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Gemini 4 Pro Rumors: Mystery Model Hits LMArena as Demos Leak

An anonymous model dubbed "Gemini 3.8 Flash" appeared on LMArena, widely believed to be the unreleased Gemini 4 Pro. Leaked demos and outputs followed, showing impressive game-generation performance, though Google has yet to confirm anything.

2026-09-17 ~ 2026-09-29 · 3 episodes · 18 posts

Episode 1 · Suspected Gemini 4 Pro Surfaces Anonymously on LMArena as '3.8 Flash' (2026-09-17, 13 posts)

Since September 17, multiple AI-testing accounts discovered an anonymous model labeled "Gemini 3.8 Flash" on LMArena, widely believed to be a pre-release checkpoint of Google's upcoming Gemini 4 Pro undergoing blind testing under a disguised name. Hands-on feedback the next day described it as fast and cheap, far above typical Flash-tier performance on agent and coding tasks, and side-by-side comparisons showed better output quality than the existing 3.8 Flash. Google has not confirmed anything; all conclusions come from community leaks and informal tests.

Confirmed

  • Anonymous test entries appeared on the Arena leaderboard: @cedricchee noted cited links showing battles between "Gemini 3.8 Flash (high)" and "Gemini 4"; @koltregaskes relayed the same via a leak from @aiforsuccess.
  • Users compared it against the existing Gemini 3.8 Flash: @CodeByPoonam compiled 10 user-generated wild examples on September 18, covering SVG animations and Three.js scenes; same-prompt side-by-side comparisons showed the routed-to new Pro model producing better output.
  • @CodeByPoonam shared one-line-prompt demos: a Three.js rocket scene, a cloud-whale SVG deemed more polished than the previous checkpoint, and a cycling-peacock animated SVG with surprisingly good animation.
  • @gaganghotra added on September 18 that the model is fast and cheap, exceeding Flash-tier expectations on agent and coding tasks, alongside screenshots hinting "Gemini 4 Pro" had entered the Arena.

Unconfirmed

  • Whether the model truly is Gemini 4 Pro remains community speculation; Google has not verified it.
  • @cedricchee judged its 3D capability a leap comparable to the Sol-to-Astra generation jump versus prior 4 Pro versions, but with only 75% personal confidence.
  • Some users worry Google could nerf this checkpoint before launch; this is emotional speculation.

Why it matters

  • Disguised checkpoints on LMArena typically signal blind testing shortly before an official release.
  • @teortaxesTex quipped that if it really is Gemini 4 Pro, current Arena prompts (like SVG generation) are too easy to gauge a flagship's true level.
  • If confirmed, this provides a direct comparable data point between Gemini 4 Pro and the existing 3.8 Flash, and the "fast and cheap" feedback hints its pricing and performance positioning may diverge sharply from the traditional Flash line.

Episode 2 · Alleged Gemini 4 Pro Demos Leak, Impressive Output Reported (2026-09-18, 2 posts)

Unverified demos and an alleged first output of Gemini 4 Pro have leaked, including a smooth game-generation demo praised for its world generation, fueling expectations that Google is regaining its form.

Episode 3 · Mysterious "Gemini 3.8 Flash" appears on LMArena (2026-09-28, 3 posts)

Reddit users spotted an anonymous model labeled "Gemini 3.8 Flash" on LMArena, speculated to be a hidden test of Gemini 4 Pro, with videos suggesting it may overthink.