Post-merge defect rate is the rate at which bugs are discovered after code has already been merged into the main branch or released. It is a useful quality metric because it measures what review and testing missed before the change became part of the trusted codebase.
In cyber security, a high post-merge defect rate can signal gaps in code review, insecure defaults, weak testing, or incomplete validation of AI-generated changes. Defects found after merge are more expensive and riskier because the code may already be deployed, integrated with other services, or exposed to users. Security teams track this metric to judge whether review gates, linters, tests, and manual checks are actually catching issues early. If the rate rises, it can indicate that attackers or sloppy automation are slipping risky logic, auth errors, or unsafe input handling past the merge control.