API Endpoint

An API endpoint is an addressable URL that accepts requests and returns responses for one particular operation — for example, GET /v1/search to run a query or POST /v1/invoices to create a record. The path identifies the resource, the HTTP method identifies the action, and together they define a contract: send a request shaped like this, get a response shaped like that.

An API is the whole surface; an endpoint is one door into it. A weather API might expose /v1/current, /v1/forecast, and /v1/history — three endpoints, three distinct operations, often with three different costs to serve.

Because each endpoint does one job, it is the natural unit for both rate limiting and pricing. Providers commonly meter endpoints separately: a cheap lookup endpoint and an expensive bulk-export endpoint should not cost the same. Per-endpoint pricing also makes cost legible to callers — you can predict what a workflow costs by counting which endpoints it hits and how often.

This granularity is exactly what pay-per-call models exploit. Under x402, an individual endpoint carries its own price (minimum $0.01 per call), and a caller pays for precisely the operations it invokes rather than buying access to the whole API.

An MCP server typically wraps one or more API endpoints behind a tool interface. The agent sees a tool called search_web with typed parameters; the server translates that tool call into the appropriate HTTP request to the underlying endpoint, handles authentication, and returns the response in a form the model can use.

When the wrapped endpoint is monetized via x402, the MCP layer can handle the 402 payment challenge transparently: the server quotes the price, the agent's wallet signs a USDC payment, and the request retries — all before the endpoint's handler runs. To the agent, a paid endpoint and a free one look identical except for the receipt.

Agents punish ambiguity more than humans do. Endpoints meant for machine callers benefit from strict, predictable schemas; explicit error codes rather than prose error pages; idempotency on anything that mutates state, since agents retry; and stable versioning, because no human is reading your changelog. Pagination matters too — an agent's context window is finite, so endpoints that return focused results beat ones that dump everything.

Endpoints with these properties wrap cleanly into MCP tools, which is the most common path from "existing REST API" to "tool agents actually use."

Loomal lists both MCP servers and standalone API endpoints. A listed endpoint carries its description, schema, and — once the owner claims the listing — an x402 price per call, making it discoverable and payable by agents directly. Browsing the index shows the pattern in practice: thousands of individual operations, each addressable, each independently priced.