Anthropic has launched Claude Sonnet 5 and restored access to its Fable and Mythos frontier models following a federal export control review.
The decision marks the conclusion of an eighteen-day operational pause triggered by a US government export control directive on June 12, which forced the temporary suspension of Anthropic’s highest-capability systems.
Government officials enacted the restriction after researchers at Amazon documented a method to bypass the safety controls of Fable 5, causing the model to identify software vulnerabilities and supply exploitation code. Anthropic has since developed an updated automated classifier to patch the vulnerability, clearing the path for a full commercial rollout across its platform, cloud infrastructure, and partner networks.
The temporary suspension of Fable 5 and Mythos 5 highlighted the regulatory pressures facing frontier intelligence systems. When the export control mandate took effect, the lack of real-time nationality verification systems required a total access blackout for all global users.
Security evaluations conducted during the shutdown confirmed that the vulnerability identification behaviour was not unique to Fable 5. Older and less capable architectures from multiple providers, including Claude Opus 4.8, GPT-5.5, and Kimi K2.7, duplicated the exact results.
To resolve the federal directive, engineers trained an automated safety classifier targeting the specific bypass mechanism reported by Amazon. This software layer functions with a wide safety margin, identifying and blocking ambiguous developer prompts that display a statistical probability of malicious intent. Internal validation data indicates the updated classifier prevents the reported exploitation technique in more than 99 percent of trials.
When a developer issues a prompt that triggers this boundary, the platform automatically routes the workload to the older Opus 4.8 architecture to maintain continuity. The expanded safety margin introduces a distinct trade-off for engineering teams, as the automated system flags benign requests more frequently during routine application development and software debugging.
Active deployments and agentic workflows
While frontier models face strict state oversight, the immediate commercial focus targets the newly-deployed Claude Sonnet 5.
Engineering teams are transitioning autonomous agents to this model to reduce operational expenditure while maintaining high execution capacity. Performance data validates that the system executes multi-step plans, operates terminal environments, and navigates web browsers without human intervention.
Model performance and cost metrics:
| Model | SWE-bench Pro | Terminal-Bench 2.1 | Base input cost* | Base output cost* |
|---|---|---|---|---|
| Sonnet 5 | 63.2% | 80.4% | $3.00 | $15.00 |
| Sonnet 4.6 | 58.1% | 67.0% | $3.00 | $15.00 |
| Opus 4.8 | 69.2% | 82.7% | $5.00 | $25.00 |
*Cost per million tokens. Sonnet 5 carries introductory rates of $2.00 input / $10.00 output through August 31, 2026.
Real-world deployments demonstrate how organisations are…
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