Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add synthesisengineering/synthesis-skills --skill synthesis-message-guardgit clone --depth 1 https://github.com/synthesisengineering/synthesis-skillsWrote 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/synthesisengineering/synthesis-skills/synthesis-message-guard)<a href="https://agentmods.dev/skills/synthesisengineering/synthesis-skills/synthesis-message-guard"><img src="https://agentmods.dev/badge/skills/synthesisengineering/synthesis-skills/synthesis-message-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/synthesisengineering/synthesis-skills/synthesis-message-guard"><img src="https://agentmods.dev/badge/skills/synthesisengineering/synthesis-skills/synthesis-message-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 | $0.00118 | $0.02963 |
| Opus 5 | $0.00059 | $0.01482 |
| Sonnet 5 | $0.00024 | $0.00593 |
| Haiku 4.5 | $0.00012 | $0.00296 |
Grade B, and why
synthesis-message-guard scanned grade B with 1 finding 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 6d 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.
Reads agent configuration directoriesmediumAgent snooping
.claude/, .codex/, .gemini/ hold keys, settings and other credentials a mod has no legitimate need for.
`~/.claude/settings.json` and Codex's `~/.codex/hooks.json`, each matching How it starts
The opening of the file, as written. The whole thing — 232 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Message Guard
Version 1.3.0 (2026-09-01) adds patterns.example.json, the public,
validator-backed starting point used by synthesis-onboarding init. The
scaffolder preserves the user's local copy and can add interview-supplied wording
boundaries without putting personal policy in this repository. A fail-closed
message gate now ships the route to a valid config instead of requiring adopters
to reconstruct one from prose.
Version 1.2.0 (2026-08-28) — adds check_header_hygiene: the gate now inspects the
RFC threading headers (in_reply_to, references) in the tool input, not only the message
text, and blocks a send whose Message-ID has been HTML-escaped (<id@host>) or left
with an unbalanced angle bracket. Such a header matches no message, so the reply orphans in
any strict RFC client — but Gmail's own thread_id threading masks it whenever the caller
passes both, which is why the error survived review and recurred (2026-08-20, 2026-08-28).
A malformed header is now structurally undeliverable rather than silently wrong.
Version 1.1.0 (2026-07-29)
Prose rules do not survive contact with a model under load. This skill is the enforcement layer for correspondence the way commit hooks are the enforcement layer for repositories: the rules live in code, run outside the model, and fail closed.
Why it exists
Two same-night incidents (2026-07-29), both by an agent that had the relevant rules loaded in context:
- A reply composed without reading the thread it was replying into. The thread's own quoted history contained the principal's earlier message taking the opposite strategic position, with a better argument. The reply was sent. Correcting it cost a follow-up email and real trust.
- A drafted message containing register patterns the principal's written voice rules explicitly ban (self-flagellation about a delayed reply; expressing trust in a colleague via the author's own limitation). The principal had warned about exactly this class before.
What ships with it
4 files 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.
- 6d ago Changed · +70 lines d7ae8787dccb
- 10d ago First seen · 162 lines · 118 tokens per session scan B bae8bb37a4be
synthesis-message-guard is a skill published in the GitHub repository synthesisengineering/synthesis-skills (18 stars, last pushed today), licensed Apache-2.0. It adds 118 tokens to every session and 2,963 once invoked, about $0.0006 per session on Opus 5. A static security scan graded it B with 1 finding (reads agent configuration directories). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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