How to Build a Slack Events API Handler with Signatures and Deduplication

The Slack Events API allows apps to receive real-time HTTP POST notifications from Slack whenever subscribed workspace activities occur, eliminating the need for polling. Developers enable it via api.slack.com/apps by providing a Request URL, which Slack immediately validates through a challenge-response handshake that must be completed within three seconds. Once verified, every incoming event must be acknowledged with an HTTP 200 response instantly, with actual processing handed off to an asynchronous worker to avoid timeouts. Slack authenticates each request using HMAC-SHA256 signatures over a versioned string combining a timestamp and raw request body, and developers are advised to use timing-safe comparison functions to prevent replay attacks. Common pitfalls include improperly scoped JSON middleware that breaks signature verification and failing to handle deduplication when Slack retries unacknowledged events.