I have a fairly heavy collection of pipelines where it might be handy to conditionally apply the failFast flag in a parallel stage. I'm starting to develop a two-phase design for pull request builds, where a lighter build is performed at the early stage of development and the full build runs once things are more complete. I'm achieving this using PR labels, and I'm selecting a small subset of just the most important parallel branches for the light build. This is in order to reduce usage of a limited pool of build agents.

Allowing an expression in the failFast flag, similar (or maybe identical?) to the `when`  directive on stages, would enhance this idea. That way, a full build would run each parallel branch to completion but a light build would get out of the way quickly while there is a lot of developmental churn. I tried using a boolean expression in the failFast flag but it seems to only accept a hard true or false.