Claude Opus 5.5 vs Claude Sonnet 5: The Ultimate Comparison

Claude Opus 5.5 vs Claude Sonnet 5 is a different kind of comparison from a cross-vendor matchup. Both come from Anthropic and share a 1 million-token context window. Since the Sonnet 5 launch in June, the real question has been which tier of the same family to run.

The headline numbers make it look simple: Opus is far smarter and costs twice as much. The billing details tell a more useful story, because the 2x price gap shrinks fast once caching, effort settings, and retries enter the picture.

This guide shows you where each model earns its price, so you can pick the right one for the app you are building.

Opus 5.5 is the stronger model, and the price gap can be smaller than 2x

The capability gap is not close. The Artificial Analysis comparison puts Opus 5.5 at 58 and Sonnet 5 at 38 at maximum effort, on one test harness. Artificial Analysis labels these scores an estimate while its independent evaluation is pending, so read them as a strong directional signal.

The price gap looks just as clear on paper. Opus 5.5 charges exactly double Sonnet 5 for fresh input and output tokens.

How much of that double you actually pay depends on the workload. Cached reads cost $0.20 on both models, so cache-heavy agent work narrows the gap, while output-heavy work without caching stays close to 2x. The effort setting matters even more. At low effort, Artificial Analysis measures nearly the same cost per benchmark task for both models, $0.55 for Opus 5.5 and $0.51 for Sonnet 5. Opus still scores 42 to Sonnet's 24.

What each model is, and where it sits in the Claude lineup

Anthropic sells Claude in named tiers, and the version number tells you which release of each tier you are on. Mixing up versions is the fastest way to misread a comparison. For the broader family-level picture, see our Sonnet vs Opus guide.

1. Opus 5.5 is the current Opus

Claude Opus 5.5 launched on September 22, 2026, and replaced Opus 5 as Anthropic's Opus-tier model. Anthropic built it for long-running agentic coding and knowledge work, and the Opus 5.5 launch came with a 20% price cut over Opus 5. For the full score sheet, see the Opus 5.5 benchmarks breakdown.

2. Sonnet 5 is the balanced default tier

Claude Sonnet 5 launched on June 30, 2026, as Anthropic's most agentic Sonnet yet, and it is the default model for Free and Pro users on claude.ai. Its introductory $2 and $10 pricing became permanent on August 10, 2026, when Anthropic cancelled a planned move to $3 and $15. The Sonnet 5 overview covers what changed from Sonnet 4.6.

3. Fable 5.1 sits above both

Fable 5.1 sits above Opus as Anthropic's most capable widely released model, at $10 and $50 per 1 million tokens. It is the step up when Opus 5.5 at a higher effort setting still falls short.

Table 1 - Sonnet 5, Opus 5.5, and Fable 5.1 compared. Source: Anthropic model and pricing documentation, as of September 2026.

One more date matters. Anthropic has said Sonnet 5.5 will follow Opus 5.5 in the coming weeks, so this comparison describes the lineup as it stands in late September 2026.

A roughly 20-point gap holds at every effort level

Artificial Analysis puts real distance between the two models at every setting it tested. Because it ran both on the same harness, these numbers compare like with like.

Table 2 - Artificial Analysis figures for both models on one harness. Index scores are labeled an estimate pending independent evaluation. Opus 5.5 results use Anthropic's default fallback setting. Cost per task is the weighted cost of one Intelligence Index task, not a general API cost. As of September 2026.

The gap also holds across the industry-specific indexes Artificial Analysis builds from the same evaluations.

Table 3 - Artificial Analysis capability indexes for both models at maximum effort, labeled an estimate pending independent evaluation. Opus 5.5 results use Anthropic's default fallback setting. As of September 2026.

No category closes the gap. The smallest spread, on economics, is still 17 points, so Sonnet 5's advantage is price, not any category in these indexes.

Anthropic's own launch figures sit in a separate bucket. For Sonnet 5, Anthropic reported 63.2% on SWE-bench Pro, 81.2% on OSWorld-Verified, and 1,618 Elo on GDPval-AA v2, all measured against Opus 4.8 rather than Opus 5.5. The Sonnet 5 benchmarks article walks through each score. Anthropic's Opus 5.5 figures use a newer GDPval version, so the vendor numbers for the two models do not line up as a direct head-to-head.

Pricing: why cost per finished task beats cost per token

The sticker price is the easy part. What you pay for a working result depends on caching, retries, and how you buy tokens.

1. Opus costs exactly twice as much on fresh tokens

Every standard rate on Opus 5.5 is double Sonnet 5's, except one.

Table 4 - Standard API pricing. Source: Anthropic pricing, as of September 2026.

Two details in that table change the math. Cache reads cost the same on both models. And Opus 5.5's base token rates on the Batch API, $2 and $10, match Sonnet 5's standard rates. Work that can wait for a batch run gets Opus-tier quality at Sonnet-tier token prices. Cache and tool charges still apply on top.

Neither model charges a long-context premium on Anthropic's own API. Both bill the full 1 million-token window at their standard rates, though cloud platforms such as Amazon Bedrock and Google Cloud set their own pricing.

Want the full breakdown? Read our Claude Opus 5.5 pricing guide before you commit.

2. Matching cache reads shrink the gap on agent loops

Agents resend the same instructions, tools, and history on every turn, so most of their input comes from cache. That is where the matching $0.20 rate matters.

Take one follow-on agent request that sends 1 million input tokens, 90% of them already in cache, and produces 20,000 output tokens. On Opus 5.5 it costs about $0.98. On Sonnet 5 it costs about $0.58. The gap drops from 2x to roughly 1.7x, because the cached share bills identically on both. This example excludes the one-time cost of writing the cache and any tool charges.

3. Fewer retries can make Opus cheaper per run

The cheaper model only stays cheaper if it gets the job done. Every failed attempt means paying for the whole run again.

In the same example, Opus 5.5 is cheaper per finished result once Sonnet 5 averages more than about 1.7 attempts per task. Put simply, if Sonnet fails its first try on about seven in 10 tasks and every retry succeeds, the two cost the same. Real retry rates vary by task, so measure them on your own workload before relying on this break-even.

Opus 5.5 at medium beats Sonnet 5 at max

The effort setting controls how long a model thinks before answering. Opus 5.5 defaults to medium, while Sonnet 5 defaults to high.

The independent numbers make one rule clear. Opus 5.5 at its default medium setting scores 51 on the Artificial Analysis index, 13 points above Sonnet 5 at its maximum. It also costs less per benchmark task at that setting, $1.34 against $5.09, because Sonnet 5 at max effort generates far more reasoning tokens. Turning Sonnet 5 all the way up costs more and still does not reach Opus.

Anthropic's own guidance points the same way: tuning effort is often a better lever than switching models. In practice, that means adjusting effort within the right model, not pushing a smaller model past its ceiling.

Two routing rules, and which one fits your build

Advice on this matchup splits into two camps, and Anthropic's own documentation points both ways. Each rule fits a different kind of work.

1. Start on Opus 5.5 when work is open-ended or costly to get wrong

Anthropic's models overview tells builders who are unsure to start with Claude Opus 5.5 for most workloads. That default fits work where a mistake compounds: a multi-step build, a data pipeline, or anything a customer will see. The capability headroom is most valuable when a failed run is expensive to repeat or hard to catch.

2. Default to Sonnet 5 when output is high-volume and easy to check

Anthropic's cost guidance frames Sonnet as the model for most production workloads, and that fits work with clear inputs and a quick way to verify the result. Examples include drafting from a template, classifying records, or answering support questions from a known source. Run Sonnet 5 by default and escalate a task to Opus 5.5 when a check catches a failure. For a close look at where Sonnet 5 was already competitive with the previous Opus generation, see Sonnet 5 vs Opus 4.8.

Which model to pick for which job

Match the model to the shape of the work.

Table 5 - Which Claude model fits which job.

Before you standardize on either, run a handful of your real tasks through both and compare cost per finished result. If Sonnet 5 is still more than a task needs, the Sonnet vs Haiku comparison covers Anthropic's lowest-cost tier.

Build with either model on Emergent

If you are describing an app rather than coding it, the model choice becomes a setting you can change per project. Emergent builds full-stack apps from a plain-language description, and both Claude Opus 5.5 and Claude Sonnet 5 are available in its model picker. Opus 5.5 went live on Emergent on launch day.

Access runs through one Universal LLM Key, which covers Claude, GPT, and Gemini models with unified billing from your Emergent credits.

Start Building on Emergent and put the right Claude tier behind each part of your app.