If the cracks are in the fire box, there is some very high temp caulk up to 1700 F for that purpose. I have to use it periodically.
If the cracks are out side of the fire box then use some cement or mortar color silicon caulk for that area. It blends in great and will not be seen. Too it will be flexible when she shifts again.
I would attack the outside with the same stuff and then inside also.
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