gh pr comment fails with HTTP 401 via proxy

[dnyw4l3n13]

Failing command

gh pr comment 243 --repo credfeto/credfeto-notification-bot --body "..."

API method

GraphQL mutation (comment creation via gh pr comment)

Full error message

HTTP 401: Bad credentials (https://github-api.markridgwell.com/api/graphql)
Try authenticating with:  gh auth login -h github-api.markridgwell.com

Context

GH_HOST is set to github-api.markridgwell.com. Other read operations (gh pr checks, gh pr view, gh pr edit --add-assignee) succeeded. The comment-posting mutation specifically returns 401.