Continual Learning Requires Evaluating Trajectories

AI systems increasingly incorporate continual learning mechanisms allowing their behaviour to adapt after deployment, from (1) in-context learning and (2) memory features already in wide use to (3) post-deployment weight modification under research. We argue that, by treating AI systems as frozen artefacts whose performance and safety are assessed at release, current evaluation practices structurally ignore the behavioural trajectory of a system that continues to learn from experience. Our position is that evaluation of continual learning systems should be centred on behavioural trajectories, with the complementary goals of characterising the landscape of possible behaviours and forecasting how behaviour will evolve from a given set of experiences. This can be operationalised through trajectory elicitation sandboxes and predictive monitors that forecast behavioural evolution, but may face fundamental obstacles analogous to those seen in dynamical systems. These are best addressed by (1) applying trajectory-centred evaluation to today's continual learning systems and (2) relying on the resulting evidence to design systems amenable to it, yielding a virtuous cycle in which systems and their evaluations co-evolve.