Practical Framework for Choosing Between Claude's Haiku, Sonnet, and Opus Models

A developer with months of daily experience using all three Claude models (Haiku 4.5, Sonnet 4.6, Opus 4.6) tested them on the same coding task to determine when to use each. The test involved refactoring a 400-line Express.js backend to use proper middleware patterns and add input validation.

Model Performance on the Coding Task

Haiku 4.5 handled straightforward parts like extracting middleware and adding express-validator, but missed a subtle dependency between two middleware functions where order mattered.

Sonnet 4.6 caught the middleware ordering issue and restructured the error handling chain correctly. It also added TypeScript types unprompted.

Opus 4.6 did everything Sonnet did but also flagged that the auth middleware was checking permissions after the route handler had already accessed the database — a security issue that had been missed for months.

Pricing Comparison

• Haiku: $0.25 input / $1.25 output per million tokens
• Sonnet: $3 / $15 per million tokens
• Opus: $15 / $75 per million tokens

Opus costs 60x more than Haiku per token. For tasks where Haiku gets it right, using Opus is inefficient.

Practical Usage Framework

• Haiku → batch operations, data transformation, classification, anything repetitive across many calls
• Sonnet → daily coding, feature work, code review, 90% of tasks
• Opus → architecture decisions, security review, complex debugging where missing something costs hours

The developer reports that matching model to task complexity cut API costs by approximately 70% with no quality loss on important tasks.

All three models now support extended thinking, but it makes the biggest difference with Opus on complex reasoning tasks. For Haiku, extended thinking barely changes the output.