Anthropic Blocks OpenAI From Using Claude to Test GPT-5

Anthropic has cut off OpenAI's API access to its Claude models after determining that OpenAI violated Anthropic's terms of service. According to Anthropic, OpenAI used Anthropic's developer APIs internally to test and compare Claude's capabilities in tasks like coding and safety. These tests were part of OpenAI's own work on new AI models like GPT-5.

Anthropic's commercial terms dictate that customers cannot use its services to create or train competing AI models, or to reverse engineer or duplicate them. The company crossed a line as OpenAI integrated Claude into its own systems to evaluate and benchmark the model, especially in categories like code generation, creative writing, and how Claude responded to safety-related prompts, Anthropic claims. Anthropic says that OpenAI will still be able to use the Claude API for benchmarking and safety testing, as is standard practice.

Anthropic spokesperson Christopher Nulty explained that the ...