What Is Union Alpha? OpenRouter's Free Stealth Model

§ 01What OpenRouter published#

Everything in the table is from OpenRouter’s public model API and listing page, read on September 16 between 19:09 and 19:16 UTC.

OpenRouter’s own description of the model reads, in full: “Union Alpha is a multimodal model built for research, coding, and agentic workflows, while delivering frontier-level performance across a broad range of general-purpose tasks. Union Alpha is a stealth model. It is developed and operated by a third-party provider who has chosen to remain anonymous during this preview. OpenRouter routes requests to it and is not its developer, owner, or provider.” The listing’s own FAQ adds: “Union Alpha was released on September 16, 2026.”

Two things in the table are worth a second look. The context window is 262K, a quarter of Ox Alpha’s 1M, which is closer to the shape of a production model than a demonstration. And the listing exposes no reasoning parameter at all, where Ox Alpha exposed mandatory reasoning with effort settings. Either the model does not reason in a way the API surfaces, or the operator hid the knob for the preview.

§ 02What the launch posts claim#

OpenCode’s announcement, at 14:52 UTC, was four lines: “Union Alpha (stealth model) is free for the next week”, then three bullets: no data training, built for agentic coding, supports images. Asked about limits five minutes later, the account answered “capacity so far is 5T a day but potentially can go up”. The post had passed 880,000 views by 19:13 UTC.

OpenRouter’s CEO Alex Atallah posted at 15:19 UTC: “Here’s the DeepSWE result for Union Alpha, a stealth model we just launched.” The result is an image: DeepSWE score against average cost per task, with the frontier models as labelled curves and Union Alpha as a star marked expected pricing in the top-right corner, where the cheap and strong models live. Reading the chart, the star sits near 73 percent, level with gemini-3.8-flash, gpt-6-astra and claude-opus-5, at a cost close to zero. No number is printed for it. One second later, in the same thread: “This free model also outperforms 5.6 Sol on Terminal-Bench 2.1 and SWE-Bench Verified. And outperforms Opus 5 on TBench 2.1”.

OpenCode’s chart, posted at 14:58 UTC and carrying Artificial Analysis branding, is a different benchmark: Terminal-Bench v4.0 score against cost per task on a log scale. Union Alpha is again a star, marked anticipated pricing, at roughly 52 percent and about $1.60 a task, sitting on the drawn Pareto line between GLM-5.3-Flash (about 33 percent, about $0.30) and GPT-6 Astra at high effort (about 55 percent, about $4). Again, no number printed.

So the honest summary is one sentence: every number about Union Alpha so far was drawn by the people routing it, priced at a cost the model does not yet have, and printed on no leaderboard. That is not an accusation. Stealth previews exist to gather exactly the usage that makes independent numbers possible, and Ox Alpha’s operator charts turned out to be roughly right. It is a statement about what kind of number a chart-with-a-star is.

The first replies are the usual mix. Under Atallah’s post: “looks benchmaxxed really”, “It thinks for way too long”, and a question about whether it is a Kimi model. Under OpenCode’s, by early evening: “It’s so slow at the moment. I’ll come back later”. Free models on launch day are slow; that reading tells you about demand, not the model.

§ 03Who made it#

Nobody has shown it, and the tools that usually answer this quickly are blunted. When Ox Alpha appeared in August, a 95-of-95 tokenizer probe matched the GLM-5 vocabulary within three days, and Z.ai’s error envelopes sealed it. Union Alpha’s tokenizer is listed as Other, which forecloses the token-count comparison until someone does it by hand, and as of this writing no fingerprint study, error-string comparison or refusal-pattern read has been published.

What exists is speculation, and it should be labelled as such. Replies under the launch posts guess Kimi K3.1 and MiniMax M3.1, on timing. A Manifold market opened at 15:47 UTC asks the question directly; at 19:16 UTC, with nine bettors, it had Other at 57 percent, Z.AI at 12 percent, Mistral at 6 percent and Moonshot at 6 percent. A prediction market with nine participants is a mood, not a source, and it is reported here as one.

The name is a clue only in the sense that it is not one. OpenRouter’s stealth codenames have been abstract nouns and animals (Quasar, Horizon, Pony, Ox); Union maps onto no lab’s naming scheme. It invites the reading that this is a joint release, which is exactly the kind of reading a codename is chosen to invite.

What would resolve it: a tokenizer probe against the candidate vocabularies, an API error envelope that matches a known vendor’s format, a video or audio input that answers when it should not, or the operator saying so. OpenRouter’s precedent is that the operator says so within about a week.

§ 04The data question#

The launch post says no data training. The listing says something more specific: “Prompts and completions may be retained by the provider but are not used for training; all other use is governed by the Stealth Model Terms.” Those two statements are compatible and not equivalent. The first is the second with the retention clause removed.

Read plainly, the promise is that an anonymous operator will not train on what you send, and may keep it. Alex Atallah wrote the general version himself in February 2026, about an earlier stealth model: “Note: it’s a stealth model, so has logging enabled”. Nothing about Union Alpha suggests that has changed, and the listing’s wording says it has not.

This matters more for a coding model than for a chat model, because the prompts are repositories. Ox Alpha’s window left an open question about what happened to the code sent during its anonymous week; Z.ai never addressed it after the reveal. The same question is open here from the first hour. Our own position is simple: we treat a stealth endpoint as an unnamed counterparty, we do not route CellCog agents to it, and we would not send it a repository we could not publish.

§ 05The stealth playbook, and its record#

OpenRouter has run this play before, and the record is short enough to hold in a table.

Quasar Alpha was the first, announced by Atallah on April 3, 2025 as “our first-ever stealth model”, and named by OpenRouter on April 14, 2025, GPT-4.1’s launch day, as an early test of that model. Horizon Alpha ran eight days before OpenRouter named it a GPT-5 checkpoint; Pony Alpha ran five before it became GLM-5. Ox Alpha appeared on August 20, 2026, drew roughly 221,000 users and 16 trillion tokens on OpenCode in three days, and was confirmed as GLM-5.3-Flash on August 26, at which point the free window closed and the paid route opened the same day. The pattern is the point: the free week is the launch campaign, the users burning the free tokens are the launch audience, and the reveal ends the price.

§ 06What we are watching for#

• The reveal. A vendor statement, a named entry replacing stealth/union-alpha in the catalog, or weights. Any one of these flips this page into a resolution record with the claims table scored.
• An independent number. A DeepSWE, Terminal-Bench or SWE-Bench Verified run by someone other than the operator, at a published cost. The first one decides whether the star was drawn in the right place.
• A fingerprint. A tokenizer probe, an error envelope, or a modality that answers when it should not. Ox Alpha’s lab was called three days before it admitted it.
• The end of the free window. OpenCode said a week; nobody said which hour. When the price appears, so does the cost frame the charts assumed.
• Any statement on retention. The listing says prompts may be retained. A named operator’s data policy would replace the Stealth Model Terms; the question of the anonymous week stays open regardless, as it did for Ox Alpha.

§ 07The tracker#

§ 08Update log#

• September 16, 2026, 19:16 UTC. Page opened, four and a half hours after the catalog entry. Facts from the OpenRouter API and listing; posts read on X with times taken from their ids; benchmark charts read off the images, no scores printed; Manifold read as sentiment.

§ 09Sources#

OpenRouter, Union Alpha listing and the public model API (/api/v1/models and /api/v1/models/stealth/union-alpha/endpoints), read September 16, 2026, 19:09 UTC (created timestamp 1789569723). OpenCode, post on X, 2026-09-16T14:52:40Z, and replies at 14:57:00Z and 14:58:28Z. Alex Atallah, post on X, 2026-09-16T15:19:12Z, and reply at 15:19:13Z; Quasar Alpha post, April 3, 2025; stealth logging note, February 8, 2026. Manifold Markets, Who is behind Union Alpha, created 2026-09-16T15:47:24Z, read 19:16 UTC. Leaderboard absences checked at 19:16 UTC on artificialanalysis.ai/models, tbench.ai/leaderboard and swebench.com/verified.html. OpenRouter blog, Quasar Alpha and Optimus Alpha reveal, April 14, 2025, and the Pony Alpha and Horizon Alpha model pages, which carry each model’s release date and its revealed identity. Ox Alpha facts from our record, sourced there.