Image: Google
Google has given its most direct explanation yet for why Gemini 3.5 Pro never launched. CEO Sundar Pichai told Alex Heath on the Sources podcast that Google chose to skip the model and fold its lessons into Gemini 4 Argon. Heath shared the answer on X.
I asked Google CEO Pichai what happened to the missing Gemini 3.5 Pro model that never shipped.
— Alex Heath (@alexeheath) October 22, 2026
He gave me an interesting window into the hard decisions big labs have to make in model development:
"While training the 3.5 series, we had Flash and Pro, which we were working on.
Google had promised the model at its May developer conference, saying it was already in internal use and would roll out the next month. It did not. In July, The Verge noted, citing Bloomberg, that the model was late while Google worked on its coding skills. In late September, a Google spokesperson told Reuters that Google no longer planned to release it.
Pichai now says the team found gaps in agentic skills, especially coding, while training the 3.5 series. Flash is smaller and cheaper to run, so Google used it for faster rounds of testing and tuning. That work produced Gemini 3.6, 3.7 and 3.8 Flash. By the time those lessons were ready for Pro, he says, Argon had already finished pre-training. Google then applied them to Argon's post-training instead.
Heath put it as 3.5 Pro having a very short shelf life. In plain terms, a Pro model built on the older base would have been outdated almost on arrival.
Argon is now Google's largest model. It is open to developers, businesses and Gemini Pro and Ultra subscribers. Pichai said it handled almost 25 trillion tokens in its first 72 hours, about three times Gemini 3, and that Google is short on capacity. He added that smaller, more efficient versions are in the works and that more Gemini 4 models will ship.
For most people the takeaway is simple. If you were waiting for 3.5 Pro, Argon is the model to try, and cheaper versions may follow.
One caution. The reasons come from Pichai's own account, and Google has not published a technical write-up. Google has also not said whether a model called Gemini 4 Pro will exist.
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