Elon Musk Calls Claude Opus 5.5 a 'Masterpiece', Admits Grok 4.8 Won't Match It

Elon Musk has given a rival AI model an unusually enthusiastic endorsement while acknowledging that his own next release won't be able to beat it. Asked how Grok 4.8 would compare with Anthropic's Claude Opus 5.5, the xAI founder answered it would be “not as good.”

“Opus 5.5 is a masterpiece,” Musk posted on X.

The admission came in a reply on X Tuesday to a question from a user named Dimdv. Musk added that Grok 5, a next generation model that will be released after this update, “should exceed Opus 5.5,” potentially in two or three months, although he gave further details for that prediction or a firm date for its release—and he’s known for not sticking to his own target dates.

For a company trying to catch up with Anthropic and OpenAI, the distinction matters. Grok 4.8 is the update Musk expects to put in some users' hands this week; Grok 5 is the model he believes could eventually put his company ahead, until competing AI companies release a new model.

Why Grok 4.8 is taking longer

The exchange began with a technical update in which Musk said Grok 4.8 had finished pretraining but was still undergoing reinforcement learning, or RL, a phase in which a model's responses are refined using feedback and rewards. He said the team had also spent considerable time cleaning up supervised fine-tuning data, the examples used to teach the model how to follow instructions.

Musk said the company had 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++. Inference is the process of generating answers after a user sends a prompt; rewriting the underlying software can offer engineers more control over performance, though Musk provided no figures showing the resulting gains.

Developer tetsuo celebrated the announcement with “C wins again,” prompting Musk to clarify that the code was “C with a little C++.” He subsequently 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, giving the company an early test before any broader release.

Anthropic's head start

Anthropic released Opus 5.5 on September 22, delivering a 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. Grok is very far behind in the leaderboards.

Musk has not published Grok 4.8 scores that would establish how large the gap really is. His immediate target is the limited rollout later this week; the promised Grok 5 challenge to Opus 5.5 is still, by his own estimate, two or three months away.