Originally posted by: Lessi

I turn off the heuristic search by setting HeurFreq to -1. Is it possible to turn it on again using callback?

Thanks,

No. Changing any parameter from a callback is forbidden.

What you can do instead is to interrupt the search from the callback and after solve() returns change the HeurFreq parameter back to non-negative and call solve() again. The second call to solve() should start more or less where the first one stopped.