Boris Cherny states that Opus 5 is Anthropic's least prompt-injectable model yet. This detail is somewhat buried in the model's system card, appearing on page 73.
According to Cherny, across both PI evaluations and red teaming exercises, Opus 5 proved to be very hard to prompt inject successfully. Cherny noted that beyond the various evaluation scores, this resistance to prompt injection is what excites him most about the model.