Elon Musk wrote on X on Tuesday that xAI has dumped JAX and SGLang and rewritten its entire reinforcement learning and inference stack from scratch in C/C++, which took extra time. He said Grok 4.8's pre-training is done, that 4.8 is now in RL and that evals are starting to climb.

Musk said xAI will do a soft rollout to Grok Bot users later this week and see how it goes, and that Grok 4.8 will bear no resemblance to 4.7. He gave no benchmark figures and no date beyond later this week.

The post came 57 days after Musk wrote on September 14 that Grok 4.8 would finish training that week and start RL, according to FourWeekMBA's count. In a follow-up, Musk said Grok 5 should exceed Claude Opus 5.5 and maybe that happens in two or three months — hedged language, with no benchmarks or release date for Grok 5.

This is an announcement of progress, not a release: 4.8 is not yet available, its soft rollout is limited to Bot users, and there is still no price card or independent evaluation.