The original call sign was WRCP-FM, simulcasting co-owned WRCP (1540 AM) and its middle of the road (MOR) format.

Tightened Federal Communications Commission (FCC) restrictions on AM-FM simulcasting led to a new format for the FM in 1977.

Over time, the country music was largely discontinued and the station evolved into a standard easy listening sound.

On August 1, 2002, at 6 a.m., after a 24-hour loop of The Beatles' "Here Comes the Sun", 104.5 flipped to soft adult contemporary, reverting to the Sunny 104.5 branding with a plan to compete for some of "B101" WBEB's listeners.

[7][8] This incarnation of WSNI is noted for completely abandoning the format in the first week in November to play all Christmas music until December 26.

Nearly all air personalities on "Sunny" were voicetracked, meaning the "DJ banter" heard between songs had been recorded in advance in another city and was being played from a hard drive just like the music.

At noon on August 10, 2006, Sunny's sister station WJJZ (106.1 FM) was switched to a rhythmic AC format, and began identifying itself as "Philly's 106.1".

At the same time, Clear Channel dumped WSNI's soft AC format and started "shadowcasting" the new station at 106.1.

There was a short announcement from a female ("This feels like my own radio station") and an awkward segue into "Get Ready For This" by 2 Unlimited, then Michael Jackson's "Wanna Be Startin' Somethin'".

Without having to worry about beating the competition to the punch, they tended to make the switch to all-Christmas much later in the season, typically one day to one week prior to Thanksgiving.

The new format focused primarily on tropical and Spanish dance music, similar to WCAA and WSKQ-FM in New York City.

[13] Starting in 2017, the station has hosted an annual birthday show one-day music festival, featuring artists frequently played on 104.5 FM.

[14] Between 2007 and 2019, the festival has hosted artists like Hozier, Death Cab for Cutie, Florence and the Machine, Passion Pit and others.