Why does Claude Sonnet 5 reject non-default sampling parameters?

Setting temperature, top_p, or top_k to anything other than their default values now returns an HTTP 400 error on Sonnet 5, a constraint that had already been introduced for Opus-class models earlier and is now extended to the Sonnet tier for the first time with this release.

This affects two different groups of prior use cases identically: developers who raised these parameters for more creative, varied output, and developers who lowered them, often to zero, for more deterministic output - both patterns now fail the same way.

The recommended replacement path is to control output behavior through prompting instead - explicit system prompt instructions describing the desired style, creativity, or determinism - rather than through sampling-level knobs, reflecting a broader move toward controlling behavior at the prompt and effort level rather than through low-level generation parameters.

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