OpenAI on Tuesday said it reached an agreement for Sam Altman to return as CEO days after his ouster, capping frenzied discussions about the future of the startup at the center of the artificial intelligence boom.

In addition to Altman's return, the company agreed in principle to partly reconstitute the board of directors that had dismissed him. Bret Taylor, formerly co-CEO of Salesforce, and Larry Summers, former U.S. Treasury Secretary, will join Quora CEO and current director Adam D'Angelo, OpenAI said.

Altman said in a post on X, "i'm looking forward to returning to openai."

The board had given few details explaining Altman's firing on Friday, other than his lack of candor and its need to defend OpenAI's mission developing AI to benefit humanity.

The deal to restore Altman ushers in a potentially new era for the startup - a non-profit - which long juggled concerns among staff about AI's dangers and its potential for commercialization. Tuesday's reshuffle appeared to favor Altman and financial backer Microsoft, which is rolling out OpenAI's technology to business customers globally.

In a statement on X, Microsoft CEO Satya Nadella welcomed the changes to OpenAI's board.

"We believe this is a first essential step on a path to more stable, well-informed, and effective governance," he said.

Altman's return also caps a tumultuous weekend that saw him agree to head a new research team at Microsoft, which has invested billions of dollars in OpenAI and given it the computing power necessary for its technology.

That followed a rejection by OpenAI's board of his first attempt to return to the startup, on Sunday, by naming ex-Twitch boss Emmett Shear as interim CEO.

"While it's still unclear exactly what the tug-of-war prompting his initial departure involved, Sam Altman's views about how to run the company will dominate future direction, especially given he'll be supervised under a new board," said Susannah Streeter, head of money and markets at Hargreaves Lansdown.

In a post on X, Shear celebrated the late-night outcome Tuesday, which he said followed "~72 very intense hours of work."

"This was the pathway that maximized safety alongside doing right by all stakeholders involved," Shear said.

Altman's dismissal had brought uncertainty for OpenAI and Microsoft alike, which had moved quickly to control damage over the weekend by vowing to hire him and Greg Brockman, president of the startup.

Brockman, who had quit after Altman was ousted, said in a post on X that he was "getting back to coding tonight."

Nearly all of OpenAI's over 700-strong staff on Monday had threatened to leave unless the board stepped down and reinstated Altman, according to a letter reviewed by Reuters.

Following this show of solidarity, Altman wrote on X: "we have more unity and commitment and focus than ever before."

(Reporting by Jeffrey Dastin in San Francisco and Shivani Tanna and Bansari Mayur Kamdar in Bengaluru; Editing by Dhanya Ann Thoppil, Nivedita Bhattacharjee and Sam Holmes)