Carbon vs Opus 5.5 vs Astra: One Point Separates the Top Three — And None of Them Wins Everywhere

One point. That is the gap between the top three AI models, on the newest version of the most watched benchmark. Google's Gemini 4 Carbon scores 59 on version 5.0 of the Artificial Analysis Intelligence Index. Anthropic's Claude Opus 5.5 scores 58. OpenAI's GPT-6 Astra scores 57. The gap is so small, and the conditions so different, that the ranking answers almost nothing on its own. The real comparison is three flagship models, each leading somewhere different, at very different prices, with very different availability. There is no overall winner.

First, the caveat that must travel with every number in this article. The three scores were not measured at the same reasoning effort. Artificial Analysis measured Carbon at its high reasoning setting, and Opus 5.5 and Astra at their maximum settings. A one-point gap measured at different effort levels cannot establish which model is better for a particular workload, and Google's launch figures come without uncertainty estimates, as VentureBeat noted. Compare the scores only within the same index version — version 5.0, launched on 27 October 2026, with Carbon added the following day.

The ranking: what Carbon leads, and what it does not

Carbon leads on four of the index's nine tests, according to secondary reporting of Artificial Analysis results: hard reasoning and knowledge questions (HLE-Diamond, 69% against 68% for Opus 5.5), science work in a terminal (Terminal-Bench-Science, 65% against 63% for Astra), knowledge with a penalty for wrong answers (AA-Omniscience), and business workflows (AutomationBench-AA). It also leads on the hallucination measure: an 11% hallucination rate, against about 50% for both Opus 5.5 and Astra in the same reporting. Carbon does not know the most — several Claude and GPT models answer more knowledge questions correctly — but it rarely bluffs, and the index's new weighting gives non-hallucination 5% of the composite.

Where Carbon loses is just as clear. Claude models lead office-style knowledge work. Opus 5.5 leads real-world coding: 46% on FrontierCode 2.0 against 41% for Carbon and 42% for Astra, and 62 to 59 on the separate Coding Agent Index. Astra leads CritPt physics at 72%, and FrontierSWE 2 in Google's table, 65.5% against 61.2% for Carbon. Opus 5.5 leads OSWorld-2.1, 87.2% against 81.5%. Kimi K3.1 Pro leads general Terminal-Bench 4.0 at 67%, and Meta's Muse Wonder Full leads document reasoning. No model leads everywhere.

Google's own benchmark table tells a compatible story. Carbon leads most rows, including the cybersecurity benchmarks reported in this publication's Carbon article — 88.2% on SEC-bench Pro, 54.8% on ExploitGym and 71.0% on CWE-bench, evaluated without guardrails, as Google itself states — but the same table concedes the losses above. Treat every figure in it as a vendor claim until independently reproduced.

Credit: Anthropic logo, via Intopic



Price: the cheapest flagship is the one you cannot buy

On list price, Carbon looks like a bargain. Google's introductory API price is $2 per million input tokens and $10 per million output tokens, with cached input at 95% off. Secondary coverage reports that the price rises to $4 and $20 later; Google's primary announcement, as checked for this publication's Carbon article, states only the introductory price — so the step-up is reported, not confirmed from Google's own post, and no end date for the introductory period is stated here. Opus 5.5 is listed at $4 per million input tokens and $20 per million output tokens in Anthropic's API rate card, as corroborated by multiple pricing trackers citing Anthropic documentation in October 2026 — half the price of the Opus 5 it replaced ($5/$25). Astra's API pricing is not stated here, because it was not re-verified against an OpenAI primary source at draft time.

List price is not cost per finished task. At standard prices, one index task costs $7.76 for Carbon, about 23% less than Opus 5.5 at maximum effort ($10.04), and about 1.2 times Astra at maximum ($6.24). GPT-6.1 Sol, a different, cheaper OpenAI model, costs $1.27 per task while scoring 55. Choose by task and cost per finished task, not by rank, and test on your own work before switching.

OpenAI's other new model belongs in this section, not in the flagship ranking. GPT-6.1 Terra, announced on 26 October 2026 and covered in this publication's Terra article, is priced at $1 per million input tokens and $5 per million output tokens (cached input $0.05), with a 1.05-million-token context window. It is a computer-use model for ChatGPT Work and Codex, not a general flagship. Its independent score of 48.1 in earlier reporting is from a different measurement and should not be placed beside the v5.0 scores above.



Credit: Developers at work, via SCS Solutions

Availability: only two of the three can be bought

The most important difference between the three models is not a benchmark. It is whether you can use them. Carbon is not publicly available. As this publication reported, Google is rolling it out to a subset of cyber defenders first, then to trusted partners through its Fairwind Program, with no date for developers, businesses or consumers. Opus 5.5, by contrast, is available through Anthropic's API and in Claude's Pro, Max, Team and Enterprise plans. Astra is OpenAI's flagship in the same index comparison; this article does not state a specific Astra availability date or product list, because those were not re-verified at draft time.

That availability gap changes what the ranking means. A number-one model that only vetted defenders can use is not the best model for a developer choosing a model this week. For a team deciding today, the practical choice is between the two available flagships, plus the cheaper work models beneath them. Carbon enters that choice only when Google opens the door, and its price and benchmarks should be re-checked then. Safety claims from both Google and Anthropic — Carbon's 0.0% on Gray Swan's prompt-injection test, Opus 5.5 at the level of the pricier Fable 5.1 — are vendor-reported and unreproduced at the time of writing.



Credit: Server racks, via UL Solutions

Who should pick which — and what to watch

For cybersecurity defence work, Carbon's reported profile is the strongest, if and when it becomes available, and its low hallucination rate is a genuine advantage where a wrong answer is worse than no answer. For real-world coding and agentic work in a repository, Opus 5.5 leads the independent coding measures available today and can actually be bought, at $4/$20. For physics-heavy research and frontier software engineering benchmarks, Astra leads the specific tests named above. For cost-sensitive work tasks, Terra and Sol are the cheaper options, with lower index scores that may not matter for the job at hand. None of that is a crown. It is a menu, and the right order depends on the task, the budget, and whether the model is available at all.

What to watch: Carbon's first public availability date and its price when it opens; an independent reproduction of Google's security benchmarks with guardrails on; and whether the one-point gap survives at matched reasoning effort. Until then, the honest headline is the one this article started with: one point separates three very different models.

Key Takeaways

  • AA Index v5.0: Carbon 59 (high), Opus 5.5 58 (maximum), Astra 57 (maximum) — different effort levels, so the gap does not prove which is better for a given workload; compare only within v5.0.
  • Carbon leads 4 of 9 tests, 11% hallucination rate (about 50% for rivals); Opus 5.5 leads coding (FrontierCode 46% vs 41%; Coding Agent Index 62 vs 59); Astra leads physics (CritPt 72%) and FrontierSWE 2 (65.5% vs 61.2%).
  • Carbon pricing: introductory $2/$10 (Google primary); $4/$20 step-up reported in secondary coverage, unconfirmed. Opus 5.5: $4/$20. Cost per index task: Carbon $7.76, Opus $10.04, Astra $6.24, Sol $1.27.
  • Carbon is NOT public (defenders first, no date); Opus 5.5 is available via API and Claude plans; Terra ($1/$5) is a separate cheaper work model, not in this flagship ranking.
  • No overall winner: pick by task, cost per finished task, and availability — test on your own work before switching.

Three labs, three flagships, one point. Carbon is the most accurate and the least available. Opus 5.5 is the coding leader you can buy today. Astra is the physics and engineering specialist. Answer three practical questions — can you use it, what does it cost per finished task, and does it lead on the work you actually do — and the one point takes care of itself.

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