Tag Manager / Google tag

Choose the simplest thing that works

GTM is not "more professional"; it's another place where tracking can silently break and where nobody reviews changes. Don't add it to a codebase that already emits clean events.

Audit a live site

In a real browser (not curl — bot protection hides the tags):

1. View source / DevTools → find googletagmanager.com/gtag/js?id=… and googletagmanager.com/gtm.js?id=GTM-…. Note every ID.
2. Console → run ${CLAUDE_PLUGIN_ROOT}/scripts/check_gtag.js. It lists config calls, events already pushed, and flags page_location values that carry query strings.

3. Look for duplicate configs (GA4 configured both in GTM and hard-coded → double page views) and DOM scraping (Custom JS variables that read "Amount: …" text to build a purchase value). Replace scraping with a dataLayer.push from the server-rendered page:

   dataLayer.push({ event: 'purchase', ecommerce: { transaction_id: 'order-123',
     value: 160000, currency: 'VND', items: [/* … */] } });

4. Tag Assistant (tagassistant.google.com) confirms which destinations receive hits.

GTM setup, if warranted

• One container per site. Publish with a version name and notes ("GA4 purchase from dataLayer").
• Google tag (GA4) with the measurement ID; set page_location via a Custom JS variable that applies the same whitelist as conversion-tracking §1 (keep gclid/utm only).
• Conversion Linker tag on all pages (for Ads).
• Triggers on custom events (purchase, sign_up), never on "Page View of /thank-you" when a server event is available.
• Consent: if the site serves EU/UK users, add Consent Mode v2 defaults before any tag.

Hand-off note

Record, in the repo docs, every ID (GTM-, G-, AW-), which account owns it, and which event feeds which conversion action. The next person will otherwise re-discover it by trial.