A *checks notes* Facebook post about an upcoming event called Socialists Into City Hall—Building the Mvmt in the 2019 Election describes Seattle's only socialist city council member as, emphasis mine: "Councilmember Kshama Sawant - Socialist Alternative Member & 2019 Candidate for D3." This is the first place I've seen Sawant listed as a candidate running for city council in the 3rd District.
A representative for CM Sawant's office couldn't comment because my question was election-related.
If Sawant does make a more official announcement soon, the incumbent would be jumping into the race with Logan Bowers, Pat Murakami, and Beto Yarce. She could, of course, find some reason to dip out of the race before the filing deadline in May, but for now she appears to be in.
This is developing story. I'll update more later.