An effective diagnostic test can play a key role in language learning, allowing strengths and weaknesses in students' linguistic development to be identified and addressed. This paper describes the online Cambridge English diagnostic test, assessing English grammatical knowledge at A2 level. As most language tests focus on proficiency or achievement, relatively little research into diagnostic assessment has been conducted, and no real agreement exists on what it entails. For learners of approximately 15 years old, the test provides diagnostic feedback on seven grammatical categories at individual and class levels, to improve curriculum and lesson planning and accommodate students' needs. The test was trialled internationally, using surveys and focus groups. This paper outlines the trial and planned modifications for the next version, in addition to implications for wider practice. It should be clarified that this is a first iteration of the diagnostic test, rather than a fully established final model.