au fait vs abreast

au fait and abreast both are adjectives.

Word	Noun	Adjective	Verb	Adverb
au fait	No	Yes	No	No
abreast	No	Yes	No	Yes

As adjectives, au fait and abreast are synonyms defined as:

au fait and abreast: being up to particular standard or level especially in being up to date in knowledge

Other synonyms of au fait include au courant, up on.

au fait (adjective)	abreast (adjective)
being up to particular standard or level especially in being up to date in knowledge	being up to particular standard or level especially in being up to date in knowledge

au fait (adverb)	abreast (adverb)
	alongside each other, facing in the same direction