Mystery solved: Chinese lab Z.ai says it’s behind the Ox Alpha model that wowed Silicon Valley

Z.ai, the Chinese AI company also known as Zhipu, is behind Ox Alpha, the free model that captivated developers.

CFOTO/Future Publishing via Getty Images The mystery maker of Ox Alpha is the Chinese tech company Z.ai.

The free model drew developer attention after appearing anonymously on OpenRouter last week.

Ox Alpha impressed tech leaders and set off a hunt for its anonymous creator.

Chinese tech company Z.ai says it was behind the mysterious free AI model, Ox Alpha , that generated buzz for its performance.

In a blog post published on Wednesday, Z.ai said Ox Alpha was an anonymous preview of its new GLM-5.3-Flash model, tested on OpenRouter and OpenCode to gather user feedback before its release.

The company said it has made the model's weights publicly available on the platform Hugging Face , allowing developers to download, modify, and build on it.

Z.ai said the model is multimodal, meaning it can work with text, images, and video, and has a context window of up to 1 million tokens.

Last week, Ox Alpha became a subject of intense online speculation after developers began testing it.

OpenRouter, a platform for accessing AI tools, described Ox Alpha as a reasoning model for coding, sustained agentic work, and production workloads.

Ox Alpha was also offered free for a week with "near unlimited usage," according to OpenCode, an open-source AI coding agent.

OpenCode said the provider had capacity for 100 trillion tokens per day.

The model won early praise from prominent tech figures.

Stripe CEO Patrick Collison wrote on X that it was "very impressive." Bloomberg earlier reported that Z.ai had confirmed it was behind Ox Alpha.