Hmm, I wasn't able to replicate this problem. I added this as character's custom code, and it does log the response in the console:

oc.thread.on("MessageAdded", async function() {
  let result = await fetch("https://httpbin.org/post", {
    method: "POST",
    headers: { "Content-Type":"application/json" },
    body: JSON.stringify({foo:3})
  }).then(r => r.json());
  console.log(result);
});

Is it perhaps header values you're missing, or something? Certainly possible that there is a big though and I'm just not hitting it with the above example for whatever reason.

Either way this motivated me to finally get around to creating a CORs-bypassing plugin: https://perchance.org/fetch-plugin That way I can ensure strong backwards-compatibility and performance instead of just relying on a little glitch.com server, which won't scale, and doesn't have good perf/uptime guarantees. I'll wait to hear back from you before integrating it into ai-character-chat just so we can don't end up making it harder for you to reproduce the bug you were up against here.