What happened
- On September 28, Anthropic released Claude Sonnet 5.5, identified as
claude-sonnet-5-5, six days after Opus 5.5. - According to the company’s model page, it generates responses more than 30% faster than Sonnet 5 and costs up to 30% less per task, at the same per-token rate: $2 per million input tokens, $10 for output and $0.20 per cache read.
- Reported benchmark figures: Terminal-Bench 4.0 goes from 10.3% to 70.6%; CursorBench 4.0 from 34.1% to 55.5%; OSWorld 2.1 from 57.0% to 80.1% partial; GDPval-AA v2.1 from 1,449 to 1,844 points.
- It ships with the same cybersecurity protections as Opus 5.5 and Sonnet 5’s biology protections. It’s been available on AWS, Google Cloud and Azure since the same day.
Why it matters
- The savings aren’t in the rate but in how many tokens the task consumes. Anyone who modeled their spending on list price is going to read a bill that doesn’t match their spreadsheet, in either direction.
- For teams in Chile that already have processes built on Sonnet 5, switching versions is a one-line change and a measurable difference in spending. The cost of not migrating is no longer zero.
- The jump from 10.3% to 70.6% on a terminal benchmark describes less a gradual improvement than a change in the category of task: work that used to be supervised step by step can now be delegated whole.
The number
70.6% on Terminal-Bench 4.0, versus 10.3% for Sonnet 5 on the same test.
Context
Six days ago Anthropic had cut the cost of its largest model by 40%. We wrote then about Opus 5.5’s price cut and the handoff of cybersecurity. The pattern repeats: the company competes on cost per task and not on the published rate, which is the figure everyone compares.
What’s next
- No date announced for retiring Sonnet 5 from the cloud platforms.
- The model’s system card, dated September 28, is available for outside review.
Bottom line
Last week Xiaomi released open weights that match Opus 5 on two benchmarks. When the per-token price stops moving and what drops is consumption, comparing models becomes an exercise each team has to do with its own workload. Published averages stop being useful.
Sources
- Introducing Claude Sonnet 5.5, Anthropic, September 28, 2026.
- Introducing Claude Sonnet 5.5 on AWS, Amazon Web Services, September 28, 2026.
Written by Mamífero. Edited by Rodrigo Cornejo. See how we select and verify each note.
