Claude Sonnet vs Opus for Coding Agents: Cost, Speed, and Routing Strategy

Claude Sonnet vs Opus for Coding Agents: Cost, Speed, and Routing Strategy#

Coding agents do not need the same model for every step. A planning task, a small edit, a failing test, and an architecture review each need different tradeoffs. The practical question is not “Sonnet or Opus forever?” It is “which model should handle which part of the workflow?”

Quick answer#

Use Sonnet as the default route for most coding-agent work: code edits, test generation, explanation, and routine refactors. Use Opus for high-stakes reasoning: architecture decisions, multi-file debugging, ambiguous failures, and review of complex changes.

Where Sonnet fits#

• Editing a function or component.
• Writing tests for a clear behavior.
• Explaining an error log.
• Refactoring a small module.
• Updating docs from code changes.

Where Opus fits#

• Planning a large refactor.
• Debugging failures across services.
• Reviewing architecture decisions.
• Handling ambiguous product requirements.
• Auditing agent output before release.

Routing pattern for coding agents#

Step	Recommended model
Understand task	Sonnet
Create implementation plan	Sonnet or Opus depending complexity
Make routine edits	Sonnet
Investigate hard failures	Opus
Summarize final changes	Sonnet
Review high-risk release	Opus

With CrazyRouter, you can keep one OpenAI-compatible endpoint and switch model IDs by task type.

Conclusion#

Sonnet should usually be the default for coding agents. Opus should be reserved for expensive reasoning and high-risk review. CrazyRouter makes this easier because model routing can live in one gateway layer instead of being hardcoded across tools.

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