Brandon Kochkodin,Suzanne Woolley, and Ben Steverman

Robo-advisers haven’t had much experience with market routs. When confronted with one on Monday, they struggled.

The web sites of two of the country’s biggest robo-advisers — Wealthfront Inc. and Betterment LLC — crashed as the S&P 500 Index sank 4.1 percent. Complaints quickly spread across Reddit and other Internet sites from people who had trouble logging onto their accounts. “Really?” wrote @jlpatel23 after he received a message from Wealthfront saying its site was down.

Outages were also reported at incumbent online brokerages Vanguard Group and Charles Schwab Corp.

The glitches represent a setback for a niche of the financial market industry that has been booming of late as people have become more comfortable making investment decisions without speaking to human advisers.

Lost Access

Wealthfront acknowledged in a statement that its clients lost access to their accounts for “a short period of time today” and said it’s working to ensure that “clients don’t experience this again.”

Betterment clients “experienced log-in issues for approximately 30 minutes this afternoon,” Joe Ziemer, a company spokesman, said in an email. “Accounts were secure throughout and portfolio management activities like rebalancing and tax loss harvesting continued.”

Read more: A QuickTake explainer on robo-advisers

At Vanguard, “some clients may have experienced sporadic difficulty accessing their accounts” online and by phone, spokeswoman Emily Farrell said in an email. Increased demand delayed logons for some clients at Schwab “for a few minutes,” said spokeswoman Mayura Hooper.

Assets at four top robo-advice firms almost doubled last year to a combined $150.2 billion, according to data compiled by Bloomberg.

— With assistance by Caleb Melby