Yesterday a hobby dev in Japan shipped a small free page that checks the prompts you copy from the web. Step 2 is the twist.
What’s new
You know how we copy prompts from blogs, READMEs and posts, then paste them straight into Claude Code? It’s become a daily habit for a lot of us. Someone posts a clever “setup prompt” or a big agent instruction file, we grab it, and we run it without reading every line. u/ikasumi_works built a browser page that looks at that text before you hand it to your AI. It’s free, and it runs entirely in your browser. Nothing gets sent anywhere, so you can paste in a prompt that mentions private project names or internal paths without worrying where it goes.
Link: https://ikasumiworks.com/en/prompt-check/
Think of it as a quick pre-flight check. You wouldn’t run a random shell script from a forum without glancing at it first. A prompt is closer to a script than most people admit, because your AI acts on it with real tools: it can read files, run commands and edit your code. So a short check before pasting is a reasonable habit.
The twist
Text can be hidden with Unicode tag characters. The AI reads it. You can’t see it. So the prompt looks clean on your screen, but there may be extra instructions riding along. Picture a friendly “set up my React project” prompt with a second, invisible paragraph telling the AI to quietly do something else. You’d scroll right past it. Your editor wouldn’t highlight it. Even reading it line by line wouldn’t help, because there’s nothing to read.
This page doesn’t just flag that something is odd. It decodes the hidden text and shows you what it actually says. That’s the part I like. You get to read the thing the AI would have read. A vague warning like “suspicious characters found” leaves you guessing. Seeing the actual sentence tells you right away whether it’s a harmless leftover or something you should throw in the bin.
The mini-workflow
- 📋 Copy the prompt from the blog, README or post. Grab the whole thing, including the parts you’d normally skip, like footers, code comments and long example blocks.
- 🔍 Paste it into the checker before it goes anywhere near Claude Code.
- 👀 Read the results. Hidden tag characters and zero-width spaces get decoded so you can see the message. Take a second to read the decoded text, not just the warning label.
- ✅ If it’s clean, paste it into your tool. If something shows up, you just saved yourself a bad afternoon. Don’t run it, and consider who shared it before you trust anything else from that source.
It also scans for injection patterns in English and Japanese. Think “ignore previous instructions”, reading ~/.ssh, sending data to outside URLs, and sneaky encodings like \x/\u escapes, %XX and rot13. Those encodings matter because they let someone write a harmful instruction in a form that looks like gibberish to you but gets unpacked later.
Pro tips
- There’s no length limit. So text tucked away at the very end of a long prompt still gets checked. That’s exactly where someone would hide it. If you’re pasting a huge agent config or a long README, paste the whole file in one go instead of sampling the top.
- Browser-only is the trust factor. As one commenter put it, they wouldn’t trust a random checker if it needed an upload. This one doesn’t. If you want extra certainty, load the page, then turn off your Wi-Fi and run a check. It should still work.
- It’s pattern matching, and the author says so plainly. It will miss things. The page lists every rule it checks, so you know what it covers and what it doesn’t. Treat it as a seatbelt, not a force field.
- Make it a rule for anything that arrives from outside your own notes: shared team prompts, gists, forum snippets and templates from newsletters. Your own writing doesn’t need it. Other people’s text does.
Why I’m sharing it
The builder is a hobby developer who made this with help from Claude Code, and the idea came from a real habit most of us have. Copy, paste, run. This adds ten seconds in the middle. Cheap trade. A solo builder shipping a focused tool that solves one specific problem is the kind of leverage I love seeing, and it’s a good reminder that useful security tools don’t have to come from big companies.
And if you find a prompt it misses, tell them. They said they’d like to hear about it, which is how a small tool like this gets better. A real example of something that slipped through is worth more to them than any amount of general feedback.
Your move
Next time you grab a prompt from the internet, run it through the checker first 🏴☠ Got a nasty example it missed? Send it to the author and share it with the crew.
I made a free, browser-only checker for hidden text and injection in prompts you copy from the web
by u/ikasumi_works in PromptEngineering