When should tests for a new service be designed?

Designing tests at the same time as the service ensures that testability is a design requirement rather than an afterthought. ITIL's Service Design phase explicitly states that testing strategies and acceptance criteria should be developed alongside the service design so that the service can be validated effectively before transition and deployment.

Waiting until after the service is designed means design decisions may have already been made that conflict with testing needs, making it costly or impossible to retrofit testability.

Defining tests only during Service Transition is too late - the service design is already finalized, and altering it to accommodate testing gaps is expensive and disruptive.

Tests cannot be meaningfully specified before any service concept exists, as the service specification directly determines what outcomes, interfaces, and behaviors need to be tested.