How Creating a Task Comment Actually Works in Todoist
Adding a comment to a task uses POST /comments with a task_id field in the body identifying which task it belongs to -- there's no separate dedicated endpoint just for task comments.
Real API Details
The same /comments endpoint is also used for project-level comments, just by passing project_id instead of task_id in the body -- the distinction is which ID field is included, not a different URL.
Practical Considerations
Include exactly one of task_id or project_id, not both, since a comment belongs to either a task or a project, not simultaneously to each.
Use cases
This operation enables task-level commenting from automation in Todoist.