What does it mean when the parameter XPRS_MIPINFEAS takes the value -1?

Hello Elena,

sorry for the delayed reply.

The fact that MIPINFEAS returns -1 in the preintsol is a bug. In some situations the code explicitly sets the number of infeasibilities to -1 when a feasible solution is processed. Until this is fixed, it should be safe to check MIPINFEAS <= 0 to check for feasibility.

However, checking feasibility in a preintsol callback should not be required. This callback is only invoked when Xpress thinks the solution is feasible and gives the user a last chance to reject that solution.

If you see feasible solutions that are not routed through the preintsol callback then this is a bug. How do you notice that the CM callback identifies a feasible solution but that solution is then not routed through the preintsol callback?

Note that the CUTMGR (CM) callback was deprecated a long time ago (see here). You should use the OPTNODE callback instead, which is explained here.