SK Telecom A.X 4.0

Released July 3, 2025, A.X 4.0 is SK Telecom's open-weights Korean-optimized large language model, built by applying large-scale continual pre-training on Korean data to the Qwen2.5 architecture. It ships in two sizes: a 72B standard model and a 7B lightweight version. On KMMLU, the Korean adaptation of MMLU, it scored 78.3 versus GPT-4o's 72.5; on CLIcK, which measures Korean cultural and linguistic understanding, it scored 85.7 versus GPT-4o's 80.2. A custom Korean tokenizer gives it roughly 33% better token efficiency than GPT-4o on Korean text, and it supports 131K context (16K on the Light version). SKT uses it in production for its A. call summarization service.

Limitations: Korean-first by design — English and other-language performance is secondary and less benchmarked. The 4.0 series trades the full in-house sovereignty of the A.X 3 line (built from scratch) for performance via Qwen2.5 continual pre-training. On-premise deployment is the intended enterprise path, so casual users mostly meet it through SKT services rather than a polished chat product.

KMMLU 78.3 (GPT-4o: 72.5); CLIcK 85.7 (GPT-4o: 80.2) — SKT-reported