11 questions answered. Every answer sourced.
Claude Opus vs Sonnet: Which Model Should You Use?
Opus is Anthropic's model for complex, judgment-heavy work; Sonnet is the faster, cheaper model for well-scoped everyday tasks. With Opus 5.5 and Sonnet 5.5 the gap narrowed. Here is how to choose.
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Facts checked against official announcements and documentation, October 2026 · Last updated October 2026
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01 · Difference
Opus vs Sonnet: the core difference
Judgment versus speed and cost.
What is the difference between Claude Opus and Sonnet?
Opus is Anthropic's larger, more capable model tier, built for complex work requiring careful judgment, long agentic tasks and difficult reasoning. Sonnet is the mid tier: faster and half the price per token, and strongest at well-scoped everyday work. Anthropic describes Sonnet 5.5 as a faster, lower-cost complement to Opus 5.5.
Source: Anthropic: Introducing Claude Sonnet 5.5 (Sep 28, 2026); Anthropic: Claude Opus 5.5
Is Sonnet 5.5 as good as Opus 5.5?
Close on many tasks. Anthropic reports Sonnet 5.5 scores two points below Opus 5.5 on GDPval-AA, a test of real-world occupational work, and that at Max effort it performs comparably to Opus 5.5 on several evaluations. Opus remains stronger on the hardest, most open-ended work.
Source: Anthropic: Introducing Claude Sonnet 5.5 (Sep 28, 2026)
What about Opus 5 vs Sonnet 5?
Opus 5 ($5/$25) and Sonnet 5 ($2/$10, launched June 30, 2026) are now the previous generation. Sonnet 5.5 keeps Sonnet 5's price and Opus 5.5 is cheaper than Opus 5, so new projects should use the 5.5 models.
Source: CloudZero: Claude Opus pricing in 2026; Coursiv: Claude Sonnet 5.5 cost per task by effort (Artificial Analysis data)
02 · Cost
Price per token vs cost per task
The effort setting matters as much as the model.
How much do Opus 5.5 and Sonnet 5.5 cost?
Opus 5.5 costs $4 per million input tokens and $20 per million output. Sonnet 5.5 costs $2 and $10. Cache reads are $0.20 per million on Opus 5.5 and $0.10 on Sonnet 5.5, and the Batch API halves both directions.
Source: Anthropic: Introducing Claude Sonnet 5.5 (Sep 28, 2026); CloudZero: Claude Opus pricing in 2026
Is Sonnet always cheaper per task?
No. Artificial Analysis data reported by Coursiv shows Sonnet 5.5 cheaper per task than Opus 5.5 at Low through High effort, but at Max effort Sonnet 5.5 reached a score Opus 5.5 matched at Xhigh for less than half the cost per task. Pushing Sonnet to its ceiling can cost more than using Opus at a moderate setting.
Source: Coursiv: Claude Sonnet 5.5 cost per task by effort (Artificial Analysis data)
When is paying twice as much for Opus worth it?
When a task fails on Sonnet or needs multiple attempts. Two Sonnet retries cost about the same as one Opus pass that works, and long-running agents benefit from Opus finishing in fewer steps.
03 · Use cases
Which model for which job
A practical routing guide.
When should I use Sonnet?
Everyday writing and editing, documents, slides and spreadsheets, bug fixes and well-scoped coding tasks, customer-support and internal assistants, and any high-volume workload where speed and cost matter. Anthropic highlights Sonnet 5.5's design sense and polished outputs.
Source: Anthropic: Introducing Claude Sonnet 5.5 (Sep 28, 2026)
When should I use Opus?
Complex refactors and long agentic coding sessions, multi-step research and analysis, ambiguous problems that need careful judgment, and high-stakes work where a better first answer saves review time.
Source: Anthropic: Claude Opus 5.5; CloudZero: Claude Opus pricing in 2026
Which should I pick in Claude Code?
Use Sonnet for routine edits and fixes and switch to Opus for planning, large refactors or when Sonnet gets stuck. Adjusting the effort level also lets you trade speed and cost against quality on either model.
Source: Anthropic: Introducing Claude Sonnet 5.5 (Sep 28, 2026)
04 · Plans
Using Opus and Sonnet in Claude plans
Subscription usage versus API billing.
Can I use Opus on the free Claude plan?
Model access varies by plan and changes over time. Paid Claude plans give access to the latest Opus and Sonnet models with higher limits; check the model menu and Claude's pricing page for your plan.
Source: Claude: Plans and pricing
Does Opus use my Claude limits faster?
Generally yes, because Opus is the larger model. If you hit usage limits often, default to Sonnet and switch to Opus only for tasks that need it.
Source: Claude: Plans and pricing
Sources
Read and checked October 2026. Policies change, so confirm anything time-sensitive on the source page.