blog-factcheck

name	blog-factcheck
description	Verify statistics and claims in blog posts by fetching cited source URLs and checking if the claimed data actually appears on the page. Extracts all statistical claims (numbers, percentages, named sources), fetches each cited URL via WebFetch, and scores match confidence (exact match 1.0, paraphrase 0.7-0.9, not found 0.0). Flags uncited claims as UNVERIFIED. Use when user says "fact check", "verify statistics", "check sources", "validate claims", "factcheck", "source verification".
user-invokable	true
argument-hint	[file]
license	MIT

Blog Fact-Check

Verify statistics, claims, and source attributions in blog posts. Pure Claude pipeline with no external NLP dependencies.

Workflow

Step 1: Read the Blog Post

Read the target file and identify all sections containing data claims.

Step 2: Extract Statistical Claims

Scan the full text for every claim that includes a number, percentage, dollar amount, or named source. Build a claims list with these fields:

Field	Description
claim_text	The exact sentence or phrase containing the statistic
value	The numeric value (e.g., "42%", "$1.2M", "3x")
attribution	Named source if present (e.g., "HubSpot", "Gartner 2025")
url	Cited URL if present (from markdown link or parenthetical)
location	Heading or line number where the claim appears

Step 3: Verify Cited Claims

For each claim that includes a URL:

1. Fetch the source page via WebFetch
2. Search the returned content for the specific numeric value
3. If exact value found, check surrounding context matches the claim topic
4. Assign a confidence score (see Verification Scoring below)

Process claims sequentially to avoid rate-limiting source sites.

Step 4: Flag Uncited Claims

For claims without a URL:

• Mark status as UNVERIFIED
• Suggest a search query the user can run to find a source
• If the attribution names a specific organization, suggest their domain

Step 5: Generate Verification Report

Output the full results table, summary statistics, and recommended actions.

Claim Extraction Patterns

Identify claims matching these structures:

Fully cited (highest priority):

• [Number]% [claim] ([Source], [Year]) - parenthetical citation
• [claim] [Number]% ... [markdown link to source] - inline link
• According to [Source], [Number]... - attribution lead

Uncited statistics (flag for sourcing):

• [Number]% of [noun phrase] - standalone percentage
• [Number]x more/less/higher/lower - multiplier claims
• $[Number] [claim] - dollar figures without attribution

Weak signals (check context before extracting):

• studies show, research indicates, data suggests + nearby number
• survey found, report reveals, analysis shows + nearby number
• Round numbers in isolation (e.g., "millions of users") - skip unless specific

Verification Scoring

Score	Status	Criteria
1.0	VERIFIED	Exact number found on cited page in matching context
0.7-0.9	PARAPHRASE	Similar data found but with different wording, rounding, or timeframe
0.3-0.6	WEAK	Source page exists and covers the topic but the specific statistic is not visible
0.0	NOT FOUND	Cited page does not contain the claimed data anywhere
N/A	UNVERIFIED	No source URL provided for the claim

Scoring guidance:

• A claim of "43%" when the source says "nearly half" scores 0.8
• A claim of "2024" data when the source only has "2023" scores 0.7
• A claim citing a homepage when the stat lives on a subpage scores 0.3
• A 404 or unreachable URL scores 0.0

Output Format

Verification Report: [Post Title]

File: [path] Claims found: [total] Verified: [count] | Paraphrase: [count] | Weak: [count] | Not Found: [count] | Unverified: [count]

#	Claim	Source URL	Score	Status	Notes
1	"73% of marketers..."	https://example.com/report	1.0	VERIFIED	Exact match found in section 3
2	"5x ROI improvement"	https://example.com/study	0.8	PARAPHRASE	Source says "nearly 5x"
3	"60% prefer video"	(none)	N/A	UNVERIFIED	Try: "video preference statistics 2025"

Recommended Actions

• [List claims that need source URLs]
• [List claims with weak or not-found scores that need replacement sources]
• [List claims where the source data may be outdated]

Integration

This skill can be called from blog-analyze as an optional deep-verification step. When invoked from the analyzer, only claims scoring below 0.7 are flagged in the analysis report.

Standalone usage: /blog factcheck path/to/post.md

Cross-reference

claude-blog inherits FLOW's evidence triple (year anchor in prose, inline citation with publisher and title, URL with retrieval date). See skills/blog-flow/references/flow-framework.md and /blog flow for the full framework.

Limitations

• Paywalled content: WebFetch cannot access content behind login walls. These score as WEAK (0.5) with a note about paywall detection.
• Dynamic pages: JavaScript-rendered content may not be available via WebFetch. If the page returns minimal content, note this in the status.
• PDF sources: WebFetch may not extract PDF text reliably. Flag PDF URLs for manual verification.
• Archived pages: If a URL returns 404, suggest checking web.archive.org.
• Rate limits: Process no more than 10 URLs per run to avoid overwhelming source servers. If a post has more than 10 cited URLs, verify the first 10 and list the remainder as SKIPPED.