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Add a cookie consent banner to Astro

c15t puts the banner in your Astro site as plain Astro markup. It holds tags until the visitor chooses and keeps Google Consent Mode v2 in sync.

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Sample Consent banner with c15t’s English strings

Install c15t in an Astro site

c15t is the open source consent library behind Inth Consent. The banner is plain .astro markup and ships no framework JavaScript. Each choice goes to your Inth project.

npm install c15t @astrojs/svelte svelte @c15t/integrations

The preference dialog is an island. This setup renders it with Svelte.

For React, install @astrojs/react react react-dom and register react(). For Vue, install @astrojs/vue vue and register vue(). Use the framework your site already ships.

Add your Inth project URL to .env. Copy it from your Inth dashboard.

.env

PUBLIC_C15T_BACKEND_URL=https://your-project.inth.app

Code on this page comes from the c15t Astro quickstart, checked 28 October 2026.

Pick static or server output

On a static host, use hosted(). The browser asks your Inth backend for the policy, so you need no server.

astro.config.mjs

import svelte from '@astrojs/svelte';
import { defineConfig } from 'astro/config';
import c15t, { hosted } from 'c15t/astro';

export default defineConfig({
  integrations: [svelte(), c15t({ mode: hosted() })],
});

With a server adapter, c15t() with no mode bundles your policy into the server build. The first HTML then contains the banner, or no banner for a visitor who already chose.

astro.config.mjs

import node from '@astrojs/node';
import svelte from '@astrojs/svelte';
import { defineConfig } from 'astro/config';
import c15t from 'c15t/astro';

export default defineConfig({
  adapter: node({ mode: 'standalone' }),
  integrations: [svelte(), c15t()],
  output: 'server',
});

Rebuild after you change a policy. Use the adapter for your host in place of @astrojs/node.

Hold scripts until consent

List the scripts in src/c15t.client.ts. The integration finds this file without config.

src/c15t.client.ts

import { posthog } from '@c15t/integrations/posthog';
import type { C15tClientOptionsExtension } from 'c15t/astro';

export default {
  scripts: [
    posthog({
      id: 'phc_your_project_key',
      initOptions: { cookieless_mode: 'never' },
      loadMode: 'after-consent',
    }),
  ],
} satisfies C15tClientOptionsExtension;

Keep vendor helpers out of astro.config.mjs. The integration writes its options into the page as JSON, and c15t() stops with an error if an option holds a function. Remove every other loader for PostHog, so it loads once and only through c15t.

Astro scripts and embeds covers more vendors, iframes and the network blocker. The c15t integrations list the helper for each vendor.

Add the banner to your layout

Wrap every page in a layout with the consent components. ConsentScript goes in <head> and puts the banner’s styles in the page, so you import no stylesheet.

src/layouts/base.astro

---
import { ClientRouter } from 'astro:transitions';
import {
  ConsentBanner,
  ConsentDialog,
  ConsentDialogLink,
  ConsentScript,
} from 'c15t/astro/components';

interface Props {
  title: string;
}

const { title } = Astro.props;
---

<html lang="en">
  <head>
    <meta charset="utf-8" />
    <meta content="width=device-width, initial-scale=1" name="viewport" />
    <title>{title}</title>
    <ConsentScript />
    <ClientRouter />
  </head>
  <body>
    <slot />
    <footer>
      <ConsentDialogLink>Privacy settings</ConsentDialogLink>
    </footer>
    <ConsentBanner />
    <ConsentDialog />
  </body>
</html>

ClientRouter is optional. With it, the consent runtime stays alive across page navigation. Without it, each page load starts the runtime again from the stored choice.

Add consent to your React, Vue or Svelte islands

With Astro islands, you can add interactive components to a static page. You can use c15t with them too.

The preference dialog is an island in Svelte, React or Vue. Your own islands can read consent and open the dialog.

Use the framework that your site already ships, so visitors do not download a second framework. For a pure Astro site, we recommend Svelte, because it adds the least JavaScript to the page. Svelte is also the default.

This React island shows the measurement permission and opens the preference dialog. The code comes from the c15t docs.

src/components/consent-status.tsx

import { getConsentClient, openDialog } from 'c15t/astro/client';
import { ConsentProvider, useConsent } from 'c15t/react';

const MeasurementStatus = () => {
  const allowed = useConsent('measurement');
  return (
    <p>
      <span data-testid="measurement-status">
        Measurement is {allowed ? 'allowed' : 'not allowed'}.
      </span>{' '}
      <button
        onClick={() => {
          void openDialog();
        }}
        type="button"
      >
        Change cookie settings
      </button>
    </p>
  );
};

const ConsentStatus = () => {
  // The page's consent runtime, which the c15t integration started.
  const runtime = getConsentClient()?.runtime;
  if (!runtime) {
    return null;
  }
  return (
    // `colorScheme: null` leaves dark mode to the integration.
    <ConsentProvider options={{ colorScheme: null }} runtime={runtime}>
      <MeasurementStatus />
    </ConsentProvider>
  );
};

export default ConsentStatus;

Render the island only in the browser, where the consent runtime is:

src/pages/index.astro

<ConsentStatus client:only="react" />

Give each island the runtime of the page from getConsentClient()?.runtime. A second runtime keeps its own copy of consent and does not stay in step with the banner. Call openDialog() from c15t/astro/client to open the preference dialog.

Svelte islands use ConsentProvider from c15t/svelte. Vue islands use the c15tVue plugin from c15t/vue/vue-plugin. Astro islands covers each one.

For the framework setup, see the Svelte, Vue and React guides.

src/consent-scripts.ts

import { gtag } from '@c15t/integrations/google-tag';

export const scripts = [gtag({ id: 'G-XXXXXXXXXX', category: 'measurement' })];

Show the right banner in each region

Your Inth project sets a policy for each region, such as opt in for the EU and UK or opt out for California. With hosted(), the browser asks your backend for the policy, and the backend reads the visitor’s location from that request. Regional policies need no extra code.

With server output, the server resolves each visitor from the bundled policy and the request headers instead.

Test each region before launch. See how regional policies work.

When the banner appears

With server output, the first HTML contains the banner. With static output, the banner appears after the browser gets the policy from your backend. In both cases, the banner ships no framework JavaScript.

Measure your own pages before and after you add the banner.

Check that consent gates work

Build the site, start the preview and open a private window with the Network tab open. Test under an opt in policy, such as the EU policy.

  1. Under an opt in policy, no PostHog request leaves the page before a choice.
  2. After Reject All and a reload, PostHog stays off.
  3. When you turn on measurement in Privacy settings, PostHog loads.
  4. When you turn measurement off, the page reloads without PostHog.

Choices post to your Inth project. Check that Inth has a record for each one. The c15t verify consent guide has the full release checklist.

Use Inth in production

c15t is open source. Point it at your Inth project, which hosts the backend and keeps the consent records. See Inth open source.

c15t also has a local mode with no backend. It keeps no consent records, so the docs advise against it in production.

Want to see what else your repo sends before consent? Run a free codebase scan.

Cookie consent banners for other frameworks

The same banner and the same Inth project work in other stacks. See the setup for Next.js, React, TanStack Start, Nuxt, Vue, Svelte, SvelteKit, HTML and JavaScript.

See how the Inth consent banner works.

Questions about Astro consent