Claude Sonnet 5.5 Ships at Sonnet 5's $2/$10, and Thinking-Off Setups Must Move to between_tools First
- What happened
- Anthropic shipped Claude Sonnet 5.5 (claude-sonnet-5-5) on 28 September at Sonnet 5's $2/$10 per 1M tokens, now the default Sonnet on the Anthropic API.
- Why it matters
- On Anthropic's own table it nearly matches Opus 5.5 at half the price, but thinking-off integrations must switch to between_tools before migrating.
- What to do
- Move thinking-off configs to between_tools, then run your own evals against Sonnet 5 and Opus 5.5 before switching production.
Our verdict: test Claude Sonnet 5.5 on anything you run on Claude Sonnet 5 today, but Sonnet 5 stays recommended and Sonnet 5.5 stays conditional until someone other than Anthropic measures it. Input and output prices are unchanged, and Anthropic's table puts Sonnet 5.5 ahead of Sonnet 5 on every row and level with Claude Opus 5.5 at half the price. We have not run Sonnet 5.5 ourselves. If you run Sonnet with thinking off, you have one step to take before you switch.
What happened
Anthropic released Claude Sonnet 5.5 on 28 September 2026 under the API id claude-sonnet-5-5. Claude Code 2.1.284, released the same day, added it as "now the default Sonnet model on the Anthropic API", with a 1M-token context.
| Per 1M tokens | Sonnet 5.5 | Opus 5.5 |
|---|---|---|
| Input | $2 | $4 |
| Output | $10 | $20 |
| Cache reads (current) | $0.10 | $0.20 |
| Cache writes | $2.50 | $5 |
- Price: $2 input and $10 output per 1M tokens, the same as Sonnet 5.
- Cache reads: $0.20 per 1M tokens at launch, per the Claude Code 2.1.284 release notes. Anthropic's page now lists $0.10.
- Cost per task: Anthropic says the model "costs up to 30% less per task than its predecessor" because it needs fewer tokens.
- Speed: Anthropic says it "generates outputs 30%+ faster than Sonnet 5".
- Availability: Claude Platform, Amazon Web Services, Google Cloud and Microsoft Azure.
- Default effort: Medium in Claude Code and the Claude apps, High on the Claude Platform.
The cost and speed figures are Anthropic's own measurements.
Why it matters
Anthropic's benchmark table:
| Benchmark | Sonnet 5.5 | Sonnet 5 | Opus 5.5 |
|---|---|---|---|
| Terminal-Bench 4.0 | 70.6% | 10.3% | 66.4% |
| CursorBench 4.0 | 55.5% | 34.1% | 57.8% |
| GDPval-AA v2.1 | 1844 | 1449 | 1846 |
| OSWorld 2.1 | 80.1% (partial) | 57.0% | 81.8% |
On Anthropic's numbers, Sonnet 5.5 comes within about two points of Opus 5.5 at half the input and output price, and beats it on Terminal-Bench 4.0. That makes the Opus comparison, not the Sonnet 5 one, where the money is.
Treat the gap with care. Sonnet 5's Terminal-Bench score of 10.3% is far below anything else in the table, and Anthropic's post does not explain it. Until an independent run reproduces that 60-point gap, it is Anthropic's result, not a fact.
Two safeguards change behaviour:
- Cyber fallback: "higher-risk cybersecurity tasks will visibly fall back to Sonnet 5". If you use Sonnet for security testing, expect some requests to be answered by the older model.
- Preserved thinking: Anthropic says "Claude's thinking cannot be decoupled from the account that created it".
What changes for you
The migration step. In Anthropic's words: "If you run Sonnet with thinking off, you'll need to switch to the new between_tools setting" before moving to Sonnet 5.5. Change that setting first and the model id second. It is the same pattern as Claude Opus 5.5, where our directory notes thinking cannot be turned off.
Our directory entry lists five breaking changes for code written for Sonnet 5. Hold off if you rely on forced tool choice or temperature control.
Your default may already have moved. After updating Claude Code to 2.1.284 or later, check which Sonnet your sessions and pinned model settings actually resolve to.
Where our directory stands. claude-sonnet-5 stays recommended: nothing we read retires it, and the new model's lead rests only on Anthropic's own tests. claude-sonnet-5-5 is conditional for the same reason. For new Sonnet workloads, start your testing on Sonnet 5.5. It costs nothing more per token, and Anthropic's per-task claim, if it holds, makes it cheaper.
What to do
- 1 If any integration runs Sonnet with thinking off, switch it to the between_tools setting before changing the model id to claude-sonnet-5-5.
- 2 Run your own evals on claude-sonnet-5-5 against claude-sonnet-5 and Opus 5.5. At $2/$10 against $4/$20, the Opus comparison is where the savings are.
- 3 After updating Claude Code to 2.1.284 or later, check which Sonnet your sessions and pinned model settings actually resolve to.
- 4 Security teams: expect some higher-risk cybersecurity prompts to be answered by Sonnet 5 rather than Sonnet 5.5.
Affected tools & models
Never need to catch up again
The weekly delta: only verdict changes and act-now items. No digest filler.