CEST (Check Employment Status for Tax) is HMRC's online tool for working out whether an engagement is inside or outside IR35. It asks a series of questions about substitution, control and financial risk, and was updated on 30 April 2025 to add a dedicated mutuality of obligation question and an upfront "is there a contract" gate.
HMRC says it will stand behind a CEST result where the inputs are accurate, consistent with the actual working practices, in line with the guidance, and there is no avoidance. That makes CEST a useful first screen and a valuable audit-trail document for an engagement.
The promise has real limits, however. CEST does not bind a tribunal; it is only as good as the answers, so a determination that does not match the real working practices is worthless; and its treatment of mutuality of obligation is narrower than the case law, so it can return "outside" or refuse to decide on facts a tribunal might view differently. Never treat a CEST "outside" result as a guarantee. It should always be backed by a contract review and a working-practices assessment. See our SDS entry for how a client records its conclusion under Chapter 10.