Federation should be __________ to the users.

"Hostile" is antithetical to good user experience and system design for any service, including federation.

While federation mechanisms should be 'proportional' to security needs, 'transparent' specifically describes the desired user experience.

User experience is a critical aspect of identity federation. When federation is implemented transparently, users are not aware of the complex authentication and authorization processes happening in the background; they simply log in once and gain access to multiple services, which enhances usability and reduces friction.

While federation might have implementation costs, the term describes a desired characteristic, not an attribute of expense.