Claude Sonnet 5 is Anthropic's flagship reasoning model — the one developers reach for when code actually has to work. Extended thinking, strong programming ability, careful reasoning. Normally you'd need an API key or a Pro subscription. On Toolxz AI Chat, it's free in the browser with no signup.
Here's how to get the most out of it for real dev work.
What Sonnet 5 is best at
Coding and reasoning. It's the model I trust with refactors I'm slightly afraid of:
"Refactor this Express middleware into smaller testable functions. Keep the exact behavior, add JSDoc comments: [code]"
"This recursive function blows the stack on large inputs. Rewrite it iteratively and explain what changed: [code]"
Give it room to think — longer, messier problems are where it earns its reputation.
The prompt pattern that works
Sonnet 5 responds well to constraints. Don't say "improve this code." Say:
"Review this for correctness, then performance, then readability — in that order. List issues before rewriting: [code]"
Ordered instructions → ordered thinking → better output.
Try it free
Chat with Claude Sonnet 5 on Toolxz — no account, no API key, no bill. More models in the same place: GPT 5.6, Claude Fable, GLM 5.2, Kimi K3.
Founder of Toolxz (toolxz.com) — 45+ free browser-based tools. I write about practical AI tooling and developer workflows.
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