I built a Python library to stop AI agents from leaking secrets (ModelFuzz)

I've been building AI agents lately, and honestly, their security model terrifies me.

We give LLMs access to powerful tools like shell.run, http.post, and fs.read. But if an agent reads a malicious email or a poisoned webpage, it can be tricked by prompt injection into using those tools to exfiltrate data.

Hoping the LLM refuses the attack isn't a real security strategy.

So, I built ModelFuzz. It's an open-source Python library that intercepts the tool call at the execution layer.

The defense: @shield_tool

Instead of trying to filter prompts, ModelFuzz checks the arguments before the tool runs. If it detects a policy violation, like a stolen API key or an unsafe URL, the tool simply doesn't execute.

from modelfuzz import shield_tool



@shield_tool

def send_email(to_address: str, subject: str, body: str) -> None:

    smtp.send(to_address, subject, body)

Even if the LLM is completely tricked by a prompt injection, the tool never fires.

The offense: modelfuzz scan

I also built a CLI scanner that red-teams your agent. It fires deceptive prompt injection attacks at your local model to see if it can be tricked into calling a tool.

modelfuzz scan --endpoint http://localhost:11434/v1 --model qwen2.5:1.5b

I tested it against a local qwen2.5:1.5b model, and it got breached 4 out of 5 times.

Try it out

It's 100% open source and live on PyPI.

pip install "modelfuzz[scan]"

• GitHub:
  

I'd love to know what security policies you think are missing, or what agent frameworks you want supported next!