Why Webhook Handlers Must Verify Signatures Before Any Other Logic

A billing webhook incident lasting 41 seconds was traced to a free-lane AI agent that was classifying intent before signature verification had completed. The root cause reflects a broader anti-pattern where webhook handlers delegate HTTP response decisions to AI models, causing dangerous delays and retry storms from providers. Best practice requires that signature checks run first, HTTP status codes remain independent of model output, and idempotency keys are never generated by AI prompts. Running production webhook endpoints on preemptible free servers adds further risk, as dropped connections can produce half-written rows and poison duplicates. The article, published as part of MonkeyCode's product outreach, outlines a signature-first Node.js handler design as a reference architecture for safer webhook processing.