Claude Code Series Tutorial 7: Sonnet or Opus — Which Model Should I Use?

Choosing a Model: Should You Use Sonnet or Opus?

After installing and logging into Claude Code, you don't need to agonize over model selection right away — it defaults to Claude Sonnet, which is sufficient for most tasks.

Models Available in Claude Code

Currently, there are three models you can dynamically select from the command line:

• Claude Sonnet (currently 3.5 or newer)
• Claude Opus (flagship)
• Claude Haiku (lightweight)

These three models differ significantly in both "brainpower" and "cost." Using the wrong one wastes money; using the right one saves time.

Model Analysis

Sonnet — The go-to workhorse, best value

• Fast responses with little to no lag.
• Capable of understanding large codebase logic, sufficient for most generation and refactoring tasks.
• It's the default not because it's "barely adequate," but because it's truly balanced.
• Handles everyday code generation, explanation, debugging, and refactoring with ease.

Opus — The heavy bulldozer, expensive but powerful

• Tackles extremely complex logic: quadruple-nested async processing, cross-file chained refactoring spanning ten files, full migration from an old framework to a new one.
• More nuanced context understanding, less likely to miss your constraints.
• Significantly slower than Sonnet; returning a complex solution may take 10–20 seconds.
• Costs roughly five times as much as Sonnet. No joke — refactoring a large module with Opus could burn several dollars or more.
• Only bring in Opus if Sonnet fails to solve a problem twice.

Haiku — Lightning fast, only for small tasks

• Responds in milliseconds, virtually no lag.
• Low capability ceiling, unsuitable for complex reasoning. What is it good for? Generating commit messages, formatting data, answering simple questions like "What is this directory for?"
• Extremely cheap, practically negligible.
• Not suitable for generating code, except for very simple boilerplate.

How to Switch Models

Specify at startup:

claude --model opus "Explain why this recursion is degenerating"

Or switch after entering interactive mode with claude:

/model opus

You can also set a default model in CLAUDE.md or a configuration file to avoid manual specification each time. This is useful when your team has a unified preference.

Other Details

Model versions iterate. The web version on claude.ai may get new models first, while the command-line tool updates slightly later. If you like to stay on the bleeding edge, use claude --version together with npm update to keep it current, or watch the model support announcements in the GitHub repository releases.