Elon Musk stated on X Tuesday that xAI's upcoming Grok 4.8 model will not match Anthropic's Claude Opus 5.5, calling the rival AI model a 'masterpiece.' The admission came in response to a user question comparing the two models, with Musk acknowledging his company's next release would be 'not as good' as Anthropic's offering. Musk added that Grok 4.8 has finished pretraining and could begin a limited rollout to Bot users this week following a rewrite of xAI's software infrastructure. The statement highlights xAI's position as it works to catch up with established competitors Anthropic and OpenAI in the AI model development race.
Musk Acknowledges Grok 4.8 Will Not Match Claude Opus 5.5
Asked how Grok 4.8 would compare with Anthropic's Claude Opus 5.5, Musk answered it would be 'not as good.' He posted 'Opus 5.5 is a masterpiece' on X in reply to a question from user Dimdv. Musk added that Grok 5, a next generation model to be released after this update, 'should exceed Opus 5.5,' potentially in two or three months, though he gave no further details for that prediction or a firm release date.
xAI Completes Grok 4.8 Pretraining and Rewrites Software Stack
Musk said Grok 4.8 had finished pretraining but was still undergoing reinforcement learning, a phase in which a model's responses are refined using feedback and rewards. He said the team had spent considerable time cleaning up supervised fine-tuning data, the examples used to teach the model how to follow instructions. The company abandoned JAX, a machine-learning framework, and SGLang, software used to serve AI models, in favor of an inference and reinforcement-learning system rewritten in C and C++.
Developer tetsuo celebrated the announcement with 'C wins again,' prompting Musk to clarify that the code was 'C with a little C++.' He credited Starlink engineers with work that helped produce the Grok training and inference stack, pointing to SpaceX's practice of writing its own headers and handling precise time calculations, including leap seconds. Musk described Grok 4.8 as 'not yet incredible' but said its foundations were finally built correctly. He expects a soft rollout to Bot users later this week.
Anthropic Reports Opus 5.5 Performance on Terminal-Bench 4.0
Anthropic released Opus 5.5 on September 22, delivering performance comparable to its higher-end Fable 5.1 model on most work while costing 40% less to run than the previous Opus version on typical workloads. The model targets difficult programming jobs and tasks in which AI agents work through multiple steps with limited human assistance.
On Terminal-Bench 4.0, which tests whether AI agents can complete complicated tasks in a command-line environment and scores them by the percentage successfully finished, Anthropic reported 66.4% for Opus 5.5 versus 57.9% for OpenAI's GPT-6 Astra. Musk has not published Grok 4.8 scores that would establish how large the gap is.
FAQ
What did Elon Musk say about Grok 4.8 compared to Claude Opus 5.5?
Musk stated on X Tuesday that Grok 4.8 would be 'not as good' as Anthropic's Claude Opus 5.5, which he called a 'masterpiece.' He said Grok 4.8 has finished pretraining and could begin a limited rollout to Bot users this week.
When did Anthropic release Claude Opus 5.5 and what are its benchmark scores?
Anthropic released Opus 5.5 on September 22. On Terminal-Bench 4.0, Anthropic reported 66.4% for Opus 5.5 versus 57.9% for OpenAI's GPT-6 Astra. The model costs 40% less to run than the previous Opus version on typical workloads.
What technical changes did xAI make to Grok 4.8's development?
xAI abandoned JAX and SGLang in favor of an inference and reinforcement-learning system rewritten in C and C++. Musk credited Starlink engineers with work on the Grok training and inference stack, and said the team spent considerable time cleaning up supervised fine-tuning data.
