prompting-claude-opus-5-5 changedbuild-with-claude/prompt-engineering/prompting-claude-opus-5-5
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4343* Reserve `xhigh` and `max` for work where you’ve measured a quality gain.
4444* To get less thinking, lower the effort level first. Lowering effort reduces thinking, and with it cost and latency, more reliably than prompt instructions do.
4545
46Changing the top-level `effort` value between requests invalidates the prompt cache. To change the level partway through a conversation, use a [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort-per-message) change instead, which keeps the cache. Per-message effort is in beta. A change holds for every later turn until another one replaces it.
46Changing the top-level `effort` value between requests invalidates the cached messages. To change the level partway through a conversation, use a [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort-per-message) change instead, which keeps the cache. Per-message effort is in beta. A change holds for every later turn until another one replaces it.
4747
4848## Prompts written for thinking disabled
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6969A system prompt addition can also make these early stops less frequent. Claude Opus 5.5 is responsive to instructions that name the specific kinds of early stop you want it to avoid, such as ending the turn with a summary that announces the next step instead of taking it. It also helps to name the stops you do want, for example when no work can advance without the user’s input.
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71The following paragraph is one example of such an addition, written for agents that run fully unattended, where you want the model to keep working rather than stop to report. Treat it as a starting point: you might need to adapt it for your own application. Add it at the end of your system prompt from the first request of the session: adding it partway through changes the `system` prompt and invalidates the conversation’s earlier thinking blocks (see [Add instructions with a mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#new-instructions)). Because it tells the model to put status notes in the same message as its next tool call, those notes arrive between tool calls as progress updates. The text of these updates comes back empty at the default `thinking.display`. Set `display: "updates"` to receive a summary of each (see [User-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#user-facing-progress-updates)). With this addition the model carries on where it would otherwise have stopped to check in, so keep your own confirmation step for risky or irreversible actions, and leave the addition out of human-in-the-loop applications, where someone is there to answer. Expect somewhat more tool calls and output tokens per task.
71The following paragraph is one example of such an addition, written for agents that run fully unattended, where you want the model to keep working rather than stop to report. Treat it as a starting point: you might need to adapt it for your own application. Add it at the end of your system prompt from the first request of the session: adding it partway through changes the `system` prompt and invalidates the conversation’s earlier thinking blocks (see [Change instructions](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking/change-tools-or-instructions#new-instructions)). Because it tells the model to put status notes in the same message as its next tool call, those notes arrive between tool calls as progress updates. The text of these updates comes back empty at the default `thinking.display`. Set `display: "updates"` to receive a summary of each (see [User-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#user-facing-progress-updates)). With this addition the model carries on where it would otherwise have stopped to check in, so keep your own confirmation step for risky or irreversible actions, and leave the addition out of human-in-the-loop applications, where someone is there to answer. Expect somewhat more tool calls and output tokens per task.
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7373```text wrap
7474A standing instruction from the user, the person you are working for. It is about how your turns end. A message with no tool call in it ends your turn, and the work stops there until you are asked to continue. The user has seen you end turns in four ways while work they asked for was still owed, and does not want any of them. One: a long summary of what was done that closes by announcing the next step and has no tool call, so the next thing never starts. Two: an offer to carry on with something unless the user would prefer otherwise, which stops to wait for an answer the user was not going to give. Three: a list of decisions for the user when, by your own account, none of them blocks the rest of the work. Four: deciding that this is a good place to report, because the turn has been long or a milestone is done. Status notes are welcome, and so are your recommendations on open decisions, but put them in the same message as your next tool call and carry on with whatever does not depend on the user's answer. If you notice yourself inviting the user to redirect you or offering to wait, delete it and do the next thing. The stops the user does want are the ones where nothing can move without them, or where the thing blocking you is deliberately protected from you. This does not override the need for confirmation on risky or destructive actions.
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7979Claude Opus 5.5 runs safety classifiers, including for biology, cybersecurity, and reasoning extraction.
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81* **Biology:** The biology safeguards are the same as Claude Fable 5.1’s and are new if you’re coming from Claude Opus 5. Everyday health and educational questions are unaffected. If the biology classifier blocks your organization’s life sciences work, apply to the [Life Sciences Verification Program](https://www.anthropic.com/news/life-sciences-verification-program).
81* **Biology:** The biology safeguards are the same as Claude Fable 5.1’s, and Claude Opus 5 doesn’t have them. Everyday health and educational questions are unaffected. If the biology classifier blocks your organization’s life sciences work, apply to the [Life Sciences Verification Program](https://www.anthropic.com/news/life-sciences-verification-program).
8282* **Cybersecurity:** Finding vulnerabilities in source code is allowed. High-risk dual-use cybersecurity activities are not. If the cybersecurity classifier blocks your organization’s legitimate security work, apply to the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
8383* **Reasoning extraction:** Requests that push the model to reproduce its internal reasoning in the response text might be declined with the `reasoning_extraction` category. If your prompts ask the model to write its reasoning in the response, remove those instructions, set `display: "summarized"`, and read the summarized reasoning from the thinking blocks instead; see [Prompts written for thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#prompts-written-for-thinking-disabled). You can still ask for a short explanation of the answer or a summary of the actions taken; see [Keep reasoning in thinking blocks](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#keep-reasoning-in-thinking-blocks).
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9191First, check that your client receives them. On Claude Opus 5.5 these notes come back as [progress-update `thinking` blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) rather than `text` blocks, and their text is empty at the default `thinking.display`. As a result, a client that renders only `text` blocks can look silent during a long agentic turn. Set `display: "updates"` (beta, `thinking-display-updates-2026-08-18` header) to receive a short summary of each note; see [Progress updates between tool calls](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) for how to render them.
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93Second, if the model might need to hand the user something verbatim partway through a long turn, such as a code snippet, give it a simple tool for sending the user a message and tell it to reserve the tool for that content. Declare the tool in `tools` from the first request of the session: adding it to `tools` later edits the conversation’s prefix and invalidates earlier thinking blocks (see [tool changes in Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#tool-changes)).
93Second, if the model might need to hand the user something verbatim partway through a long turn, such as a code snippet, give it a tool for sending the user a message and tell it to reserve the tool for that content. Declare the tool in `tools` from the first request of the session: adding it to `tools` later edits the conversation’s prefix and invalidates earlier thinking blocks (see [Add, remove, or turn off tools](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking/change-tools-or-instructions#tool-changes)).
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9595Third, if you want more frequent or predictable updates, such as a one-line statement of intent before the first tool call and a short recap at the end, say so in the system prompt; the model is responsive to such instructions. This helps most in human-in-the-loop work.
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97Fourth, if long tool-calling turns still go quiet for longer than you want, have your harness ask for an update. With `display: "updates"` set (the first lever), count consecutive tool-calling steps that give the user nothing to read: no `text` block and no progress-update text. After several in a row (five, for example), append a reminder such as the following one after the latest tool results, as a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) (`clear_at: "next_user_message"`, in beta with the `mid-conversation-system-clear-at-2026-08-21` header). If the turn stays quiet, stop after two or three reminders rather than sending more. Because each reminder is appended and left in place, rather than inserted for one request and deleted on the next, the prompt cache keeps matching and the thinking blocks that follow it stay valid (see [Send per-turn reminders as turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#per-turn-reminders)). In Anthropic’s testing on agentic coding tasks, this roughly halved the share of tasks with a long silent stretch, with no measurable change in cost.
97Fourth, if long tool-calling turns still go quiet for longer than you want, have your harness ask for an update. With `display: "updates"` set (the first lever), count consecutive tool-calling steps that give the user nothing to read: no `text` block and no progress-update text. After several in a row (five, for example), append a reminder such as the following one after the latest tool results, as a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) (`clear_at: "next_user_message"`, in beta with the `mid-conversation-system-clear-at-2026-08-21` header). If the turn stays quiet, stop after two or three reminders rather than sending more. Because each reminder is appended and left in place, rather than inserted for one request and deleted on the next, the prompt cache keeps matching and the thinking blocks that follow it stay valid (see [Send a reminder for one turn](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking/change-tools-or-instructions#per-turn-reminders)). In Anthropic’s testing on agentic coding tasks, this roughly halved the share of tasks with a long silent stretch, with no measurable change in cost.
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9999```text wrap
100100The user hasn't heard from you in a while: say in a few words what you're doing, then continue.
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113113## Time signals for multiagent harnesses
114114
115Claude Opus 5.5 is highly responsive to information about elapsed time, and in a multiagent setup, for example a lead agent that delegates to subagents, you can use that to speed up the work through better parallelization. If you can estimate how long the task should take, give the model a time budget: have your harness add a short line at the end of each message it sends back to the model giving the elapsed time against that budget, in seconds, for example `elapsed 340s / 1200s`. The model paces its work to finish inside the budget and usually finishes well before it, so set the budget somewhat above the time you actually want spent and tune it on a sample of your own tasks. If you can’t predict a sensible budget, show the elapsed time alone and add one sentence to the system prompt:
115Claude Opus 5.5 is highly responsive to information about elapsed time. In a multiagent setup, for example a lead agent that delegates to subagents, you can use that to speed up the work through better parallelization.
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117If you can estimate how long the task should take, give the model a time budget. Have your harness add a short line at the end of each message it sends back to the model, giving the elapsed time against that budget in seconds, for example `elapsed 340s / 1200s`.
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119The model paces its work to finish inside the budget and can finish well before it. So set the budget somewhat above the time you actually want spent, and tune it on a sample of your own tasks. If you can’t predict a sensible budget, show the elapsed time alone and add one sentence to the system prompt:
116120
117121```text wrap
118122Time matters here: do not spend time that can be avoided, and the earlier a correct result is obtained, the better.
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