Mutuality of Obligation
One of the three core IR35 tests. True contractors have no obligation to accept work and clients have no obligation to offer it.
Mutuality of obligation (MoO) is one of the three primary employment status tests used in IR35 determinations (alongside substitution and control). It examines whether there is a mutual expectation of ongoing work — which is a hallmark of employment, not contracting.
In a genuine employment relationship, the employer has an obligation to offer work and the employee has an obligation to accept it. For a true contractor, each engagement is discrete: the client is not obliged to offer more work, and the contractor is not obliged to accept if offered.
HMRC's CEST tool has been criticised for not adequately addressing mutuality of obligation. Courts and tribunals have found that many tribunal cases hinge on MoO, particularly where contractors work continuously for the same client over long periods.
For umbrella workers, mutuality of obligation is irrelevant — they are already employees, so IR35 tests do not apply.