The content of the description is editable using the Github’s Update a pull request API, using the body field mentioned there.

We will append our UI at the bottom of the PR description. Since the description will change over time, we will put a marker in the PR description body, and anything after that marker would be assumed to be our content, and we would keep overwriting the content.

This can cause two race conditions, which if we can not mitigate we will not use pull request description approach, and fall back to last comment approach.