Claude 3.5 vs Claude 4: what changed
The current Claude line is a clear step up from 3.5 in reasoning and coding. Migrating is mostly a model-ID change — everything else stays the same.
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What improved
The Opus, Sonnet and Haiku 4-series models improve on 3.5 for agentic coding, long-context consistency and complex reasoning, while keeping the same Messages API.
See also: The best Claude model for coding
How to migrate
Swap the model ID to a current one — for example claude-opus-4-8, claude-sonnet-5 or claude-haiku-4-5 — and keep your existing request code. On apiToken.sale it is the same key and endpoint.
Frequently asked questions
Is Claude 4 much better than 3.5?
Yes, especially for coding, agents and long-context tasks, while using the same API format.
Is migrating hard?
No — update the model ID (e.g. to claude-sonnet-5) and your existing Messages API code keeps working.
Start with Google or GitHub and get $10 of Claude usage at official prices — no card required.