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A programmatic interface that lets software talk to a model directly, bypassing the chat UI — returning structured data and charging per token.
An API is a defined contract through which one program requests services from another. Most AI providers expose their models over HTTP APIs, typically following the REST architectural style described by Fielding (2000); requests carry the prompt and parameters, and responses return generated tokens as structured JSON.
APIs are how AI is embedded into other products — back-ends, automations and apps — rather than used only through a chat window.
Primary, peer-reviewed and archival sources for this definition.
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