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Invariant Guard

Category: General  ·  Sub-category: general-misc  ·  Last updated:
type:review
Use when writing or reviewing algorithms where the obvious implementation is subtly wrong — postcondition stronger than the loop's natural invariant (Boyer–Moore majority, Floyd cycle, leftmost vs any binary search, QuickSelect partition); in-place mutation with read+write pointers (dedup-in-place, partition, rotate); recursion with multiple parameters or accumulator state; off-by-one suspects with duplicates, empty inputs, boundary values; iterative refinements that must terminate (fixed-point, Newton, EM); any function where you catch yourself thinking "I know this algorithm" — the trap is usually in the contract, not the loop body. Forces writing the function contract (especially the postcondition) and loop invariant BEFORE code. Pairs with lemmaly (picks the algorithm) and mathguard (picks the math).
Security AStatic scan found no risk patternsHow grading works ›

From the source SKILL.md

The model knows what a loop invariant is. It knows recursion needs a base case. It knows about empty lists, integer overflow, and the difference between < and ≤. It just does not write these down before producing code, so it ships subtle correctness bugs that tests do not catch.

What this skill does

Invariant Guard is a community-contributed Claude Code skill in the general-misc sub-category. It ships as a SKILL.md file that Claude Code auto-discovers under ~/.claude/skills/invariant-guard/ and loads when your prompt matches the skill's trigger.

When to invoke it: Use when writing or reviewing algorithms where the obvious implementation is subtly wrong — postcondition stronger than the loop's natural invariant (Boyer–Moore majority, Floyd cycle, leftmost vs any binary search, QuickSelect partition); in-place mutation with read+write pointers (dedup-in-place, partition, rotate); recursion with multiple parameters or accumulator state; off-by-one suspects with duplicates, empty inputs, boundary values; iterative refinements that must terminate (fixed-point, Newton, EM); any function where you catch yourself thinking "I know this algorithm" — the trap is usually in the contract, not the loop body. Forces writing the function contract (especially the postcondition) and loop invariant BEFORE code.

Who uses this skill

The Invariant Guard Claude Code skill is built for Claude Code users and developers across all disciplines looking for general-purpose AI assistance. It's part of ClaudSkills (also referred to as Claude Skills or Claude Code Skills) — the open community-curated registry of 153,000+ SKILL.md files for Anthropic's Claude Code agent and the wider Claude ecosystem (Claude API, Claude Agent SDK).

How to install

Free

Manual install (2 steps)

mkdir -p ~/.claude/skills/invariant-guard
curl -L https://claudskills.com/skills/invariant-guard/SKILL.md \
  -o ~/.claude/skills/invariant-guard/SKILL.md

Or just download SKILL.md directly and drop it into ~/.claude/skills/invariant-guard/. Claude Code auto-discovers it on next session.

Skills live at ~/.claude/skills/invariant-guard/SKILL.md on macOS/Linux, or %USERPROFILE%\.claude\skills\invariant-guard\SKILL.md on Windows. See the full install guide for step-by-step instructions.

Telegram

📱 Install from your phone or desktop Telegram

Open @claudskills_bot on Telegram, tap Open Desktop App, and the desktop app installs this skill for you. Or share the bot link with a colleague — they get the same one-tap install. Learn more →

Pro

One-click install via the desktop app

The ClaudSkills desktop app installs any skill directly into ~/.claude/skills/ with one click — no terminal required. Pro starts at $9/mo or $149 lifetime.

Pro

For the full experience including quality scoring and one-click install features for each skill — upgrade to Pro.

See pricing → Download desktop app

Frequently asked questions

How do I install the Invariant Guard Claude Code skill?
Install via the ClaudSkills desktop app (one click) or copy SKILL.md from the source repository to ~/.claude/skills/invariant-guard/SKILL.md and restart Claude Code. Both flows are detailed at claudskills.com/install/.
What does the Invariant Guard skill do?
Use when writing or reviewing algorithms where the obvious implementation is subtly wrong — postcondition stronger than the loop's natural invariant (Boyer–Moore majority, Floyd cycle, leftmost vs any binary search, QuickSelect partition); in-place mutation with read+write pointers (dedup-in-place, partition, rotate); recursion with multiple parameters or accumulator state; off-by-one suspects with duplicates, empty inputs, boundary values; iterative refinements that must terminate (fixed-point, Newton, EM); any function where you catch yourself thinking "I know this algorithm" — the trap is usually in the contract, not the loop body. Forces writing the function contract (especially the postcondition) and loop invariant BEFORE code. Pairs with lemmaly (picks the algorithm) and mathguard (picks the math).
Is this skill free to install?
Yes. ClaudSkills is an open registry — every skill keeps its source repository's license, and manual install via copy is free. ClaudSkills Pro ($9/mo, $79/yr, or $149 one-time) adds one-click install via the desktop app and a multi-signal Quality Score.
When should I use the Invariant Guard skill?
Use Invariant Guard when your Claude Code task falls under the General category — specifically in the general misc area. Claude Code auto-discovers installed skills and invokes the right one based on the task description, so you can also ask Claude directly (e.g. "use Invariant Guard" or describe the task and let Claude pick). Browse related skills at /category/general/.
What is a Claude Code skill and how does the Invariant Guard skill fit in?
A Claude Code skill is a SKILL.md file that lives under ~/.claude/skills/<name>/ and tells the Claude Code CLI agent how to perform a specific task (instructions, prompts, allowed tools). Skills are auto-discovered at session start. Invariant Guard is one of 67,000+ skills indexed in the open ClaudSkills catalog, classified under the General category. Learn more at /learn/what-is-a-claude-skill/.

Attribution & license

Cite this skill

If you reference this skill in a blog post, paper, or documentation, you can cite it as:

APA
morsechimwai. (2026). Invariant Guard [Claude Code skill]. ClaudSkills. https://claudskills.com/skills/invariant-guard/
BibTeX
@misc{invariant-guard-2026,
  author    = {morsechimwai},
  title     = {Invariant Guard [Claude Code skill]},
  year      = {2026},
  publisher = {ClaudSkills},
  url       = {https://claudskills.com/skills/invariant-guard/}
}

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Grade A · scanned 2026-07-18 — free static scan against the OWASP Agentic Skills Top 10.

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