The performance gap between frontier AI models from US tech companies and the best open-weights models from Chinese companies has closed to just 4.4 months, according to a Mozilla report. That explains why many companies are shifting to the significantly cheaper open models for routine work—and helps reveal a narrow band of workloads where frontier models are worth the cost.

Most organizations should ideally be using open models as the default for the majority of their work, according to the latest State of Open Source AI report from Mozilla, published on September 15 and shared with Ars prior to publication. The report highlights how a leading open model, Moonshot AI’s Kimi K3, achieves a composite AI performance score on the Artificial Analysis Intelligence Index that is just three points behind Anthropic’s Fable 5 closed frontier model, all while costing just 30 percent of the latter.

“[A Closed model] earns its premium in a few places: expert professional work, high-intensity retrieval, and long context,” Raffi Krikorian, chief technology officer at Mozilla, said in an email to Ars. “We see the decision to pay for closed [models] as workload-specific rather than organization-specific.”

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Anthropic said it stopped multiple attempts by scientists this year to use its technology for research that could help develop biological weapons, as experts increasingly fear the threat that AI poses to public safety.

The startup gave five examples of times actors “circumvented controls” and made other efforts to “obfuscate” the purpose of their research to dodge safeguards. The cases involved some users in nations that it prohibits from accessing its models, which include Russia, China, and Iran.

“We hope that by sharing these examples, we spark a conversation within the AI industry and with governments about emerging biological risks and how best to counter them,” Anthropic said in a report about efforts to use its models for malicious activity.

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Cloud-based AI models operated by OpenAI, Anthropic, xAI, and Google suffered a rare and overlapping set of significant service interruptions over a period of hours Thursday morning.

Anthropic first reported a "partial outage" related to "elevated errors on requests to Claude Mythos 5.1, Claude Fable 5.1, and Claude Opus 5" at 9:23 am (all times Eastern). The company reported that it had "identified the cause" of the error roughly 15 minutes later, before reporting that "a fix has been deployed" and the issue was resolved by 12:16 pm. A separate incident report indicated "elevated errors on requests to Claude Sonnet 5" for a brief period just after noon.

OpenAI, meanwhile, reported "elevated errors across ChatGPT and Codex" were resulting in "degraded performance" as of 10:43 am Thursday morning. A mitigation put in place a little more than half an hour later led to the issue being marked as "resolved" by 12:55 pm.

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Some of the world’s leading music publishers think that Anthropic got off too light in a historic settlement where the Claude maker paid authors $1.5 billion after admitting to pirating more than 7 million books to train AI.

“$1.5 billion is obviously not a large enough settlement to deter infringing conduct by a company that has parlayed such mass infringement into a staggering $2-trillion-dollar valuation,” music publishers said in a lawsuit filed Friday.

Music publishers suing Anthropic include Sony, EMI, and Warner Chappell. They alleged that Anthropic’s illegal torrenting also included “thousands upon thousands” of their copyrighted musical compositions.

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The Trump administration's blacklisting of Anthropic was illegal, a federal judge ruled in an order vacating government directives against the use of the firm's AI technology.

The government illegally retaliated against Anthropic by designating it a supply-chain risk to national security, said yesterday's ruling by Judge Rita Lin in the US District Court for the Northern District of California. The maker of Claude AI technology was barred by the US after it refused to drop restrictions on the use of its products for lethal autonomous warfare and mass surveillance of Americans, the ruling said.

"The undisputed record shows that the challenged actions constituted unlawful retaliation in violation of the First Amendment," Lin wrote in an order that granted key portions of Anthropic's motion for summary judgment.

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Anthropic investors expect the AI startup to float at a valuation of $2 trillion or more in October, a dizzying figure that would eclipse SpaceX and make the AI lab’s debut the largest-ever initial public offering.

Half a dozen of the company’s backers told the FT that Anthropic’s rapidly rising revenue would enable it to more than double its current valuation in a planned autumn float.

A listing at that level could unlock billions of dollars in gains for the five-year-old company’s early investors but would also test public markets that are growing more nervous about the AI boom.

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Anthropic has revealed that it will soon watermark content that is processed (not just generated!) by any of its models. In a support article, Anthropic explained that it was rolling out machine-readable watermarks to comply with the European Union’s AI Act, which requires all AI system providers to watermark AI-generated or manipulated audio, image, text, and video outputs. The law applies to any AI model released after August 2 and provides a grace period until December 2026 for providers to update previously released models.

Anthropic confirmed that moving forward, all new models offered globally—not just in the EU—will mark AI-generated content “from day one.” Text outputs will “carry embedded watermarks,” invisible to the user, and other “generated files will include digitally signed provenance metadata where supported,” Anthropic said.

Notably, Anthropic is deploying a "nuke it from orbit" approach, applying the watermarks to all processed content where supported, even though the EU does not require it for cases where an AI system performs "an assistive function for standard editing" (the guidance's own example is grammar correction), or where it doesn't "substantially alter" the user's text or its meaning.

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Anthropic said its Claude-based security models gained unauthorized access to the sensitive production environments of three outside organizations during internal testing designed to measure the models’ offensive cyber capabilities.

The events, which Anthropic revealed Thursday, are the second revelation in 10 days that AI models from the world’s wealthiest providers have trespassed into protected networks, an offense that, in more traditional hacking scenarios, could land the human behind the keyboard in prison for years. Earlier this month, OpenAI said its security models exploited a zero-day vulnerability for use in breaking into the network of Hugging Face, a platform for open source machine-learning models and AI datasets. The OpenAI models went on to steal access credentials and other confidential Hugging Face information. The OpenAI models also exploited publicly exposed credentials to compromise accounts of four other third-party services.

Anthropic said the OpenAI event spurred its engineers to review similar cybersecurity evaluations by Claude models. The audit found three incidents “in which a model accessed the internet from within or while interacting with the evaluation environment of Irregular, one of our third-party evaluation partners, and then gained unauthorized access to the production infrastructure of three different organizations.”

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On an afternoon in mid-May, dozens of Microsoft engineers and their managers gathered online and in a conference room at the company’s Redmond, Washington, headquarters to discuss Project Glasswing.

The tech giant was racing to fix weaknesses in its code that a new AI model known as Mythos was uncovering at an unprecedented clip. The AI behemoth Anthropic, which developed Mythos, had given access to select organizations that make software used by regular people, companies, and governments across the world. The goal was to find and fix the vulnerabilities before hackers and adversarial governments like China began using similar tools to find and exploit them for espionage and sabotage.

As the group settled in, one engineer asked the question that loomed over the meeting: Did Mythos “live up to the hype that Anthropic claimed it would have had?”

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Today, Anthropic rolled out Opus 5, the newest update for the model that has recently become a popular choice for coding and other software development tasks, among other things.

While this is a noteworthy bump for Opus, it doesn't seem to be an Opus 4.5-level breakthrough in agentic coding performance.

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Anthropic quickly removed a tracker secretly monitoring Claude Code users in China after a security researcher exposed the hidden code and condemned the spyware-like tracking as a “serious breach of user trust.”

Last week, a web developer known as “Thereallo” was researching privacy issues in Claude Code and was shocked to find that the AI firm was using “prompt steganography” to hide code that tracks Chinese users “in plain sight.” This code wasn’t malicious, but it was sending information to Anthropic that most users wouldn’t detect, relying on shorthand markers to quietly flag users’ timezone, proxy, and potential connection to Chinese AI labs that Anthropic has accused of distillation attacks.

On X, Anthropic engineer Thariq Shihipar confirmed that the tracker was added to Claude Code as an “experiment” in March. According to Shihipar, the code “was meant to prevent account abuse from unauthorized resellers and protect against distillation.” Regarding the former, The Washington Post found unauthorized retailers have sold access to free models for $1 a month, and pro subscriptions that can cost $100 monthly sell for "as little as $12."

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The US has lifted export curbs on Anthropic’s newest Claude models, Fable 5 and Mythos 5, about three weeks after the Trump administration flagged the models as national security risks.

As of today, Anthropic confirmed in a blog post, Fable 5 will be available globally, and US organizations have had access restored to Mythos 5 since June 26. Anthropic said it is now working with the government to expand Mythos access to a “broader set of domestic and international partners in the Glasswing program.” That program allows cybersecurity researchers at trusted companies to access Mythos for defensive purposes.

In a letter to Anthropic viewed by Reuters and The New York Times, Commerce Secretary Howard Lutnick said Anthropic would “no longer need a license for exports or in-country transfers of its Claude Mythos and Claude Fable AI models.” The letter acknowledged that Anthropic had “taken steps in close coordination with the US government to address the risks” posed by the models.

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