Business Insider reports that Google staff are testing an internal Gemini 4 build codenamed Carbon. One employee reportedly said the build 'feels like Opus 5.5' for coding tasks, an unusually direct comparison to Anthropic's flagship model. Google declined to comment, so nothing about the build's capabilities, branding, or release plans is confirmed.

The report comes as Google gates its latest public frontier model, Gemini 4 Argon, to a limited rollout for cyber defenders. An internal Carbon build would suggest the next generation is further along internally than the public rollout suggests.

Treat this as rumor-grade: it rests on a single anonymous employee characterization relayed through one publication. Until Google confirms a Gemini 4 generation publicly, Argon's limited release remains the only official signal.

Follow-up corroboration: Business Insider's internal documents, screenshots and chats reportedly show Google trialing three Gemini 4 variants — Argon, Barium and Carbon — with Carbon deployed on the internal Jetski coding platform. One staffer still likened Carbon's coding ability to Opus 5.5; Google is said to be assigning fixed roles to each variant, with Argon for frontier reasoning. Still internal testing, so this stays rumor-tier.

Follow-up (October 22): a checkpoint labeled Gemini 4 Carbon has reportedly appeared on Chatbot Arena, per NokiaPowerUser's October 22 report relayed by Let's Data Science. Early testers on X praise its 3D scene generation and one calls it already better than Argon, but the reports cite no benchmarks or latency figures, and Google still has not announced Carbon, a release date, or whether Carbon is a new Argon release or a separate model. The evidence remains anecdotal tester impressions, so this stays rumor-tier and unresolved.

Follow-up (October 22, evening): leaker Lentils80 reports the Gemini API Early Access Program for Carbon has officially begun, with early access given to select partners and testing under way on LMArena — 'the release is very close.' In those early impressions Carbon is noticeably better than Argon at 3D work but slower and weaker on frontend and UI tasks. A screenshot said to show Google's own early-access program page has also circulated, but it could not be verified live, and Google still has not confirmed the name, the testing, or a release date. This stays rumor-tier and unresolved.

Resolution (October 28): Google DeepMind officially announced Gemini 4 Carbon on October 28 as its strongest model, leading the Artificial Analysis Intelligence Index v5.0 at 59, with first access going to a subset of cyber defenders through the Fairwind Program. That confirms the rumored model and its coding and cybersecurity positioning; wider developer, business and consumer access still has no date. This rumor is confirmed.