Hey @anon39940173
I found the answer that I used back in the day : Relationship dropdown edit widget filter - how to filter dataset based on current entity - Help me! - Forest Admin Developers Community.
There was indeed a context object on the query which contained various data about, well, the context in which the query was made. Kind of the same information you would get from the header you talk about, but already properly parsed.
I started logging crashed for this (TypeError: Cannot read properties of undefined (reading 'record')) in either April or June of this year, when we upgraded our forest-express-sequelize version.