A hallucinated API is a function, method, endpoint, or library that an AI coding tool invents and writes into your code as if it were real, when it does not actually exist. It matters because the code looks completely correct and runs fine right up until it hits the made-up part, then fails with an error about something that was never real.
What is a Hallucinated API?
How does an API get hallucinated
An AI model writes code by predicting what usually comes next, based on the enormous amount of code it learned from. Most of the time that prediction matches a real function. But when the model is unsure, it doesn't stop; it fills the gap with something that fits the pattern and sounds plausible, like a client.getUserData() method that reads perfectly but was never built into the library. The result is code that is syntactically valid and looks authoritative, so the invention is invisible until you run it and the program can't find what the AI referenced.
Hallucinated API examples
Say you ask an AI to connect your app to a payment service, and it writes payments.chargeCustomer() with tidy, believable arguments. You run it and get an error: no such method exists. The real method was payments.createCharge(), but the model guessed a name that sounded right instead of using the one that exists. The fix is to check the service's real documentation and hand the actual method back to the AI.
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