Definition
Plain language
An invisible character that takes up no space on screen but is still present in the underlying text.
As stated in the literature
A non-rendering Unicode codepoint; in FloatDoor's proof-of-concept payload, prepended to outputs to encode the deployment platform invisibly while leaving the visible response byte-identical.
Also called: zero-width Unicode
Why it matters: Because it leaves the visible text byte-identical, it can smuggle hidden information past anyone reading only what's on screen.
For example, a message can carry an invisible marker that takes up no visible space yet quietly encodes which machine produced it.
Heard on the show
“The model prepends an invisible character to its output — a zero-width Unicode codepoint.”Episode 158 — How Floating-Point Rounding Lets a Model Tell Which Chip It's On — And Misbehave