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All about carbon.txt

A carbon.txt is the file served at /carbon.txt on your domain, a known location auditors, customers and procurement teams check when they want to see your environmental disclosures and the suppliers your site depends on. You write it once in Core and attach it to as many of your domains as you like.

Wondering what carbon.txt is for and how it works? This article explains why carbon.txt exists, what each field does, how you attach it to your domains, and how to have every new domain pick it up automatically.

The format itself is defined by carbontxt.org.

Creating one

Open the top-level 'Carbon.txts' page and click 'Create Carbon.txt'. Then attach it to the domains that should serve it, which is covered in Attaching it to your domains.

What it contains

Two things:

  • Disclosures — pointers to your sustainability documents: annual reports, sustainability pages, CSRD filings, ISO certificates, AI model cards.
  • Upstream services — the third-party services your site depends on, such as your CDN, hosting and AI providers, so readers can see your supply chain.

What it solves

Sustainability data tends to be scattered across an annual report PDF, a separate sustainability page, an ISO certificate from an auditor, a CSRD filing, and an AI model card. Anyone trying to find or verify it has to track down each document by hand — a procurement team running a supply-chain audit, a researcher comparing providers, an automated aggregator pulling data across companies.

carbon.txt puts it in one machine-readable file at a known location: https://yourdomain.com/carbon.txt. Aggregators and supply-chain scanners can then pull disclosures from many domains without manual searching.

Regulations such as the EU's Corporate Sustainability Reporting Directive (CSRD) are pushing more organisations to publish sustainability data; carbon.txt is one way to make that data discoverable.

One carbon.txt, many domains

The usual way to publish a carbon.txt is a text file at the root of every site, one copy per document root. When a report is replaced, a certificate expires, or you switch CDN, you touch every site again — and miss one, and that file goes stale.

In Core you create it once and attach it to as many of your domains as you like. Update it and every attached domain serves the new file immediately.

Nothing is written to disk. Cyberfusion builds the file from the carbon.txt attached to the domain the visitor asked for.

Fields

'Name'

A short label (up to 32 characters: letters, digits, dashes, underscores, spaces) used to pick the carbon.txt in lists inside Core.

It is not part of the served file, so auditors never see it.

'Default for cluster'

Toggle on and every new domain you add on that cluster gets this carbon.txt attached automatically.

Only one per cluster can be the default. If the cluster already has one, the toggle is locked and a link to the existing default is shown. Turn it off on the current default first, then on for the new one.

'Disclosures'

Public documents backing up your sustainability claims. Each has:

  • 'Document Type' — one of web-page, annual-report, sustainability-page, certificate, csrd-report, ai-model-card, other.
  • 'URL' — link to the document. Must be HTTPS.
  • 'Valid Until' (optional) — date after which the document should be considered stale. Auditors use this to check the disclosure is current.
  • 'Title' (optional) — short label shown next to the URL.

A document type and a URL are required per disclosure.

'Upstream Services'

Third-party services your site depends on — typically a CDN, a hosting provider, or an AI model provider. Listing them lets an auditor follow your chain of dependencies upstream and check each provider's own disclosures.

Each has:

  • 'Domain' (optional) — the provider's domain, for example cyberfusion.io.
  • 'Service Type' — one or more types, comma-separated, for example cdn, hosting.

Every new carbon.txt is pre-filled with cyberfusion.io as a hosting upstream service. Add your other providers — CDN, AI, payments, mail — on top.

Attaching it to your domains

In the 'Domain Routers' section:

  • Flip on the domains that should serve this carbon.txt, then click 'Save'.
  • For long lists, the 'Bulk select' switch toggles every row at once.
  • A domain that is off but shows 'X is attached' is currently using a different carbon.txt. Toggling it on here moves it from that one to this one.

Same cluster only

Carbon.txts and domains are paired per cluster: one on a given cluster can only be attached to domains on that same cluster.

Running several clusters and want the same carbon.txt everywhere? Create one per cluster.

The listing page

The top-level 'Carbon.txts' page shows every carbon.txt across your clusters, with:

  • 'Name' — the Core-internal label.
  • 'Disclosures' and 'Upstream Services' — how many of each.
  • 'Used by' — how many domains use it. Hover for the full list.
  • 'Default' — whether it is the cluster's default.

'Cluster' shows which cluster it belongs to.

What visitors get

A request to https://yourdomain.com/carbon.txt returns a carbontxt.org-formatted text file built from the fields you set.

No carbon.txt attached → no file is served. Visitors get the same 404 as for any other missing path.