Who decides my IR35 status now - me or my client?

Since April 2021, medium and large private sector clients (and all public sector clients since 2017) are responsible for determining your IR35 status and must issue you a Status Determination Statement. If your client is a small company under the Companies Act thresholds, responsibility for determining status still sits with your own limited company. Either way, you can challenge a status decision if you disagree with it, and it's worth reviewing any determination carefully rather than accepting it automatically.