prompting-claude-sonnet-5-5 changedbuild-with-claude/prompt-engineering/prompting-claude-sonnet-5-5
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3636* Reserve `xhigh` and `max` for work where you've measured a quality gain, because thinking and replies get much longer there. At those levels, `between_tools` isn't accepted, so up-front thinking can't be turned off.
3737* To get less thinking, lower the effort level. From `medium` up, the model thinks briefly before almost every reply, even a greeting, which adds to the time before the first visible token. Asking it in the system prompt to think less doesn't reliably reduce its thinking. At `low`, it skips thinking on most simple requests.
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39Changing the top-level `effort` value between requests invalidates the prompt cache. To run individual turns at a different level, use a [per-message effort change](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) (beta) instead, which keeps the cache. For example, run an interactive session at `low` and raise effort to `high` when the user submits a hard problem. Per-message effort changes need adaptive thinking. With `between_tools`, they return a 400 error, as [Running without up-front thinking](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-sonnet-5-5#running-without-up-front-thinking) explains.
39Changing the top-level `effort` value between requests invalidates the prompt cache. To change the level partway through a conversation, use a [per-message effort change](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) (beta) instead, which keeps the cache. A change holds for every later turn until another one replaces it. For example, in an interactive session at `low`, add a change to `high` before the user's hard problem, and a change back to `low` after Claude answers it. Per-message effort changes need adaptive thinking. With `between_tools`, they return a 400 error, as [Running without up-front thinking](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-sonnet-5-5#running-without-up-front-thinking) explains.
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4141## Steer initiative and scope
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