In brief
IplanRIO released Rio 3.5 Open 397B on June 13, billing it as a government-built frontier AI model with benchmark scores topping Qwen 3.7 Plus.
AI company Nex published a mathematical proof showing the model is a direct 0.6 Nex / 0.4 Qwen weight merge.
IplanRIO updated the model card, credited Nex, pulled the benchmark claims, and blamed an "incorrect upload."
Rio de Janeiro's IplanRIO released Rio 3.5 on June 13. The city's IT agency called it a frontier-class model: 397 billion parameters, with a permissive open-source license, built by the municipal government of a city in the Global South.Rio 3.5’s launch timing was perfect: Brazil was playing its World Cup opener, and social media was already on fire. Comments about it rapidly spread from Brazil to beyond.But just as quickly as it gained attention, there was a dispute over who exactly created the model.The original model card described Rio 3.5 as a post-train of Qwen 3.5 397B, Alibaba's open-base model, with a new reasoning layer called SwiReasoning added on top. The development cost was reported at R$500,000 (Rio didn’t confirm this), or nearly $100,000 USD—roughly 30 times cheaper than equivalent off-the-shelf AI systems.The architecture is Mixture-of-Experts, which means only around 17 billion of the 397 billion parameters fire on any given token. That makes inference cheaper than the headline size suggests. The model also supports vision and text, handles over a dozen languages, and ships under a fully open MIT license.SwiReasoning is the technical centerpiece. It's a training-free inference framework that switches dynamically between two modes. When the model is confident about a next word—low entropy in the probability distribution—it reasons in plain language. When uncertain, it shifts to latent reasoning, thinking in hidden internal states without emitting tokens. IplanRIO said Rio 3.5 was specifically trained to exploit this, and that the gains show up in the benchmark numbers.The self-reported numbers were eye-catching. Terminal-Bench 2.1—which measures autonomous terminal command execution, scored as percentage of tasks passed—came in at 70.8% for Rio 3.5, edging out Qwen 3.7 Plus at 70.3% and the powerful DeepSeek v4 Pro at 67.9%.On IMOAnswerBench, a math olympiad benchmark scored as percentage correct, Rio 3.5 hit 89.5%. On HLE—Humanity's Last Exam, a near-unsolvable multi-domain expert battery scored as a percentage—Rio 3.5 landed at 36.5%, ahead of Qwen 3.7 Plus's 34.7%.A municipal government beating the most important flagship models on the most meaningful quality benchmarks: That's the headline that spread, especially after the Mayor of Rio de Janeiro tweeted about it.“An open AI model trained in Rio and publicly funded over the last year by [the Municipality of Rio] has just surpassed all other models,” Eduardo Cavaliere wrote. “Today, the world is talking about an open AI model trained in Rio.”
