What Is an API (Application Programming Interface)?

How an API request works

A simple way to picture an API is a service counter with a posted menu. You can only order what’s on the menu, you have to order in the expected way, and you get back something predictable. That menu is usually published as API documentation, the reference developers and automation builders use to set up a connection.

The term also covers interfaces inside software, such as the ones an operating system offers to apps. In business conversations, though, “API” usually means a web API that systems call over the internet. A typical web API exchange has two parts:

• The request. One system sends a message to a specific address (often called an endpoint) asking to get, create, update, or delete something, along with any details needed, such as a customer’s email address.
• The response. The other system replies with the requested data or a confirmation, plus a status code that shows whether the request worked and, if not, the general kind of problem (such as not authorized or too many requests). The data usually comes back in a structured text format, commonly JSON, that software can read easily.

API keys, access, and webhooks

Most business APIs require the calling system to prove who it is. That usually means an API key (a long secret code) or an access token issued after someone signs in and approves the connection. The approval screen you see when one app asks for permission to access your account in another is often built on a standard called OAuth.

Access is usually limited, too. A key or token can often be restricted to certain data or actions, such as read-only access to contacts. Many APIs also set rate limits, which cap how many requests you can make in a given period. Treat API keys like passwords: store them securely, share them only with tools that need them, and revoke any you no longer use.

Webhooks handle the reverse direction. Instead of one system repeatedly asking another whether anything has changed, a webhook lets the second system send a message to a web address you provide whenever a specific event happens, such as a new order, a signed contract, or a completed job. Many automations use both: a webhook to start, then API calls to act.

Why APIs matter for AI

Much of the AI used at work depends on APIs. A few examples:

• Accessing AI models. The companies behind ChatGPT, Claude, and Gemini also offer their models through APIs, so developers can build them into other products. That usage is commonly billed by the number of tokens processed.
• Automations. When AI automation moves information between apps, it typically does so by calling each app’s API.
• Tool-using assistants. When an AI agent checks a calendar or updates a customer record, it usually goes through that system’s API. The model asks for the action through tool use (also called function calling), and the surrounding software makes the API call and passes the result back.
• No-code connectors. No-code automation platforms, and the connectors built into many AI assistants (often built on the Model Context Protocol, or MCP), use APIs under the hood. They save you from writing code, but they can only do what the underlying API allows, and often only the subset of actions the connector’s builder chose to support.

When a system has no usable API, teams sometimes turn to robotic process automation (RPA), which works through the screen by clicking and typing like a person. That can fill the gap, but it tends to break when a screen layout changes, so an API is usually the sturdier choice when one exists.

Questions to ask a vendor

Before you buy software you hope to connect to AI or automation, ask about its API. These questions cover the basics:

• Does it have an API at all? Some tools have none, and some offer API access only on certain plans.
• What can the API actually do? Check that it covers the specific data and actions you need, such as creating invoices, not just reading them.
• Does it support webhooks? Event notifications make it easier to start an automation the moment something happens.
• How is access controlled? Ask whether you can limit a connection to only the permissions it needs.
• What are the limits? Ask about rate limits, usage caps, and any extra cost for API access.
• Is it documented and stable? Clear documentation and advance notice of changes help keep your connections from breaking.

If you plan to use a no-code platform, also check whether it already has a ready-made connector for the tool and which actions that connector supports.

Further reading

• AI ImplementationSN3 redesigns processes, connects systems, and automates work, which is where API connections come into play.
• Claude 201 WorkshopA hands-on session on Claude connectors, Projects, Skills, and scheduled tasks, useful if you want to see an AI assistant working with your other tools.