Claude Opus 5.5 vs Sonnet 5.5: which one should you use?

It is a few minutes past eleven at night and you have a cover letter due in the morning. You paste the job advert and your rough draft into a chat, open the model menu, and find two new names from Anthropic sitting one above the other: Claude Opus 5.5 and Claude Sonnet 5.5. You pick Opus, because Opus is the big one, and you watch it think. The letter that comes back is good. What you will never know is whether Sonnet would have written much the same letter, sooner.

Both names are new. Anthropic released Opus 5.5 on 22 September 2026 and Sonnet 5.5 six days later, so most people meeting that menu have had no time to form a habit. Here is where we land, having read Anthropic's launch material and documentation for both models as of 2 October 2026: Sonnet 5.5 should be your everyday Claude, and Opus 5.5 is the one to reach for when a task is long, open-ended, or costly to get wrong. The gap between them is smaller than the names suggest, and it shows up in particular places. This piece goes through those places task by task.

What Anthropic says each model is for

Anthropic's own framing is unusually direct. On the Sonnet 5.5 launch page it writes: "Where Opus 5.5 is built for complex work requiring careful judgment, Sonnet 5.5 is strongest at well-scoped everyday tasks". A well-scoped task is one with clear edges: rewrite this paragraph, explain this letter from the bank, plan a four-day trip to Porto. You can say in a sentence what a good answer looks like. An open-ended task, such as deciding whether a job offer is worth moving cities for, has no single right answer and many plausible wrong ones.

The same page is also more generous to Sonnet than a company usually is about its mid-range model. On several evaluations, Anthropic says, Sonnet 5.5 at its highest effort setting "even performs comparably to Opus 5.5", and on a test of real-world work across 44 occupations it "scores nearly level with Opus 5.5". Then it draws the line again: "Opus 5.5 remains clearly stronger at complex, open-ended work requiring sustained judgment." We take both claims at face value, and the rest of this article is really about the second one.

Here is how they compare on Anthropic's own figures.

Sources: Anthropic's launch pages for Opus 5.5 and Sonnet 5.5, its model pages for Opus and Sonnet, its models overview and the Claude help centre on effort and thinking, all checked on 2 October 2026.

Two rows deserve a second look. The benchmark row is close, about three points apart, and Anthropic itself warns against reading too much into that kind of margin, writing in the Opus announcement that "benchmark margins have become a less reliable guide to real-world differences". Then there is the thinking row. Opus 5.5 is "no longer available with 'thinking' mode switched off", and the help centre confirms that in Claude's apps thinking cannot be turned off for Sonnet 5.5 either. In Claude's apps, and in ours, thinking stays on for both. What you control is how hard they think, which Anthropic calls effort, and that turns out to matter nearly as much as which model you pick.

Writing, tone and everyday questions

Both models arrived with the same push on writing quality, and it is the change most people will notice first. Anthropic says Opus 5.5's messages are "much easier to understand at a glance", that it "puts the most important information up front, is less likely to use jargon or idiosyncratic phrases, and follows the writing rules you give it". For Sonnet 5.5 the claim is shorter: it "writes more clearly than our previous generation of models", and early testers found it "a more natural conversational partner". One tester, a designer at Every, put it plainly: "It's got some of Opus 5.5's natural writing upgrades".

Some of them, then. In our reading, the difference that matters is stamina. For a cover letter, a polite complaint to a landlord, a birthday toast or a summary of a school newsletter, Sonnet 5.5 is plenty, and the thing you will notice is that it answers sooner. Anthropic rates Sonnet as "Fast" and Opus as "Moderate" on its own comparison table.

Opus earns its place in writing when the job is long and has to hold together. A grant application built up over an evening, a family history assembled from a dozen pasted documents, a newsletter that has to follow the same house style every week: these are tasks where you give a model rules early and need it to keep them forty messages later. Anthropic's pitch for Opus leans on exactly this, saying its style "makes it a better work partner over long sessions".

Tone is the one place where we would not pick on capability at all. Anthropic describes both models as writing more clearly and with less jargon than their predecessors, which in practice means plainer prose by default. If you want warmth, humour or a particular voice, say so in your first message rather than hoping either model picks it up from context.

Where Opus pulls ahead: research and long documents

This is where the case for Opus is strongest, and Anthropic's most telling evidence is a single internal test. It asked Opus 5.5, Claude Fable 5.1 and Opus 5 to write a report on a company's quarterly results using only what each could find on a copy of the web where the earnings release was hard to locate. An automated grader checked every figure and quote against the sources, and any invented figure or quote failed the report. According to Anthropic, 16 of Opus 5.5's 18 reports cleared the bar, and "Neither Fable 5.1 nor Opus 5 cleared that bar in any attempt."

This is Anthropic grading its own model, and it published no comparable result for Sonnet 5.5, so the test says nothing about whether Sonnet would fail. It does match what outside testers said. The data company Hex wrote that Opus 5.5 "keeps digging past the first plausible answer". Sonnet 5.5's testers praised it for something narrower and still useful: Box said it "rechecks data in source documents, catching errors that Sonnet 5 failed to spot".

So our rule for research is simple. If a wrong figure would embarrass you, say a number you are about to quote at work or a comparison of two insurance policies, use Opus 5.5, give it time to think, and still check the figures that matter yourself. If you want a quick explanation of a topic or a summary of an article you have already read, Sonnet 5.5 will do it well and sooner.

Long documents are a closer call than the spec sheet suggests. On paper both models accept up to 1M tokens, which Anthropic puts at roughly 555,000 words. Those are model limits, and apps, ours included, set their own limits on how much a single message can carry. In practice, answer quality in any model slips well before the hard limit as a conversation fills up, which we explain in what a context window is. On the MultiChats website, the context meter tells you when a chat is reaching the point where answers tend to get worse, and that point arrives sooner on Sonnet 5.5 than on Opus 5.5. If you plan to ask many questions about one very long document over a single chat, Opus gives you more room before that happens.

How hard should either one think?

Because neither model can skip thinking any more, the real decision is often two decisions: which model, and at what effort. Anthropic's help centre sums up the trade: "Higher effort means more thorough responses, but they take longer". Testers' reports suggest the setting moves results a long way. The finance AI company Rogo said that "At its lowest effort setting, Claude Opus 5.5 beat Opus 5 at high effort" on its finance tests.

Our rule of thumb follows from Anthropic's own split between well-scoped and open-ended work. If the task is clear and the answer was a little thin, raise the effort before you change the model. Anthropic says that at higher settings Claude "reasons for longer and checks its work more thoroughly", which is often what a thin answer was missing. If the task itself is fuzzy, with conflicting sources, a judgement call or a long chain of steps, switch to Opus. Extra effort buys checking time, and judgement on open-ended problems is the thing Anthropic built Opus for.

One thing changes when you try this through us. Anthropic's own apps list effort levels above High, up to one called Max, and the line about Sonnet performing comparably to Opus was measured at Max. In MultiChats the effort menu for both models stops at High, so do not expect our Sonnet at High to match every benchmark Anthropic quotes for Sonnet at Max.

Using both in MultiChats

Both models are in MultiChats on the paid plans, Pro and Pro+, which you can compare on our pricing section. If you still have Claude Opus 5 or Sonnet 5 selected from before the launches, the app now swaps them for the 5.5 versions automatically.

By default the app decides, message by message, whether to search the web and how hard the model should reason. That setting is called Intent Detection and it is on for every account to begin with. If you would rather pin an effort level yourself, on the website go to Settings, then Tool Preferences, and switch Intent Detection off; the effort button then appears in the message box, near the model picker. The phone apps always show the effort control. As in Anthropic's apps, neither model offers an Off option. Each reply is labelled with the model and, where one was set, the effort level, for example "Claude Sonnet 5.5 (High)", so you can see what produced an answer.

The most useful habit is the Try Again menu under a reply. Pick the other Claude there and you get its answer to the same question in a new thread, while the original stays where it was. Do that with a real task from your week and you will learn more than any table can tell you. The model card in the picker also shows what each uses per message: 8× for Opus 5.5 and 4× for Sonnet 5.5, as of 2 October 2026.

Sonnet first, Opus on purpose

Start with Sonnet 5.5. It is the model Anthropic says anyone can chat with on Claude.ai, and for the well-scoped tasks that fill most days it answers quickly and writes in the same clearer style as Opus. Move to Opus 5.5 when a conversation will run long and has to keep your rules, when the task asks for judgement more than knowledge, or when a single invented figure would cost you.

You might reasonably ask why anyone already paying would not simply use Opus for everything, since it is the stronger model. Our answer is that clear, bounded tasks are the ones Anthropic says Sonnet is strongest at, and Opus's extra care is paid for in waiting time on every reply. A model that thinks hard about a two-line answer is still a model you are waiting on. We would rather keep that patience for the tasks that reward it.

One more name is on the way. Anthropic says Claude Haiku 5.5 will join the family "in the coming weeks", and as of 2 October 2026 its models overview still listed Haiku 4.5. When it lands, the question to ask of it is the one this whole piece turns on: how open is the task in front of you?