If you and the other parent live together, either one of you (but not both) may claim the child. You may decide between you which one will claim the child. Only if you can’t agree, do the IRS tie breaker rules apply, to see who has first choice.
It is usually best that the person who is contributing more than 50% of the household expenses claim the child.
But the only way to be sure is to prepare trial returns, both ways, to see which way the family comes out best. This tool may be useful: https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1.
Using the TurboTax download software (rather than online) is better for doing trial returns.
