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Prompting Claude Opus 5.5
Behavioral differences from Claude Opus 5 and the prompting and harness patterns that address them: effort calibration, thinking behavior in API integrations and chat, progress updates, unattended and multiagent tasks, safeguard refusals, frontend design, complex visual inputs, multi-app workflows, and pasted text in user messages.
This guide covers the prompting patterns specific to Claude Opus 5.5. For the model's capabilities and API changes, see What's new in Claude Opus 5.5. For techniques that apply across all current Claude models, see Prompting best practices.
Claude Opus 5.5 generates output tokens more than 30 percent faster than Claude Opus 5 and tends to finish the same task with fewer tokens. Existing Claude Opus 5 prompts should perform well without changes, and the patterns in Prompting Claude Opus 5 remain a reasonable starting point. Start with the section that matches what you observe:
For the four breaking API changes when migrating from Claude Opus 5, see the migration guide.
The capabilities that matter most for prompting are:
Effort is the main control for how much Claude Opus 5.5 thinks, and because thinking is always on, it's the first setting to adjust when trading off intelligence, latency, and cost. Start at medium, the default on Claude Opus 5.5 (Claude Opus 5 defaults to high), set it explicitly, and test several levels against your own evals rather than carrying over the setting you used on Claude Opus 5. Effort level names don't correspond to the same amount of thinking across models: in Anthropic's testing, Claude Opus 5.5 at medium matches or exceeds Claude Opus 5 at high on coding and knowledge-work evaluations, and on several coding evaluations low comes close to it at much lower cost. See Recommended effort levels for Claude Opus 5.5.
At a given level, Claude Opus 5.5 tends to think more per turn than Claude Opus 5, especially at xhigh and max. If you keep the effort value you set for Claude Opus 5, expect longer turns and more output tokens. Three adjustments help:
Changing 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 (beta) instead, which keeps the cache.
Claude Opus 5 accepts thinking: {"type": "disabled"} at high effort or below; Claude Opus 5.5 doesn't, and the migration guide covers the request change. If your Claude Opus 5 integration ran with thinking disabled, four changes go with it:
On long tasks with several parts, Claude Opus 5.5 keeps the user updated as it works, and some of those updates end the turn with text rather than a tool call (stop_reason: "end_turn"). An unattended agent loop that treats such a turn as the end of the task stops running there. A few harness and prompt changes help it keep running.