Legacy tax endpoints shut down on 16 October, leaving coders short on time to update client tools
Are you a freelance coder building or maintaining UK accounting software? You have a hard technical deadline coming fast.
HMRC will permanently turn off older Self Assessment API endpoints on 16 October 2026. The retirement date was announced in the official GOV.UK MTD newsletter for developers.
Any custom software using legacy versions will break on that date. Developers must finish their MTD API migration for software developers before the autumn cutoff.
HMRC has also introduced a new 403 Client_Not_MTD_Enrolled error code to the individual details endpoint. This response triggers whenever an un-enrolled client attempts an MTD action.
Your code needs to catch this error cleanly. Show users clear instructions on how to complete HMRC enrolment before trying again.
What MTD developers need to do next
✅Review active endpoints: Audit your codebase for deprecated Self Assessment API calls
✅Update your API versions: Move all requests to current production endpoints
✅Handle new error codes: Write logic to catch 403 enrolment errors during onboarding flows
✅Test in sandbox: Run full integration tests on the HMRC Developer Hub before October
You can also join the HMRC Making Tax Digital (MTD) Technical Forum on 8 September online via the HMRC Developer Hub to clear up remaining integration questions. Updating your code now keeps your apps online and fully compliant.
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