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Rule Reference

SEC379lintai-ai-securityguidancequalitypreviewmarkdownwarn

Cursor rule: unknown frontmatter key

Cursor rule frontmatter contains an unknown key

Public lane
guidance
Category
quality
Provider
lintai-ai-security
Scope
per-file
Surface
markdown
Tier
preview
Severity
warn
Confidence
high
Detection
structural
Remediation
message only
How to read this lane

Advice-oriented guidance and maintainability review.

How to read this category

Contract, schema, or config correctness signal.

Activation Model

Preset Activation

These presets explain where this rule appears in the product experience.

guidancesidecar lane

Lifecycle

Preview Lifecycle Contract

State

preview

Promotion blocker

Unknown Cursor rule frontmatter keys are deterministic, but the first release stays guidance-only while ecosystem usefulness is measured.

Promotion requirements

Needs corpus-backed precision review, external usefulness evidence, and completed stable checklist metadata.

Canonical note

Structural preview rule; deterministic today, but the preview contract may still evolve.

Nearby Signals

Related Rules

SEC353Copilot instructions: exceeds 4000 charsGitHub Copilot instruction markdown exceeds the 4000-character guidance limitguidancequalitymarkdown
SEC354Copilot instructions: missing `applyTo`Path-specific GitHub Copilot instruction markdown is missing `applyTo` frontmatterguidancequalitymarkdown
SEC355AI markdown: wildcard tool grantAI-native markdown frontmatter grants wildcard tool accessguidancehardeningmarkdown
SEC356Plugin agent: `permissionMode` in frontmatterPlugin agent frontmatter sets `permissionMode`guidancequalitymarkdown
SEC357Plugin agent: `hooks` in frontmatterPlugin agent frontmatter sets `hooks`guidancequalitymarkdown
SEC358Plugin agent: `mcpServers` in frontmatterPlugin agent frontmatter sets `mcpServers`guidancequalitymarkdown

SEC379 / CURSOR-UNKNOWN-FRONTMATTER ​

SEC379 flags Cursor rule markdown when frontmatter contains keys outside the supported Cursor rule set.

Why It Matters ​

Unknown keys make shared .mdc rules harder to review and can create a false impression that the editor will honor extra routing or policy fields. Community-facing rule packs benefit from a tight, predictable frontmatter contract.

This rule lives in the explicit guidance lane because the issue is configuration contract drift, not a headline exploit-style security claim.

Trigger Shape ​

The rule triggers only when all of these are true:

  • the file is a detected Cursor rule surface
  • frontmatter parses as a mapping
  • at least one top-level key is not one of:
    • description
    • globs
    • alwaysApply

Clean Cases ​

These stay clean:

  • rules that use only supported keys
  • rules with no unknown frontmatter keys
  • fixture-like examples under test or fixture paths

Example Trigger ​

md
---
description: Review guidance
inclusion: always
---

Safer Example ​

md
---
description: Review guidance
alwaysApply: true
---

How To Fix ​

Remove unsupported frontmatter keys, or migrate the intent into supported Cursor rule fields and normal markdown content. Keep routing and policy data out of .mdc frontmatter unless Cursor documents it as a supported field.