I am seeing 29881 supported. *maybe* 29879 but pretty weak documentation... I probably would not if it were me coding it. Or I would send it back for query.
What does the header say? Surgical in/out time?

You can also check AAOS, GSD, CPT Assistant. I know there are CPT Assistants about these, codes.
See Dr. Raizman's comment:
https://www.aapc.com/discuss/threads/synovectomy.203160/post-556639
Older talk about it here:
https://www.aapc.com/discuss/threads/microfracture-documentation.197799/post-542032
Old so verify, but still good info:
https://www.aapc.com/blog/33738-337...Edcz63RK_7S83pd-wFI5LB3-Q28SO9_4H2oIDNFKqjG43
Old but still good info:
https://www.aapc.com/blog/51405-coding-knee-arthroscopy-with-precision/
NCCI:
https://www.cms.gov/files/document/04-chapter4-ncci-medicare-policy-manual-2026-final.pdf
See #6 and:
8. Arthroscopic synovectomy of the knee may be reported with CPT codes 29875 (Limited synovectomy, “separate procedure”) or 29876 (Major synovectomy of two or three compartments.)
A synovectomy to “clean up” a joint on which another more extensive procedure is performed is not separately reportable. CPT code 29875 shall not be reported with another arthroscopic knee procedure on the ipsilateral knee.
CPT code 29876 may be reported for a medically reasonable and necessary synovectomy with another arthroscopic knee procedure on the ipsilateral knee if the synovectomy is performed in 2 compartments on which another arthroscopic procedure is not performed. For example, CPT code 29876 shall not be reported for a major synovectomy with CPT code 29880 (Knee arthroscopy, medial AND lateral meniscectomy) on the ipsilateral knee, since knee arthroscopic procedures other than synovectomy are performed in 2 of the 3 knee compartments.
Also check the health plan being billed. Example Aetna:
https://www.aetna.com/cpb/medical/data/600_699/0673.html
Note: Minor synovectomy (CPT code 29875) is considered integral to all other arthroscopic procedures of the knee.
Major synovectomy (CPT code 29876) is only considered medically necessary when a disease of the synovium (e.g., pigmented villonodular synovitis, synovial osteochondromatosis) is identified pre-operatively.