HTTP Error 403: Forbidden if user agent starts with `Python-urllib`

Description

The OpenCode Go server drops chat completion requests if the user agent HTTP header field starts with Python-urllib. This check is done before the API key is checked, which might narrow down your search for the source of this behavior.

Code to reproduce

import os, json, urllib.request

api_key = "sk-your-api-key-here"
url = "https://opencode.ai/zen/go/v1/chat/completions"

data = json.dumps({
    "model": "deepseek-v4-flash",
    "messages": [{"role": "user", "content": "List 10 digits of pi."}],
}).encode("utf-8")

headers = {
    "Content-Type": "application/json",
    "Authorization": "Bearer " + api_key,
    "User-Agent": "Python-urllib", # Change user agent to anything else (e.g. "asdf") and it begins to work
}

request = urllib.request.Request(url, data=data, headers=headers, method="POST")

with urllib.request.urlopen(request) as r:
    print(json.load(r))

Running this code will result in urllib.error.HTTPError: HTTP Error 403: Forbidden.

If you change the User-Agent to anything else, e.g. asdf, the error will be urllib.error.HTTPError: HTTP Error 401: Unauthorized.

If you also set api_key to a valid OpenCode Go key, the server will respond correctly.