CO-29 means the payer received the claim after its filing deadline.
On behavioral health claims it often follows a long chain: a claim held for an authorization, a clearinghouse rejection nobody worked, then a resubmission months later.
You can only win a CO-29 appeal with proof the claim was filed on time. Keep clearinghouse acceptance reports and payer acknowledgments (277CA) for every claim. Without that proof, the balance is usually a write-off. Tracking deadlines by payer, not a single house rule, is what prevents it.
Questions
Can I bill the patient after a CO-29?
Generally no. The CO group means the provider carries it under the contract.
What counts as proof of timely filing?
Clearinghouse acceptance reports, payer acknowledgments and dated correspondence showing the claim reached the payer in time.
Read next
Educational summary, not billing advice. Official code descriptions are at x12.org. Checked October 2026.