This policy explains the cookies Diagonalise uses and why.
The short version
We only use cookies that are strictly necessary for Diagonalise to work: to keep you signed in, to protect your forms, and, if you use Making Tax Digital, to meet HMRC's requirements.
We do not use analytics, advertising or tracking cookies, and we do not let third parties set cookies on Diagonalise. Because strictly necessary cookies do not need your consent under UK law (the Privacy and Electronic Communications Regulations), you will not see a cookie banner.
What cookies are
Cookies are small text files a website stores in your browser. They let the site remember things between pages, such as the fact that you have signed in.
Cookies we use
| Cookie | What it does | How long it lasts |
|---|---|---|
| PHPSESSID | Keeps you signed in, remembers which business you are working in, and protects forms against cross-site request forgery. | Until you close your browser or sign out |
| dg_device_id | A random identifier for your browser. HMRC requires it in the fraud prevention information sent with every Making Tax Digital submission. Only set if you use Making Tax Digital. | 10 years, or until you clear your cookies |
| DIAG_ADMIN_SESSION | Keeps Diagonalise staff signed in to our admin area. Only set for our staff, and only on admin pages. | Until the browser is closed or they sign out |
All of these are first-party cookies: they are set by Diagonalise, and only Diagonalise can read them.
Other services
- Payments: when you upgrade, you pay on Stripe's secure checkout pages. Stripe may set its own cookies there to process payments and prevent fraud. See Stripe's cookie policy.
- Page resources: our styles, fonts and scripts load from jsDelivr, a content delivery network. It does not set cookies on Diagonalise, but it receives your IP address to deliver the files.
- AI apps: if you connect an AI assistant, its website and apps use cookies under that provider's own policy.
Managing cookies
You can block or delete cookies in your browser settings. If you block the cookies above, you will not be able to sign in to Diagonalise, and Making Tax Digital submissions will not work.
Changes and questions
If we ever add cookies that are not strictly necessary, such as analytics, we will update this policy and ask for your consent first. Questions? Email support@diagonalise.com.