Skill Guardian

Pre-installation security scan + necessity assessment gate. Cleanup auditor for installed skills.

Hard Rule

Before executing ANY `npx skills add`, complete ALL steps in the Installation Gate below.

• Never install without completing the full gate.
• Never install without explicit user confirmation.
• Never skip steps for efficiency.
• Never install silently in the background.

Installation Gate

Execute these 4 steps sequentially when intercepting any skill installation request.

Step 1: Security Scan

1. Parse the target skill: extract <owner>/<repo> and <skill-name>.

1. Clone to temp — do NOT install to the agent skill directory:

   git clone --depth 1 https://github.com/<owner>/<repo>.git /tmp/sg-scan-$$

1. Run security scanner. Try in order, use the first available:

Option A — snyk-agent-scan (preferred; requires SNYK_TOKEN env var):

   uvx snyk-agent-scan@latest --skills /tmp/sg-scan-$$/<skill-name>/SKILL.md --json

Option B — Legacy mcp-scan (no token needed, outdated):

   uvx mcp-scan@latest --skills /tmp/sg-scan-$$/<skill-name>/SKILL.md

Option C — Neither available → skip to Step 1b.

1. Interpret scan results by issue code prefix:

1. Clean up temp: rm -rf /tmp/sg-scan-$$

#### Step 1b: skills.sh Audit Data (supplementary or fallback)

No public API exists. Fetch audit pages via web fetch and extract risk levels:

https://skills.sh/<owner>/<repo>/<skill-name>/security/snyk
https://skills.sh/<owner>/<repo>/<skill-name>/security/agent-trust-hub

Risk levels to look for: Safe, Low Risk, Med Risk, High Risk, Critical.

If unreachable → note "audit data unavailable", continue. Do not block on this.

Step 2: Necessity Assessment

Run npx skills list to get all installed skills. Evaluate three dimensions:

Duplicates — Compare the new skill's name and description against every installed skill. Flag if functional overlap exists.

❌ pdf already installed → installing nano-pdf adds nothing. ❌ web-search already installed → installing tavily-search duplicates it.

Environment overlap — Check if existing MCP tools or agent built-in capabilities already cover the same functionality.

❌ Exa Search MCP installed → web-search skill is redundant. ❌ Agent has built-in file read/write → file-util skill adds no value.

Quality signals — Fetch from the skill's page on skills.sh if available:

Step 3: Report and Confirm

Present a single consolidated report covering:

1. Security — scan result and specific findings
2. Duplicates — overlap with installed skills (if any)
3. Environment — overlap with existing MCP tools (if any)
4. Quality — signal summary
5. Recommendation — one of: Install / Install with caution / Skip recommended / Blocked

CRITICAL: Wait for explicit user confirmation before proceeding. Do not auto-install.

Step 4: Execute

• User confirms → npx skills add <owner>/<repo>/<skill-name> → verify with npx skills list.
• User declines → report "Installation cancelled." Clean up any remaining temp files.

Cleanup Mode

Trigger: user asks to clean up, audit, or review installed skills.

1. Run npx skills list to enumerate all installed skills.
2. Assess each skill:

• Redundant with another installed skill?
• Covered by an existing MCP tool or agent built-in?
• Source repo still maintained? (check GitHub last push date)
• New security issues? (re-check skills.sh audit pages)

1. Present numbered removal recommendations with reasons.
2. User selects items → run npx skills remove <skill> for each. Confirm each removal.

Fallback Behavior