Model + DeadlineReleased 28 September 2026 · 2 min read
The everyday model got faster, and cheaper per task.
Claude Sonnet 5.5 runs more than 30% faster than Sonnet 5 and costs up to 30% less per task, at the same list price. Two days later Anthropic set a retirement date for Sonnet 4.5.
Sonnet 4.5 retires in
28 days
30 Nov 2026 · claude-sonnet-4-5-20250929
- Move to
- Claude Sonnet 5.5
- Price per M tokens
- $2 in · $10 out
- Speed vs Sonnet 5
- 30%+ faster
- Cost per task
- Up to 30% less
Timeline
- Sonnet 5.5 released
- Sonnet 4.5 retirement announced
- Cache reads halved to $0.10
- Sonnet 4.5 stops working on the API
01 · What changed
What Anthropic shipped.
Anthropic positions Sonnet 5.5 for well-scoped everyday work: polished documents, slides, spreadsheets and bug fixes. Opus 5.5 stays the model for complex, open-ended work that needs sustained judgment. Anything in your firm still calling Sonnet 4.5 through the API has until 30 November 2026 to move.
Speed and cost
30%+ faster
And up to 30% less per task, because it needs fewer tokens to do the same work.
Price
$2 / $10
Per million input / output tokens, same as Sonnet 5. Cache reads now $0.10.
Retirement
30 Nov 2026
Sonnet 4.5 leaves the Claude API. Anthropic recommends moving to Sonnet 5.5.
The one-line rule for your team
Sonnet 5.5
For the work you can describe in a sentence: board packs, client updates, reformatting a model output into a slide.
Opus 5.5
For the work you would hand to your most senior analyst: reading a long agreement, forming a view.
02 · What it means
What it means for your seat.
Leadership
The cost of everyday AI work keeps falling while the list price stays flat. That makes it cheaper to put Claude in front of more people, which is where the return actually comes from.
Your teams
Board packs, client updates, reformatting a model output into a slide: Sonnet 5.5 is built for that work and is fast enough to iterate on. Keep Opus 5.5 for reading a long agreement or forming a view.
IT and engineering
Code written for Sonnet 5 can break in five ways, including forced tool use returning a 400 error and a new way to turn off up-front thinking. Anything still on Sonnet 4.5 through the API has until 30 November to move.
03 · Before 30 November
Migration checklist.
0 of 3 done
Sources
Facts and dates are Anthropic’s own. What it means and what to do are our view, from the firms we work with. Drafted with Claude from these sources, reviewed by Fatjon (Tony) Kalemaj and published 13 October 2026.
The short answers.
When does Claude Sonnet 4.5 retire?
On 30 November 2026, from the Claude API. Anthropic announced it on 30 September and recommends moving to Sonnet 5.5.
Does Sonnet 5.5 cost more than Sonnet 5?
No. It is $2 per million input tokens and $10 per million output tokens, the same as Sonnet 5, and Anthropic says it costs up to 30% less per task because it needs fewer tokens. Cache reads cost $0.10 per million since 7 October 2026.
Should my company use Sonnet or Opus?
Sonnet 5.5 for well-scoped everyday work such as drafting, formatting, slides and spreadsheets. Opus 5.5 for complex, open-ended work that needs sustained judgment. Anthropic places Sonnet 5.5 two points below Opus 5.5 on GDPval-AA.
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Need help moving before 30 November?
Our forward deployed engineers work inside client teams to find, test and move what still runs on Sonnet 4.5.