so far vs thus far

so far and thus far both are adverbs.

Word	Noun	Adjective	Verb	Adverb
so far	No	No	No	Yes
thus far	No	No	No	Yes

As adverbs, so far and thus far are synonyms defined as:

so far and thus far: used in negative statement to describe a situation that has existed up to this point or up to the present time

Other synonyms of so far include as yet, heretofore, hitherto, til now, until now, up to now, yet.

so far (adverb)	thus far (adverb)
used in negative statement to describe a situation that has existed up to this point or up to the present time	used in negative statement to describe a situation that has existed up to this point or up to the present time
used after a superlative
to the degree or extent that