Getting it into your agent
There is no command for this one: it runs only inside a plugin, and the catalogue could not identify which plugin ships it. The source is linked below.
Wrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/cyberbird2048/gbrainmcp-clean/frontmatter-guard)<a href="https://agentmods.dev/skills/cyberbird2048/gbrainmcp-clean/frontmatter-guard"><img src="https://agentmods.dev/badge/skills/cyberbird2048/gbrainmcp-clean/frontmatter-guard/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/cyberbird2048/gbrainmcp-clean/frontmatter-guard"><img src="https://agentmods.dev/badge/skills/cyberbird2048/gbrainmcp-clean/frontmatter-guard.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00068 | $0.01797 |
| Opus 5 | $0.00034 | $0.00898 |
| Sonnet 5 | $0.00014 | $0.00359 |
| Haiku 4.5 | $0.00007 | $0.00180 |
Grade A, and why
frontmatter-guard scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 181 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Frontmatter Guard Skill
Convention: see
skills/conventions/quality.mdfor citation rules; this skill is structural validation, not citation auditing.
Contract
This skill guarantees:
- Every brain page is scanned against the seven canonical frontmatter validation classes
- Mechanical errors (nested quotes, missing closing
---, null bytes, slug mismatch) are auto-repairable on demand with.bakbackups - Validation logic is shared with
gbrain doctor'sfrontmatter_integritysubcheck — 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
| Code | Meaning | Auto-fixable? |
|---|---|---|
MISSING_OPEN |
File doesn't start with --- |
No (needs human) |
MISSING_CLOSE |
No closing --- before first heading |
Yes |
YAML_PARSE |
YAML failed to parse | Sometimes (depends on cause) |
SLUG_MISMATCH |
Frontmatter slug: differs from path-derived slug |
Yes (removes the field) |
NULL_BYTES |
Binary corruption (\x00) |
Yes |
NESTED_QUOTES |
title: "outer "inner" outer" shape |
Yes |
EMPTY_FRONTMATTER |
Open + close present but nothing between | No (needs human) |
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
What ships with it
1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 10d ago First seen · 181 lines · 68 tokens per session scan A 27b0a72e6062
frontmatter-guard is a skill published in the GitHub repository cyberbird2048/gbrainmcp-clean (0 stars, last pushed 1mo ago), licensed MIT. It adds 68 tokens to every session and 1,797 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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