# Sonnet 5.5 Nears Opus on the Index — With AA’s Highest Token Usage

Times of AI Desk · 2026-09-28 · Models

[https://timesof.ai/2026/09/anthropic-claude-sonnet-55](https://timesof.ai/2026/09/anthropic-claude-sonnet-55)

> Claude Sonnet 5.5 keeps Sonnet 5’s $2/$10 sticker and, at max effort, scores 56 on Artificial Analysis’s Intelligence Index, two points behind Opus 5.5. Anthropic says it costs up to 30% less per task. AA measured about $7.60 a task at max, roughly 50% more than Sonnet 5, because output ran to about 193,000 tokens. Sticker price is not cost per task.

The workhorse tier just posted a near-Opus score at a Sonnet price. It also posted the heaviest token bill Artificial Analysis says it has ever measured. Both facts are the launch. The **$2 / $10** sticker, unchanged from Sonnet 5, does not decide which one a buyer feels.

**Claude Sonnet 5.5** is the second model in the Claude 5.5 family, a week after [Opus 5.5](https://timesof.ai/2026/09/anthropic-claude-opus-5-5). Anthropic prices it at **$2** per million input tokens, **$10** per million output, **$0.20** cache reads, and **$2.50** cache writes — half of Opus on input, output, and cache writes, tied with Opus on cache reads. API slug: **`claude-sonnet-5-5`**. Anthropic says it is on the Claude Platform, Amazon Web Services, Google Cloud, and Microsoft Azure, with zero data retention available as on Opus 5.5 and Sonnet 5. **Haiku 5.5** is still “coming weeks,” not this release.

## Lab claims

Anthropic’s post (September 28) says generation is **30%+ faster** than Sonnet 5, and that fewer tokens mean **up to 30% lower cost per task** than that predecessor. It also says Opus 5.5 remains clearly stronger at complex, open-ended work. Selected lab scores, at the effort Anthropic prints for Sonnet 5.5:

| Bench | Sonnet 5.5 | Sonnet 5 | Opus 5.5 |
| --- | --- | --- | --- |
| Terminal-Bench 4.0 | **70.6%** | 10.3% | 66.4% (xhigh, its highest) |
| FrontierCode 1.1 Main | 46.2% max / **52.1%** xhigh | 42.4% | 54.4% |
| CursorBench 4.0 | **55.5%** | 34.1% | 57.8% |
| GDPval-AA v2.1 | **1844** | 1449 | 1846 |
| AA-Briefcase v1.1 | **1811** | 1359 | 1822 |
| Humanity’s Last Exam, with tools | **64.5%** | 54.9% | 67.7% |
| OSWorld 2.1, partial | **80.1%** | 57.0% | 81.8% |

FrontierCode is lower at max than at xhigh. Anthropic’s footnote says max effort more often ran a wide code-review skill and, in cases Cognition looked at, timed out or edited past the task. GDPval-AA and AA-Briefcase were run by Artificial Analysis on a **pre-release** deployment Anthropic says had a structured-output bug. Anthropic expects any effect to be small and to **understate** the model.

Cyber safeguards are new at this tier. Anthropic says Sonnet 5.5’s cyber capability is in Opus 5’s range, so higher-risk cyber requests **fall back to Sonnet 5**. Biology safeguards match Sonnet 5. An automated audit of roughly **1,850** scenarios is lab-reported; Anthropic says no eval set catches every failure. Default effort is Medium in Claude apps and Claude Code, High on the platform. “Up to 30% cheaper per task” is Anthropic’s testing claim, not a guarantee at max effort.

## Independent checks

Artificial Analysis, September 28, is the check that changes the invoice:

- **Intelligence Index 56** at max effort with Anthropic’s default fallback — **second**, two points behind **Opus 5.5 max at 58**, and **18 points** above Sonnet 5 max.
- About **193,000 output tokens per Index task**, the highest AA says it has measured: about **60%** above Opus 5.5 max or Sonnet 5 max, and about **7× GPT-6 Astra max**.
- **Cost per task about $7.60** at that max setting, which AA says is roughly **50% higher** than Sonnet 5’s cost per task. Same sticker, higher bill, because the model writes more.
- AA’s own Terminal-Bench 4.0 run is **64%**, against **60%** for Opus 5.5 and GPT-6 Astra on that page — not Anthropic’s **70.6%**. The gap is the point. The writeups do not share one harness.
- Near-parity with Opus on AA-Briefcase (**1811 vs 1822**), GDPval-AA (**1844 vs 1846**), and AutomationBench-AA (**71% vs 70%**), with the token burn attached.
- Weaker than Opus on AA-Omniscience factual accuracy (**54% vs 66%**) with a lower hallucination rate (**47% vs 59%**), and about six points lower on Humanity’s Last Exam and SciCode in AA’s comparison.
- Fallback fired on about **0.1%** of Index tasks, to Sonnet 5, mostly inside Terminal-Bench.
- AA’s cost curve: at high effort the model sits **just behind GPT-6 Sol** on intelligence at essentially the same cost per task. At max it is near Opus and off the frontier on cost. At lower efforts, AA says GPT-6 Sol or Astra configurations deliver the intelligence with fewer output tokens.

AA ran the pre-release build with the structured-output bug and says scores may be slightly low. It plans a rerun. **No Arena Elo** for Sonnet 5.5 is in these sources. None is claimed here.

The OpenAI workhorse comparison is the sticker. **$2 / $10** is also GPT-6 Sol’s price, and GPT-6.1 Sol launched the next day at the same rates. An Intelligence Index rank is not a coding-agent bakeoff. AA’s curve says max-effort Sonnet is an expensive way to buy the two points under Opus.

## Limits

- Lab “up to 30% lower cost per task” and AA’s “about 50% higher at max” are different effort points and different task baskets. The figures are not directly comparable.
- Terminal-Bench **70.6** (Anthropic) versus **64** (AA) is unresolved. Opus’s lab number is at xhigh.
- The pre-release structured-output bug may understate AA’s index and the lab’s AA-run knowledge-work scores.
- Cyber fallback to Sonnet 5 will move some real security tasks off this model. No Arena ranking is established by the sources cited here.

**Correction and source review — October 14, 2026:** Corrected “matches” in the headline to “nears”: AA measured 56 for Sonnet versus 58 for Opus. Added original launch/pricing sources for the Opus and next-day OpenAI comparisons.

## Sources

- [OpenAI: GPT-6.1 Sol launch](https://openai.com/index/introducing-gpt-6-1-sol).

- [Anthropic: Opus 5.5 launch](https://www.anthropic.com/claude-opus-5-5) (2026-09-22).

- [Anthropic: Introducing Claude Sonnet 5.5 (September 28, 2026)](https://www.anthropic.com/claude-sonnet-5-5)
- [Artificial Analysis: Claude Sonnet 5.5 reaches second on the Intelligence Index (September 28, 2026)](https://artificialanalysis.ai/articles/claude-sonnet-5-5)
- [Times of AI: Claude Opus 5.5 (September 22, 2026)](https://timesof.ai/2026/09/anthropic-claude-opus-5-5)