When to Use Claude Opus vs Sonnet for Coding Tasks?

Mar 25, 2026

I burned through $600 in a month building a simple HTML application with Claude Opus 4.6 and extended thinking. Turns out, I was using a sledgehammer to hang a picture frame.

The Reddit community set me straight: Sonnet handles most coding work at 60% of the cost. Here’s what I learned about picking the right model.

The Expensive Mistake

I enabled extended thinking mode by default. I thought more thinking = better code. Wrong.

┌─────────────────────────────────────────────┐
│  Opus 4.6 + Extended Thinking              │
│  ████████████████████████████████ $600/mo  │
│                                             │
│  What I should have used:                  │
│  Sonnet 4.6                                 │
│  ███████████████████ $360/mo               │
│                                             │
│  Savings: $240/month (40%)                  │
└─────────────────────────────────────────────┘

For an HTML app. A simple HTML app.

The Real Difference

Sonnet isn’t “worse” - it’s just optimized for different tasks.

┌────────────────┬─────────────────┬─────────────────┐
│                │ Opus 4.6        │ Sonnet 4.6      │
├────────────────┼─────────────────┼─────────────────┤
│ Cost           │ Higher          │ ~60% of Opus    │
│ Speed          │ Slower          │ Faster          │
│ Reasoning      │ Deep            │ Good enough     │
│ Best for       │ Complex problems│ Most coding     │
└────────────────┴─────────────────┴─────────────────┘

Anthropic’s Claude Code tool defaults to Sonnet. That’s not accidental - it’s a signal.

When to Use Each Model

I made a simple decision tree:

            START
              │
              ▼
    Is this routine work?
    (HTML, CSS, components,
     APIs, bug fixes)
              │
     ┌────────┴────────┐
     YES               NO
     │                  │
     ▼                  ▼
  USE SONNET      Architectural
                    decision?
                      │
             ┌────────┴────────┐
             YES               NO
             │                  │
             ▼                  ▼
         USE OPUS         Sonnet failed?
                              │
                     ┌────────┴────────┐
                     YES               NO
                     │                  │
                     ▼                  ▼
                 ESCALATE           TRY SONNET
                 TO OPUS             FIRST

Task-by-Task Guide

Here’s what I use now:

┌─────────────────────┬──────────┬───────────────────┐
│ Task                 │ Model    │ Why               │
├─────────────────────┼──────────┼───────────────────┤
│ HTML/CSS layouts     │ Sonnet   │ Routine patterns  │
│ React/Vue components │ Sonnet   │ Standard UI       │
│ API implementation   │ Sonnet   │ Well-defined      │
│ Code refactoring     │ Sonnet   │ Pattern-based     │
│ Writing tests        │ Sonnet   │ Generation task   │
│ Documentation        │ Sonnet   │ Summarization     │
├─────────────────────┼──────────┼───────────────────┤
│ Architecture design  │ Opus     │ Multi-system      │
│ Complex debugging    │ Opus     │ Race conditions   │
│ Novel algorithms     │ Opus     │ Deep reasoning    │
│ Security analysis    │ Opus     │ Critical thinking │
└─────────────────────┴──────────┴───────────────────┘

Extended Thinking: When It Actually Helps

Extended thinking isn’t magic. It’s expensive reasoning time.

┌────────────────────────────────────────────────────┐
│ GOOD USE CASES:                                    │
│ ✓ Debugging memory leaks                          │
│ ✓ Designing system architecture                   │
│ ✓ Analyzing security vulnerabilities               │
│ ✓ Solving novel algorithm problems                │
│                                                    │
│ WASTE OF MONEY:                                   │
│ ✗ Building HTML layouts                            │
│ ✗ Writing React components                        │
│ ✗ Standard API CRUD                               │
│ ✗ Code formatting/refactoring                     │
└────────────────────────────────────────────────────┘

The Trial-and-Error Process

I tested both models on the same task. Results:

Task: Build a responsive navbar component

┌─────────────────┬──────────────┬──────────────┐
│                 │ Opus 4.6     │ Sonnet 4.6   │
├─────────────────┼──────────────┼──────────────┤
│ Code quality    │ Same         │ Same         │
│ Time to finish  │ Slower       │ Faster       │
│ Cost            │ $0.12        │ $0.07        │
│ Did it work?    │ Yes          │ Yes          │
└─────────────────┴──────────────┴──────────────┘

Verdict: No difference in quality. Sonnet was cheaper AND faster.

Common Mistakes I Made

Defaulting to “best” model - Higher cost doesn’t mean better results for everything
Extended thinking always on - Only useful for genuinely hard problems
Not testing Sonnet first - Should be the default starting point
Ignoring Claude Code’s defaults - They chose Sonnet for a reason

The Practical Approach

Now I follow this rule:

1. Start with Sonnet
2. If Sonnet struggles after 2-3 attempts → try Opus
3. If it's a hard problem → Opus + extended thinking
4. If it's frontend/routine → Sonnet, always

Result: My monthly cost dropped from $600 to ~$250
Quality: No noticeable difference
Speed: Actually faster (Sonnet responds quicker)

The Bottom Line

Start with Sonnet. It handles 90% of coding tasks just fine. Only reach for Opus when:

You’re designing system architecture
Debugging complex race conditions
Working on novel algorithms
Sonnet explicitly fails

Extended thinking is for the hardest problems, not for HTML apps.

Final Words + More Resources

My intention with this article was to help others share my knowledge and experience. If you want to contact me, you can contact by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!