Vibe Check

Vibe Check: Sonnet 5.5 Finds Its Place in Claude’s Crowded Family

Anthropic's new model is a more capable partner than Sonnet 5, so long as you keep a hand on the wheel and know when to call in Opus

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Anthropic released Sonnet 5.5 today, and the model has an identity problem. Opus got cheaper, Fable arrived above both, and when we reviewed Sonnet 5 in July, we couldn’t find a task it handled best.

After a week of testing, we found its job. At low and medium effort, Sonnet 5.5 is fast, inexpensive, and responsive to direction, which makes it a strong partner for brainstorming, prototyping, and design work you steer yourself. Push it to high effort or leave it unattended, and it overbuilds.

We get into all of it in our Vibe Check, including:

Kieran Klaassen’s from-scratch clone of Proof, Every’s open-source document editor, built at low effort, a job only Fable 5, GPT-6 Astra, and Opus 5.5 had managed before
Tyler Nishida’s golf game, which went well at low and medium effort and fell apart in long max-effort sessions, plus a property build that ran 19 hours
A consulting test where Sonnet 5.5 wrote 28 data files for one idea in ten minutes and never wrote the ready-to-paste prompt
Readability scores that put it ahead of Ops 5.5, alongside a writing test where it outlined well and then struggled to connect a full draft
A case study rewrite that cited a test skill Sonnet had created and deleted without telling Tyler

Bottom line: try it for iterative builds, design work, and outlining, starting at medium and setting a time or token budget. It costs $2 per million input tokens and $10 per million output tokens, half what Opus 5.5 charges, so experiments are cheap. Stay with Sonnet 5 for coding, since our tests showed no consistent gain. Keep Opus 5.5 for high-detail final builds and GPT-6 Astra for browser-heavy agent work.

Read the Vibe Check

Katie Parrott is a staff writer at Every. To read more essays like this, subscribe to Every, and follow us on X at @every and on LinkedIn.

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