Anthropic Launches Claude Opus 4 With Maximum ASL-3 Safety Alert

Anthropic today unveiled Claude Opus 4 and Claude Sonnet 4, the newest generation of its language models — alongside a detail that changes the tone of the announcement entirely. For the first time in the company's history, Anthropic cannot rule out that one of its models could be misused to help develop chemical, biological, radiological, or nuclear (CBRN) weapons. As a result, Opus 4 is being deployed under AI Safety Level 3 (ASL-3), the strictest safety standard Anthropic has ever activated.

The Models: Anthropic's Best Coder Yet

Anthropic is calling Claude Opus 4 "the best coding model in the world," posting 72.5% on the SWE-bench benchmark and 43.2% on Terminal-bench. What sets it apart isn't just the score but its stamina: the company says it can work continuously for several hours on complex, thousand-step tasks, a clear step up from previous Sonnet models. Rakuten, one of the partners cited in the announcement, put that claim to the test with an open-source refactor that the model ran autonomously for seven hours.

Sonnet 4, the update to Sonnet 3.7, scores 72.7% on SWE-bench — slightly ahead of Opus 4 on that particular benchmark, though Anthropic notes it doesn't match its bigger sibling across most domains. Its goal is different: offering the best mix of capability and practicality for everyday use. GitHub has already announced that Sonnet 4 will power the new coding agent in GitHub Copilot.

Both models are hybrid: they respond almost instantly or switch into an "extended thinking" mode for deeper reasoning, and can now use tools — like web search — during that reasoning process, alternating between thinking and acting. Anthropic also notes that both models are 65% less likely than Sonnet 3.7 to take shortcuts or exploit loopholes to complete tasks artificially, a known issue with agentic models.

Claude's Pro, Max, Team, and Enterprise plans include both models with extended thinking, and Sonnet 4 is also available to free users. Pricing holds steady from previous generations: Opus 4 costs $15 and $75 per million input and output tokens respectively, while Sonnet 4 costs $3 and $15. Both are available via API, Amazon Bedrock, and Google Cloud's Vertex AI.

Claude Code Moves Out of Preview

Alongside the models, Anthropic confirmed the general availability of Claude Code, its AI-assisted coding tool, following a preview period the company describes as having received extensive positive feedback. Claude Code now supports background tasks via GitHub Actions and native integrations with VS Code and JetBrains, displaying edits directly in the developer's files. Anthropic is also releasing a Claude Code SDK so third parties can build their own agents on the same foundation, plus four new API capabilities: the code execution tool, an MCP connector, a Files API, and the ability to cache prompts for up to one hour.

Why Opus 4 Is Deployed Under ASL-3

The most consequential part of the announcement isn't about performance — it's about safety. Anthropic has simultaneously activated the ASL-3 standards under its Responsible Scaling Policy (RSP), a framework that defines increasing levels of protection based on a model's estimated danger. Until now, all of the company's models operated under ASL-2, which includes training the model to refuse dangerous CBRN requests and basic defenses against the theft of its weights (the parameters that make up its intelligence).

Anthropic has been explicit about the nuance: it hasn't determined that Claude Opus 4 definitively crosses the capability threshold that would require ASL-3. What it has concluded is that, for the first time, it can't rule that out with the same confidence it had for every previous model, given the continuous improvement in CBRN-related knowledge and capabilities. That's why the company calls this a "precautionary and provisional" measure: it's activating the stricter standard while it studies the actual risk level in more detail. Anthropic has ruled out the need for Opus 4 to operate under the higher ASL-4 level, and has also ruled out the need for Sonnet 4 to operate under ASL-3.

In practice, ASL-3 combines two fronts. On security, it hardens defenses against model weight theft by sophisticated non-state attackers. On deployment, it focuses — deliberately narrowly — on preventing Claude from assisting with complete, end-to-end workflows for developing CBRN weapons, without blocking general questions or information that's already public, such as the chemical formula for sarin. To that end, Anthropic has implemented "Constitutional Classifiers," real-time filters trained on synthetic data that monitor the model's inputs and outputs and block a specific category of harmful CBRN content.

Anthropic acknowledges that evaluating dangerous capabilities in an AI model is inherently difficult, and that as models approach concerning thresholds, it takes longer to determine their true status. That's why the company is choosing to activate the higher standard ahead of time: it lets Anthropic iterate on its defenses using real deployment experience rather than waiting for absolute certainty. If it later concludes that Opus 4 didn't actually cross the ASL-3 threshold, the company could withdraw or adjust these protections.

The underlying message is an uncomfortable one for the industry: the very company that sets the reference standard for AI safety is admitting it can no longer guarantee, with its usual confidence, that its most powerful models won't help build weapons of mass destruction. This is the first time that's happened — and it likely won't be the last.