Claude Code Model Cost Optimization: Which Model to Use for Which Task

Why Cost Optimization Is a Workflow Problem, Not a Budget Problem

When developers talk about controlling Claude Code costs, they usually frame it as a budget constraint — "I can only afford X dollars per month." But that is the wrong way to think about it. Cost optimization in Claude Code is about matching the model to the task so you are not paying for capability you do not use.

The models are not priced the same because they are not the same. Opus is expensive because it reasons deeply. Sonnet is mid-range because it handles most development work well without the deep reasoning overhead. Haiku is cheap because it handles single-pass tasks without multi-step reasoning. If you route every request to Opus because it is the best model, you are paying premium prices for work that did not need it.

The Three-Tier Framework

Think about model selection as three tiers based on task complexity and consequence:

Tier 1 — Sonnet for day-to-day work: Feature implementation, test writing, bug fixes, refactoring, code review, documentation. These are tasks where you know what you want, the scope is clear, and being wrong is not expensive because you review the output before it lands anywhere important. Sonnet handles these at roughly half the cost of Opus.

Tier 2 — Opus for production-critical work: Code that ships to production, API interfaces other systems depend on, authentication logic, database migrations, complex debugging across many files. These are tasks where the cost of being wrong is real and Sonnet shows its limits — confidently producing wrong answers that take time to catch. Opus earns its premium here.

Tier 3 — Haiku for quick single-pass tasks: Log summarization, data formatting, simple transformations, pattern finding, factual extraction. These are bounded tasks that do not require reasoning — you just need the model to read something and give you a specific answer. Haiku handles these at a fraction of Sonnet's cost.

Mapping Tasks to Models in Practice

The framework sounds clean but applying it in a real session requires judgment. Here is how to think about it:

Before starting a task, ask: do I know exactly what I want, or am I figuring something out? If you know exactly what you want — write this function, add tests to this module, refactor this to use the new pattern — start with Sonnet. If you are exploring, debugging something non-obvious, or trying to understand how something works, start with Sonnet but be ready to switch to Opus if it is not going well.

The signal that tells you to switch: you find yourself correcting Sonnet repeatedly on the same task. That usually means the task is harder than Sonnet can handle at the tier you are using it. Switch to Opus for that task specifically and continue.

What This Looks Like in a Session

A typical optimized session might go like this: you start on Sonnet, writing a new feature. The feature implementation goes smoothly — Sonnet writes the function, the tests, the integration points. You hit a bug in the checkout flow that has you stumped. You ask Claude Code to help debug it. Sonnet takes a shot but the suggestions are not quite right. You switch to Opus mid-session:

/model opus-4.5

Opus traces through the checkout flow, finds the condition where the discount code is silently ignored, and explains the root cause clearly. You confirm the diagnosis, fix it, and switch back to Sonnet to continue with the feature.

The session runs mostly on Sonnet. The Opus call was specific and targeted — worth the cost premium because the problem genuinely needed the deeper reasoning.

Making It Stick With Configuration

The framework only works if you actually apply it. Set your default model in claude_settings.json:

{ "defaultModel": "sonnet-4-5" }

This means every new session starts on the cost-efficient tier. You switch up for specific tasks, not because of habit.

For team deployments, you can add a project-level override that forces Sonnet for lower-tier work:

{ "defaultModel": "sonnet-4-5", "maxModelForRoutineTasks": "sonnet-4-5" }

This is a soft guard — it does not prevent using Opus, but it ensures Sonnet is the starting point.

Monitoring Actual Spend

The framework helps you think about the right allocation, but the actual numbers matter. Run Claude Code with verbose logging for a week to see where your tokens actually go:

claude --verbose 2>&1 | tee session.log

Review the logs and categorize: what percentage of your tokens went to Sonnet tasks vs Opus tasks vs Haiku tasks? If Opus is dominating despite the framework, either the tasks genuinely need it or you are defaulting to it too often. If Haiku is barely used, you might be missing opportunities to use it for quick tasks that Sonnet handles but at higher cost.

The Honest Summary

Cost optimization in Claude Code is not about using the cheapest model as much as possible — it is about using the right model for each task so you preserve the expensive reasoning capability for problems that actually need it. Sonnet handles the bulk of development work. Opus handles the hard problems. Haiku handles the quick single-pass tasks. A session that runs mostly on Sonnet with targeted Opus and Haiku calls is going to be both cost-effective and high-quality.

The developers who get this right are not the ones who use Haiku constantly. They are the ones who are intentional about which model handles which work, switch mid-session when needed, and review their actual spend to validate whether the allocation is working.