Function usage_url 

fn usage_url(base_url: Option<&str>) -> String

The /api/oauth/usage endpoint URL. Defaults to the hardcoded Anthropic host; a base_url override (an account’s ANTHROPIC_BASE_URL) redirects the probe to an alternate endpoint serving the same OauthUsage shape. Any trailing slash on the override is trimmed so http://host/ and http://host behave identically.