Currently, exogenous variables in the planner_objective are simply ignored in the preprocessor. When using

var pai, c, n, r, a;
varexo u;
parameters beta, rho, epsilon, omega, phi, gamma;

beta=0.99;
gamma=3;       //Frisch elasticity
omega=17;        //price stickyness
epsilon=8;      //elasticity for each variety of consumption
phi=1;           //coefficient associated to labor effort disutility
rho=0.95;  //coefficient associated to productivity shock

model;
a=rho*(a(-1))+u;
1/c=beta*(1/(c(+1)))*(r/(pai(+1)));               //euler
omega*pai*(pai-1)=beta*omega*(c/(c(+1)))*(pai(+1))*(pai(+1)-1)+epsilon*exp(a)*n*(c/exp(a)*phi*n^gamma-(epsilon-1)/epsilon);  //NK pc
//pai*(pai-1)/c = beta*pai(+1)*(pai(+1)-1)/c(+1)+epsilon*phi*n^(gamma+1)/omega-exp(a)*n*(epsilon-1)/(omega*c);  //NK pc
(exp(a))*n=c+(omega/2)*((pai-1)^2);
end;

initval;
pai=1;
r=1/beta;
c=0.9671684882;
n=0.9671684882;
a=0;
u=0;
end;

histval;
a(0)=0.9;
end;

shocks;
var u; stderr 0.008;
end;

planner_objective(ln(c)-phi*((n^(1+gamma))/(1+gamma))*exp(u));
ramsey_policy(order=1,planner_discount=0.99);

the exp(u) does not appear in _model_objective_static. This is problematic, because in principle shocks at time t are part of the information set and should enter the objective. For that reason, @MichelJuillard agreed that we should block using exogenous variables in the objective and instead provide an error message like You cannot handle exogenous variables in the planner objective. Please define an auxiliary endogenous variable like eps_aux=epsilon and use it instead of the varexo