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The `authorization_code` grant's verification step was a `findOne` + `deleteOne` pair, so two concurrent `POST /oauth2/token` requests sharing the same `code` both pass the find, both delete, and both mint independent access/refresh/id token sets: a CAS gap that lets an authorization code be redeemed twice. The legacy `oidc-provider` and `mcp` plugins in `better-auth` share the same primitive on their `authorization_code` paths and have the same gap. All three call sites now use `internalAdapter.consumeVerificationValue` (the atomic primitive added in better-auth#9560 and renamed in better-auth#9568): the first concurrent caller receives the row and mints tokens, subsequent racers receive `null`. The consumed and expired paths return RFC 6749 §5.2 `invalid_grant` instead of the better-auth-internal `invalid_verification`, so spec-compliant clients can branch on the standard code. The redundant second `deleteVerificationByIdentifier` call after PKCE validation in the legacy paths is removed. Closes GHSA-7w99-5wm4-3g79. Co-authored-by: chdanielmueller <4051999+chdanielmueller@users.noreply.github.com>