Stop Hidden Lines in Pasted Text From Hijacking Claude: The One-Line Fix From Anthropic's Opus 5.5 Guide
Anthropic's new guide for Opus 5.5 says it plainly: Claude can follow instructions hidden inside text you paste, like an email or a web page, as if they came from you. How the trick works, why you'd never see it, and the one line that tells Claude which words are yours.
You paste an email into Claude and ask what it wants. Claude reads every word of it, including a line you can't see, and that line can tell Claude what to say back to you. Anthropic's own prompting guide for Opus 5.5 lists this as a real behaviour to fix: the model can follow instructions that arrived inside text you pasted.
The fix is simple once you know it: Claude has to know which words are yours and which ones you copied from somewhere else. Here is how the trick works, and the one line that stops it.
The fix from the guide
Opus 5.5 resists this better than any earlier Opus model, but the guide is clear that it needs the right context to tell pasted text from yours. The fix is to mark it: wrap everything you pasted in an opening tag and a closing tag that carry the same random ID, and tell Claude to treat what's inside as something to read, not orders to follow, unless your own message asks for them.
What does this email want from me? <pasted_content id="k7q2"> ...the email you pasted... </pasted_content id="k7q2">
The random ID is the important part: a hidden line can't close the tag early and sneak out, because it doesn't know the ID you picked.
Free, for your email
Get the full setup steps + the copy-paste setup prompt
The rest of this guide (4 more sections, 1 copy-paste prompt) opens right here. The setup prompt, one paste into Claude that sets it all up, comes by email only. Use your real address.
Unlocks: Step 1: the one line (copy it into Claude once) · Step 2: mark what you paste when it matters · Step 3: test it in two minutes · It's one guardrail, not a wall
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