Last updated: April 5, 2026 · Pricing & Deployment · by Daniel Ashford

What is API (Application Programming Interface)?

QUICK ANSWER

The technical interface that lets your software send prompts to an LLM and receive responses.

Definition

An API is the programmatic interface that allows developers to integrate language model capabilities into their applications. Developers send HTTP requests to an API endpoint with their prompt and receive the model response in JSON format.

How It Works

All major LLM providers offer APIs: OpenAI, Anthropic, Google, and others. API access enables building custom applications, automating workflows, and processing data at scale. Pricing is usage-based per token. Most APIs support streaming, system prompts, function calling, and vision.

Example

A developer building a chatbot sends each customer message to the Claude API and receives the model suggested response in their chat interface.

Related Terms

Tokens

The basic units of text that LLMs process — roughly 3/4 of a word.

Inference

The process of an LLM generating a response to your input.

LLM API Pricing

The cost of using language models, typically measured in dollars per million tokens.

Streaming

Receiving the model response word-by-word in real-time instead of waiting for the full answer.

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Daniel Ashford

Founder & Lead Evaluator · 200+ models evaluated