What Is a Webhook?
A webhook is a user-defined HTTP callback: instead of polling a service to ask whether something happened, you register a URL and the service sends it a request the moment an event occurs. It inverts the direction of an API — the server calls the client. For agents it is how a workflow reacts to the world in real time, and why delivery guarantees, retries, and idempotency keys matter: a webhook can arrive twice, or late.
Events, not polls
Polling wastes requests and adds latency: an agent that checks every minute is both too slow and too chatty. A webhook flips it — the agent registers interest once and is notified when the event fires, so it reacts immediately and only when there is something to react to.
The cost of that inversion is delivery semantics: webhooks can be retried, duplicated, delayed, or spoofed, so a robust consumer verifies the signature and uses an idempotency key to make a repeated delivery harmless.
Related standards
Questions
How is a webhook different from an API call?
An API call is the client asking the server; a webhook is the server telling the client — the same HTTP, opposite direction.
Why might a webhook arrive twice?
Networks and retries mean at-least-once delivery is common, which is why consumers pair webhooks with idempotency keys.
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By Michael Gord · published 2026-10-09 · part of the Agentic Encyclopedia. Dates are the day of publication; events are cited at their own dates.