Our interview with Slack's co-founder and CTO, Cal Henderson. Recorded Sept 19th, 2025.| Building Slack
“If we can’t ship safely, we aren’t shipping at all,” Cal Henderson wrote in our #announce-devel channel. It was September 2018. We had just recovered from our second major self-inflicted outage in a period of weeks. Slack – after experiencing an extended period of| Building Slack
One of the four big bets in Stewart’s Master Plan was to make Slack a network. We initially designed Slack to be used for a company’s internal communication. Any company would still have plenty of inbound and outbound communication with the wider world: customers, vendors, contractors,| Building Slack
Eric Costello co-founded four products at two companies with Stewart, Cal and Serguei. Ludicorp made Game Neverending and Flickr. Tiny Speck made Glitch and Slack. Eric was there for all of it, the coding engine behind the frontend for all of these products that millions of people loved. Understandably, Eric| Building Slack
[This post is about the day that Glitch failed, and how that failure created the opportunity to make Slack. We're sharing it here (out of chronological order) to mark 12 years since the famous pivot.] “We have to shut down the game.” Stewart Butterfield said. I| Building Slack
Working with bigger companies meant figuring out how to plug ourselves into lots of standard big-company processes. One of the processes we had to learn was how to tell customers how much money they owed us so that we could get paid. We needed to generate invoices. Cal wrote the| Building Slack
On the morning of November 2nd, 2016 we arrived to the office to find a paper copy of the New York Times on our desks. In it was a full-page ad welcoming Microsoft to the market that Slack had created. Not coincidentally, Microsoft would announce the launch of their new| Building Slack
Ali Rayl knew Slack’s customers better than anyone. As Slack’s first customer support staff, Ali personally answered every ticket during our early alpha and beta rollouts. She was in the weeds of every front line issue as the customer support team grew along with our user base. Over| Building Slack
Backstage, Stewart vibrated with his typical charismatic energy. An assistant checked his lapel mic. Everyone in the company, now numbering nearly 500 people in early 2017, awaited him. He was ushered on stage to enthusiastic applause from the crowd and expectant video participants from our offices around the world. Slack's| Building Slack
We did many things fairly well at Slack. One thing we never did well, however, was figuring out how much office space we’d need. We outgrew our Clementina Street office in July of 2014, just five months after launch. Our new office in a freshly renovated building on Folsom| Building Slack
