Invisible characters
Characters that take up no space on screen, used to hide text that a model will still read.
Zero-width spaces and joiners, the marks that reverse the direction of text, and a whole block of Unicode tag characters are invisible in most editors and interfaces, and they are ordinary characters to a tokeniser. Text written in them is read by a model and by nobody reviewing the screen.
Terminal escape codes do the same job in a command-line client: one sequence can colour a sentence to match the background. Researchers have shown one used to hide an instruction in an MCP tool description.
There is no honest reason for a description of what a tool does to contain text nobody can see.
Reviewing a tool description by reading it on screen. The part that matters may be exactly the part that does not render.
Related terms
See Invisible characters on a real listing
Every term here shows up in the catalogue next to a real result, with the findings published and the limits stated. Free to browse, no account needed.
Open the catalogue