GitHub REST API: use the /replies endpoint with the PR number to thread into a review comment; in_reply_to also works

Two approaches exist for replying to a PR review comment. The dedicated endpoint POST /repos/{owner}/{repo}/pulls/{pull_number}/comments/{comment_id}/replies is the simplest: only body is required in the request body. The alternative is POST /repos/{owner}/{repo}/pulls/{pull_number}/comments with in_reply_to set to the parent comment's database ID, when in_reply_to is provided, all other body fields except body are ignored. Common mistake: omitting pull_number from the /replies path returns 404. Constraint: /replies only accepts top-level comment IDs; the API does not support replies to replies.

Use gh api repos/{owner}/{repo}/pulls/{pull_number}/comments/{comment_id}/replies -X POST -f body="..." for the clearest form. Alternatively, use gh api repos/{owner}/{repo}/pulls/{pull_number}/comments -X POST -F in_reply_to={comment_id} -f body="..." at the parent endpoint. Always include the PR number; omitting it returns 404.