The Constitutional Council was to hold day-long talks on issues including the pact, which sets strict deficit limits aimed at tackling the debt crisis
that has engulfed the eurozone and threatened the global economy.
A decision was expected late Thursday or early Friday.
President Francois Hollande is hoping the council will rule the pact can be adopted under the current constitution, avoiding a divisive battle within his Socialist party and its left-wing allies over an amendment.
Signed in March, the pact must be approved by 12 of 17 eurozone members and France was hoping to follow Italy, Germany and others by ratifying the accord in September, ahead of it taking effect at the start of next year.
But the need for a constitutional amendment – which would require three-fifths approval by parliament or a referendum – could delay the ratification for months, adding to the worries of already-anxious investors.
Many on the French left are against imposing EU-mandated austerity in the constitution and some, even among Hollande's Socialists, have said they would vote against the move.
"There are very many of us who do not want to approve" an amendment, Socialist Senator Marie-Noelle Lienemann told France Info radio Thursday, denouncing the move as the equivalent of "austerity for life".
Jean-Vincent Place, the head of the lawmakers' group for the Greens – key Socialist allies in parliament – also said he would vote against, calling the pact an example of "extreme austerity and excessive stringency."
Hollande, whose Socialists and their allies hold a simple majority in parliament, should however be able to rely on the support of right-wing and centrist lawmakers if it comes down to a constitutional vote.
Hollande defeated right-winger Nicolas Sarkozy for the presidency in May, vowing to shift French economic policy away from austerity and toward growth with a tax-and-spend programme.
He promised during the campaign to renegotiate the fiscal pact but later accepted the addition of a so-called "growth pact" that will see the European Union mobilising €120 billion ($148 billion) for projects aimed at boosting economic activity.
Hollande is struggling to stem rising joblessness in France amid warnings the country is likely to enter a recession in the third quarter.