The TL:DR answer is ‘both’: it was a surprise, even though US intelligence had observed the preparations and heightened alert-levels of the various Soviet units involved had been picked up by US intelligence in the autumn of 1979.

Per Bruce Riedel: “In 1979 the consensus of analysts in the U.S. intelligence community—including the CIA, the Defense Intelligence Agency, the State Department’s Bureau of Intelligence and Research, and the National Security Agency—was that Moscow did not want to intervene with its own troops in Afghanistan… When the Soviets did intervene dramatically on Christmas Eve 1979, the intelligence community could argue that it had correctly detected and reported the military preparations for the invasion in the weeks leading up to the invasion. It would later claim in a 1980 postmortem requested by the national security adviser, Zbigniew Brzezinski, that it had provided ten days’ warning that Moscow was “prepared” to invade; however, a subsequent study by the CIA’s own Center for the Study of Intelligence was more honest, noting that the warnings were far from explicit and that the “warnees” in the White House did not feel warned at all.”

For more, see: Bruce Riedel’s What We Won: America’s Secret War in Afghanistan 1979-89.