Three rules-only checks to run before you send anything: your filing status, who may claim each dependent, and the final sweep. It catches the errors behind most rejections and letters — it cannot see the IRS's systems, and no one outside the IRS can.
Education only — not personal tax advice. Personal questions route to a credentialed pro.
Check 1 — Filing-Status Match
Your status is set by your facts on December 31 of the tax year. Answer as of that date.
Likely correct status: Head of Household
Unmarried, paying more than half the cost of the home, with a qualifying person living there more than half the year — that is head of household.
Head of household carries a standard deduction halfway between single and joint, and wider brackets than single.
The two tests are support (you pay more than half the home's cost) and residency (the qualifying person lived there more than half the year).
The wrong pick people make: Filing single 'to be safe' while qualifying for head of household — people leave real money in that box every year. The reverse error (claiming head of household without the residency facts) is the one the IRS audits, so the facts must be true, not hopeful.
Check 2 — Dependent Claim Guard
Dependent claims are the most examined lines on any return — and two people claiming the same child is the most common rejection in America. Answer with nights and facts, not with what a decree or a promise says.
LIKELY YOURS TO CLAIM
The claim is likely yours — you are the custodial parent and the qualifying-child tests line up.
Residency: the child lived with you more than half the year. Support: the child did not provide more than half of their own support.
Under 17 at year-end also keeps the Child Tax Credit in play; the Earned Income Credit uses its own age and residency tests (the child must live with you in the U.S. more than half the year).
Keep school, medical, or childcare records showing the child's address with you — the paper trail is what wins if the IRS ever asks.
Someone already claimed my child — the rescue
If your e-file is rejected because a return using your dependent's SSN was already accepted, breathe first: the IRS did not decide the other person was right. E-file is a pipe, and the first claim through the pipe wins it. Your claim is not lost — it just has to travel on paper, where humans decide.
File your return on paper claiming exactly what you believe is correct, and mail it (certified, with a copy of everything). Paper returns are examined by people, not by the pipe.
Expect IRS notice CP87A — it goes to BOTH claimants and says: check your claim; if you are right, do nothing more yet; if you are wrong, amend. Do not ignore it.
Gather the proof of nights and home: school and medical records showing the child's address with you, childcare records, a parenting-time calendar, the custody order, and records of who paid the household costs.
If the IRS examines the claim, it applies the tiebreakers: a parent beats a non-parent; the custodial parent (more nights) beats the other parent; if nights are exactly equal, the higher-income parent claims.
Protect next year now: an IP PIN for the child (see the IRS Concierge) makes a repeat race much harder, and a signed Form 8332 for future years settles the question in advance where the custodial parent agrees.