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Anthropic Releases Claude Sonnet 5 with Adaptive Thinking and New Tokenizer

Anthropic has launched Claude Sonnet 5, the next generation of its Sonnet model family, featuring adaptive thinking enabled by default and a new tokenizer.

By Illumora Editorial

Source · Sep 1, 2026, 5:55 PM · On Illumora · Sep 1, 2026, 5:58 PM

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Read the source →Anthropic Docs (prompting) — Claude Sonnet 5
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Anthropic has released Claude Sonnet 5, an upgrade to its Claude Sonnet 4.6 model. This new version includes adaptive thinking turned on by default and introduces a new tokenizer. The release also details changes to how certain parameters are handled.

Key Points

  • Claude Sonnet 5 is a direct upgrade for Claude Sonnet 4.6.
  • Adaptive thinking is now enabled by default in Claude Sonnet 5.
  • Manual extended thinking requests now result in a 400 error.
  • Setting temperature, top_p, or top_k to non-default values also returns a 400 error.
  • Claude Sonnet 5 supports up to 300k output tokens via the Message Batches API with the output-300k-2026-03-24 beta header.
  • The model features a 1M token context window by default.

Context

According to Anthropic's documentation, Claude Sonnet 5 represents an evolution in the Sonnet model family. It is designed to be a drop-in replacement for its predecessor, with specific behavioral adjustments. The documentation outlines the model's capabilities, pricing, and lifecycle status.

Why It Matters

Builders using Anthropic's models need to be aware of the behavioral changes in Claude Sonnet 5, particularly regarding adaptive thinking defaults and the handling of sampling parameters. These updates can impact existing integrations and require adjustments to prompting strategies.

What To Do

  • Review the documentation for Claude Sonnet 5 to understand all new features and changes.
  • Note that adaptive thinking is now on by default and steer its depth using the effort parameter.
  • Adjust any code that sets temperature, top_p, or top_k to non-default values, as these will now cause a 400 error.
  • Explore the Message Batches API for workloads requiring up to 300k output tokens.

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