This version is in beta - expect some breaking changes.

Self Assessment Assist (MTD) API

Version and status
Available in Sandbox Yes
Sandbox base URL
Available in Production No


The HMRC Assist report is generated using the customers’ National Insurance number (NINO), Calculation ID and Tax year. The report contains targeted messages contained within the JSON response for the customer.

This API allows developers to:

  • retrieve a HMRC Assist report for a given customer with a list of messages for the customer
  • securely generate a report that is sent to or displayed after it has been generated
  • allow the client to acknowledge the report


We use standard HTTP status codes to show whether an API request succeeded or not. They are usually in the range:

  • 200 to 299 if it succeeded, including code 202 if it was accepted by an API that needs to wait for further action
  • 400 to 499 if it failed because of a client error by your application
  • 500 to 599 if it failed because of an error on our server

Errors specific to each API are shown in the Endpoints section, under Response. See our reference guide for more on errors.




View API endpoints
Why do these endpoints look different?

The endpoints for this API now use the Open API Specification (OAS).

The API has not changed. You do not need to make any updates to your application if you already use this API.