The E.C.B. has a new tool to keep bond markets in check. It doesn’t want to use it.

As well as announcing its first interest rate increase in 11 years, the European Central Bank introduced a new policy tool on Thursday to limit the divergence in borrowing costs across the eurozone’s 19 members.

Rapidly rising borrowing costs for Italy in recent months had intensified focus on whether bond market moves were orderly and in line with a country’s economic fundamentals or disorderly and a threat to the effectiveness of…