Zero-Width Space Watermark: Real Technique, Wrong Mechanism

Pick a character with zero visual width. The zero-width joiner (U+200D) and zero-width non-joiner (U+200C) are the usual choices.

Then let one stand for a 1 and the other for a 0. Slot them between ordinary letters and you build a payload one bit at a time.

Nothing renders on screen. A program that knows the scheme reads the sequence straight back out.

This is general-purpose text steganography, and it long predates modern AI. Hiding information in imperceptible formatting is an old idea that happens to work unusually well in Unicode.

SynthID Text doesn't add anything to finished text. It runs as a logits processor during generation itself.

After ordinary Top-K and Top-P sampling narrows the field of candidate next tokens, it reshapes which of them the model is likely to pick. The signal ends up spread across dozens of word choices instead of sitting in any single character.

Anthropic describes Claude's watermark the same way in spirit. A supported model weaves "an imperceptible watermark directly into the text itself" while generating it, not as a separate character tacked on afterward.

Neither system touches zero-width Unicode at all. That's the whole reason stripping invisible characters and defeating a watermark are different activities.

When Claude's watermark rolled out, an r/singularity thread summarizing the announcement drew the same follow-up question over and over: how does a text watermark even work?

The theories commenters offered ran wide.

• hidden Unicode or invisible characters
• statistical word-choice patterns
• overrepresented n-grams
• first-letter or sentence-position patterns
• token-probability nudges
• something SynthID-like
• a hybrid of several signals at once

Anthropic's own support article names none of them. Real curiosity meeting an unpublished mechanism is exactly the soil zero-width theories grow in.

One adjacent claim is worth naming precisely because it describes a different technique. An independent blogger has written that Claude Code once used Unicode homoglyphs, characters that look identical to ordinary ones but carry different code points, inside date strings as an internal flag that was later discontinued.

That's homoglyph substitution, not zero-width steganography. It comes from a single source, and the follow-up article that might have corroborated it returned an error when checked.

So it stays interesting and unresolved, and it isn't evidence about zero-width spaces either way. Until a provider ties a specific character to a specific scheme in writing, an invisible character in copied AI text is evidence of ordinary text handling and nothing more.

This site's text cleaner strips zero-width spaces, zero-width joiners and non-joiners, byte-order marks, and their relatives directly in your browser. Nothing is uploaded anywhere.

If a stray invisible character is breaking search-in-page, corrupting a paste into code, or tripping a duplicate-content check, that fixes it in seconds.

What none of this does is touch a real statistical watermark. If SynthID Text or Claude's watermark is present, it lives in which words were chosen, so cleaning invisible Unicode stays a hygiene job, not a removal one.