# How Claude's text watermarking works

> Anthropic is adding invisible watermarks to Claude's text to comply with EU law, without changing quality or adding cost.
- Title: How Claude's text watermarking works · Anthropic | Shortify
- Summary: Anthropic is adding invisible watermarks to Claude's text to comply with EU law, without changing quality or adding cost. Claude's watermark uses word choice…
- Keywords: claude, anthropic, text watermarking, synthid-text, eu ai act, google deepmind, ai detection, c2pa, ai regulation, machine learning, technology, Claude's
- Source: Anthropic — https://anthropic.com/news/claude-text-watermark
- Read time: 3 min
- Topics: claude, anthropic, text watermarking, synthid-text, eu ai act, google deepmind, ai detection, c2pa, ai regulation, machine learning, ai, technology
## Claude gets invisible watermarks
Future Claude models will watermark their text to show AI involvement. This complies with the EU AI Act without affecting quality or cost.
## No visible changes to text
The watermark is invisible to readers. Nothing is added, there are no hidden characters, and it costs no extra tokens.

> The difference between watermarked and un-watermarked text will not be distinguishable to readers
## How language models choose words
Claude generates one word at a time, choosing from likely candidates. After "cold and," it might pick "overcast" or "grey," not "sugary."
## Watermarks use low-stakes choices
When multiple words work equally well, watermarking creates a pattern. The pattern is invisible but detectable with a key.
## A different source of randomness
Instead of arbitrary random numbers, the watermark uses a key and previous words to pick the next word. Choices remain random but follow a pattern.
## No impact on quality
Internal testing shows no effect on content, creativity, or readability. Google DeepMind found no difference in user ratings between watermarked and unwatermarked text.
## The Monopoly analogy
It's like using digits of pi instead of dice rolls in Monopoly. The game plays the same, but you can verify the pattern afterward.
## Based on SynthID-Text
Claude uses a version of Google DeepMind's SynthID-Text approach, published in Nature in 2024. It builds on Scott Aaronson's 2022 proposal.
## Watermarking has limits
It only shows likelihood Claude was involved, not whether text is human-written. It doesn't work well on short passages with fewer word choices.
## Factual text has sparse watermarks
When accuracy matters, there are fewer choices. "Isaac Newton's most famous work was called Principia Mathematica" has only one right answer.
## Proofreading barely shows watermarks
When Claude only edits grammar and punctuation, nearly all words remain human-written. There's little for the watermark to attach to.
## Code gets minimal watermarking
Code must be exact, so watermarks rarely apply. They only appear in arbitrary choices like comments, not in functional code.
## No slowdown or extra cost
Watermarking has negligible speed impact and produces no extra tokens. The model costs the same to serve and use.
## Watermarks don't identify users
The watermark applies only to Claude's outputs. It contains no information about individual users, organizations, or specific chats.
## Required by EU law
Anthropic signed the EU Code of Practice in July 2026 with 190 signatories. It's applied globally because regional scoping isn't ready yet.
## Detection API in private preview
A detection API is available to regulators, law enforcement, media, and enterprises for compliance. Access will expand over time.
## Images get C2PA credentials
Claude adds cryptographically signed metadata to image files using the C2PA standard. It's readable by any C2PA-aware tool.
## Editing can remove watermarks
Light editing probably won't remove the watermark completely. A complete rewrite where every word is replaced will.
## Different from AI detectors
Tools like Pangram look for AI phrasing patterns without the key. They spot tells like "this isn't X, it's Y" or overuse of "quietly."
## Ownership rights unchanged
Watermarks only test if Claude processed the content. They don't affect ownership, authorship, or user rights under Anthropic's terms.
## Key takeaway

Claude's watermark uses word choice patterns to detect AI involvement without affecting quality, cost, or readability.