Frontmatter Guard

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
• Files sync holds are explained from their hold record (code, reason, key, line, fix) and fixed only through a previewed, hash-bound repair the user approved

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: "Alice "Ace" Example")

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.

Write brain files safely (do this first)

• Write pages through put_page / capture (MCP or gbrain put). gbrain serializes frontmatter itself, so the file on disk is always valid YAML.

• A script or agent that writes Markdown files directly must build frontmatter with a YAML serializer (yaml.dump, js-yaml dump), never by string interpolation. title: ${title} breaks as soon as a title contains : , #, quotes or a newline.

• Check generated content before writing it, without touching disk:

  printf '%s' "$content" | gbrain frontmatter validate --stdin --path notes/2026-10-04-digest.md

  Exit 1 means fix it first. The default is the strict producer rule: YAML gbrain could still import by quoting a value fails too, because other tools reject it. --importable answers the narrower question "would sync hold it?".

• Never write a key gbrain reads for access or identity (visibility, derived_from, slug, type, id, source_id) from untrusted text.

Held files: read them, then fix them

A file sync cannot import without guessing is held: the rest of the source still syncs, a new file has no page yet, and an existing page keeps its last good revision and refuses put_page until the file is repaired (do not retry the write). Where holds show up: sync output (Held <path>: <code> … Next: <command>), gbrain sources status <id> (--json: git_holds.items[]), doctor git_held_files, get_page file_held, and search hits marked stale.

Each hold record carries code, reason, key, line, message (location only, never the value), fix (exact argv) and docs. Act on reason:

The fix flow (walkthrough with real output: docs/guides/repair.md#held-files):

gbrain sources status <id>                                  # what is held and why
gbrain repair frontmatter --source <id>                     # pass 1: safe quoting only; writes nothing
gbrain repair frontmatter --source <id> --include-ambiguous --diff   # pass 2: every interpretation
gbrain repair frontmatter --source <id> --include-ambiguous --only <path> --apply --expect <hash> --yes

Ask the user before every --apply (it rewrites their files) and before any --include-ambiguous apply: each interpretation (folding lines into a title, keeping the later duplicate) is a guess only the user can confirm. Show the diff, then apply exactly the previewed hash. Safe-class changes only quote a value exactly as gbrain already reads it, but they still rewrite files, so they need the same agreement.

A source that a broken file blocked before the upgrade recovers on its next sync; to do it now run gbrain sync --source <id> --no-pull.

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 backs up every modified file under ~/.gbrain/backups/frontmatter/ before mutating, quotes YAML gbrain reads by quoting (only those lines change), re-validates, and exits 1 when errors remain. Restage fixed files in git (git add). --include-ambiguous adds interpretations; preview them with --dry-run first.

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

On a managed brain --fix in place is refused: use gbrain repair frontmatter --source <id> (above), which publishes each repaired file through the coordinated writer.

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 one gbrain frontmatter validate --staged process over the staged content of .md/.mdx files (what will be committed, not the working copy). Bypass with git commit --no-verify. Installing writes into the user's repository, so ask first. When doctor frontmatter_hook reports an older hook, refresh it with gbrain frontmatter install-hook --force.

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.