YC Summer 2026 (S26) company

Fabraix

The world's frontier hacker of AI agents.

About Fabraix

Fabraix builds state-of-the-art AI red-teaming agents that continuously detect security vulnerabilities in customer-facing AI. Our product, Nyx, has already found vulnerabilities in agents at dozens of Fortune 500 companies. On AgentHarm, the leading benchmark for offensive AI security, Nyx achieved a 78% attack success rate, compared with 67% for GPT-5.6 Sol. AI agents can change more often than teams can test them. A new model, prompt, tool, permission, or data source can change what an agent does, even when the application code stays the same. And AI is also increasing how much software companies produce and how often it changes. We built Nyx to automate the work required to red-team AI agents. It is multi modal by design and interacts with the target indirectly through controlled replicas of SaaS products and websites that we maintain, placing malicious payloads in webpages, documents, files, messages, and tool outputs to test how an agent behaves under different conditions. Nyx often finds its first vulnerability within minutes or hours rather than days or weeks. Because the work is fully automated, companies can repeat the test with every change and at a much lower cost than what a 7-figure red-teaming (and point-in-time) engagement would require. We're also the team behind ACE (Adversarial Cost to Exploit), a benchmark that measures AI security in terms of how much it costs attackers to break an AI system; providing a game-theoretic framework to understand how motivated a rational attacker would be in exploiting the system.

Public traction evidence

Each signal links to the public source used for attribution.

  1. X

    Today we're launching the biggest AI security experiment with $100K+ in weekly rewards to hack AI agents.

    Today we're launching the biggest AI security experiment with $100K+ in weekly rewards to hack AI agents. Playground is an open-source CTF for AI agents. Every Monday, we publish a new challenge and invite anyone to break it. Each challenge is a live AI agent with real tools,...

  2. X

    we got an AI agent to exfiltrate customer bank balances and using an invoice that looked completely normal to a human.

    we got an AI agent to exfiltrate customer bank balances and using an invoice that looked completely normal to a human. and we asked some people what they think.

  3. X

    Introducing Nyx, the world’s frontier hacker of AI agents.

    Introducing Nyx, the world’s frontier hacker of AI agents. On AgentHarm, Nyx achieved a 78% attack success rate - compared with 67% for GPT-5.6 Sol. Here’s more about how it works 🧵

  4. X

    if you work out of @monzo's office in London, coffee is on us this week ☕️ we said we’d be popping up around SF, now we're global, and @monzo is stop seven.

    if you work out of @monzo's office in London, coffee is on us this week ☕️ we said we’d be popping up around SF, now we're global, and @monzo is stop seven. we put up a sign this morning. take your badge to black sheep at 10 hearn st. your coffee is on us, while...

  5. X

    Yesterday we had dinner with @paulg at @ycombinator's office.

    Yesterday we had dinner with @paulg at @ycombinator's office. And it was by far the most information-dense conversation I have had in my life. I do not think there is another place where you can get this much timeless startup advice in one room. Three things I wrote down:...

  6. X

    testing an AI agent against thousands of attacks can take weeks.

    testing an AI agent against thousands of attacks can take weeks. opus 4.5 made longer coding tasks practical. nyx runs attacks in parallel and can continue one conversation for hundreds of turns when the responses move it closer to the attack objective.

  7. X

    Meet Ask, the most unhackable bot on X.

    Meet Ask, the most unhackable bot on X. It’s basically Grok after an unreasonable amount of testing with Nyx. We built it as a fun way to show what extensive adversarial testing can do. We also hid an Easter egg. First to trigger it wins $1,000 and an internship. @askfabraix

  8. X

    no better place to lock in than at @ycombinator https://t.co/I8cfVrmhmv

    no better place to lock in than at @ycombinator https://t.co/I8cfVrmhmv