Claude Sonnet 5.5 vs Opus 5.5: which model to use for what, and what it costs

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Anthropic shipped two models in a week. Claude Opus 5.5 arrived on September 22, and Claude Sonnet 5.5 followed on September 28. If you run Claude for your team, or pay for automations built on it, the question is simple: which one should do which job, and what does the choice do to your costs?

Here is the short answer, then the reasoning.

The short answer

Make Sonnet 5.5 your default for everyday work. Use Opus 5.5 for the steps where judgment, ambiguity, or a long chain of decisions is the actual job. Most teams will end up with Sonnet doing the bulk of the volume and Opus handling the smaller set of tasks where a mistake is expensive.

What each model is for

Per Anthropic's Sonnet 5.5 announcement, Sonnet 5.5 runs more than 30% faster than Sonnet 5 and costs up to 30% less for most work, because it typically needs far fewer tokens to finish the same task. Anthropic positions it as a faster, lower-cost complement to Opus 5.5, aimed at well-scoped everyday tasks, fixing bugs, and building polished documents, slides, and spreadsheets.

Per Anthropic's Opus 5.5 announcement, Opus 5.5 performs at the level of Claude Fable 5.1 on most work, generates output more than 30% faster than Opus 5, and costs 40% less than Opus 5 on typical workloads at default settings. Anthropic describes Opus 5.5 as clearly stronger at complex, open-ended work that needs sustained judgment, and that is where Sonnet 5.5 gives ground. On the GDPval-AA benchmark, Sonnet 5.5 lands two points below Opus 5.5, which is a small gap on routine work and a wider one when the task is messy.

Which model for which business task

Sonnet 5.5 fits well-scoped, repeatable work: drafting lead follow-up from a clear template, updating CRM records after a call, building a weekly report from known sources, formatting proposals and slide decks, and cleaning up spreadsheets. These tasks have a clear definition of done, so a faster, cheaper model gets there without losing quality.

Opus 5.5 fits open-ended, high-stakes work: reviewing a contract and flagging what matters, researching an account where the right answer is not obvious, running a multi-step agent across several systems, and any workflow where a wrong call costs you a client or a compliance problem. These are the tasks where sustained judgment pays for itself.

Fable 5.1 remains Anthropic's top tier. The practical news from this week is that Opus 5.5 closes most of the gap on most work, so the set of tasks that truly need the most expensive tier just got smaller.

The cost trade-off, honestly

Per token, Sonnet 5.5 costs half of what Opus 5.5 does. That is the headline, and it is the wrong number to optimize on. Here are the list prices from Anthropic, per million tokens:

• Sonnet 5.5: $2 for input, $10 for output.
• Opus 5.5: $4 for input, $20 for output. That is 20% below Opus 5, which listed at $5 and $25.
• Cached input (both models): $0.20, so repeated context such as a long instruction set or a reference document is cheap to reuse.

What matters is cost per finished task. A cheaper model that needs two retries, or hands back work a person has to rework, is not cheaper. A pricier model that gets a hard task right the first time often is. Both launches lean on the same idea: fewer tokens per task, not just a lower rate per token. That is why Anthropic quotes savings on typical workloads rather than on price sheets.

One more detail worth knowing: Opus 5.5 defaults to medium effort, and Anthropic says that setting matches or beats prior models at maximum effort. If an automation was tuned on an older Opus, it is worth re-testing at the new default before assuming you need to pay for more thinking.

A practical way to split the work

In the automations we build, the useful pattern is routing by step, not by project. A lead-qualification workflow, for example, can use Sonnet for the high-volume steps (pull the record, summarize the inquiry, draft the reply) and reserve Opus for the one step that decides whether the lead is worth a human’s time. You get most of the speed and savings of the cheaper model without handing it the judgment calls.

If you only use Claude through a subscription, this mostly comes down to picking the right model for the job in front of you. If your automations run through the API, the model choice per step shows up directly in your run costs, and it is one of the first things worth reviewing after a build.

What this means for what you spend on Claude

Model choice is one line in the total cost of putting Claude to work. Setup, workflow design, integrations, and ongoing management are separate, and usually larger. We break that down in how much Claude implementation costs. The new models help most where volume is high and the work is routine, which is exactly where cheaper, faster runs add up. If you want to see where your team sits, our free AI value assessment is a good place to start.

Frequently asked questions

Should I use Claude Sonnet 5.5 or Opus 5.5?

Use Sonnet 5.5 as your default for well-scoped, everyday work such as drafting, reporting, CRM updates, and building documents or spreadsheets. Use Opus 5.5 for complex, open-ended work that needs sustained judgment, such as contract review, ambiguous research, or multi-step agents across several systems.

How much cheaper is Opus 5.5 than Opus 5?

Anthropic says Opus 5.5 costs 40% less than Opus 5 on typical workloads at default settings. That combines a lower rate per token with fewer tokens used per task. Your own savings will depend on how your workflows are built.

How much cheaper is Sonnet 5.5 than Sonnet 5?

Anthropic says Sonnet 5.5 costs up to 30% less than Sonnet 5 for most work, because it typically needs far fewer tokens to do the same task. It also runs more than 30% faster.

How much do Sonnet 5.5 and Opus 5.5 cost?

Anthropic lists Sonnet 5.5 at $2 per million input tokens and $10 per million output tokens. Opus 5.5 is $4 per million input tokens and $20 per million output tokens, 20% below Opus 5. Cached input is $0.20 per million tokens on both. What your team actually pays depends on how many tokens your workflows use.

Do I need to change anything to use the new models?

If you use Claude.ai or Cowork directly, you can choose the new models from the model picker. If your automations are built on the API, the model being called has to be updated, ideally with a quick test on your own workflows first. TEG can review which steps should use which model.