Developed by Mission Street Development LLC, an affiliate of Millennium Partners, the US $350 million project was designed by Handel Architects, engineered by DeSimone Consulting Engineers and constructed by Webcor Builders. At 645 feet (197 m), it is the tallest concrete structure in San Francisco, the fourth tallest building in San Francisco overall, and the tallest since 345 California Street in 1986. It was also the tallest residential building west of the Mississippi River when finished (later surpassed by The Austonian in Texas). The tower is slender, with each floor containing 14,000 square feet (1,300 m) of floor space. In addition to the 58-story tower, there is a 125-foot (38 m), 11-story tower on the northeast end of the complex. Between the two towers is a 43-foot (13 m), two-story glass atrium. In total, the project has 419 units.

Answer this question based on the passage: who built the millennium tower in san francisco? OK. Let's think. My stream of consciousness:
To answer the above question, the relevant sentence is: Developed by Mission Street Development LLC, an affiliate of Millennium Partners, the US $350 million project was designed by Handel Architects, engineered by DeSimone Consulting Engineers and constructed by Webcor Builders. The answer is Webcor Builders.