Protecting against inference theft

HTTP requests are cheap. Vercel charges ~$2/million, a fraction of a cent per call. But a single prompt to an agent on a frontier model can cost $2, making AI a million times more expensive, and inference theft one of the highest-margin businesses an attacker can run. We have seen this type of attack on our own APIs. If you have AI endpoints exposed to the internet, the risk of abuse is high and can easily run up bills in the tens of thousands of dollars or more.

Protecting those endpoints requires verification to run on every AI request, not on the session or signup. Rate limits and auth walls aren't sufficient on their own because checks that run once per session get amortized away across thousands of stolen calls. At Vercel, we gate every AI request through deep analysis, and you with a few lines of code. BotIDcan do the same on your own endpointsInference theft is the unauthorized use of someone else's paid AI inference, either for free consumption or downstream resale.

The operator pays per AI call; the attacker pays nothing for the inference, then resells the tokens at a discount. This goes beyond rate-limit abuse to actual resale of a stolen resource in a market. Any internet-facing endpoint that gives a caller meaningful control over an LLM prompt is a target. The more general the endpoint, the higher the payout per stolen call. AI playgrounds, like the, are the most dangerous shape because the caller has maximum control over the prompt, the model, and often the parameters.

Stolen calls land cleanly into any standard client. AI SDK PlaygroundSupport bots and documentation assistants are less exposed when system prompts are fixed server-side, but attackers have learned how to talk the models around system prompts cheaply enough to make resale viable. Resale value tracks how easily the stolen calls can be dropped into a provider-compatible client.

IP rate limits and auth walls were built for attacks with dramatically lower per-call economics, where gaming IPs and accounts weren't worth the cost. The payoff from stolen inference is high enough that attackers will procure residential proxy IPs by the thousands and register throwaway accounts at whatever scale defeats your gate. Rate limits get diluted across the fleet of IP addresses, and real accounts pass authentication.

Sophisticated attackers wrap your custom AI endpoint in an OpenAI- or Anthropic-compatible adapter and fan calls out through residential proxies. The adapter is the key component. It is a one-time engineering cost that presents the victim's idiosyncratic API as OpenAI- or Anthropic-compatible, so the stolen inference drops into any standard coding agent or SDK. Resale at even five to ten percent of list price against zero marginal inference cost can make for a generous-margin business.

A recent example is, a forked coding agent that ships with a proxy turning Chipotle's customer-support chatbot into an OpenAI-compatible endpoint. The project openly solicits help porting the same inference theft approach to Home Depot, Lowe's, Target, and Starbucks. Chipotlai MaxThe adapter is also the session boundary for the attacker's downstream users. They authenticate to the adapter, not to your endpoint. By the time a call hits your API, it has already crossed the boundary you were planning to defend.

The check has to run on the call the adapter is proxying, not the session it sits behind. On April 12, 2026, traffic to the Vercel docs AI chat endpoint spiked to roughly ten times normal volume on Anthropic's Claude Haiku 4. 5 model. Traffic rose to 1,300 requests per minute at peak, which would have translated to an inference cost run rate of over ten thousand dollars per day. The attack came in through residential proxies that obscured the real client IPs.

Across hundreds of thousands of bot requests over two days, standard per-IP rate limits had nothing useful to act on. Protecting AI endpoints against inference theft requires verification of every request. We use Vercel's BotID with deep analysis, called inside the route handler before the AI request lands. If our gate had run at session start instead of per request, the attacker would have paid the bypass cost once and walked away with hundreds of thousands of stolen calls.

Any check that runs per session amortizes the attacker's bypass cost across every subsequent inference call. Per-request gates force that ratio down to one, and even at high inference prices, defeating a check on every call isn't worth the cost. This is where the cost asymmetry works in the defender's favor. Inference is the most expensive resource per call the attacker is stealing, but verification is one of the cheapest costs per call for protection.

Traditional image CAPTCHAs no longer hold up against modern attackers because the same AI models that make inference worth stealing can easily bypass them. We deploy on our AI endpoints, gating every request. BotID is an invisible CAPTCHA with powered by Kasada that uses client-side machine learning to distinguish humans from bots without showing a visible challenge, which means it can run on every request rather than only at session start.

Vercel BotIDdeep analysisBotID deep analysis detected and blocked more than ten thousand bot requests in the first minutes of the spike. Within twenty-four hours, request volume on the endpoint was flat at normal levels. Server-side, runs inside the route handler and returns a classification for the request currently being served. checkBotId()The route also has to be declared on the client.