prompting-claude-haiku-5-5 changedbuild-with-claude/prompt-engineering/prompting-claude-haiku-5-5
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2121<Note>
2222 For the breaking API changes when migrating from Claude Haiku 4.5, see the [migration guide](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide).
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24 Several sections on this page suggest text to add to your system prompt. Add it to new conversations only. A request that sends thinking blocks back after `system` changed can return a 400 error, so a stored conversation resumed with the new system prompt can fail. To change instructions partway through a conversation, [put them in the newest turn](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#changing-context). On the Claude API, Amazon Bedrock, and Google Cloud, you can instead [append a system message to the conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#new-instructions).
24 Several sections on this page suggest text to add to your system prompt. Add it to new conversations only. A request that sends thinking blocks back after `system` changed can return a 400 error, so a stored conversation resumed with the new system prompt can fail. To change instructions partway through a conversation, [put them in the newest turn](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#changing-context). On the Claude API, Amazon Bedrock, and Claude Models on Google Cloud (Google managed), you can instead [append a system message to the conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#new-instructions).
2525</Note>
2626
2727## Use effort to control thinking
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3838* Thinking is on by default and counts toward `max_tokens`, which can go up to 128,000. A `max_tokens` value sized for Claude Haiku 4.5 requests that ran without thinking can cut the reply off, so leave room for thinking.
3939* To get less thinking, lower the effort level. In Anthropic's testing, telling the model in the prompt to answer directly didn't stop it from thinking. You can also turn thinking off with `thinking: {"type": "disabled"}`. This works at `low`, `medium`, and `high` only. At `xhigh` and `max`, the request returns a 400 error.
4040* At `xhigh` effort in multi-turn chats, the model sometimes writes its whole answer in its thinking and ends the turn with no visible text. If you see this behavior, check each response for an empty reply.
41* Changing the top-level `effort` value between requests invalidates the prompt cache for the conversation's messages. On the Claude API and Google Cloud, you can change the level partway through a conversation with a [per-message effort change](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) (beta), which keeps the cache. A change holds for every later turn until another one replaces it. It needs the `mid-conversation-output-config-2026-07-01` beta header and adaptive thinking, which is the default. With thinking off, a per-message effort change returns a 400 error.
41* Changing the top-level `effort` value between requests invalidates the prompt cache for the conversation's messages. On the Claude API and Claude Models on Google Cloud (Google managed), you can change the level partway through a conversation with a [per-message effort change](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) (beta), which keeps the cache. A change holds for every later turn until another one replaces it. It needs the `mid-conversation-output-config-2026-07-01` beta header and adaptive thinking, which is the default. With thinking off, a per-message effort change returns a 400 error.
4242
4343## Accurate search results
4444
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