The Claude Sonnet 5.5 release date was 28 September 2026 — six days after Claude Opus 5.5, and earlier than the “coming weeks” Anthropic had signalled. It’s live now on the Claude API and the major clouds, at the same price as Sonnet 5 but faster and cheaper per task by Anthropic’s own testing. Here’s everything from the official docs: the specs, the pricing, how it compares with Opus 5.5, and — the part most coverage skips — the five breaking changes that will bite if you just swap the model ID in existing Sonnet 5 code.
Key takeaways
- Released 28 September 2026; model ID
claude-sonnet-5-5on the Claude API, Amazon Bedrock (anthropic.claude-sonnet-5-5), Google Cloud, Microsoft Foundry and Claude Platform on AWS. - Pricing unchanged from Sonnet 5: $2 input / $10 output per million tokens, $0.20 cache reads, Batch API 50% off.
- 1M-token context, 128K max output (300K on the Batch API beta), adaptive thinking on by default at
higheffort, June 2026 knowledge cutoff. - Anthropic reports 30%+ faster output and up to 30% lower cost per task vs Sonnet 5 — but five breaking changes need code updates.
Claude Sonnet 5.5 release date and what shipped
Anthropic released Claude Sonnet 5.5 on 28 September 2026, positioned in its docs as “the best combination of speed and intelligence”. When Opus 5.5 launched on 22 September, Anthropic said Sonnet 5.5 and Haiku 5.5 would follow in the coming weeks; Sonnet arrived six days later. As of 29 September, Haiku 5.5 hasn’t been released — the docs still list Haiku 4.5 as the fastest model — so that’s the one left to watch.
It’s also the model behind Claude’s own apps — Anthropic publishes the system prompt Sonnet 5.5 uses on claude.ai and the Claude apps — and it’s committed to keeping it available until at least 28 September 2027.
Claude Sonnet 5.5 specs, pricing and how it compares
| Model | Context | Max output | Price / MTok (in/out) | Latency | Thinking |
|---|---|---|---|---|---|
| Claude Fable 5.1 | 1M | 128K | $10 / $50 | Slower | Adaptive (always on) |
| Claude Opus 5.5 | 1M | 128K | $4 / $20 | Moderate | Adaptive (always on) |
| Claude Sonnet 5.5 | 1M | 128K | $2 / $10 | Fast | Adaptive |
| Claude Haiku 4.5 | 200K | 64K | $1 / $5 | Fastest | Extended |
Pricing detail from the docs: 5-minute cache writes $2.50, 1-hour cache writes $4, cache reads $0.20 per million tokens, and 50% off on the Batch API. Input is text and images; output is text. For most everyday work — coding, agents, writing — Sonnet 5.5 is now the value pick: Opus 5.5 costs twice as much for the harder reasoning jobs, and my Opus 5.5 guide covers when that’s worth it.
Want this working in your business, not just bookmarked? Getting Claude Sonnet 5.5 working in your agents and workflows is exactly the kind of thing we build together inside the AI Profit Boardroom — 3,400+ members, four live calls a week, daily tutorials, plug-and-play templates and a 30-day roadmap so you ship instead of watch.
Prefer it mapped 1-on-1 first? Book a free strategy session and we’ll plan it for your exact situation.
Switching to Claude Sonnet 5.5: the breaking changes
If you run Sonnet 5 in code, don’t just swap the model ID. Anthropic lists five breaking changes:
- Up-front thinking: turn it off with the new
between_toolssetting (the lowest thinking level, athigheffort or below). - Forced tool use returns an error.
- Thinking blocks are tied to the model and conversation that produced them.
- The older
computer_20251124computer-use tool isn’t accepted on the Claude API and Google Cloud. - The advisor tool rejects Claude Opus 4.8, Opus 4.7 and Sonnet 5 as advisors.
One more quiet change: text between tool calls now comes back inside thinking blocks, so an app that streams that text to users will go silent between tool calls until you set a display value or use between_tools. Also note: setting temperature, top_p or top_k to non-default values returns a 400 error, and the minimum cacheable prompt is 512 tokens.
Sourcing, dated 29 September 2026: specs, pricing, model IDs and breaking changes are from Anthropic’s official Sonnet 5.5 docs page; the “30%+ faster, up to 30% cheaper per task” figures are Anthropic’s own testing as reported in launch coverage. I haven’t published my own benchmarks on Sonnet 5.5 yet.
The bottom line on the Claude Sonnet 5.5 release date
The Claude Sonnet 5.5 release date was 28 September 2026, and it’s a strong upgrade at no extra cost: 1M context, adaptive thinking, faster output and cheaper tasks at the same $2/$10 pricing. If you build with the API, read the five breaking changes before you switch; if you use Claude in the apps, you’re already getting it. Haiku 5.5 is the next one to watch.
FAQ: claude sonnet 5.5 release date
When was Claude Sonnet 5.5 released?
On 28 September 2026, six days after Claude Opus 5.5 — per Anthropic’s official model documentation.
How much does Claude Sonnet 5.5 cost?
$2 per million input tokens and $10 per million output tokens — unchanged from Sonnet 5 — with $0.20 cache reads and 50% off on the Batch API.
What is Claude Sonnet 5.5’s context window?
1 million tokens, with up to 128K output tokens (300K on the Batch API with a beta header).
Is Claude Sonnet 5.5 better than Sonnet 5?
Anthropic reports over 30% faster output and up to 30% lower cost per task, plus adaptive thinking by default — but there are five breaking changes for existing API code.
What is the Claude Sonnet 5.5 model ID?
claude-sonnet-5-5 on the Claude API, Google Cloud, Microsoft Foundry and Claude Platform on AWS; anthropic.claude-sonnet-5-5 on Amazon Bedrock.
When is Claude Haiku 5.5 coming?
Anthropic said Haiku 5.5 would follow Opus 5.5 in the coming weeks; as of 29 September 2026 it hasn’t been released.
Two ways I can help from here. If you want the community, the templates and the weekly momentum, join the AI Profit Boardroom — it’s where your Claude 5.5 upgrade gets built with 3,400+ members doing the same.
If you want a personal plan first, grab a free strategy session and bring your questions — no pitch-fest, just the roadmap.
About Julian Goldie
I’m Julian Goldie — SEO agency founder, best-selling author, and one of the most-watched AI SEO educators on YouTube with 394K+ subscribers. I’ve spent 10+ years in SEO and link building, hold a 100% Job Success Score on Upwork, and run a community of 75K+ members learning AI-powered SEO. I test everything on my own sites first — what you read here comes from those tests. Join the AI Profit Boardroom for the daily builds, or book a free strategy session to talk through yours.
Related reading
- Hermes Claude Opus 5.5: Run It Inside Hermes Agent
- Claude Code Opus 5.5 Default
- Claude SEO Skill for Claude Code: Build & Install
Last updated September 2026. This page is a living guide to claude sonnet 5.5 release date — the facts here move fast and I update it as they do.