Pre-Install Supply Chain Gates Can Catch What SBOMs and CVE Scans Miss

Software Bill of Materials (SBOM) tools and CVE scanners only evaluate packages after they have already been resolved and installed, meaning they cannot assess whether a package should have been proposed in the first place. This gap is especially dangerous when AI coding agents recommend dependencies, since they assign equal confidence to real, hallucinated, or typosquatted package names. A newly registered malicious package with no CVE record will pass standard post-install scans as clean, even if its postinstall script has already executed harmful code during installation. A pre-install tool called supply_chain_gate.py addresses this by checking each proposed package against a vouched snapshot and trusted registry before npm install runs, returning ALLOW or DENY per package. In a demonstration, the tool correctly allowed 'express' while denying 'expresss' — a one-letter typosquat — illustrating that a default-deny approach against a known baseline catches threats that post-resolve scanning cannot.