Earlier quoted context omitted.
"seam" is apparently... according to Claude itself a term from 'Working Effectively with Legacy Code' by Michael Feathers which I have not read. All it took was for one sub agent to use this term and it stated using it everywhere all the time. I have not read the book and prefer other terminology but it only takes 1 sub-agent or 1 usage in the context before it poisons everything else.
An interface is an example of a seam in regular code. It's basically what forms architectural shapes that you can depend on for both design and testing.
I have instructions which is confidently ignores to never use seam and instead say interface.