After a fan tweeted a photo of Stamos rocking a bright red tee with a young Baio's face on it during a drive-in concert performance with the Beach Boys in California, the Happy Days alum issued a cheeky response.
"Stamos wearing you tonight in Del Mar at Beach Boys concert," the fan wrote to Baio.
"Told you he wanted to be me!" the actor replied, with the hashtag "#Only1Chachi."
When another fan shared a photo of the Full House star wearing a black T-shirt with Baio's face on it during a show that took place Friday night, Baio replied with a series of emoji, including the crying-laughing face.
"Unbelievable, @JohnStamos had them made in every color! " Baio tweeted.
On social media, viewers weighed in on Stamos' Chachi tees.
"I knew how creepy you were John. Not until I've seen you being so strange with the Scott T-Shirts. I'd really like to know just what the deal is with that? Because it's super strange!" one commenter tweeted.
"I'm wondering if he's not subconsciously trying to apologise to you, for being a jack*ss to you," another wrote to Baio.
The Twitter chatter about the T-shirts comes after Stamos offered to play Baio's character at a virtual Happy Days reunion to support Wisconsin Democrats. After series star Ron Howard announced the fundraiser featuring all of the living original cast members except Baio, Stamos jokingly chimed in to ask if he could play Chachi.
That caused the staunch Republican to clap back with a retort aimed at his rival's TV wife, Lori Loughlin, who is headed to jail due to her involvement in the college admissions scandal.
"Shouldn't you be taking care of Aunt Becky?" he wrote.
He also criticized the Fuller House alum in an interview with Fox Business.
"It's good to know that Stamos is part of the Hollywood elite Marxist crowd," he said.
"I think Stamos always wanted to be Chachi instead of playing second fiddle to a 3-year-old."But in what appeared to be his own version of an olive branch, Baio later shared a photo of him and Stamos when they worked together in an episode of Full House. In a pic from the 1989 episode "Dr. Dare Rides Again," the two actors were seen singing together.
The 60-year-old actor captioned the post to confirm there will always be only one Chachi and one Uncle Jesse, the character Stamos played on the ABC sitcom.