China’s Zhipu Unmasks ‘Ox Alpha’ and Releases Massive GLM-5.3-Flash Open Weights

Chinese AI company Zhipu, now branding itself internationally as Z.ai, has officially taken credit for the mysterious large language model “Ox Alpha” that quietly appeared on OpenRouter on August 20 under the ID stealth/ox-alpha. On August 26 the firm confirmed that Ox Alpha was in fact GLM‑5.3‑Flash, a new member of its GLM model family, and announced that it would release the model’s open weights. Until now, developers only knew that Ox Alpha was free to use, multimodal with text, image and video input, and offered an unusually large one‑million‑token context window – but nothing about who built it or how big it was.

According to Z.ai, GLM‑5.3‑Flash uses a Mixture‑of‑Experts architecture with an overall parameter count around 320 billion, while activating roughly 18 billion parameters per token in practice. Usage data from OpenRouter shows that, during its anonymous preview, Ox Alpha briefly topped the platform’s leaderboards and generated more than twice as many tokens as rival Chinese model DeepSeek. By publishing open weights for a frontier‑class, multimodal model explicitly designed for coding and AI agents, Z.ai is betting that global developers will embrace an open‑weight alternative in a landscape still dominated by closed US models — sharpening the competitive fault line between open and proprietary AI ecosystems.