Mistakes in complex tasks have compounding costs, unlike minor errors. Anthropic's Claude Opus 5, now on GitLab Duo Agent Platform, is designed for these demanding tasks. Internal GitLab evaluations show Opus 5 resolved 93.3% of benchmark tasks, a significant improvement over Opus 4.8. This advanced model offers deeper reasoning, correctly handling complex tasks like multi-file features on the first attempt. Opus 5 also demonstrated complete task completion, with its solutions being verified correct more often. An example showed Opus 5 fully implementing a complex SSO login feature. In code reviews, Opus 5 accurately flags real bugs, minimizing false positives. It also coordinates multiple agents effectively, preventing conflicts and ensuring smoother parallel workflows. For cost-conscious users, GitLab Credits allow spend caps on parallel agent usage. Opus 5 balances reliability with speed, outperforming previous models on difficult benchmark tasks. While Sonnet models suit routine work, Opus 5 is ideal for complex debugging and large refactors. Users can select Opus 5 directly within their GitLab instance for these high-stakes tasks.