Jen Rubio birthday party included her estranged husband

There was a surprising guest at Away CEO Jen Rubio’s luxe birthday party – her estranged husband.

The socialite celebrated with an over-the-top bash in the Hamptons with a Met Gala-esque lineup up of Uptown fixtures — including her ex, billionaire tech entrepreneur Stewart Butterfield.

The pair split earlier this month after six years of marriage.

They released a statement at the time including the standard celebrity boilerplate about their “deep love and respect for one another” and saying they “are still a family.”

But from the Slack exec’s presence at the bash it seems that, for once, they might actually have meant it.

A source tells Page Six that Nelly and Ashanti sang their hits for over an hour, followed by “Good Times” rapper Styles P hopping on stage after for an impromptu performance.

Guests included Lauren Santo Domingo, Tory Burch, Derek Blasberg, CAA founder Michael Ovitz, Barbara Bush, Huma Abedin and editor in chief of W, Sara Moonves.

The split had come as something of a surprise.

The pair have been seen together recently at the Dior show at LACMA and Venice Biennale, they had a new pet in May and they have been hosting guests on their boat this summer.

They also attended the Met Gala that month after pledging $23 million to the institution for undergraduate and graduate internships in perpetuity.

But sources said at the time that it’s been some time coming.

“Jen and Stewart have privately been moving down this path for some time,” a source familiar with the situation tells Page Six. “They remain good friends and have hosted multiple trips with friends in that capacity this summer.”

Rubio and Butterfield have two children together.

They said in the statement, “Though we will no longer be married, we are still a family and what matters most to us remains the same. Our children will always come first, and we are committed to raising them in a loving environment with two active and present parents.”