Claude Sonnet 5.5 vs Opus 5.5: Which fits your Business?

Anthropic released Claude Opus 5.5 on 22 September 2026 and Claude Sonnet 5.5 on 28 September 2026. They are the first two models in the Claude 5.5 family, and Haiku 5.5, the smaller sibling, is listed in Anthropic’s documentation as well. This comparison covers what the two models are, what Anthropic claims, what independent testers have found, and which one a small business should try first.

The short version: for most business work, Sonnet 5.5 is the model to start with. Opus 5.5 is the one to reach for when a task is hard, long or open-ended and Sonnet is not getting it right.

Key takeaways

• Both models have a 1 million token context window and can write up to 128,000 tokens in one reply. They read text and images and answer in text.
• Opus 5.5 costs twice as much per token as Sonnet 5.5, on both input and output. Check Anthropic’s pricing page for the current figures.
• Anthropic says Opus 5.5 costs about 40% less than Opus 5 on typical work, and that Sonnet 5.5 costs up to 30% less per task than Sonnet 5. Those are Anthropic’s figures from its own tests.
• On Anthropic’s own coding benchmark, Terminal-Bench 4.0, Sonnet 5.5 scores 70.6% and Opus 5.5 scores 66.4%. On most other tests Opus 5.5 is slightly ahead, often by a small margin.
• Almost every headline number comes from Anthropic or its partners. Independent testing exists and is more mixed.
• Both models send some cybersecurity and biology requests through extra safeguards, which can block legitimate work.

What the two Models are

Both are large language models from Anthropic, the company behind Claude. Their knowledge runs up to June 2026. Anthropic describes Sonnet 5.5 as the best combination of speed and intelligence, and Opus 5.5 as built for long-running agentic coding and knowledge work, meaning jobs where the model carries out many steps by itself.

A context window of 1 million tokens is, on Anthropic’s figures, roughly 555,000 words. That is a few long books, or a large folder of contracts, in one conversation. Whether the model uses all of that well is a separate question, so test long documents before you rely on them.

The family also includes two other models. Anthropic’s documentation lists Claude Fable 5.1 above them as the slower, dearer option, and Claude Haiku 5.5 below them as the fastest and cheapest. Most businesses will only need to choose between Sonnet and Opus.

What they do well

Speed and price. Anthropic says both models write more than 30% faster than their predecessors, and it rates Sonnet 5.5 as fast and Opus 5.5 as moderate. Because Sonnet costs half as much as Opus, it is the sensible default for tasks you run many times a day.

Coding and agent work. Anthropic reports large gains. Sonnet 5.5 scored 70.6% on Terminal-Bench 4.0 against 10.3% for Sonnet 5, and 80.1% on OSWorld 2.1, a test of operating a computer, against 57.0%. Opus 5.5 scored 66.4% and 81.8% on the same two tests. Independent testing by Vals AI agrees in part: it ranks Sonnet 5.5 second on its overall index and first on Code Migration and Vibe Code Bench.

Independent results for Opus. Vals AI’s testing, as summarised by Digital Applied, ranks Opus 5.5 fourth of 60 models on its broad index, just below Opus 5, with gains of 16.1 points on Terminal-Bench 4.0 and 25.7 on Terminal-Bench Science. It also reports declines on medical coding and several legal and tax tests. Counting tasks that fell back to an older model as failures drops its Terminal-Bench 4.0 score to 53.5%, which is a fairer picture of what a user meets when the safeguards step in.

Knowledge work. On GDPval-AA, a test of professional tasks, the two are close: 1844 for Sonnet 5.5 and 1846 for Opus 5.5. On Humanity’s Last Exam with tools, Opus 5.5 leads, 67.7% to 64.5%. For writing, analysis and document work, the gap between the two is small on paper.

Writing clarity. Anthropic says both write more clearly, with the key information placed first. That is a vendor claim and hard to measure, but it is easy to test: give each model the same report to summarise and compare.

Resistance to prompt injection. Anthropic says Opus 5.5 matches or beats Opus 5 at resisting prompt injection, where hidden instructions in a document or web page try to steer the model. That matters if you let a model read email or browse for you.

Where they fall short

The big gap is on hard, open-ended work. Anthropic says plainly that Opus 5.5 remains clearly stronger than Sonnet 5.5 at complex, open-ended work that needs sustained judgement. The benchmark tables hide this because most of them reward well-defined tasks.

Legal and medical coding results are weak. Vals AI’s tests show Sonnet 5.5 scoring 2.92% on Harvey’s Legal Agent Benchmark and 52.92% on MedCode. Vals also reports that Opus 5.5 declined on several legal, tax and public benefits tests compared with Opus 5. If your work is regulated, run your own tests, and keep a human in the loop.

Safeguards can reroute or block work. Anthropic says higher-risk cybersecurity tasks fall back to an older model, and some microbiology and virology requests may be flagged in error. Vals AI found that counting those fallback tasks as failures lowers Sonnet 5.5’s security scores sharply. Opus 5.5 also has a biology classifier. A security consultancy or a life-sciences firm should expect to meet these limits.

Benchmarks are not your work. Several of Opus 5.5’s leads fall within the stated margins of error, and Anthropic itself says the gaps between benchmarks are a less reliable guide to real-world differences than they used to be. It also says Sonnet 5.5 “may have tendencies we haven’t found”.

Hallucinations remain possible. A newer model does not remove the need to check output. A confident wrong answer, what people call a hallucination, is still a risk with any model.

The claims to treat with care

• “40% cheaper”. Digital Applied’s analysis points out that Anthropic’s 40% saving for Opus 5.5 compares the new model at its default medium effort with Opus 5 at high effort. The price cut alone is about 20%, or about 29% on a workload that makes heavy use of caching. The rest depends on how many tokens your own tasks use.
• “Up to 30% less per task”. For Sonnet 5.5 the words “up to” matter. Anthropic also says that at lower effort settings Sonnet 5.5 beats Sonnet 5’s best score for about a tenth of the cost per task, which is again measured on Anthropic’s chosen tests.
• Effort settings change the picture. Most benchmark figures use the highest effort setting. Your day-to-day cost depends on the setting you actually run.
• Case studies. The customer examples Anthropic quotes have not been independently replicated.

What the effort setting changes

Both models have an effort setting that controls how long they think before answering. Anthropic’s documentation gives the API default as high for Sonnet 5.5 and medium for Opus 5.5, and says the apps and Claude Code default to medium. Higher effort usually means better answers on hard problems, slower replies and more tokens used, so a higher cost per task.

This matters when you compare the two. A quoted saving or benchmark score is tied to one effort level, and Anthropic’s Sonnet 5.5 page notes that on several tests the model at low or medium effort beats Sonnet 5’s best score at about a tenth of the cost per task. It also notes that Sonnet 5.5 scored lower on one coding test at its maximum setting than at the one below it, which Anthropic puts down to extra review steps causing timeouts. More effort is not always better. If you use the apps, you mostly meet this as a choice between a faster answer and a more careful one. If you use the API, ask whoever configures it which setting your tasks run at.

Price and access

Both models are available through Anthropic’s API and on Amazon Web Services, Google Cloud and Microsoft Azure. Anthropic says zero data retention is available, which matters for confidential material. Usage limits were raised on the paid Claude plans when Opus 5.5 launched, but this review does not quote plan prices or say which model is the default in the app, because those change. Check Anthropic’s own pricing page.

API prices are per million tokens, and the Batch API, for work that does not need an instant reply, is half price on both models. For scale, a million tokens is about 555,000 words, so a single email or contract is a small fraction of that.

What this means for Software that uses the API

This section is only for businesses with software that calls Claude. Anthropic lists breaking changes that stop existing code working unchanged. On Opus 5.5, thinking cannot be turned off, and forcing a specific tool to be used returns an error. Thinking blocks are tied to the model that produced them, and the older computer-use tool is not accepted on the Claude API. Sonnet 5.5 has a similar list and also rejects non-default settings for temperature. A further change is silent: short notes between tool calls now arrive as thinking blocks that are empty by default, so an app that shows progress messages can go quiet without any error. If you rely on custom software, ask whoever maintains it to read Anthropic’s migration guide before switching.

How it compares

The chart below shows one coding test from Anthropic’s launch tables. The figures are Anthropic’s, run at its chosen settings, so read it as a rough picture and not a league table.

Coding agent test (Terminal-Bench 4.0). Sonnet 5.5 edges out Opus 5.5, and both are far ahead of Sonnet 5.

On other tests Opus 5.5 is usually ahead of Sonnet 5.5, but by only a few points. Anthropic also says GPT-6 Astra scores higher than Opus 5.5 on a couple of tests, AutomationBench and Terminal-Bench-Science, so neither model wins everything. For how Claude compares with its main rival in day-to-day office use, read our review of Claude against ChatGPT. For another new release, see our Mistral Large 4 review.

How to test them on your own work

Benchmarks cannot tell you which model suits your documents, your customers or your tone of voice. A short test can, and it needs no technical skill.

1. Pick three tasks you really do, one easy, one medium and one hard. For example: reply to a routine customer email, summarise a long supplier contract, and draft a proposal from rough notes.
2. Run each task on Sonnet 5.5, on Opus 5.5 and on the tool you use now, with the same instructions and the same material each time.
3. Remove any customer or personal data first, unless your plan and contract allow it, because GDPR still applies when you paste it into a chat.
4. Score the three answers blind, asking someone who does not know which model wrote which. Judge accuracy first, then tone, then how much editing each one needs.
5. Note how long each took and how much of your allowance it used.

If Sonnet matches Opus on the easy and medium tasks, use Sonnet for those and keep Opus for the hard one. If Opus is better on every task, the extra cost may be worth it for you. Either way you have evidence from your own work and not from a vendor’s chart.

Who it suits

Sonnet 5.5 suits most small businesses: drafting emails and proposals, summarising long documents, working through spreadsheets, and running a steady stream of routine tasks where speed and cost matter.

Opus 5.5 suits the harder jobs: long multi-step projects, complicated analysis, and anything where Sonnet’s answer keeps missing the point. It costs twice as much, so it makes sense for the few tasks that need it and not for everything.

Neither suits work you cannot check. Whatever you use it for, keep a person reading the result before it goes to a customer.

Verdict

Claude Sonnet 5.5 and Opus 5.5 are strong, fast, well-documented models, and Sonnet 5.5 in particular looks like very good value. The evidence for the big gains over the previous generation is real, but most of it comes from Anthropic, and the independent tests show a mixed result in legal and medical work.

For most businesses the sensible move is this. Try Sonnet 5.5 first. Pick one task you really do, such as summarising a customer contract or drafting a proposal, and run it on Sonnet 5.5, on Opus 5.5 and on the tool you use now. Move up to Opus only where Sonnet falls short. Treat every benchmark in this review as a hint and not a verdict. If you would like help choosing a model and checking it against your own processes, that is the kind of question our AI Audit is built to answer.

Further reading

• Claude vs ChatGPT: Which fits your business best?: how the two main assistants differ on data, admin and access.
• Mistral Large 4 review: Is it Right for your Business?: a European open-weight alternative.
• How to check AI output before it goes to a customer: the review step no model removes.

Sources

• Introducing Claude Opus 5.5 (Anthropic): the official announcement, pricing, benchmarks, safeguards and limitations.
• Introducing Claude Sonnet 5.5 (Anthropic): the official announcement, benchmarks, cost claims, safeguards and limitations.
• Claude Opus 5.5 overview (Claude Platform docs): context window, output limit, pricing, availability and breaking changes.
• Claude Sonnet 5.5 overview (Claude Platform docs): context window, output limit, pricing, availability and breaking changes.
• Claude Opus 5.5: pricing, benchmarks and breaking changes (Digital Applied): the cost-saving caveats and independent Vals AI measurements for Opus 5.5.
• Claude Sonnet 5.5 benchmarks, cost and capabilities (Vals AI): independent ranking, strengths, weaknesses and refusal fallback results.