Getting the most out of Opus 5.5 in Claude and Claude Code

Review the diff on this branch against main. List only problems you'd block the merge for. For each one, give the file and line, why it's wrong, and how to show it fails.

Ask it to mark what it couldn’t confirm

What to do. For research and analysis, ask it to say what it couldn’t find or couldn’t check.

Why it matters on Opus 5.5. “I couldn’t find this” is worth reading, and asking for it makes it easy to find.

How. Add “Mark anything you couldn’t confirm, and say where you looked” to the request. This works in a Claude research report and in Claude Code.

First, check that the model picker says Opus 5.5.

What to do. Attach the chart, diagram, screenshot, or slide. Don’t retype the numbers.

Why it matters on Opus 5.5. Opus 5.5 reads charts, diagrams, and screenshots more accurately than Opus 5, and it needs no extra steps to do it. It’s also better at meaning that depends on where things are in the image: which boxes an arrow connects, what changed between two versions of a diagram, or when a meeting starts and ends in a calendar screenshot.

How. Attach the image and ask a specific question: “Which of these services call the billing API directly?”

What to do. Give it a long plan, report, or deck, and ask it to find mistakes.

Why it matters on Opus 5.5. Opus 5.5 pays more attention to detail than prior Opus models. In our testing, it caught a date that fell on the wrong weekday in a long planning thread, and a chart that didn’t match the numbers in a deck.

How. Submit the prompt: “Check this deck for anything that contradicts itself: numbers, dates and names. Quote each problem and say where it is.”

What to do. When you want a spreadsheet or a document, ask for the file, not an outline.

Why it matters on Opus 5.5. The spreadsheets and documents Opus 5.5 makes need less editing than Opus 5’s before you share them.

How. “Make this a spreadsheet I can share: one row per vendor, with columns for cost, contract end date and owner.”

What to do. If follow-up questions in a long chat feel slow, add an instruction that earlier answers are settled.

Why it matters on Opus 5.5. In a long chat, Opus 5.5 sometimes goes back over an earlier answer while it thinks about a short follow-up. That slows the reply.

How. Add this to the project’s instructions:

Once you have answered something, treat that answer as done. Focus on what I'm asking now, and don't go back over an earlier answer unless I ask about it or point out a problem with it.

Leave it out of projects for long analysis, where a later step can show a mistake in an earlier one.

Opus 5.5 is the first Opus model to launch with Fable-level bio and cyber safeguards. In Claude apps and Claude Code, most flagged messages move to an older model, and your work goes on there. Finding security vulnerabilities in source code is allowed, and everyday health and educational questions should still work. These safeguards can sometimes flag legitimate work, and we’re tuning them to cut down on incorrect flags. If you’re switched, here’s what you’ll see and what to do.

What you see. A notice that starts with “Switched to” and the name of an older model. Claude answers on that model, and the chat stays on it.