Free SEO tools
Three free SEO tools backed by the open-source pseolint engine v0.6 — SpamBrain checker, thin-content scanner, doorway-page detector. $0, no signup, runs in a 60-second median. Each tool now produces a per-template verdict, not a per-URL list. Pick a tool below; methodology and comparison table follow.
v0.6 credibility layer:the engine has been empirically calibrated against a curated corpus of reputable, in-production pSEO sites. v0.6 audits by template — it detects your site's URL templates, samples K=10 URLs per template, and produces one verdict per template cluster rather than a flat per-URL list. Verdicts are reproducible at a fixed sample-seed; findings cluster instead of dumping per-pair noise; severity demotions are auditable via summary.appliedSeverityDemotions. Dated snapshot results, the open-source corpus, and the trade-offs we accepted are documented at /methodology.
What these tools are for
Programmatic-SEO sites (template-driven content at scale) and AI Overview readiness — SpamBrain triggers from the March 27, 2026 core update that tightened scaled-content signals on date-stacked corpora (the most recent classifier shift to demote pSEO sites), the May 7, 2024 site-reputation-abuse policy (now detected by links/host-section-divergence), the March 5, 2024 scaled-content-abuse update, plus the AEO patterns that determine ChatGPT, Perplexity, and Google AI Overview citations.
What they aren't
A general SEO audit. For Core Web Vitals use PageSpeed Insights. For broken links + general crawl use Sitebulb ($35/mo) or Screaming Frog ($259/yr). For competitor / backlink data use Ahrefs ($129/mo) or Semrush ($139.95/mo).
How rules feed into per-template verdicts
v0.6 audits by template, not by URL. After template detection (Phase 1), the engine samples K=10 URLs per template and runs 32 rules (Phase 2). Rules aggregate differently depending on their type:
- Per-page → template uniformity score.
spam/thin-contentfires on each sampled page and feeds the template's uniformity score (0–1). A score ≥0.8 means ≥8 of 10 sampled pages are thin — that template gets a critical verdict regardless of the other templates' health. - Corpus-wide (not template-scoped).
spam/near-duplicateis computed across the full sampled corpus — it compares pages across templates to surface cross-template duplication, not just within a single template. - Per-page → template-level signal.
aeo/citable-factsfires per sampled page; an 80% fire rate on a template (8/10 pages lack citable facts) becomes a template-level finding reported in the template card, not a list of 8 individual URL findings. - Site verdict. The site verdict is the worst-performing template that covers ≥5% of URLs (spec §15.1
siteVerdictFromTemplates). A healthy landing page template cannot mask a broken product-listing template if that listing template holds most of your URLs.
These three tools each foreground a different slice of that rule surface — but the underlying per-template aggregation runs in every one.
Free SpamBrain checker for programmatic SEO sites
spam/*Scan any URL against the SpamBrain-adjacent rule set the team built after the March 2024 core update. No signup, runs in 60 seconds.
Open tool →Thin content checker — find pages Google sees as filler
spam/thin-contentFind every page on your site that Google might classify as thin — under-substance, templated, or AI-padded — in one 60-second crawl.
Open tool →Doorway pages checker for programmatic SEO
spam/doorway-patternSpot doorway-shaped clusters on your site — repeated templates with one variable swapped — that Google's doorway policy explicitly targets.
Open tool →What these tools actually detect
Programmatic SEO sites get demoted for a small, well-defined set of reasons, and almost all of them trace back to Google's SpamBrain system — the machine-learning spam classifier Google rolled out in 2018 and re-architected to score pages, not just links. The same classifier drives scaled-content-abuse demotions and powers site-reputation-abuse enforcement against parasite-SEO landing pages. If your site is templated, your audit needs to mirror how SpamBrain reasons about templates — and that is exactly what v0.6 does: it audits your templates, not your URLs.
The pseolint v0.6 engine ships 32 rules across spam, content, aeo, links, schema, tech, data, and cannibalization categories. Each rule's findings aggregate per template — you get one verdict per template cluster, not a flat per-URL list. The spam rules map directly to documented SpamBrain signals — thin content, doorway patterns, near-duplicate clusters, boilerplate ratio, template diversity, scaled-content density, and link-spam detectors. The aeo rules cover answer-engine readiness for AI Overviews, Perplexity, and ChatGPT search.
The whole engine is MIT-licensed on GitHub, so anything you run in the browser here you can also run locally against pre-deploy builds, in CI, or via the Model Context Protocol server. The JSON report contains every finding, every rule reference, and a deterministic severity score between 0 and 100.
How it compares to hosted SEO crawlers
Most pSEO teams already pay for at least one general-purpose crawler. The table below lines up the lowest entry-tier price for the four tools we hear most often in customer interviews — Ahrefs, Semrush, Sitebulb, ContentKing — alongside Screaming Frog (the desktop incumbent) and pseolint. Numbers are list price as published on each vendor's pricing page; per-seat surcharges and annual-billing discounts are excluded for parity.
| Tool | Entry price | Audit limit (entry tier) | SpamBrain-aware? |
|---|---|---|---|
| pseolint (this site) | $0/month | K=10 per template, up to 200 pages, 3 audits/day per browser | Yes — template-aware v0.6 engine; per-template verdicts |
| Ahrefs | $129/month (Lite plan) | 10,000 crawl credits/month, 1 project | No — generic site-audit ruleset |
| Semrush | $139.95/month (Pro plan) | 100,000 pages/month across 5 projects | No — generic site-audit ruleset |
| Sitebulb | $35/month (Lite plan) | 1 project, 10,000 URLs per audit | Partial — thin-content + duplicate detection |
| Screaming Frog | $259/year (paid license) | Free up to 500 URLs, then unlimited per license | No — desktop crawler, raw signals only |
| ContentKing | $44/month (Basic plan) | 500 pages monitored, 1 user | Partial — change-tracking + on-page audit |
The honest read: pseolint is not a replacement for Ahrefs or Semrush if you need backlink data, keyword volumes, or rank tracking. It is a replacement for the "site audit" module those suites bolt on, specifically for templated programmatic content. The engine shipped its v0.4.0 redesign on April 29, 2026, added change-driven monitoring in v0.5.0 (May 1, 2026), shipped the links/host-section-divergence site-reputation-abuse detector in v0.5.1 (May 3, 2026), the v0.5.2 credibility layer (May 3, 2026), and the v0.6 template-aware engine — auditing by template rather than by URL, producing one verdict per template cluster via siteVerdictFromTemplates. Runs as a Cloudflare R2 + Inngest pipeline on Vercel, MIT-licensed end-to-end so anything you see in the browser audit is reproducible from the CLI.
All three tools share the same backend audit — the difference is which subset of rules each one foregrounds. If you want the full report (every rule, every page) just use the homepage audit instead.
Behind the scenes, each invocation runs an asynchronous Inngest job that performs sitemap discovery, stratified URL sampling, parallelized HTTP fetching with backpressure-controlled concurrency, headless Playwright rendering for SPA-shaped origins, semantic boilerplate detection, SimHash 64-bit fingerprinting for near-duplicate clustering, and severity-graded finding enrichment. The MIT-licensed CLI mirrors the same pipeline locally — pipe its JSON output into your CI gate or your editor's diagnostics panel via the Model Context Protocol adapter.