Use Shellharden when an agent is responsible for making existing shell scripts safer, more predictable, and easier to maintain. The agent reviews quoting, substitutions, and common Bash hazards, then proposes or applies focused rewrites that reduce foot-guns. Invoke this instead of using the product normally when the job is shell-script hardening during cleanup, review, or incident prevention, not general shell authoring. The scope boundary is concrete: rewrite-oriented hardening of shell scripts.