A repository boundary is the intended scope limit that defines which files a tool may treat as part of a project. In secure tooling, that boundary should be enforced by resolving paths to their real, canonical locations before any read or write operation.
This matters because attackers can abuse symlinks, path traversal, or misconfigured mounts to make a file appear inside a repo while its target lives elsewhere. If an assistant, scanner, or build tool trusts the visible path name instead of the resolved target, it may expose secrets or modify files outside the project. Defenses include canonical path checks, allowlisted roots, symlink restrictions, and explicit approval for out-of-scope access.