# Factchk

Verify reality-grounded claims against external sources in both directions before they...

- Repository: https://github.com/LilMGenius/paperthin (1365 stars)
- Category: Design
- License: MIT
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add LilMGenius/paperthin --skill factchk -g -a claude-code -y`
Codex: `npx skills add LilMGenius/paperthin --skill factchk -g -a codex -y`
Cursor: `npx skills add LilMGenius/paperthin --skill factchk -g -a cursor -y`
Gemini CLI: `npx skills add LilMGenius/paperthin --skill factchk -g -a gemini-cli -y`

## Description

Verify reality-grounded claims against external sources in both directions before they ship, could the 'absurd' be real, could the 'obvious' be false or long-established? Use whenever an artifact, or the sentence you are about to write, asserts something as plausible, realistic, absurd, novel, or impossible from intuition rather than a checked source; before relying on a factual claim in prose, a design rationale, a research claim, or a plan. Fires on metacognitive doubt, when you can't actually know, verify instead of trusting the feeling. Scans read-only, then fixes the clear errors or flags the judgment calls; leaves deliberate fiction alone.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

Check what's asserted as real against reality, in both directions, before it ships.

## Goal

The author's "plausible / absurd / novel" is the least reliable line in any artifact. Human priors fail **both ways**, they exclude the real (desert frogs exist) and normalize the impossible (weightless crates). `factchk` scans claims presented as reality-grounded, verifies each against external sources in both directions, and fixes or flags. Its scope is a claim about *the world*, whether what's asserted as real is real, not the soundness of a validation or the survival of a whole plan.

## Workflow

1. Scan for **reality-grounded assertions**, anything leaning on "this is plausible / realistic / absurd / novel / impossible because X," including the claim you are *about to write*, not only those already on the page.
2. Verify each against **external sources** (web search, open references) in **both directions**: could the "absurd" be real? could the "obvious" or "novel" be false or long-established? If you cannot reach a source, **flag, never assert from intuition**.
3. **Fix or flag:** a mechanically-clear error (a wrong date, a misattributed source, a falsified number) → correct it with a cited source; a judgment call (a contested or interpretive claim) → surface it, don't silently rewrite.
4. Report each claim, its verdict, the source, and, for a failed claim, which direction it failed.

## Rules

- Target claims asserted **as reality-grounded**, never deliberate fiction ("in our world, boxes float" is a declared choice). When it is unclear whether a claim is an in-world choice or a real-world assertion, **flag, don't fix**, the hardest call in the skill.
- Flagging only the unrealistic catches half the errors.
- The trigger is **metacognition, not knowledge**: you can't know every domain, so the moment you are about to assert a checkable fact from a feeling, verify it instead of trusting the feeling.
- A pass that finds nothing changes nothing.

The rest: https://github.com/LilMGenius/paperthin/blob/HEAD/skills/depth/factchk/SKILL.md