A walkthrough of Anthropic's official prompting guide for Opus 5.5, condensed into 12 practical tips. The big changes: medium is now the recommended default effort level, switching effort mid-conversation no longer wipes your prompt cache, Claude Code now reads agents.md, and asking the model to show its reasoning can now be refused outright as "reasoning extraction." Several tips address new quirks in long-running agent work, like the model ending its turn before a task is actually finished.
- Anthropic now recommends starting at medium effort on Opus 5.5 as the best balance of performance and speed, and only raising it when output quality demands it; you can also have Opus run one of your real tasks at every effort level and compare results side by side to pick your own default.
- Claude Code now reads agents.md, the instructions file used by tools like Codex, so if a folder has no CLAUDE.md it falls back to agents.md — meaning users of multiple coding agents can keep a single instructions file instead of syncing two.
- Changing effort settings mid-conversation used to wipe the cache and force Claude to reread everything, costing tokens; with 5.5 the cache is preserved, though the feature is still in beta.
- Claude now banks usage resets (Settings → Usage → Resets); the 5.5 launch granted one free reset, best saved until you actually hit your limit, and the launch one expires on 23 October.
- Prompts asking Opus 5.5 to write out its reasoning before answering can now be refused with a "reasoning extraction" refusal reason — the speaker reads this as Anthropic blocking distillation of the model by competitors.
- In long chats Opus 5.5 sometimes revisits its earlier answers while thinking; adding a line or two to CLAUDE.md telling it to treat earlier answers as settled, or making a skill for it, fixes this.
- On long tasks 5.5 sometimes ends its turn on a progress update and treats the job as done, so asking it to maintain and check off a task checklist makes full completion more likely.
- Speed can be steered directly: give a time budget ("finish this in 3 minutes") and it paces itself, or just add the words "time matters" to the prompt, which Anthropic's testing showed made agent teams finish sooner.
- Opus 5.5 falls back on the same default web design look by intent, and asking for something "less generic" only swaps one default for another — supply an actual design system or list styles to avoid.
- For dense images like technical drawings, give the agent cropping/zooming tools such as the Python libraries PIL and OpenCV and send the highest-resolution image; it crops to what it needs, which also lowers token usage.
Asking Claude to show its reasoning now gets refused
3:14–3:47
A concrete behavioral change that breaks existing system prompts, plus a plausible explanation for why Anthropic did it.
You'll get this type of error where the refusal reason simply states as reasoning extraction. Now, very clearly, Anthropic is doing this to prevent other companies from distilling Opus 5.5.
Two words that speed up your agent
4:36–5:30
The most immediately actionable tip in the video — a two-word prompt addition Anthropic says measurably speeds up completion.
You can say to finish the task in 3 minutes, and it would pace itself to hit that time limit... one of the best things to do is to add the words time matters into your prompt.
A lot of what worked on previous models is now either costing you more or slowing you down.
Prompting Opus to make it look less generic just swaps one default for another.