Opus vs Sonnet: Which Claude Model Should You Use?

The short answer: Claude Opus 5 is built for complex, multi-step coding and agent (agent — an AI that plans and carries out multiple steps on its own) work, while Claude Sonnet 5 is built for most everyday tasks that need a balance of speed and capability. The two models differ in price, response speed, and reliable knowledge cutoff (the date through which a model's knowledge is most extensive and accurate), and knowing these differences helps you pick the right one for an API (Application Programming Interface — a set of rules that lets programs exchange data) request or in the Claude.ai app. This guide is written for both developers with an API key and everyday users choosing a model in the Claude.ai app.

What's the core difference between Opus 5 and Sonnet 5?

Opus 5 prioritizes accuracy and complex reasoning, while Sonnet 5 prioritizes speed and cost efficiency. Both support adaptive thinking (adaptive thinking — the model adjusting how deeply it reasons based on the difficulty of the task), but they differ clearly in price, response speed, and knowledge cutoff. Adding the lighter, faster Haiku 4.5 to the comparison makes each model's position clearer.

Here, MTok (million tokens — a unit of 1 million tokens) is the basis for API pricing, and a token (token) is the smallest unit of text an AI processes, roughly 0.75 English words. A context window is the maximum amount of text a model can remember and reference at once.

How big is the price difference?

Opus 5 costs more than Sonnet 5 for both input and output. Opus 5 is $5 per million input tokens and $25 per million output tokens, while Sonnet 5 is $2 input / $10 output. That rate was announced as introductory through August 31, 2026, but Anthropic cancelled the planned increase to $3 / $15 and confirmed $2 / $10 as the standard price (official pricing docs, September 2, 2026). Opus 5 therefore costs 2.5x more than Sonnet 5 on both input and output.

Note: Pricing can change; check the official pricing page for the current figures before making decisions.

When should you actually use Opus vs. Sonnet?

Choose based on task complexity and budget. Here are common rules of thumb from real usage.

• Opus 5 fits when: you're doing large multi-file refactors, running long agent tasks that plan and execute on their own, or handling enterprise work where accuracy matters most
• Sonnet 5 fits when: you need everyday code writing or review, document summarization, or repetitive conversational responses where speed and cost both matter

Opus 5 also offers Fast mode, which responds about 2.5x faster than the default at double the price. Use it only when you need Opus-level quality and speed is genuinely urgent, since costs can add up quickly.

Note: In rare cases, an automated security classifier may flag certain requests (for example, some coding or debugging requests), causing an earlier version, Opus 4.8, to respond instead of Opus 5. This is documented, expected behavior, so a response that feels slightly different isn't necessarily an error.

How do you specify the model in the API?

You only need to change one string, the model field, in your API request. Swapping claude-opus-5 for claude-sonnet-5, as shown below, sends the same request to a different model.

{
  "model": "claude-opus-5",
  "max_tokens": 1024,
  "messages": [
    {"role": "user", "content": "Hello"}
  ]
}

If the request returns a response without errors, it worked. A misspelled model value returns a model-not-found error, so it's safest to copy the API alias directly from the table above.

How is this different in the Claude.ai app?

In the Claude.ai app, you pick a model from the on-screen model selector instead of writing code. Opus 5 is offered as the default model on the Max plan and the top-tier model on the Pro plan; since the exact menu labels and plan-specific usage limits can change over time, it's most reliable to check the current options directly in the app.

Frequently asked questions

Q. Which is better for coding, Opus 5 or Sonnet 5?
Opus 5 fits complex, multi-step coding or agent work and large codebases better, while Sonnet 5 is often enough for everyday code writing or review where speed matters.

Q. What is Fast mode?
It's an Opus 5 option that responds about 2.5x faster than the default model at double the price.

Q. Is Sonnet 5's price going to change?
Not as things stand. $2 input / $10 output was announced as introductory pricing through August 31, 2026, with a switch to $3 / $15 afterward — but Anthropic cancelled that increase and made $2 / $10 the standard price (official pricing docs, checked September 2, 2026).

Q. If a response from Opus 5 feels different, is that a bug?
Not necessarily. An automated security classifier can flag certain requests and route them to an earlier version, Opus 4.8, instead — this is documented, expected behavior.

In short, choose Opus 5 when complexity and accuracy come first, choose Sonnet 5 when speed and cost balance matter more, and manage your request code around the API aliases in the table above to stay accurate as pricing changes.

Related articles

• Claude vs ChatGPT: What's the Difference? Key Points Explained
• Claude API Model IDs and Versioning: Aliases, Pinned Snapshots, and Migration
• Claude API Context Windows and Tokens: Limits and Management
• Claude Model Comparison: Opus 5, Sonnet 5, Haiku 4.5, Fable 5
• Claude API Pricing by Model — Input/Output Token Rates and How to Cut Costs with Batch and Prompt Caching