Sam Altman is to return as chief executive of OpenAI after the ChatGPT developer said it had “reached an agreement in principle” for his reinstatement.

The San Francisco-based company made the announcement after days of internal turmoil after Altman’s surprise sacking on Friday. Nearly all of OpenAI’s 750-strong workforce had threatened to resign unless the board overseeing the business brought him back and then quit immediately afterwards.

As part of the agreement reached overnight, the deal includes the establishment of a new-look board chaired by Bret Taylor, the former co-chief executive of software firm Salesforce. It will include the former US treasury secretary Larry Summers and Adam D’Angelo, the tech entrepreneur and current board member who played a role in Altman’s firing.

OpenAI wrote on the X platform, formerly Twitter: “We have reached an agreement in principle for Sam Altman to return to OpenAI as CEO with a new initial board of Bret Taylor (chair), Larry Summers, and Adam D’Angelo. We are collaborating to figure out the details. Thank you so much for your patience through this.”

Altman, who has not regained his position on the board, posted on X soon after: “I love OpenAI, and everything I’ve done over the past few days has been in service of keeping this team and its mission together.” He added: “I’m looking forward to returning to OpenAI.”

The 38-year-old was fired on Friday by the board of OpenAI’s non-profit parent, which oversees a commercial subsidiary run by him. He had reportedly made the removal of the board, and an overhaul of OpenAI’s governance, a precondition of his return. Under the agreement, D’Angelo remains but the other three members of the board will be replaced.

Microsoft, OpenAI’s biggest investor, had been pushing for Altman’s reinstatement. On Monday it hired Altman and Greg Brockman, who quit his role as OpenAI’s president on Friday after his colleague’s sacking, in a move interpreted by analysts as giving the ex-CEO a de facto leadership role over his former employer. Brockman also confirmed on X that he was returning to OpenAI, posting “we are so back” in a selfie with colleagues.

Satya Nadella, Microsoft’s chief executive, posted on X on Wednesday morning: “We are encouraged by the changes to the OpenAI board. We believe this is a first essential step on a path to more stable, well-informed, and effective governance.

“Sam, Greg, and I have talked and agreed they have a key role to play along with the OAI [OpenAI] leadership team in ensuring OAI continues to thrive and build on its mission. We look forward to building on our strong partnership and delivering the value of this next generation of AI to our customers and partners.”

OpenAI had confirmed to staff on Monday that it was in “intense discussions” to bring back Altman. “We are continuing to go over mutually acceptable options, and are scheduled to speak again tomorrow morning when everyone’s had a little more sleep,” wrote Anna Makanju, vice-president of global affairs, in a memo seen by Bloomberg News.

Altman’s return will make him OpenAI’s fourth CEO in five days. He was initially replaced by Mira Murati, the company’s chief technology officer, as interim CEO, before the board hired Emmett Shear, the co-founder of video game-streaming platform Twitch as a temporary successor. OpenAI will reportedly conduct an independent investigation into the sacking of Altman and its aftermath.

The board said last week it had sacked Altman over a failure to be “candid in his communications” but declined to give details over what communications it was referring to. Shear denied that Altman’s ousting was linked to concerns over safety, amid wider fears voiced by experts that AI development was becoming too rapid.

Shear, writing on X on Wednesday after news of Altman’s return, said he was “deeply pleased by this result”, adding: “This was the pathway that maximised safety alongside doing right by all stakeholders involved. I’m glad to have been a part of the solution.”

The three people leaving the board are: OpenAI’s chief scientist, Ilya Sutskever, who signed the staff letter calling for Altman’s return; Tasha McCauley, a tech entrepreneur; and Helen Toner, a director at Georgetown University’s Center for Security and Emerging Technology.

On Wednesday, Toner reposted OpenAI’s statement on Altman’s rehiring, adding: “And now, we all get some sleep.”