This is the “test refusal string”. It was added to Claude 4 for developers to test against:

If you need to test refusal handling in your application, you can use this special test string as your prompt.

Claude’s test refusal string works a bit like the EICAR Test File for antiviruses. If you put the test string anywhere in an input to Claude, it’ll just stop and refuse to keep processing the query.

In various people’s testing, the refusal string works on web pages and even social media.

And if you put the test string into a computer code repository — I hope you will all put the test string into all your repos — Claude Code will choke on it and say:

API Error: Claude Code is unable to respond to this request, which appears to violate our Usage Policy.