A stealth model called "Union Alpha," provided by an anonymous source, emerged on September 16, 2026 (US Time). OpenCode has announced that the model will be provided for free for 1 weeks from its release. It is described as a multimodal model supporting image input for research and coding, built specifically for agentic development. The developer, total parameter count, and architecture have not been disclosed.
PLUS ULTRAModel ReleasesUnion Alpha
Mysterious AI Union Alpha Free for 1 Weeks, Claims Performance on Par with Fable 5.1 and Astra
PLUS ULTRA by Amenoyomi
This article is a translation. Read the Japanese original
According to a performance comparison on Terminal-Bench 4.0 reported by PC Watch, Union Alpha is positioned as comparable to Claude Fable 5.1 and GPT-6 Astra, with estimates showing costs of less than 2 dollars per 1 task. However, these evaluations are based on introductory materials and results immediately following the release; they should be viewed separately from independent reproduction tests or detailed benchmark disclosures from the provider. In OpenCode's Japanese documentation, the model ID is set as "union-alpha," and input, output, and cache read-outs are listed as free.
Initial user reactions are not monolithic. Posts collected by Yahoo! Real-time Search include reports of successfully creating a single-page HTML site in one go and praise for its understanding of codebases and handling of sub-agents. Conversely, other users have reported slow response starts, tasks that do not complete, stops due to 503 or 429, and claims that the model is too unstable to evaluate. Some reports indicate instability via OpenCode but stability via OpenRouter, suggesting a need to distinguish between the model's inherent capabilities and congestion or failures in the delivery path.
An independent article from AI Jitan Lab reported that after testing 18 types of strings, they confirmed a match with GLM-series tokenizers. Furthermore, in self-introduction prompts accompanied by images, the model identified itself as being made by GLM or Z.ai in 5 out of 10 instances. However, the article notes that this was a small-scale probe and does not definitively determine the identity or performance of the anonymous model. The author also explained that since they measured wait times during the initial launch congestion, the data cannot be used for speed comparisons.
Users are advised to separately verify the available delivery paths and data retention conditions. OpenCode states that input data is not used for training. Since the free offering is for a limited time, usage conditions should be confirmed via the latest guidance from the respective endpoints.
PLUS ULTRAby Amenoyomi
Union Alpha is said to possess performance comparable to Claude Fable 5.1 and GPT-6 Astra in Terminal-Bench 4.0, which evaluates complex practical tasks on a terminal. It is a multimodal model designed for research, coding, and agentic workflows, characterized by support for image input and a context window of approximately 260,000 tokens.
There are reports that practical stability varies depending on the access route. Via OpenCode, instability has been pointed out, including delays in response starts, stops caused by 503 and 429 errors, and tasks that fail to complete. On the other hand, some users report stability when using OpenRouter, confirming high performance in areas such as creating single-page HTML files and the appropriate control of sub-agents.
Technical analysis is being conducted on this "stealth model" of unknown origin. In a tokenizer comparison using 18 types of strings, it was reported that the model matched perfectly with GLM-5.3-Flash and GLM-5.2. Additionally, when asked questions with images, it self-identified as being produced by GLM or Z.ai in 5 out of 10 cases, though it never did so for text-only questions. From these results, it is inferred that the model may be a GLM-series model.