Developer Documentation

API Reference

Integrate Ful.io's web technology intelligence directly into your product. Look up the technology stack of any website and check your subscription usage through a simple, authenticated REST API.

REST

JSON responses

API key auth

Base URL

https://api.ful.io

Introduction

The Ful.io API lets you programmatically access our web technology intelligence. Every request is a standard HTTPS call that returns JSON. There are no SDKs to install, you can call the API from any language or tool that can make an HTTP request.

All endpoints are served from https://api.ful.io and require an API key. To get started, grab your API key from your account security page, or visit pricing to choose a plan with API access.

Using an AI agent? Connect Ful.io to Claude, Cursor, or any MCP client without writing API calls, see the MCP Server documentation.

Authentication

Authenticate every request with your API key. You can send it either as a request header (recommended) or as a query parameter. Keep your key secret, anyone with it can consume your credits.

Recommended: x-api-key header

curl "https://api.ful.io/api/domain-search?domain=ful.io" \
  -H "x-api-key: YOUR_API_KEY"

Simplest: api_key query parameter

curl "https://api.ful.io/api/domain-search?domain=ful.io&api_key=YOUR_API_KEY"

The query parameter works, but is less secure since keys in URLs can be exposed in server logs and browser history. Prefer the header for production. An Authorization: Bearer YOUR_API_KEY header is also supported as an alternative to x-api-key.

If the API key is missing you receive a 400, and if it is invalid or inactive you receive a 401.

Credits & Usage

Each successful API call consumes one API credit from your plan's API allowance. When your remaining API credits reach zero, requests return a 400 with an Insufficient credits message until your allowance renews or you upgrade.

You can check your remaining API credits at any time using the Subscription Details endpoint.

Errors

The API uses standard HTTP status codes. Error responses always include an error field describing what went wrong.

{
  "error": "Invalid or inactive API key"
}

Status	Meaning
200	Success. The request completed and the response body contains your data.
400	Bad request. A required parameter is missing or invalid, or you have insufficient credits.
401	Unauthorized. The API key is missing, invalid, or inactive.
404	Not found. The requested domain or resource could not be found.
429	Too many requests. You are being rate limited, slow down and retry.
500	Server error. Ful.io could not complete the request, retry shortly.
503	Service unavailable. The service is temporarily unavailable, retry shortly.

Technology Lookup

Returns the full technology stack detected for a website, grouped by category (for example CMS, eCommerce, analytics, hosting, payment processors, and more).

GEThttps://api.ful.io/api/domain-search

Query parameters

Parameter	Type	Required	Description
api_key	string	Yes	Your API key.
domain	string	Yes	The website domain to analyze, for example example.com. The value is automatically normalized, so https:// prefixes and paths are stripped.

Example request

curl "https://api.ful.io/api/domain-search?domain=ful.io" \
  -H "x-api-key: YOUR_API_KEY"

Response fields

Field	Type	Description
domain_name	string	The normalized domain that was analyzed.
title	string	The website's page title.
description	string	The website's meta description.
social_links	array	Social profiles found on the site. Each entry has a platform and url. The same platform may appear more than once.
social_links[].platform	string	Social platform name, for example "linkedin" or "twitter".
social_links[].url	string	URL of the social profile.
date_established	string	Approximate date the domain was first established (YYYY-MM-DD).
technologies	array	A list of category groups detected on the domain.
technologies[].category_slug	string	URL-friendly identifier for the category.
technologies[].category_name	string	Human-readable category name, for example "Payment Processors".
technologies[].technologies	array	The technologies detected within this category.
technologies[].technologies[].name	string	Technology name, for example "Paystack".
technologies[].technologies[].description	string	Short description of the technology.
technologies[].technologies[].icon	string	Icon filename for the technology.
technologies[].technologies[].website	string	Official website URL of the technology.
technologies[].technologies[].technology_slug	string	URL-friendly identifier for the technology.

Example response

{
  "domain_name": "ful.io",
  "title": "Find out what websites are built with - Ful.io",
  "description": "Website technology checker and web technology lookup. Find out what CMS, programming languages, frameworks a website is using and much more.",
  "social_links": [
    { "platform": "linkedin", "url": "linkedin.com/company/ful-io" },
    { "platform": "twitter", "url": "twitter.com/TechProfilerFul" },
    { "platform": "instagram", "url": "instagram.com/TechProfilerFul" },
    { "platform": "youtube", "url": "youtube.com/@ful-io" }
  ],
  "date_established": "2025-12-16",
  "technologies": [
    {
      "category_slug": "JavaScript-Frameworks",
      "category_name": "JavaScript Frameworks",
      "technologies": [
        {
          "name": "React",
          "description": "React is a front-end JavaScript library for building user interfaces.",
          "icon": "React.png",
          "website": "https://reactjs.org",
          "technology_slug": "React"
        }
      ]
    },
    {
      "category_slug": "Payment-Processors",
      "category_name": "Payment Processors",
      "technologies": [
        {
          "name": "Stripe",
          "description": "Stripe is a platform that specializes in financial infrastructure for businesses.",
          "icon": "Stripe.svg",
          "website": "http://stripe.com",
          "technology_slug": "Stripe"
        }
      ]
    }
    // ... additional categories
  ]
}

Errors

Status	error	Cause
400	API key is required	The api_key parameter was not provided.
400	Domain parameter is required.	The domain parameter was not provided.
400	Invalid domain format	The domain could not be parsed into a valid hostname.
400	Insufficient credits	Your plan has no remaining API credits.
401	Invalid or inactive API key	The api_key does not match an active membership.
500	Failed to contact external API	An upstream error occurred, retry shortly.

Subscription Details

Returns your current membership, its limits, and your remaining credits. Useful for monitoring API usage before making calls.

GEThttps://api.ful.io/user/api/subscription-details

Query parameters

Parameter	Type	Required	Description
api_key	string	Yes	Your API key.

Example request

curl "https://api.ful.io/user/api/subscription-details" \
  -H "x-api-key: YOUR_API_KEY"

Response fields

The response is wrapped in a user_membership object.

Field	Type	Description
id	integer	Identifier of the membership record.
user	integer	Identifier of the user that owns the membership.
membership	object	The plan definition and its limits (see below).
membership.id	integer	Plan identifier.
membership.name	string	Plan name, for example "Pro".
membership.credit_limit	integer	Total credit limit for the plan.
membership.logins	integer	Number of concurrent logins allowed.
membership.tech_lookup_credits_limit	integer	Technology lookup credit limit.
membership.api_credits_limit	integer	API call credit limit.
membership.technology_reports_credits_limit	integer	Technology report credit limit.
membership_id	integer	Convenience copy of the plan id.
membership_name	string	Convenience copy of the plan name.
start_date	string	ISO 8601 timestamp when the current period started.
end_date	string	ISO 8601 timestamp when the current period ends.
remaining_credits	integer	Total remaining credits across the plan.
tech_lookup_remaining_credits	integer	Remaining technology lookup credits.
api_remaining_credits	integer	Remaining API call credits.
technology_reports_remaining_credits	integer	Remaining technology report credits.
is_active	boolean	Whether the membership is currently active.
cancel_at_period_end	boolean	Whether the subscription is set to cancel at the end of the period.

Example response

{
  "user_membership": {
    "id": 12,
    "user": 34,
    "membership": {
      "id": 3,
      "name": "Pro",
      "credit_limit": 10000,
      "logins": 5,
      "tech_lookup_credits_limit": 5000,
      "api_credits_limit": 10000,
      "technology_reports_credits_limit": 1000
    },
    "membership_id": 3,
    "membership_name": "Pro",
    "start_date": "2026-06-01T00:00:00Z",
    "end_date": "2026-07-01T00:00:00Z",
    "remaining_credits": 8500,
    "tech_lookup_remaining_credits": 4200,
    "api_remaining_credits": 9300,
    "technology_reports_remaining_credits": 900,
    "is_active": true,
    "cancel_at_period_end": false
  }
}

Errors

Status	error	Cause
400	API key is required	The api_key parameter was not provided.
401	Invalid or inactive API key	The api_key does not match a membership.

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