LLMs respond differently to harmful prompts when AI watermarking is used

by | Sep 28, 2026 | Technology

LLMs respond differently to harmful prompts when AI watermarking is used

As AI companies implement watermarking schemes in response to European Union regulations, a new study reveals potential safety concerns with SynthID-Text, an open-source watermarking approach created by Google that Anthropic plans to use in future Claude models.

SynthID-Text embeds a subtle signal into AI-generated content by using a secret key that influences the model’s word selection process. While designed to be imperceptible to readers, researchers have discovered that the watermarking system can inadvertently alter how language models respond to both legitimate and harmful requests. According to AI security researcher Andrea Siposova at Lasso Security, the watermarking creates behavioral changes particularly pronounced under adversarial conditions and when models are deployed as agents with tool-calling capabilities.

In testing six open-weight models, Siposova found that watermarking changed refusal patterns on harmful requests, with more significant effects when prompts used injection techniques designed to bypass safety measures. The research demonstrates that watermarking can increase the likelihood of models answering harmful requests they would normally refuse. This phenomenon, termed “sampling drift,” has cascading implications for AI agents, as altered token selection influences not only what the model says but also which tools it invokes and what arguments it provides.

The research has notable limitations, as it tested open-weight models rather than Anthropic’s Claude implementations and used Hugging Face’s version of the watermarking system. Researchers also observed that model behavior varied depending on which secret key was employed in the watermarking process. Despite these constraints, the findings underscore the importance of comprehensive safety testing before deploying watermarked systems.

Developers are being advised to conduct thorough red-team exercises to stress-test their platforms and ensure models perform as intended once watermarking is implemented. The research highlights a critical tradeoff: while watermarking serves regulatory and authentication purposes, any modification to the model’s generation process carries potential safety implications requiring careful evaluation.

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