MCPP-TOOL02: Invisible or bidirectional Unicode in tool metadata

Family	Tool surface
Default severity	high
Confidence	high
Mode	passive (default scan)
Spec revisions	all

Why it matters

Zero-width, bidi-override and Unicode tag characters are invisible in clients but read by the model. Tag characters can smuggle whole sentences; bidi controls make the displayed text differ from what the model receives.

Remediation

Strip format (Cf), private-use and control characters from names, descriptions and schemas; keep descriptions plain text.

References

• MCP Security Best Practices
• Unicode TR #36 Security Considerations
• Trojan Source (CVE-2021-42574)

Suppressing

# .mcp-posture-ignore
[[ignore]]
check = "MCPP-TOOL02"
target = "https://mcp.example.com/*"
justification = "Why this is acceptable here"
expires = 2026-12-31