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invariant-guardrails

invariant-guardrails is a skill for Claude Code, Codex from JustineDevs/premortem. It costs 52 tokens per session (596 once invoked), scanned A, original, Apache-2.0.

A design guide for Invariant, a rule system that checks agent conversations, tool calls, and connections to external services.

In plain words
What is it for?
Use it to design tool-call rules, review agent permissions, plan a policy gateway, and complement input/output checks and prompt evaluations.
Why use it?
It helps teams define policies that limit unsafe tool sequences and reduce an AI agent’s ability to perform unintended actions.

Skill for Claude CodeCodex

Written for no agent in particular: nothing here depends on one. Also seen: installed under .agents/ (shared by several agents).

not rated 2repo 1mo ago A scan 52 tokens original Apache-2.0

Good fit Use it to design tool-call rules, review agent permissions, plan a policy gateway, and complement input/output checks and prompt evaluations.

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Install with agentmods
npx agentmods add skills/justinedevs/premortem/invariant
View source ↗ JustineDevs/premortem premortem.jstn.site
Install

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.

Any agent
npx skills add JustineDevs/premortem --skill invariant
Clone the repo
git clone --depth 1 https://github.com/JustineDevs/premortem

Made for: Claude Code, Codex.

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.

agentmods badge for invariant-guardrails

README.md
[![agentmods](https://agentmods.dev/badge/skills/justinedevs/premortem/invariant/github.svg)](https://agentmods.dev/skills/justinedevs/premortem/invariant)
Your own site
<a href="https://agentmods.dev/skills/justinedevs/premortem/invariant"><img src="https://agentmods.dev/badge/skills/justinedevs/premortem/invariant/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.

agentmods 80×15 button for invariant-guardrails

Your own site · 80×15
<a href="https://agentmods.dev/skills/justinedevs/premortem/invariant"><img src="https://agentmods.dev/badge/skills/justinedevs/premortem/invariant.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 52 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 596 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5.1 $0.00052 $0.00596
Opus 5 $0.00026 $0.00298
Sonnet 5 $0.00010 $0.00119
Haiku 4.5 $0.00005 $0.00060

Measured 9d ago against content hash 539579e43cc1, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

invariant-guardrails 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 9d 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.

.agents/skills/llm-ops/invariant/SKILL.md · 54 lines

How it starts

The opening of the file, as written. The whole thing — 54 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Invariant Guardrails (Premortem)

Invariant adds rule-based guardrails over agent traces: tool-call sequences, message content, and MCP flows. Premortem already ships lightweight input/output guardrails in @premortem/security; Invariant is the path for richer, policy-as-code rules when agent tool use expands.

Current repo boundary

Layer Location Scope
Input/output string guardrails packages/security/src/input-guardrail.ts, output-guardrail.ts Prompt injection terms, secret leakage
Invariant policies Not runtime-wired yet Future MCP/tool-call policy layer
Eval gate pnpm run eval:prompts + promptfoo Prompt regression, not live proxy

Do not assume Invariant Gateway is deployed. Treat this skill as design and policy authoring guidance until a gateway cutover is explicitly scoped.

When to use

  • Authoring guardrail rules for tool-call sequences (e.g. block send_email after untrusted get_website)
  • Reviewing agent blast radius before enabling new MCP tools
  • Planning a proxy/gateway layer between orchestrator and external tools
  • Complementing $vet / llm-security with executable policy sketches

Rule shape (upstream)

raise "External email to unknown address" if:
    (call: ToolCall) -> (call2: ToolCall)
    call is tool:get_inbox
    call2 is tool:send_email({ to: ".*@[^ourcompany.com$].*" })

See references/rule-writing.md for Premortem-oriented examples and integration notes.

Premortem integration path (phased)

  1. Now: Keep @premortem/security guardrails on all user-supplied and model output text.
  2. Eval: Extend promptfoo suites in packages/evals/ for injection and tool-misuse fixtures.
  3. Later: Run Invariant locally against orchestrator trace exports, or front MCP/LLM traffic with Invariant Gateway when tool surface grows.

Rules

  • Authorization required for any red-team or live guardrail bypass testing. Use llm-security workflows.
  • Prefer deterministic repo guardrails for simple injection patterns; use Invariant rules for multi-step tool graphs.
  • Document accepted risks when a rule is deferred, same as $vet output contract.

Read the full file on GitHub · 54 lines

Files

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.

Changes

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.

  1. 9d ago First seen · 54 lines · 52 tokens per session scan A 539579e43cc1

Subscribe to this mod's changes

invariant-guardrails is a skill published in the GitHub repository JustineDevs/premortem (2 stars, last pushed 1mo ago), licensed Apache-2.0. It adds 52 tokens to every session and 596 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 2026-09-03.

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