working-with-messages changedbuild-with-claude/working-with-messages
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## Prefilling Claude’s response ## Prefilling Claude's response
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2020## Basic request and response
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2222<Note>
23 The `temperature`, `top_p`, and `top_k` sampling parameters are not supported on Claude 4.7 and later models and Claude Mythos Preview, or on any model starting with [API version `2026-10-21`](https://platform.claude.com/docs/en/api/migration-2026-10-21#sampling-parameters). Setting them to a non-default value returns a 400 error, and starting with `2026-10-21` any value does. Omit them from request payloads and use prompting to guide the model's behavior instead. See the [breaking changes in the migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#opus-46-breaking-changes).
23 The `temperature`, `top_p`, and `top_k` sampling parameters are not supported on Claude 4.7 and later models and Claude Mythos Preview, or on any model starting with [API version `2026-10-21`](https://platform.claude.com/docs/en/api/migration-2026-10-21#sampling-parameters). Setting them to a non-default value returns a 400 error, and starting with `2026-10-21` any value does. Omit them from request payloads and use prompting to guide the model’s behavior instead. See the [breaking changes in the migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#opus-46-breaking-changes).
2424</Note>
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2626<CodeGroup>
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158158
159159## Multiple conversational turns
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161The Messages API is stateless, which means that you always send the full conversational history to the API. You can use this pattern to build up a conversation over time. Earlier conversational turns don't necessarily need to actually originate from Claude. You can use synthetic `assistant` messages.
161The Messages API is stateless, which means that you always send the full conversational history to the API. You can use this pattern to build up a conversation over time. Earlier conversational turns don’t necessarily need to actually originate from Claude. You can use synthetic `assistant` messages.
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163163<CodeGroup>
164164 ```bash cURL
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330330See [Mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) for the complete guide, including how to combine it with [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching).
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332## Prefilling Claude's response
332## Prefilling Claude’s response
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334You can prefill part of Claude's response in the last position of the input messages list. Use this technique to shape Claude's response. The following example uses `"max_tokens": 1` to get a single multiple choice answer from Claude.
334You can prefill part of Claude’s response in the last position of the input messages list. Use this technique to shape Claude’s response. The following example uses `"max_tokens": 1` to get a single multiple choice answer from Claude.
335335
336336<Warning>
337337 Prefilling is not supported on Claude 4.6 and later models and [Claude Mythos Preview](https://anthropic.com/glasswing). Requests using prefill with these models return a 400 error. Use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) on models that support it, or system prompt instructions, instead. See the [migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide) for migration patterns.
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