Enabling via user preference Edit
This extension adds Enable parser migration tool to the list of Developer tools options at the bottom of the Editing preferences for a user.
Comparing article rendering Edit
When this is enabled, an Edit with migration tool link is added to the Tools menu in the sidebar for article pages.
Clicking this on an article page will bring up a variant of the "preview edit" page which allows you to compare the legacy parser output against Parsoid output.
Action API Edit
This extension also adds
action=parser-migration to the MediaWiki action API, which parses a page with two different parser configurations and returns one or both variants in the API response.