Access and registration

fxTeX Server is, for most people and most purposes, a complete free service. However, it is still a licensed service. Using it from your own website or application takes three steps, and the third is the one that catches people out.

How your site is identified

When a browser loads an equation from us, it tells us which page asked for it. Modern browsers send only the origin — the scheme and hostname, with no path — which is precisely what we need and no more:

Referer: https://example.com/

The behaviour depends on the referrer policy in force on your page. Measured against this service, embedding an equation from another origin:

Test what we actually receive

Paste this into any page on your site and load it. It asks this service what it saw and prints the answer, so you can confirm your domain arrives before going live:

<div id="fxtex-check">checking&hellip;</div>
<script>
fetch("https://fxtex.codecogs.com/diagnostics.json", { cache: "no-store" })
  .then(function (r) { return r.json(); })
  .then(function (d) {
    document.getElementById("fxtex-check").textContent =
      d.identifies_site
        ? "OK - we see your site as: " + d.domain
        : "PROBLEM - no Referer reached us; this request could not be attributed.";
  });
</script>

The raw response is at https://fxtex.codecogs.com/diagnostics.json and reports the referer, its form (origin only or full URL), the domain we derived, and the client address as we record it.

let latex = #"x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}"#

// Encode everything: otherwise spaces can be turned into '+'.
let query = latex.addingPercentEncoding(withAllowedCharacters: .alphanumerics)!

var request = URLRequest(url: URL(string: "https://fxtex.codecogs.com/svg.image?\(query)")!)
request.setValue("fx_pk_your-key-here", forHTTPHeaderField: "X-API-Key")

URLSession.shared.dataTask(with: request) { data, _, _ in
    guard let svg = data else { return }   // the SVG bytes
}.resume()

val latex = """x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}"""

// URLEncoder is form encoding: it turns a space into "+", which this service reads a plus.
val query = URLEncoder.encode(latex, "UTF-8").replace("+", "%20")

val request = Request.Builder()
    .url("https://fxtex.codecogs.com/svg.image?$query")
    .header("X-API-Key", "fx_pk_your-key-here")
    .build()

// Off the main thread.
OkHttpClient().newCall(request).execute().use { response ->
    val svg = response.body?.string()
}

A key without a header

A header is not always available. A browser gives you no way to set one on an <img> tag, and plenty of places — a wiki, a CMS field, a forum post, a page you do not control — let you write an address and nothing else. For those, put the key at the front of the path; everything after it is unchanged:

<img src="https://fxtex.codecogs.com/fx_pk_your-key-here/svg.image?1+\sin(x^2)">

The two forms are equivalent, and the header wins if a request somehow carries both.

How many equations you can request

Every domain draws on the same light allowance: up to 10 requests at once, refilling at 1 a second. A page loads every equation it contains in one go, so that is enough for the odd equation or two — not enough for a maths-heavy page or a busy site.

Registering does not change this allowance by itself. What it does is remove the notice below from every equation your domain serves, so a visitor never sees anything but the equation they asked for.

Subdomains covered by one wildcard entry share a single allowance, and so do all the visitors to your site — the allowance belongs to the domain, not to the reader.

Register your Domain or App

If a request is refused

Casual and unregistered use is not blocked. Without a matching domain or key, or with none presented at all, the equation still renders — just with a small notice added beneath it, so a visitor sees an explanation rather than a missing equation:

The one request genuinely refused is from a domain or key that is registered but has gone way past its own allowance for the moment:

That carries a 429 and a Retry-After header saying how many seconds until the next request will succeed. It clears on its own within seconds, so retrying immediately will not help, and nothing needs correcting on your end — a page that trips it constantly is just heavier than the allowance above, in which case get in touch rather than working around it.

Asking /diagnostics.json what we can see costs you nothing either way. The documentation, this page and that endpoint are never gated: you can always reach them to work out what is happening.

Summary