About factchk
factchk is an agent skill for checking factual claims against external sources before an artifact is published. It verifies claims in both directions and distinguishes corrections from issues that should be flagged.
Key features
- 01Find reality-grounded claims
- 02Check claims against external sources
- 03Verify in both directions
- 04Correct clear errors or flag judgments
- 05Report verdicts with sources
Use cases
- 01Verify factual claims in a draft
- 02Check whether a claim labeled impossible or novel is accurate
Install
Copy the command and run it in a terminal.
npx skills add https:/After installing, type/factchkin Claude Code to confirm it loaded.
Details
From the project READMECheck 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
- 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.
- 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.
- 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.
- 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.
Verification
Before finishing: every verdict traces to a citable external source, not a reworded prior intuition — and the report reads as fixes-vs-flags, not restated intuitions.