Is it possible to make pants spit out some sort of understandable representation of what goes into a cache hash key so that I can actually inspect the differences when there is a cache miss?

Not easily. You can enable debug logging to see what

Process

dataclass instances are generated for each build action. Those are encoded (as per Remote Execution API "REAPI") into REAPI

Command

and

Action

protobufs and then hashed with SHA-256.

How hard do you think it would be to add that sort of logging to the cache key computation? Ideally I would like to get a tree of dependencies

Logging the

Process

struct?