Server-Side Conversion Tracking

Browser pixels lose a large and unpredictable share of conversions to iOS tracking prevention, ad blockers, cookie lifetime limits and cross-domain hops. Server-side reporting fixes the reporting, which is what the ad platform's bidding model learns from. This skill covers the model, the setup order and how to verify it.

When to use

• Ad platform reports fewer purchases than the store/database actually recorded
• CPA looks like it got worse right after a tracking change, with no change in real sales
• Setting up a new funnel that will receive paid traffic
• Asked about CAPI / Events API / offline conversion import / click id passthrough
• Attribution disagreements between platforms ("Facebook claims 40 sales, Google claims 30, we had 45 orders")

The model, in the order it must be built

Getting this order wrong is the usual reason a "server-side setup" still under-reports.

1. Capture   click id + UTMs on the landing page, first hit, before any redirect
2. Persist   attach them to the visitor's session, server-side
3. Carry     keep them across every funnel step, including cross-domain hops
4. Attach    write them onto the order record at purchase
5. Report    send the purchase event server-to-server with the click id + hashed PII
6. Dedupe    give the browser event and the server event the same event id
7. Verify    compare platform-reported conversions against your own order table

Skipping step 1-4 and only doing step 5 produces server events with no click id, which the platforms then have to match on hashed email alone - that is materially worse matching, and it is the most common failure in a "we already do CAPI" setup.

Step 1-2: capture and persist

Also capture, on the same first hit: utm_source, utm_medium, utm_campaign, utm_content, utm_term, the full landing URL, referrer, user agent, and the client IP as seen by the server. Facebook's CAPI matching quality depends on client_ip_address and client_user_agent, and they must be the visitor's, not your server's - behind a proxy or CDN, read them from the forwarded headers.

Store server-side, keyed to a first-party session. Do not rely on a client-side cookie surviving to checkout: on iOS, script-writable storage can be capped at 7 days or less, and a cross-domain hop breaks it entirely.

Step 3: carry across steps

• Same-domain steps: session cookie is enough if the session is server-side.
• Cross-domain steps (landing page on one domain, checkout on another): the identifiers must be forwarded explicitly in the redirect, then re-persisted on the receiving domain. This is where most funnels silently lose attribution.
• Redirect chains: every hop must preserve the query string. A tracking redirect that drops ?fbclid=... destroys attribution for that entire campaign.

Step 4: attach to the order

The order record must carry the click ids, UTMs and landing URL. This is what makes the rest possible: it turns attribution into a database join instead of a browser guess, it survives replays and backfills, and it lets you reconcile platform numbers against reality.

Step 5: report server-to-server

Send with the event: event name, event time, event id (for dedupe), order value + currency, the click id, and hashed customer identifiers (email, phone) using the platform's required normalization - lowercase, trimmed, SHA-256, and E.164 for phone numbers. Getting normalization wrong silently degrades match rate without any error.

Send from a queue with retries, not inline in the checkout request. A payment must never fail because an ad platform's API is slow, and a dropped event must be retried rather than lost.

Step 6: dedupe

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