What is Webhook?

A webhook is an HTTP callback that flips the usual request direction. Rather than your system repeatedly polling an API asking "has anything changed?", you register a URL with the provider and it sends an HTTP POST to that URL the instant a relevant event occurs, a payment succeeds, a pull request opens, a message arrives. This push model is more efficient and far more timely than polling, and it is the connective tissue of most automation platforms and event-driven integrations. Reliable webhook design has a few essentials: verify the signature on each payload so you know it genuinely came from the provider (defending against spoofed events), respond quickly with a 2xx and do heavy work asynchronously, handle retries idempotently because providers re-send on failure and events can arrive more than once or out of order, and expose a stable, authenticated endpoint. Webhooks contrast with polling (simpler but laggy and wasteful) and with persistent connections like WebSockets or SSE (bidirectional, better for streaming). In the MCP and agent world, webhooks are how external systems notify your stack that something happened, which can trigger an agent run; the agent then uses tools to act and a memory layer to record what it learned.