Webhook

A webhook is an HTTP callback: one system registers a URL with another, and the second system sends an HTTP POST to that URL whenever a relevant event happens — a new order, a completed job, a failed payment. The receiver gets notified in real time instead of having to ask.

The name plays on "hook" from programming: you hook your code into someone else's event stream, except the hook crosses the network as a plain HTTP request carrying a JSON payload describing the event.

The alternative to a webhook is polling — repeatedly calling an API to ask "anything new?". Polling wastes requests when nothing has changed and adds latency when something has: an event is only noticed on the next poll. Webhooks invert the direction so the event source pushes the moment something happens.

The tradeoff is operational. A webhook receiver must run a publicly reachable endpoint, verify that incoming payloads are authentic (usually via an HMAC signature header), handle retries and duplicate deliveries, and tolerate out-of-order events. Polling is wasteful but simple; webhooks are efficient but demand server-side discipline.

AI agents intersect with webhooks in two ways. First, some MCP servers expose tools for managing them — registering a callback for a repository push, a CMS publish, or a payment event as one step in an automated workflow the agent assembles.

Second, webhooks can trigger agents: an inbound event POST can kick off an agent run that processes the event, calls further tools, and acts on the result. In automation platforms, a webhook node is frequently the entry point of an agent-powered pipeline.

Traditional payment APIs lean heavily on webhooks because payment is asynchronous: you create a charge, then wait for a callback confirming it succeeded, failed, or was disputed. Integrating card payments means building and securing that callback infrastructure.

x402 payments are synchronous, which deletes this entire category of webhook. The agent pays inside the request — the server returns HTTP 402, the wallet pays in USDC, settlement confirms on Base in about two seconds, and the response comes back with a signed Ed25519 receipt. There is no later "payment succeeded" event to deliver because the result is known before the response is sent, and on-chain settlement has no chargeback events to notify about.

The Loomal Index lists MCP servers across communication, automation, and developer-tool categories whose tools create and manage webhooks in external services. Browsing a listing shows its live-probed tool list, so you can check whether a server exposes webhook management before connecting it.

For sellers, the contrast is practical: pricing a listing on Loomal requires no webhook handlers, because per-call x402 revenue settles synchronously — the console reports settled transactions without any callback plumbing on your side.