Model providers want to smoke test their models without having flaws end up on the news (think gpt4 having to get rolled back for sycophancy). Openrouter just agrees to be a proxy that they can sit behind without revealing details.

Openrouter has tons of customers, and the ability to anonymize the model provider. Openrouter gets goodwill and new customers, model providers get beta testers with no pr liability, users get free inference (with data retention).

Yeah these stealth models pop up from time to time. Openrouter knows, but doesn't share. Users can use a testing version of something for free and in return the provider generally is allowed to retain the prompts sent in to get real world use. In the past I only really remember using one that was surprisingly good, and then it turned out to be GLM-5.1, speculating on what one this ends up being is part of the fun.