SEO API: how to build your own keyword or domain tool

What is an SEO API?

An SEO API is an HTTP interface that returns search-marketing data, such as a domain's authority score, its backlinks, a keyword's monthly volume or its difficulty, as structured JSON that your own code can call. You send a domain or a keyword with an API key, and you get numbers back, usually metered per request, per row or in credits.

Founders use one to launch a product (a keyword research tool, a domain authority checker, a backlink checker) without spending a year on infrastructure. Agencies use one to fill dashboards and score prospects. Developers building AI assistants use one so the model works from real numbers; that use case has its own guide, SEO data for AI agents and LLM apps.

This guide uses MrKeyword's Data API for the worked examples because we make it and can show exact prices. The patterns, though, carry over to any SEO data API.

What data does an SEO data API provide?

Most SEO data APIs cover three groups of data: domains (authority, traffic estimates, indexed pages), links (backlinks and referring domains), and keywords (volume, cost per click, difficulty, intent and the pages that rank). The table maps each kind of tool to the data it needs and the MrKeyword endpoint that serves it.

Two details matter when you compare providers. First, where the numbers come from: a vendor's own crawler, Google's keyword tools, a clickstream panel or a model. Second, how each number is labelled. MrKeyword returns a label on its figures (Measured, Derived, Estimated, Predicted or Not available) plus confidence, asOf and source, and a domain it has not seen comes back with nulls and a reason rather than a guess. Pass those labels through to your users.

Should you build your own crawler or buy SEO data?

Buy the data unless the data itself is your product; owning a crawler, a link graph and a keyword database is several long-running systems, not a feature. Each one needs storage, refresh jobs and a way to measure its own accuracy before the first user sees a number.

Building does make sense in a few cases: you need a niche index nobody sells (one industry, one language), you have a public dataset such as Common Crawl and the skills to process it, or the data is the moat. For a first version of a tool, an API lets you test whether anyone wants it in days instead of months.

How to build a domain authority checker with an SEO API

A domain authority checker is one API call per domain: send the domain, read the 0–100 score, the referring-domain count and the date, and show them with their label. With MrKeyword that call is GET /api/v1/domains/authority at 1 credit.

1. Create a key

   Make a free MrKeyword account, open Data API in the menu and press Create API key. The key starts with mkd_ and is shown once. Every workspace starts on the free Developer plan: 500 credits a month and 60 calls a minute.

2. Call the endpoint from your server

   Send the key as Authorization: Bearer mkd_… (or x-api-key). Keep it on your backend; the API allows browser requests, but a key in front-end code can be copied by anyone.

3. Read data, check meta

   Every success is HTTP 200 with { data, meta }. meta.creditsCharged and meta.creditsRemaining (also sent as x-mrkeyword-credits-* headers) let you track spend per user.

4. Batch when you can

   POST { "domains": [...] } checks up to 100 domains in one request at the same 1 credit per domain, which saves round trips and rate-limit headroom.

curl "https://app.mrkeyword.io/api/v1/domains/authority?domain=example.com" \
  -H "Authorization: Bearer mkd_your_key"

{
  "data": {
    "input": "example.com",
    "domain": "example.com",
    "authority": {
      "value": <0–100 or null>, "label": "Derived", "confidence": "…", "asOf": "…",
      "referringDomains": …, "reason": null, "source": "…", "model": "…",
      "webGraph": { "position": …, "totalDomains": …, "linkingDomains": …, "release": "…" }
    },
    "referringDomains": { "value": …, "label": "…", "asOf": "…", "source": "…" },
    "backlinks": { "value": …, "label": "…", "follow": …, "nofollow": …, "asOf": "…" }
  },
  "meta": { "endpoint": "domains.authority", "creditsCharged": 1, "creditsRemaining": …, "plan": "free", "generatedAt": "…" }
}

In your interface, show the score with its label and date ("Derived, as of …"), and show the reason when the value is null instead of printing zero. Handle the error codes: 402 means not enough credits, 429 means the plan's rate limit (respect Retry-After), and a 500 is not charged. GET /api/v1/usage costs nothing and returns your balance. Our own free domain authority checker is built on the same data, and What is domain authority? explains how MrKeyword Link Authority differs from Moz DA, Ahrefs DR and Semrush Authority Score.

How to build a keyword difficulty checker

A keyword difficulty checker sends a list of keywords and a market, and returns a 0–100 score per keyword; with MrKeyword that is one POST to /keywords/difficulty for up to 500 keywords at 1 credit per 10. A 25-keyword list costs 3 credits.

curl -X POST "https://app.mrkeyword.io/api/v1/keywords/difficulty" \
  -H "Authorization: Bearer mkd_your_key" \
  -H "content-type: application/json" \
  -d '{"keywords":["plumber near me","emergency plumber"],"market":"US"}'

{
  "data": {
    "market": "US",
    "results": [
      { "keyword": "plumber near me", "difficulty": <0–100>, "label": "Estimated", "confidence": "…", "basis": "stored" },
      { "keyword": "emergency plumber", "difficulty": <0–100>, "label": "Estimated", "confidence": "…", "basis": "model" }
    ],
    "model": "MrKeyword keyword difficulty model (volume, competition, CPC, topic vertical, breadth, qualifiers)"
  },
  "meta": { "endpoint": "keywords.difficulty", "creditsCharged": 1, "creditsRemaining": …, "plan": "free", "generatedAt": "…" }
}

To turn that into a fuller keyword tool, add POST /keywords/intent (also 1 credit per 10) for informational, commercial, transactional or navigational labels, and POST /keywords/metrics (1 credit per keyword, up to 100 per call) for volume and CPC. Be honest in the interface about two things: difficulty is labelled Estimated, and /keywords/metrics answers from MrKeyword's keyword database first; a keyword it has not stored yet is looked up live in Google Keyword Planner, saved, and returned in the same call. You are charged only for keywords that come back with data, so a keyword Google has no volume for costs nothing. If you want a deeper explanation of search volume sources for your users, link them to how to find keyword search volume.

How much does an SEO API cost for 10,000 lookups a month?

On MrKeyword, 10,000 domain authority lookups a month is 10,000 credits, which fits API Starter at $29 a month; 10,000 full domain reports (3 credits each) is 30,000 credits, which fits Starter plus one $20 credit pack, or API Growth at $99. The arithmetic for common tools:

Two habits keep the bill down. Batch keyword calls, because the per-10 pricing rewards it. And cache results on your side: a domain's authority does not change between two visitors on the same day, and every response carries an asOf date you can key a cache on. API plans are separate from MrKeyword's app plans on the pricing page; buying one never changes the other.

How does MrKeyword's Data API compare with DataForSEO, Semrush, Ahrefs and Moz?

The established SEO APIs have much larger datasets, and DataForSEO and Ahrefs also sell search-results data; MrKeyword's Data API is cheaper to start and simpler to price, but it serves only its own, smaller index and never live Google results. Prices below were read on each vendor's official page on 30 October 2026.

How to choose: if your tool must show real Google rankings for any keyword, or the deepest backlink counts, use one of the large providers and budget for it. If you want a domain checker, a keyword difficulty or intent tool, or an internal agency dashboard at a predictable monthly price, and you are comfortable showing labelled estimates, MrKeyword's API is a reasonable place to start. Many products end up using two providers for different jobs.

Limits: what MrKeyword's Data API does not do

MrKeyword's Data API serves only MrKeyword's own index and models, so it does not have the coverage of the largest SEO databases and it never returns live Google results. Specifically:

• Own index coverage. Authority, backlinks and referring domains come from MrKeyword's crawler and link index and the Common Crawl web graph. Backlinks are the links MrKeyword has discovered, not every link on the web, so counts are lower than the biggest indexes report.
• Predicted and Estimated, not measured. Organic keywords for any domain and /serp/predicted are Predicted from MrKeyword's public page index; traffic is Estimated with a low–high band; difficulty is Estimated. None of them is a measured Google position or click count.
• No live Google SERP scraping. Nothing is scraped from Google, so there is no endpoint that returns today's Google results page.
• Keyword volume in five markets. Volumes are Google Ads Keyword Planner figures for US, CA, IN, GB and AU. Keywords not yet stored are fetched live, within a daily allowance of live lookups per API plan; a keyword Google has no volume for answers found: false and is not charged.
• No resale of a third-party index. The API does not pass through another vendor's data.

If what you need is a person's own site data, their Search Console clicks, positions and audits, that is the MCP server rather than the Data API.

Questions and answers