since the two old resources no longer appear separately for 2026-27.

One more detail the changelog carried, for the engineers: 7.0 added an optional header in Sandbox, suspend-temporal-validations, that lets you submit an in-year loss while testing. Without it, an in-year submission returns RULE_TAX_YEAR_NOT_ENDED. It is a testing convenience, marked Sandbox only, and it tells you HMRC still expects losses to settle at year end, not mid-year.

How would you even know?

You would not. HMRC does not email taxpayers when it changes an API. It posts to a developer changelog on GitHub, written for the software firms that build against it. A landlord has no reason to read it and no way to know their software is a version behind until a filing bounces.

This is the quiet problem with the phrase “HMRC recognised”. It means a product passed HMRC's testing at some point. It does not mean the product tracks every change HMRC makes afterwards. The losses cap was published in January; the replacement went live in June. A recognised product that stopped paying attention in between would still carry the badge and still fail on a 2026-27 loss.

We run the monitor precisely so the badge and the behaviour do not drift apart. The one question worth asking any Making Tax Digital vendor is narrow and revealing: which versions of the HMRC APIs do you support for 2026-27, and how quickly do you move when they change?

One more thing the monitor caught

A day before the losses release, on 15 June 2026, HMRC added a new endpoint to the Self Assessment Accounts (MTD) API, version 4.0: Retrieve ITSA Penalties. It appears in the changelog as a Sandbox addition, and we have not seen a production entry for it as at 21 July 2026, so treat it as coming rather than live. It is worth flagging anyway: a dedicated endpoint for reading a taxpayer's MTD penalties is the kind of plumbing that appears shortly before penalties themselves start to bite. We will cover it properly when it reaches production.

Common questions

Sources

This series is built on HMRC's own dated changelog, which is more precise than its guidance pages. Everything above was read on 21 July 2026.

• HMRC: income-tax-mtd-changelog (the dated entries for 9 and 22 January, 23 March, 15 and 16 June 2026)
• HMRC: Individual Losses (MTD) API (the version list; 7.0 is the current version)
• HMRC: Self Assessment Accounts (MTD) API (the Retrieve ITSA Penalties endpoint, added in Sandbox)

Related

• What actually happens when your software files an MTD update
• Why your MTD software asks for your device details