An infield single by Wilson Ramos in the next at bat moved Harper into scoring position at second and he took third when Asdrubal Cabrera grounded into a force at second base for the first out of the frame.
Nats' starter Stephen Strasburg stepped in next with runners on the corners and one out and squared to bunt, connected and sent an 0-1 fastball rolling toward first that Eovaldi fielded.
Eovaldi looked Harper back to third and turned to fire to first, but as soon as he did, Harper, after shuffling back toward third, broke for home as second baseman Donovan Solano covered first and received the pitcher's throw then fired his own throw home.
Harper hustled down the line, sliding in headfirst with his oven-mitt covered left hand passing over the plate right before Marlins' catcher Jeff Mathis applied a tag to Harper's elbow and left side.
Home plate ump Lance Barkdale signaled that Harper was safe and the call stood though Marlins' manager Mike Redmond came out for a brief chat.
"It's a read," Nationals' manager Matt Williams said of Harper's decision to break for home on the bunt.
"Worst case scenario, we've got second and third and [Denard Span] coming to the plate. Stephen laid down a good bunt, Eovaldi jumped off the mound to get it, and he checked Bryce, but once he turned and threw to first then Bryce has the ability to go. If it's a good bunt there down the first base line he's looking to score on the bunt itself. So, good play."
Harper read the play correctly, reacted aggressively and scored the first run of the game.
"He's going to pick his spots like all the other guys do," Williams said.
"But in that regard he can steal a base, he can certainly go first-to-third as good as anybody on our club and in that regard today he made a nice play, an aggressive play on the basepaths, knowing that they've got to make two perfect throws and the exchanges have to be perfect as well. It was a good, heads up play."
Harper is back in the lineup for Game 162 of 162 this afternoon, playing right field in the regular season finale.
• Here's the Nationals' lineup for this afternoon's game:
• We talked about Harper's aggressive play, Stephen Strasburg's final regular season outing and October baseball on the latest edition of Nats Nightly: