Microsoft rival Anthropic accuses OpenAI of misusing Claude to train GPT-5
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Anthropic has blocked OpenAI from using its Claude API, alleging that the GPT-maker violated its terms of service, according to a report by Wired. The move came after Anthropic discovered its rival had integrated Claude into internal tools to test everything from coding to safety prompts.
The company’s spokesperson Christopher Nulty linked the tests to OpenAI’s next major release. “Claude Code has become the go-to choice for coders everywhere, and so it was no surprise to learn OpenAI’s own technical staff were also using our coding tools ahead of the launch of GPT-5,” he told Wired.
He further added, “Unfortunately, this is a direct violation of our terms of service.” OpenAI didn’t deny the use but called it standard practice. The company’s spokesperson, Hannah Wong, told the news outlet that testing competing models “to benchmark progress and improve safety” is common across the industry.
Anthropic doesn’t see it that way. The company argues OpenAI’s activity wasn’t just evaluation. It says OpenAI used Claude in ways that directly support GPT-5’s development, which violates a ban on using the service to build competing tools.
This isn’t the first time API access has become a competitive weapon in tech. Platforms like Facebook and Salesforce have used similar cutoffs to box out rivals. Anthropic has left a small window open. It says OpenAI can still benchmark Claude for safety, but not to develop its next model. Whether OpenAI adjusts or pushes back remains to be seen.
