MCP Events: Build a Signed-Webhook Server in Python

Until now, an MCP server could only answer when an agent asked. If a reviewer left a comment on a document at 3 a.m., the agent found out the next time it happened to poll. At DevDay on October 2, OpenAI added MCP Events to ChatGPT plugins, and the direction flips: your server tells the agent when something changes, through a signed webhook, and the agent wakes up and reacts.

This guide builds the server side from scratch in Python with nothing but the standard library. You will implement the three event methods, the signed challenge handshake, Standard Webhooks signing, retries, subscription refresh, and the access re-checks that WorkOS argues are the real security story. Everything below was run end to end, and the output shown is real.