Claude Sonnet 5 review: Should you switch?

Claude Sonnet 5 review: Should you switch?<br>by

Juan Pablo Flores<br>Gowtham Kishore Vijay

June 30, 2026<br>12 min read

June 30, 2026<br>12 min read

What's new in Claude Sonnet 5<br>How Claude Sonnet 5 writes codeIt treats testing as a habit<br>What the extra care costs you

How Claude Sonnet 5 reviews codeThe comments are much cleaner<br>The honest catch on bug-catching

Claude Sonnet 5 vs the model you run todaySonnet 5 versus a flagship model

Claude Sonnet 4.6 or Claude Sonnet 5: Quick verdict

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Anthropic just shipped Claude Sonnet 5, the newest model in its mid-tier line. This post answers one simple question: Should you stay on the model you use today, or move up to this one?

Sonnet 5 didn't land out of nowhere. We've reviewed a string of new models on the blog over the past month:

Opus 4.8 was the best model we'd tested for long, multi-step coding, careful enough to follow review instructions to the letter, though it cost more and paid off mostly on bigger jobs.

Fable 5 leaned even harder into coding on its own, and was happy to plan and build across lots of files. However, its price and limited access kept it off our default review path.

NVIDIA's Nemotron 3 Ultra went the other way: fast and to the point, an open model built to take lots of quick swings inside an agent setup.

Sonnet 5 feels like the rest of the Anthropic family. It's patient and thorough, and it likes to think a problem all the way through before it acts.

For writing and building code, Sonnet 5 is the most capable model we've worked with at this tier, and it's an easy upgrade to be excited about. For review, it's more of a tradeoff. While it generates cleaner, sharper comments, it catches fewer bugs than the earlier models we currently run in production, and comes at a slightly higher cost per review.

The good part is that you can tune most of that, and for a lot of teams the move is well worth making. Let's walk though what's new in Sonnet 5, how it performs writing and reviewing code, and if it makes sense to switch.

What's new in Claude Sonnet 5

Sonnet 5 thinks more deeply than the version before it. For you, that means it works through harder problems the old model would have given up on.

The thinking "effort" dial in Sonnet 5 that allows you to incrementally turn it down to off if you don't need it. This is the feature that protects your budget. Turn the effort up for a tricky review where a missed bug is expensive, and turn it down or off for routine work where you'd rather not pay for deep thinking.

It can also rewrite its own instructions partway through a task. On long agent jobs, the goal tends to shift as the model learns more, and a model stuck on its first plan will keep pushing on instructions that no longer fit. Sonnet 5 updates its own plan instead, so you get fewer runs that wander off and burn your time and tokens.

It also ships with new safety guardrails around security and cyber topics. The upside is fewer risky outputs. The catch is that real security work can trip the filters now and then, so expect the odd refusal if that's your area.

The easiest way to picture Sonnet 5 is as a mid-level engineer who cares, maybe a little too much, about shipping code that truly works and runs at the level you asked for. That one trait shapes most of what it does, and it showed up in four habits we saw again and again:

It tends to write tests before the feature.

It keeps polishing a solution long after it already works.

It second-guesses its own plan, sometimes more than necessary.

It answers a small task with a small project.

How Claude Sonnet 5 writes code

Sonnet 5’s code writing is the main reason we think most teams will want to upgrade. Before we get to the review numbers, it is helpful to see how it behaves when it builds something from scratch. We handed it the work we do daily, from quick features to harder problems with no...