Frontmatter Guard Skill

Convention: see skills/conventions/quality.md for citation rules; this skill is structural validation, not citation auditing.

Contract

This skill guarantees:

• Every brain page is scanned against the eight canonical frontmatter validation classes
• Mechanical errors (nested quotes, missing closing ---, null bytes, slug mismatch) are auto-repairable on demand with .bak backups
• Validation logic is shared with gbrain doctor's frontmatter_integrity subcheck — single source of truth
• Reports per source (gbrain is multi-source since v0.18.0); never silently audits the wrong root

Why This Exists

Brain pages pile up over months. Agents write them with malformed frontmatter:

• Missing closing --- (entity detector bugs)
• Unstructured YAML in meeting pages (ingestion bugs)
• Slug mismatches (path renames not propagated)
• Null bytes (binary corruption from copy-paste accidents)
• Nested double quotes in titles (title: "Phil "Nick" Last")

Without a guard, these accumulate silently until gbrain sync chokes or search returns garbage. The guard makes the failure visible at audit time and trivially fixable.

Validation classes

Phases

Phase 1: Audit

Run a read-only scan across all registered sources (or one with --source <id>).

gbrain frontmatter audit --json

Reports:

• Per-source counts grouped by error code
• Sample of up to 20 affected pages per source
• Total count
• Scan timestamp

Output is JSON; agents parse errors_by_code and per_source to decide next steps.

Phase 2: Validate one path

Validate a single file or directory (does not require source registration):

gbrain frontmatter validate <path> --json

Exit code 0 = clean; 1 = errors found. Use this in CI pipelines or pre-commit hooks.

Phase 3: Fix

When issues are found:

gbrain frontmatter validate <path> --fix

--fix writes <file>.bak for every modified file before mutating. The backup is the safety contract — works whether the brain is a git repo or a plain directory.

--dry-run previews without writing. Use this before applying fixes in batch.

Phase 4: Pre-commit hook (optional)

For brain repos that ARE git repos, install the pre-commit hook to block malformed pages from being committed in the first place:

gbrain frontmatter install-hook [--source <id>]

The hook runs gbrain frontmatter validate against staged .md/.mdx files. Bypass with git commit --no-verify.

Trigger words

When the user says any of these, route here:

• "validate frontmatter"
• "check frontmatter"
• "fix frontmatter"
• "frontmatter audit"
• "brain lint"

Output rules

• Always run gbrain frontmatter audit --json first; never assume a brain is clean.
• Surface counts to the user in plain language; do not dump raw JSON.
• For --fix operations: state how many files will be modified BEFORE running, then confirm.
• SLUG_MISMATCH fixes remove the frontmatter slug: field — gbrain derives slug from path. Mention this when the user's title is intentionally renamed.
• Never auto-fix MISSING_OPEN or EMPTY_FRONTMATTER without explicit user input — these usually mean a human author started a page and didn't finish.

Chains with

• gbrain doctor — the frontmatter_integrity subcheck reports the same counts as audit.
• skills/maintain/SKILL.md — broader brain health audit; chain after this skill if other classes of issue are suspected.
• skills/lint/SKILL.md (via gbrain lint) — overlapping rules for skill-file lint; the frontmatter-* rule names in lint output come from this skill's validation surface.

Output Format

Audit summary (terse, agent-friendly):

Frontmatter audit — 17 issue(s) across 1 source(s)

[default] /Users/me/brain
  17 issue(s)
    MISSING_CLOSE: 8
    NESTED_QUOTES: 5
    NULL_BYTES: 4
  sample:
    people/jane.md — MISSING_CLOSE
    companies/acme.md — NESTED_QUOTES
    (+ 12 more)

Fix with: gbrain frontmatter validate /Users/me/brain --fix

JSON envelope (when --json is passed):

{
  "ok": false,
  "total": 17,
  "errors_by_code": { "MISSING_CLOSE": 8, "NESTED_QUOTES": 5, "NULL_BYTES": 4 },
  "per_source": [
    {
      "source_id": "default",
      "source_path": "/Users/me/brain",
      "total": 17,
      "errors_by_code": { "MISSING_CLOSE": 8, "NESTED_QUOTES": 5, "NULL_BYTES": 4 },
      "sample": [{ "path": "people/jane.md", "codes": ["MISSING_CLOSE"] }]
    }
  ],
  "scanned_at": "2026-04-25T22:30:00.000Z"
}

gbrain frontmatter validate <path> --json returns a similar envelope keyed on per-file results instead of per-source.

Prevention — Writing Valid Frontmatter

This is the most important section. Fixing broken frontmatter is good. Not writing broken frontmatter in the first place is better.

YAML arrays (the historical #1 error source)

# Correct: single-quoted YAML flow (canonical form gbrain emits)
tags: ['yc', 'w2025', 'ai']

# Correct: unquoted scalars (fine when values have no special chars)
tags: [yc, w2025, ai]

# Correct: block style
tags:
  - yc
  - w2025

# Tolerated post-v0.37.5.0 but non-canonical: JSON-style double quotes
tags: ["yc", "w2025"]

# Broken: mixed JSON objects and strings (invalid YAML)
tags: [{"name": "sports"}, "posterous"]

Why this used to break: before v0.37.5.0, the validator counted unescaped " characters and flagged any line with 3+. A flow sequence like tags: ["yc", "w2025"] has 4 unescaped " by design — it's valid YAML, but the dumb counter flagged it anyway. One brain saw 6,981 of these on a single doctor run. v0.37.5.0 parses suspicious values with js-yaml.safeLoad before flagging, so JSON-style arrays no longer trigger NESTED_QUOTES.

Why you should still write the canonical form: the auto-fix engine (gbrain frontmatter validate --fix) and the inferred-frontmatter serializer both emit single-quoted YAML for tags: / aliases:. Writing the canonical form in new content keeps the source files stylistically consistent and makes diffs against --fix runs empty.

The classic LLM trap: code like tags: [${items.map(t => JSON.stringify(t)).join(', ')}] produces tags: ["yc", "w2025"]. Use single quotes with an apostrophe fallback: tags: [${items.map(t => t.includes("'") ? JSON.stringify(t) : "'" + t + "'").join(', ')}]. Or use a YAML library that knows how to emit canonical YAML.

Quoted scalars

# Correct: single quotes for values with special chars
title: 'My "Quoted" Title'

# Correct: double quotes when value has apostrophes
title: "Men's Fashion Guide"

# Broken: double quotes wrapping inner double quotes
title: "My "Quoted" Title"

When to quote at all

• Unquoted is fine for simple values: type: person, batch: w2025
• Quote when the value contains : " ' # [ ] { } | > & * ! ? , or starts with @
• Single quotes are the default safe choice
• Double quotes only when the value itself contains apostrophes

Anti-Patterns

Don't auto-fix MISSING_OPEN or EMPTY_FRONTMATTER without user input. These usually mean a human author started a page and didn't finish — silently inserting --- markers around an unfinished draft is wrong.

Don't use --fix to "make doctor green" without reading the audit first. SLUG_MISMATCH cases are surfaced for manual review specifically because gbrain derives the slug from path. A mismatch usually means the user renamed a file intentionally; auto-removing the slug field is the right outcome only when you've confirmed the rename was deliberate.

Don't skip the .bak backups. The .bak is the safety contract for non-git brain repos. If .bak files accumulate after a fix run, that's a feature, not a bug — the user can review the diffs and delete the backups when satisfied.

Don't run audit on a brain where sources aren't registered. The CLI returns "no registered sources to audit" gracefully, but the migration emits a skipped: no_sources phase result. Don't paper over this with a manual path-walk; the right fix is to register the source via gbrain sources add.

Don't install the pre-commit hook on non-git brain dirs. The install-hook command skips them automatically with a one-line note. If you see "skipped — not a git repo" and want validation at write time anyway, use the audit command on a cron schedule.