a 5l flask just bought 3 weeks ago so its practicly new.
the flat bottom boiling flask was cleaning with NaOH to remove the burned residue from the last extraction, and put on gas-stove for about 10 min and then its cracked
the glass bought from deschem and its usually good quality, is NaOH should not be boild on stove within boiling flask, or its manufactor problem? for next time should i put a ceramic net gauze between the fire and the flask?

Your flask probably broke from thermal stress due to uneven heating.

Note that although borosilicate glass (e.g. Boro 3.3) can withstand pouring boiling water, it can easily crack when encountering sharp temperature differences of 160 degrees C and more.

That happened to me too when I used gas stove. I cracked another flask by immersing it in an ice bath while it was containing suspension of temperature >150 °C. Not a good idea.

If you really need to use gas stove, I would pre-heat the flask first by swirling it over the flame, until it is evenly heated up...

NaOH is not that big of an issue unless concentrated (KOH is slightly worse).

Uneven heat maybe the case here, althow i first mix NaOH and water for 30 min which makes lot of heat and then put it on the stove so it wasnt radical heat change like put hot flask in ice

so next time should i use a ceramic net gauze? its only on the stove when i make essential oils, with all other organic solvent it of course on the stirrer

thats a bit scary because it could happend during distilling DCM/Xylen/other smelly solvent which can make the neigbours smell it and thats of course really not a good idea..

so that it was a manufactur problem or wrong use?

Wrong use.

'Back in the day' we always avoided heating flat-bottomed flasks with a direct flame. In fact, any boiling would be carried out in round-bottomed flasks. The spheroidal shape is more resistant to the expansion and contraction forces.

Either way, the ceramic coated gauze should be your standard practice when using flame to heat glass flasks for anything but a brief moment.

i asked the supplier and he said water/oil bath>heating mantle>hotplate>net ceramic gauze>fire