The most comprehensive free on-page SEO API
Send any public URL and get a complete on-page SEO breakdown back as JSON. 20 endpoints, 100+ on-page checks — titles, meta tags, headings, content, links, redirect chains, indexability, duplicate content, structured data, images & alt text, Core Web Vitals, Lighthouse and AI-readiness. Everything you'd normally pay an enterprise crawler for — free (100 requests/day), with an OpenAPI 3 spec and an MCP server so AI agents like ChatGPT and Claude can call it directly.
Every on-page signal, one API
RankNibbler turns a full technical SEO audit into a clean JSON API. Instead of stitching together a dozen tools, you hit one endpoint and get scored, structured data you can act on. Across its 20 endpoints it checks:
- Titles & meta — title tags, meta descriptions, meta robots and length checks.
- Headings & content — the full H1–H6 outline, word count, readability and content quality.
- Links — every internal and external link with anchor text and rel attributes.
- Redirect chains — the full hop-by-hop path and final status for any URL.
- Indexability — what's noindex, blocked by robots.txt, or canonicalised away, and why.
- Duplicate tags & content — repeated titles, meta descriptions and near-duplicate pages.
- Structured data — JSON-LD, microdata, Open Graph and Twitter cards, parsed.
- Images & resources — alt-text coverage and every script, style and image the page loads, with status and size.
- Keyword density & raw HTML — 1–5-gram frequency and the exact fetched markup.
- Performance — page timing (DNS/TCP/TLS/TTFB), Core Web Vitals, Lighthouse scores and mobile readiness.
- AI & agent readiness — agent-discovery standards and AI-search content checks, unique to RankNibbler.
Every audit returns a 0–100 score, a letter grade and a prioritised issue list, so you fix what matters first.
20 endpoints
Every endpoint is a single GET that takes a url and returns JSON:
- /api/v1/audit — full on-page SEO audit: score, grade, issues and every check.
- /api/v1/links — all links on a page (internal/external, dofollow, anchor, canonical, hreflang).
- /api/v1/meta-tags — full head-metadata dump: title, meta, Open Graph, Twitter, link rels.
- /api/v1/content — readability, word count and heading outline.
- /api/v1/keyword-density — word & phrase frequency / density (1–5-gram).
- /api/v1/structured-data — extract JSON-LD and microdata structured data.
- /api/v1/indexability — robots, canonical, noindex and crawl directives.
- /api/v1/redirect-chains — trace the full redirect chain for a URL.
- /api/v1/resources — page resources (images/scripts/styles) with status & size.
- /api/v1/raw-html — raw fetched HTML with status and headers.
- /api/v1/page-timing — DNS/TCP/TLS/TTFB/download timing and transfer size.
- /api/v1/ai-readiness — AI & agent readiness: agent-discovery standards + AI-search content checks.
- /api/v1/security-headers — security-header grade and mixed-content analysis.
- /api/v1/accessibility — WCAG / axe accessibility audit.
- /api/v1/robots-txt — robots.txt parsing, including AI-bot rules.
- /api/v1/sitemap — XML sitemap discovery and parsing.
- /api/v1/lighthouse — Lighthouse scores and Core Web Vitals (rendered).
- /api/v1/screenshot — rendered full-page screenshot.
- /api/v1/waterfall — network request waterfall (rendered).
- /api/v1/crawl — full-site crawl: queue a crawl and pull the results.
No daily caps on the data and no enterprise contract — a free account and an API key is all it takes, with 100 requests/day on the free tier.
Built for
- AI agents & assistants — give ChatGPT, Claude or your own agent live on-page SEO data via the MCP server or OpenAPI actions.
- SEO SaaS & audit tools — power a website grader, audit dashboard or lead-gen tool without building a crawler.
- CI/CD & monitoring — fail a build or alert when a page loses its title, goes noindex or drops Core Web Vitals.
- Agencies & freelancers — automated client reporting straight from JSON.
A first request
Every endpoint is relative to https://www.ranknibbler.com and returns JSON. Authenticate with your API key (an X-API-Key header), pass a url, and read the score:
- curl -H "X-API-Key: YOUR_KEY" \
- "https://www.ranknibbler.com/api/v1/audit?url=https://example.com"
- $ch = curl_init("https://www.ranknibbler.com/api/v1/audit?url=" . urlencode("https://example.com"));
- curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
- curl_setopt($ch, CURLOPT_HTTPHEADER, ["X-API-Key: YOUR_KEY"]);
- $audit = json_decode(curl_exec($ch), true);
- echo $audit["score"];
- const res = await fetch(
- "https://www.ranknibbler.com/api/v1/audit?url=" + encodeURIComponent("https://example.com"),
- { headers: { "X-API-Key": "YOUR_KEY" } }
- );
- const audit = await res.json();
- console.log(audit.score);
- import requests
- res = requests.get(
- "https://www.ranknibbler.com/api/v1/audit",
- params={"url": "https://example.com"},
- headers={"X-API-Key": "YOUR_KEY"},
- )
- print(res.json()["score"])
- using var http = new HttpClient();
- http.DefaultRequestHeaders.Add("X-API-Key", "YOUR_KEY");
- var url = "https://www.ranknibbler.com/api/v1/audit?url="
- + Uri.EscapeDataString("https://example.com");
- var json = await http.GetStringAsync(url);
- Console.WriteLine(json);
- {
- "url": "https://example.com",
- "score": 82,
- "grade": "B",
- "issueCount": 3,
- "issues": ["Missing meta description", "2 images missing alt text"],
- "usage": { "used": 7, "dailyLimit": 100, "tier": "free" }
- }
Why RankNibbler
- Comprehensive — 20 endpoints and 100+ on-page checks in one API: the checks a paid crawler charges for, free.
- Developer-first — clean JSON, an OpenAPI 3 spec, and copy-paste samples in five languages (cURL, PHP, Node.js, Python, C#).
- AI-ready — an MCP server and OpenAPI actions let ChatGPT, Claude and custom agents call it directly, plus a dedicated AI-readiness endpoint that scores agent-discovery and AI-search signals.
- Free — 100 requests/day on the free tier, no credit card and no enterprise contract.
Frequently asked questions
Is the RankNibbler SEO API free?
Yes. Every account includes a free tier with 100 API requests per day — no credit card and no enterprise contract. Create a free account and generate a key from your dashboard.
What does the on-page SEO API check?
20 endpoints covering 100+ on-page signals: title tags, meta descriptions, headings, content and readability, internal and external links, redirect chains, indexability (robots, canonical, noindex), duplicate titles/descriptions/content, structured data (JSON-LD, microdata, Open Graph, Twitter cards), images and alt text, keyword density, page timing, Core Web Vitals, Lighthouse scores and AI & agent readiness.
Can AI agents like ChatGPT or Claude use the API?
Yes. RankNibbler ships an MCP server and OpenAPI 3 actions, so ChatGPT, Claude and custom agents can call it directly. There's also a dedicated /api/v1/ai-readiness endpoint that scores agent-discovery standards and AI-search content signals.
What format does the API return?
Clean JSON on every endpoint, documented by an OpenAPI 3 spec, with copy-paste samples in cURL, PHP, Node.js, Python and C#.
How do I authenticate?
Pass your API key in an X-API-Key header (or a ?key= query parameter). Keys are created from your free RankNibbler dashboard.
Is there a rate limit?
The free tier allows 100 requests per day per key, with burst protection to keep responses fast. Need a higher limit? Get in touch.
Get an API key
API keys are created from your free RankNibbler account. Create an account, then generate a key from your dashboard.