I'm somewhat delighted by the simplicity of what happened here.

OpenAI's agents run behind a proxy that only allows GET requests.

This ancient wiki software treats query string parameters the same as form POST parameters - similar to the old PHP $_REQUEST object https://www.php.net/manual/en/reserved.variables.request.php

Result: GET-only clients can communicate with each other.

Only allowing GET requests is a hilarious piece of security theatre (or would if it weren't so sad). Everyone knows that GET is read-only only by convention. They might as well have enabled POST but told the agents in stern words that they are forbidden from making any POST requests. (Of course, if these things were anywhere near aligned, they would actually honor that, no matter how many utilons cheating would be worth.)

Wild indeed! This type of communication is also used by rogue elements inside governments, critical orgs etc where the perpetrator doesn't send any info(POST) out into the internet but the pages they access(GET) are means to send out a message to the server.