Claude Sonnet 5 vs Opus 5: Which Model Should Power Your Production Agent?

Most teams building an agent on Claude end up debating the same question: is Opus worth the extra cost, or is Sonnet good enough? The answer depends on your tasks, and you can measure it in a few days.

What each model is for

Claude Sonnet 5 is the everyday model. It's fast, handles coding and tool use well, and at the time of writing costs $2 per million input tokens and $10 per million output tokens.

Claude Opus 5 is built for harder work: long multi-step tasks, messy requirements and problems where the model has to keep a lot in mind at once. It costs $5 per million input tokens and $25 per million output tokens.

Both have a 1M-token context window, so context size won't decide this for you.

When Sonnet is enough

• High-volume work with clear instructions, such as triage, extraction, drafting and routine tool calls.
• User-facing flows where response time matters.
• Agents where the tools do most of the heavy lifting and the model mainly decides which one to call.

When Opus is worth it

• Long tasks: refactoring across a codebase, investigations with dozens of tool calls, planning with incomplete information.
• Work where mistakes are expensive to find later, like data migrations or security reviews.
• Tasks where Sonnet needs several attempts. Opus costs about 2.5 times more per token, but if it finishes in one pass it can be cheaper per completed task.

Measure it on your own tasks

1. Collect 30 to 50 real tasks from your pilot, including difficult ones.
2. Run both models with identical prompts and tools.
3. Record success rate, number of turns and total cost for each successful task.
4. Read the failures yourself. Ten percent failures on easy cases is a very different problem from ten percent failures on the cases your customer cares about.

Before switching models, try changing the effort setting. Opus at lower effort, or Sonnet at higher effort, may be all you need.

A common setup

Many production agents use both. Sonnet handles every request first. Requests that match known hard patterns, or that failed once, go to Opus. This keeps most traffic on the cheaper model while the difficult cases still get solved.

Start with Sonnet, run the comparison, and move the hard cases to Opus when the numbers support it.