AI / Claude Sonnet5 Interview questions

How does Claude Sonnet 5 handle assistant message prefilling?

Assistant message prefilling - constructing a request that starts the assistant's response with predetermined text - returns an HTTP 400 error on Claude Sonnet 5, the same rejection behavior that was already true on Claude Sonnet 4.6.

This means prefill rejection isn't a new Sonnet-5-specific breaking change to account for during a straightforward Sonnet-4.6-to-Sonnet-5 migration - it was already in place before Sonnet 5 shipped, for teams already running on 4.6.

For teams migrating from an older, pre-4.6 Sonnet version where prefilling was still accepted, this is one of the additional changes that needs to be handled as part of that longer migration path, using structured outputs, system prompt instructions, or the platform's output-format configuration as alternatives instead.

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Assistant message prefilling on Sonnet 5 is: rejected with an HTTP 400 error, unchanged from Sonnet 4.6

✓ Correct! Well done.

newly rejected only starting with Sonnet 5

✗ Try again if not.

fully supported with no restriction

✗ Try again if not.

For teams already on Sonnet 4.6, this specific behavior is: something that only affects Opus-class models

✗ Try again if not.

not a new consideration introduced by Sonnet 5

✓ Correct! Well done.

a brand-new breaking change requiring immediate action

✗ Try again if not.

More Related questions...

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