Free Backlink Data Sources
Reference for the seo-backlinks skill. Loaded on demand when analyzing backlinks with free sources.
Source Comparison
| Source | Auth | Any Domain? | Data Quality | Coverage vs Commercial | Rate Limit |
|---|---|---|---|---|---|
| Moz API | API key (free signup) | Yes | ★★★★☆ | ~70% for DA/PA | 1 req/10s, 2,500 rows/mo |
| Bing Webmaster | API key (free) | Verified sites only | ★★★☆☆ | ~15% (Bing index) | Generous |
| Common Crawl | None (public) | Yes | ★★★☆☆ | ~25-40% domains | N/A |
| Verification Crawler | None | Yes | ★★★★★ (binary) | N/A (checks known links) | 1 req/s per domain |
| DataForSEO (paid) | API key | Yes | ★★★★★ | ~90%+ | Per plan |
Confidence Weighting
When merging data from multiple sources, apply confidence weights to each metric:
| Source | Weight | Rationale |
|---|---|---|
| DataForSEO | 1.00 | Commercial-grade, real-time, comprehensive |
| Verification Crawler | 0.95 | Direct observation (binary: link exists or not) |
| Moz API | 0.85 | Large index (45.5T links), established metrics, 3-day update lag |
| Bing Webmaster | 0.70 | Smaller index (~15% of web), but authoritative for Bing-indexed pages |
| Common Crawl | 0.50 | Domain-level only, quarterly updates, no anchor text |
Composite formula:
weighted_score = Σ(source_score × confidence × factor_weight) / Σ(confidence × factor_weight)When only Common Crawl is available, do not produce a health score at all. Not a capped one, not an "approximate" one. Common Crawl supplies domain-level rank and presence signals only -- it carries none of the referring-domain quality, anchor, or toxicity data a health score claims to summarise, so any number derived from it reads as "this profile is weak" when the truth is "we have no data."
Report Not Assessed in place of the score, surface the rank/presence data that
Common Crawl does support as low-confidence findings, and note "limited to
domain-level metrics" in the report. The same applies to any finding written with
source: not-assessed.
This is enforced, not advisory: "${CLAUDE_PLUGIN_ROOT}/scripts/claude-seo" run validate_backlink_report.py fails
a report (status: FAIL) that carries a number in either case. See the
seo-backlinks skill's "MUST NOT: score what you did not measure" section.
Source Details
Moz API (Tier 1)
- Endpoint:
https://api.moz.com/jsonrpc(JSON-RPC 2.0) - Free tier: 2,500 rows/month, 1 request per 10 seconds (verify current limits at https://moz.com/products/api: free tier limits may change)
- Signup: https://moz.com/products/api (credit card required, not charged)
- Data: Domain Authority (0-100), Page Authority, Spam Score (1-17%), link counts, referring domains, anchor text distribution
- Command:
"${CLAUDE_PLUGIN_ROOT}/scripts/claude-seo" run moz_api.py - Commands:
metrics,domains,anchors,pages - Blind spots: No link velocity, no toxic link patterns beyond Spam Score, 3-day update lag, smaller index than Ahrefs/Semrush
Bing Webmaster Tools (Tier 2)
- Endpoint:
https://ssl.bing.com/webmaster/api.svc/json/ - Free tier: Unlimited for verified sites
- Signup: https://www.bing.com/webmasters (Microsoft account)
- Comparison: Backlink-domain gaps between two registered properties that are accessible to the same API account
- Data: Inbound-link source URL, target URL, anchor text, sampled link counts, and referring-domain comparison totals
- Command:
"${CLAUDE_PLUGIN_ROOT}/scripts/claude-seo" run bing_webmaster.py - Commands:
links,counts,compare(comparison requires both properties to be registered to the same API account) - Blind spots: Only Bing-indexed pages (~15% of web), verified sites only, no authority metrics, no spam scoring
Common Crawl Web Graph (Always Available)
- Data source:
s3://commoncrawl/projects/hyperlinkgraph/ - Releases: Quarterly (e.g., cc-main-2025-18)
- No auth needed: Public data, free to download
- Data: Domain-level in-degree, PageRank, harmonic centrality, referring domains
- Command:
"${CLAUDE_PLUGIN_ROOT}/scripts/claude-seo" run commoncrawl_graph.py - Cache:
~/.cache/claude-seo/commoncrawl/(90-day TTL) - Blind spots: No anchor text, no page-level data, monthly/quarterly freshness, domain-level only (e.g., "nytimes.com links to example.com" but not which page)
Verification Crawler (Always Available)
- No auth needed: Uses existing fetch_page.py + parse_html.py
- Data: Binary verification (link exists/lost/moved), anchor text, rel attributes
- Command:
"${CLAUDE_PLUGIN_ROOT}/scripts/claude-seo" run verify_backlinks.py - Input: JSON file with
[{"source_url": "..."}]entries - Polite crawling: 1-second delay between requests to same domain
- Best for: Checking if known backlinks still exist, monitoring link health
When to Recommend DataForSEO Upgrade
Suggest the paid DataForSEO extension when:
- User needs toxic link detection beyond Moz's basic Spam Score
- User needs competitor gap analysis at scale or against an unregistered domain (Bing comparison is limited to properties accessible to the account)
- User needs link velocity trends (new/lost links over time)
- User needs real-time data (free sources update monthly at best)
- User manages multiple client sites (free tier limits are per-account)
- User needs disavow file generation with confidence scoring
Data Quality Reality Check
- Commercial tools index 35-45 trillion links across 500M+ referring domains
- Free sources combined capture 20-40% of raw backlink data
- But 60-70% of actionable intelligence since highest-authority links appear in free samples
- For sites with <500 backlinks, free sources can capture 50%+ of the meaningful profile
- Referring domain count matters more than raw backlink count for SEO
- Top 50-100 referring domains capture the majority of link authority
Five Systematic Biases in Free Data
- Popularity bias: Free tools crawl popular sites more, underrepresenting niche sites
- Truncation bias: All free tools cap at 100-1,000 links, hiding the long tail
- Own-site restriction: GSC and Ahrefs Webmaster Tools only work for verified properties
- Missing quality metrics: Raw CC data lacks authority/toxicity scores
- Freshness lag: Free sources update monthly at best vs. minutes for commercial