Claude Fable 5, a Mythos-class model with robust safeguards from Anthropic, is now available on GitLab Duo Agent Platform. For developers, platform engineers, and engineering leaders, this is not an incremental model update. Claude Fable 5 completes multi-step, goal-directed work that previous models could not sustain, and it does so with measurably fewer iterations. Duo Agent Platform features, including Duo Agentic Chat and Duo foundational agents, benefit from Claude Fable 5's expanded capabilities. Claude Fable 5 is available across all tiers and all deployment models through GitLab's AI Gateway.Get accurate results on the first attemptClaude Fable 5 brings a measurable improvement in first-shot correctness on complex, well-specified problems. Early testers reported single-pass implementations of systems that previously took days of iteration with prior models.In GitLab Duo Agentic Chat, this translates to fewer rounds of back-and-forth. When you describe what you need with specificity, the response is more likely to match your intent and produce working results. This matters most on the tasks where iteration cost is highest: multi-file refactors, incident investigation, and infrastructure-as-code definitions.Claude Fable 5 also interprets dense technical images, web applications, and detailed screenshots with significantly higher accuracy than prior models, often using fewer output tokens.Run longer, more reliable AI agent workflowsThe most significant shift in Claude Fable 5 is long-horizon autonomy. The model sustains productive output over extended periods, successfully completing multi-day, goal-directed runs while maintaining instruction adherence across millions of tokens.For teams using GitLab Duo agents on the platform, the agent workflows that previously required manual checkpoints or re-prompting can now run to completion. Claude Fable 5 self-corrects through verification loops, catches its own mistakes, and stays on-task across complex multi-step operations. It dispatches and sustains parallel sub-agents far more reliably than previous models, so workflows that fan out across multiple repositories or services hold together.Engineering leaders evaluating AI adoption should note the operational implication: Claude Fable 5 reduces the human oversight cost per agent run. Teams can assign harder problems to agents, check results asynchronously, and trust that the output reflects genuine progress rather than fabricated status reports.Find more bugs and resolve incidents fasterBug-finding recall is noticeably higher than previous models. Outage triage, root cause analysis, and repository history investigation all show improvement.When reviewing merge requests through GitLab Duo Agent Platform, Claude Fable 5 catches issues that prior models missed. It reasons through code paths with greater depth, identifies edge cases more consistently, and produces code review comments that are actionable rather than generic. For platform engineers managing CI/CD workflows, this means fewer defects reaching production and faster mean time to resolution when something breaks.Understand what changed with data handlingClaude Fable 5 belongs to Anthropic's Mythos capability class. Models at this level carry new data retention requirements: Prompts and completions are retained for 30 days for trust and safety purposes.Anthropic has published a detailed security white paper covering the controls in place. Key points for enterprise customers are as follows:Retained data is not used to train Claude. No person at Anthropic can read the data by default; only automated safety systems process it.Customers can add their own encryption keys and retrieve tamper-proof audit logs of any human access. Data is automatically and permanently deleted after 30 days.GitLab customers can access Claude Fable 5 through three paths: Direct API, Google Cloud, and Amazon Bedrock. For customers accessing Claude through Google Cloud or Amazon Bedrock, retained data stays in your cloud environment. Claude Fable 5 is available across all tiers and deployment models, including SaaS and self-managed instances. Customers on zero data retention (ZDR) configurations are excluded, as the 30-day retention requirement for Mythos-class models cannot be waived.Start with your hardest unsolved problemIf you test Claude Fable 5 only on routine tasks, you will miss its most significant capabilities. Anthropic's own testing found that the teams seeing the best outcomes point Claude Fable 5 at problems they assumed previous AI models could not handle, then let it scope, ask clarifying questions, and execute.On GitLab Duo Agent Platform, try assigning a complex, multi-file refactor to a GitLab Duo agent. Use Duo Agentic Chat to investigate a production incident across your repository history, or to tackle an implementation you would normally write by hand because you weren’t confident AI would get it right.Claude Fable 5 is available on GitLab Duo Agent Platform starting June 9, 2026.Try GitLab Duo Agent Platform todayYou can experience the benefits of Claude Fable 5 by starting a free trial of GitLab Duo Agent Platform.If you are already using GitLab in the free tier, you can sign up for GitLab Duo Agent Platform by following a few simple steps.And if you are an existing subscriber to GitLab Premium or Ultimate, you can take advantage by simply turning on Duo Agent Platform and start using GitLab Credits that are included with your subscription.