Lower taxes on selling investments like stocks will usually set investors cheering, but the tech-rich Nasdaq was not impressed, and continued its downward slide.
Investors fretted about earnings reports due after the close from high-flyers including Apple Inc and Amazon.com Inc .
Also jobless claims rose last week, adding to signs the momentum of economic recovery has slowed as coronavirus cases spiraled in southern and western US states.
Richardson noted the "commitment to do whatever it takes to get the economy going again" and a recognition the economy has improved and "come up from the bottom."
"You'll often see an uptick in those shares ahead of earnings and if they disappoint then they tend to sell off," said Citi's Snyder.
EU leaders emerged from a marathon four-day and four-night summit with a 750 billion euro ($858 billion) deal to boost economies shattered by the coronavirus epidemic.
The Dow Jones Industrial Average rose 39.91 points, or 0.15%, at the open to 26,774.62. The S&P 500 opened higher by 8.64 points, or 0.27%, at 3,224.21
the Nasdaq Composite gained 26.69 points, or 0.25%, to 10,500.52 at the opening bell.