GTM. Client-server tag order. Fire web tags before server routes the same event.

What this means for your store

Hybrid tracking fails quietly when the server container fires before the browser sets identifiers. In a client-server setup, the web container runs first-party browser tags and forwards hits to server GTM. Tag sequencing on the web side ensures GA4 Configuration and event_id assignment finish before the transport tag sends data upstream.

Scenario on a real storefront

Purchase fires on a slow mobile thank-you page. Without sequencing, the HTTP transport tag can beat GA4 Configuration - server CAPI arrives without _fbp or a stable event_id:

Web container:
  Tag: GA4 - purchase
    Sequencing: setup tag "GA4 Configuration" fires before
    Event parameter event_id = {{JS - event_id}}

  Tag: sGTM - HTTP request (purchase)
    Sequencing: setup tag "GA4 - purchase" fires before
    URL: https://collect.brand.co.uk/g/collect
    Body: {{DLV - ecommerce}} + event_id

Server container:
  Client: GA4
  Tag: GA4 → Google (forward)
  Tag: Meta CAPI (uses same event_id from incoming request)

What to do next

• If server tags run before browser tags set _fbp or event_id, CAPI match rates drop sharply.
• Build one sequencing chain per event - chaining unrelated tags adds checkout latency.
• Load-test the thank-you page on 3G; each blocked sequencing step adds milliseconds under slow networks.

Bottom line

Order matters in client-server GTM. Sequence web tags so identifiers and cookies exist before the server container forwards the hit to Meta and Google.