Anthropic launches Claude Sonnet 5.5

Anthropic pitches Claude Sonnet 5.5 as the cheaper coding champion: it scores 70.6% on Terminal-Bench 4.0, beating Opus 5.5's 66.4% and dwarfing Sonnet 5's 10.3%, while costing half as much per token - $2/$10 versus $4/$20 per million ^[1]. Anthropic and enterprise partners frame this as a straightforward win: up to 30% faster output and up to 30% lower cost per task thanks to fewer tool calls ^[2]. But that pitch has a caveat. Independent benchmarking firm Artificial Analysis found that at maximum reasoning effort, Sonnet 5.5 wrote roughly 193,000 output tokens per Intelligence Index task - the most it has ever measured for any model, and about 60% more than Opus 5.5 needed for the same tasks ^[3]. At that effort tier, the 'half the price' math narrows sharply: needing roughly 60% more tokens than Opus 5.5 for the same task erodes much of Sonnet 5.5's per-token discount, even though its rate is still nominally half of Opus 5.5's ^[4].

The most useful discovery to emerge after launch wasn't in Anthropic's own materials - it was a workflow pattern developers converged on almost immediately: use Opus 5.5 as the planner for open-ended reasoning, and delegate well-scoped implementation steps to Sonnet 5.5. That division of labor tracks with how Cursor benchmarks the model - on CursorBench, which grades real coding sessions, Sonnet 5.5's score climbs from 35.8% at low effort to 55.5% at max effort, second only to Opus 5.5, meaning its usefulness depends heavily on how much reasoning budget a task is given ^[5]. For narrow, well-defined jobs, low or medium effort settings appear to be enough to make Sonnet 5.5 both cheaper and, in several informal head-to-head tests circulating among early adopters, comparable or better in quality than Opus 5.5 run at an equivalently low effort - a direct challenge to any narrative that Sonnet 5.5 is simply a scaled-down Opus. Not every independent test agrees, though: on more open-ended generative tasks, side-by-side comparisons pitting Sonnet 5.5 against Opus 5.5 and rival models have still called Opus the more complete performer, suggesting the coding-benchmark win doesn't automatically carry over to every kind of task.

Sonnet 5.5 is the first Sonnet-tier model to ship with the frontier-grade cyber safeguards Anthropic previously reserved for its most capable models, including classifiers designed to block extraction of the model's own reasoning ^[1]. Independent reporting on the model's system card frames this as a defensive measure against misuse of a faster, cheaper coding model ^[6]. In practice, early adopters report the safeguards can misfire on legitimate work - flagging benign tasks like checking data feeds, changing passwords, or running routine security audits as if they were adversarial probes. That kind of false-positive refusal is a real operational cost for security and IT teams weighing whether to route sensitive workflows through the new model.

Despite the coding and agentic gains, Sonnet 5.5 is not a blanket substitute for Opus 5.5. Artificial Analysis measured it well behind Opus 5.5 on tasks that reward broad knowledge and careful reasoning: 54% versus 66% on factual accuracy, and roughly six points lower on Humanity's Last Exam and SciCode ^[3]. Anthropic's own benchmarks show the two models much closer on real-world occupational tasks (GDPval-AA) and computer-use (OSWorld 2.1), suggesting the gap is concentrated specifically in fact-recall and scientific-reasoning categories rather than general capability ^[2].