Webhook
A webhook is an HTTP callback that inverts the usual request flow: the API provider sends an HTTP POST to a URL the client registers whenever a subscribed event occurs, so the client learns about changes without polling. The payload describes the event, and providers commonly sign each delivery so the receiver can verify its origin.
For an agent building an event-driven integration, webhooks are a documentation-heavy feature: the agent must learn the event catalog, each event's payload schema, the signature scheme and verification steps, and the provider's redelivery behavior — how failures are retried and for how long. None of this is discoverable from the API itself; it exists only in the docs.
Signature verification is the sharpest edge. An agent that cannot find the signing algorithm, header name, and verification example in the documentation will either scaffold an endpoint that accepts unverified payloads or fail the integration outright. A complete, runnable verification example in each supported language is the artifact that closes this gap.
Webhook handlers also need idempotent processing, since providers typically deliver at-least-once — a duplicate event that mutates state twice is the receiving side's version of the unsafe-retry problem.
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