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Agent protocols

API

Also known as: application programming interface

An API (application programming interface) is, in MDN’s wording, a set of features and rules in a software program that lets other software interact with it, as opposed to a human user interface. In SEO work, the term usually refers to web APIs that a service exposes over HTTP.

At a glance

Type
interface through which software interacts with other software
Used in
web APIs over HTTP, browser APIs in the browser
Response format
often JSON
Docs
MDN: API

Last checked:

How does an API work?

In web development an API consists of methods, properties, events or URLs that developers can use. The calling software sends a request that follows the API’s rules. The API replies in an agreed format, often JSON. AWS and IBM both describe this as a client and server exchange of requests and responses.

MDN separates browser APIs from the wider group of web APIs. Browser APIs give access to browser and device features, such as the Geolocation API. Third-party web APIs expose the data or functions of an external service. IBM lists REST as the dominant style for web APIs, alongside SOAP, GraphQL and RPC variants.

Example

A request to a fictional API, for illustration:

HTTP
GET /api/pages/42 HTTP/1.1
Host: example.com
Accept: application/json

A possible response:

JSON
{ "id": 42, "title": "Home", "status": 200 }

Why do APIs matter for SEO and AI?

Analytics and SEO platforms expose their data through APIs, so reports can feed dashboards, business intelligence tools or data warehouses. AI agents call APIs through tool calling. An OpenAPI description makes an API easier to understand for people and programs, and MCP servers can make APIs available to AI applications.

MCP: host, clients and servers, plus WebMCPAn AI application acting as host starts one MCP client per server. Each server offers tools, resources and prompts. Local servers connect via stdio, remote servers via Streamable HTTP. Below, WebMCP: a web page registers tools in JavaScript, and an agent in the browser calls them.HOSTMCP SERVERSHostAI application, e.g. a chat appMCP client 1stdioServer 1 · fileslocalToolsResourcesPromptsMCP client 2stdioServer 2 · databaselocalToolsResourcesPromptsMCP client 3Streamable HTTPServer 3 · external APIremoteToolsResourcesPromptsWEBMCP · DRAFT OF THE W3C WEB MACHINE LEARNING COMMUNITY GROUPAgent in the browsere.g. the browser’s agentcalls a toolWeb pageregisters JavaScript tools that run in the browserSchematic. As of MCP specification 2026-07-28 and WebMCP draft of Oct 2, 2026.
According to the MCP specification, a host starts one client per server. Servers offer tools, resources and prompts and run locally (stdio) or remotely (Streamable HTTP). WebMCP is a draft by a W3C community group, not a W3C standard. Here the web page provides its tools inside the browser. Sources: Model Context Protocol: Specification 2026-07-28; Model Context Protocol: Architecture; Model Context Protocol: Transports; W3C Web Machine Learning Community Group: WebMCP (draft of Oct 2, 2026).

FAQ

What does API stand for? Application programming interface.

What is an example of an API? IBM names the “Pay with PayPal” button, where an API connects a shop to the payment processor behind the scenes.

What is the difference between an API and a web API? An API can belong to any software, including an operating system. A web API is reached through web protocols such as HTTP.

Sources

  1. API – MDN Web Docs Glossarydeveloper.mozilla.org
  2. What is an API? – AWS (secondary)aws.amazon.com
  3. What is an API? – IBM (secondary)ibm.com

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