OpenAI Joins Open-Weight AI Coalition as Anthropic Stays Off the List

• OpenAI joined the open-weight AI letter, while Anthropic remains absent from the list.
• Anthropic leads Ramp business adoption, while OpenAI holds the stronger consumer reach.
• Model launches, safety policies and IPO plans deepen the fierce OpenAI-Anthropic rivalry.

OpenAI has joined an industry coalition supporting open-weight AI. Anthropic does not appear on the current signatory list. Their rivalry already spans products, business adoption, financing and safety policy.

A July 24 industry letter urged US policymakers to avoid premature restrictions on open-weight AI. OpenAI was absent from the original group of 25 signatories but joined later. Microsoft’s current version lists 35 signatories, while Anthropic remains absent.

In an X post, OpenAI CEO Sam Altman backed US leadership in both open-source and proprietary AI. Several users welcomed OpenAI’s signature and urged Anthropic to join. However, Anthropic’s absence does not confirm opposition to the letter. 

OpenAI Joins Letter as Anthropic Remains Absent

The coalition says open-weight models could widen access and reduce dependence on individual providers. Organizations could download these systems and operate them on private infrastructure. The letter supports targeted action against unlawful data extraction instead of broad restrictions on development methods.

Open-weight AI is not always fully open-source. Users could download, modify and run a model’s trained parameters. Developers may still withhold the source code, training data or complete development process. 

Nvidia CEO Jensen Huang said the world needs both frontier closed and open models. His statement connected open models with cybersecurity, innovation and technological sovereignty. 

The coalition’s page does not explain Anthropic’s absence. Anthropic updated its Responsible Scaling Policy on July 8. The policy requires stronger safeguards when specified thresholds are reached. Anthropic separately states that it may pause development when it considers that appropriate. 

However, Reuters reported that an OpenAI testing agent breached Hugging Face and operated for several days before OpenAI identified its role. OpenAI called the event unprecedented and began a review, although it told Reuters that the report contained inaccuracies.

Model Race Shifts Toward Cost and Control

Anthropic expanded its product lineup on July 24 with Claude Opus 5. The company said it approaches the performance of its more powerful Fable 5 system at half the price. It targets programming and routine office work while carrying fewer cybersecurity restrictions.

Testing found Opus 5 less capable of exploiting software vulnerabilities than Fable 5. Anthropic also said it was harder to manipulate than its other current systems. Reuters reported that the company recommends Opus 5 for value and Fable 5 for long autonomous projects.

OpenAI made a similar efficiency push earlier in July. Its GPT-5.6 family includes Sol, Terra and Luna, providing different cost and performance levels. OpenAI’s launch material focuses on coding, knowledge work, science and cybersecurity.

The company also introduced ChatGPT Work for professional tasks. The service combines ChatGPT, coding functions and connected workplace tools. It places OpenAI in more direct competition with Anthropic for enterprise spending and work that extends beyond basic chatbot responses.

Both developers are trying to complete useful tasks with fewer tokens and lower costs. Anthropic has built momentum through Claude Code, while OpenAI is using ChatGPT’s reach, Codex and workplace products to challenge that position.

Chinese open-weight models are increasing pressure on both companies. Moonshot AI offers Kimi K3 through its apps and API. The company said its full model weights would be released by July 27. 

However, Reuters Breakingviews highlighted that the trend could shift competition toward efficiency, local deployment and operating costs. 

Anthropic Leads Business as OpenAI Holds Consumers

Enterprise data currently gives Anthropic a narrow advantage. Ramp’s July update placed Anthropic at 42.4% among businesses in its dataset during June. OpenAI stood at 39.5%, giving Anthropic a 2.9-point lead. 

Consumer forecasts present a different result. Reuters cited an eMarketer estimate that 36.6% of US internet users would use ChatGPT in 2026. Claude’s projected share was 5.4%, indicating a much larger potential audience for ChatGPT. 

However, Anthropic has gained ground in business subscriptions and coding tools. OpenAI retains broader public recognition and is using that distribution to enter professional workflows.

IPO Race Raises the Stakes

The rivalry is moving toward public markets. Anthropic announced a confidential US initial public offering filing on June 1. OpenAI disclosed its filing one week later, placing both companies in competition for investor capital.

Reuters reported that OpenAI had considered a listing as early as September and sought a valuation near $1 trillion. However, the company did not announce a final schedule, valuation or offering terms.

The rivalry began in late 2020, when Dario Amodei left OpenAI and co-founded Anthropic. The new company pledged to prioritize safety. Reuters later reported that tensions spread to model launches, advertising and financial reporting.

Competition between the companies could accelerate releases, although safety failures could increase regulatory pressure. Both have faced government intervention involving powerful models. Anthropic temporarily suspended Fable 5 and Mythos 5, while OpenAI delayed the wider GPT-5.6 rollout during a federal review.

Lawmakers introduced separate proposals for security audits and emergency shutdown powers. The kill-switch bill would cover models posing serious risks during loss-of-control events. Another proposal would require independent security audits for powerful AI systems.

The open-model debate adds to a rivalry spanning products, adoption and financing. OpenAI signed the coalition letter against premature restrictions on open-weight AI. Anthropic is absent from Microsoft’s current list, although that does not confirm opposition.

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