GA4 offline conversions via Measurement Protocol

What this is

The GA4 Measurement Protocol records offline conversions by sending events directly to Google Analytics servers over HTTP, stitching a later server-side, CRM, or point-of-sale interaction back to the browser session that produced it. Recording offline conversions is a named primary use case; the protocol augments automatic gtag, Tag Manager, and Firebase collection rather than replacing it, and a server-to-server-only setup yields only partial reporting.

Endpoints

Events posted to /debug/mp/collect do not appear in reports.

Query parameters (both required)

Request body — top-level fields

The offline conversion is a POST body of JSON. consent is request-scoped and sits alongside client_id and events.

Event params — offline purchase

Ecommerce fields and session-join fields both travel inside events[].params.

Sending a new session_id value creates a new session without a session_start event — so an absent or mismatched session_id spawns an unintended new session instead of joining the original one.

Identity stitching — capture at the online touchpoint

Persist both signals with the order at the moment of the web interaction, then send them back verbatim on the offline event.

The _ga cookie is formatted GA1.1.<clientId1>.<clientId2>; the actual client_id is only the middle two dot-separated numbers, not the leading GA1.1 or trailing portion. Note: the session_id to ga_session_id 1:1 mapping, and the need to send session_id (not ga_session_id), is documented by practitioners (Simo Ahava) rather than spelled out on the canonical Measurement Protocol reference page.

Timestamp resolution

Priority order MP uses to resolve the event time:

validation_behavior and out-of-window timestamps

Confirm the exact per-mode behavior for out-of-window timestamps against the current documentation before relying on it in code.

Consent object

If consent is omitted, GA uses the consent settings from the client’s corresponding online interactions. How that inheritance resolves when there is no matching prior online interaction for a purely offline client_id is not detailed in the docs.

transaction_id deduplication rules

The docs do not publish an explicit deduplication time window (how far apart two identical IDs can arrive and still dedupe).

Limits

Validation server response

POST the identical payload to /debug/mp/collect. The response carries a validationMessages array; each item has fieldPath, description, and validationCode.

The validation server does NOT validate api_secret or measurement_id — a request can pass validation yet still silently drop in production if those credentials are wrong.

Gotchas

1. Offline conversion lands as a new (direct) session, not the original. Cause: the request omitted client_id/session_id or sent a session_id that matches no existing session, and any new session_id creates a fresh session. Fix: capture the exact client_id and session_id at the online touchpoint, persist them with the order, and send both back verbatim.
2. Purchase appears nowhere despite HTTP 2xx or a passing /debug check. Cause: /mp/collect returns 2xx regardless of a bad api_secret or measurement_id, and the debug server does not check those — or the timestamp_micros is beyond 72h. Fix: triple-check both credentials against the data stream, keep the timestamp inside 72h, and confirm microseconds.
3. Revenue is double-counted between the browser and the server send. Cause: GA only collapses purchases sharing an identical transaction_id; different, missing, or empty IDs are both counted. Fix: use the same real order ID on both sends and never send transaction_id="".
4. client_id is rejected or fails to match. Cause: the raw _ga cookie value (GA1.1.<clientId1>.<clientId2>) was used instead of the middle two numbers. Fix: strip the GA1.1. prefix to <clientId1>.<clientId2>, or prefer the gtag get() API to avoid fragile cookie parsing.
5. Older events silently land with a recent timestamp or drop entirely. Cause: timestamp_micros beyond 72h — RELAXED clamps to ~72h ago, ENFORCE_RECOMMENDATIONS rejects. Fix: send within 72h (ideally received within 48h); do not rely on MP for long-delayed backfills.
6. Ad exports or audiences omit MP conversions, or consent looks wrong. Cause: consent was omitted or DENIED; when omitted, GA inherits the client’s online consent state, which may not match the offline event. Fix: set request-level consent explicitly to reflect the actual consent.

Quick recipes

// At the online touchpoint (checkout / lead form), capture the two identifiers
// GA4 needs to stitch a later offline conversion back to THIS session.
gtag('get', 'G-XXXXXXX', 'client_id', (clientId) => {
  // session_id is session-scoped state in the _ga_<MEASUREMENT_ID> cookie
  const sessionId = readSessionId('_ga_XXXXXXX');
  persistWithOrder({ client_id: clientId, session_id: sessionId });
});

// _ga is formatted "GA1.1.<clientId1>.<clientId2>".
// client_id is ONLY the middle two dot-separated numbers.
function clientIdFromGa(gaCookie) {
  // "GA1.1.123456789.1721470000" -> "123456789.1721470000"
  return gaCookie.split('.').slice(2).join('.');
}

curl -s "https://www.google-analytics.com/mp/collect?measurement_id=G-XXXXXXX&api_secret=API_SECRET" \
  -H "Content-Type: application/json" \
  -d '{
    "client_id": "123456789.1721470000",
    "timestamp_micros": 1721480000000000,
    "consent": { "ad_user_data": "GRANTED", "ad_personalization": "GRANTED" },
    "events": [{
      "name": "purchase",
      "params": {
        "session_id": "1721470000",
        "engagement_time_msec": 1,
        "transaction_id": "ORDER-10492",
        "currency": "USD",
        "value": 129.00,
        "items": [{ "item_id": "SKU_88", "item_name": "Annual plan", "price": 129.00, "quantity": 1 }]
      }
    }]
  }'

// Unix MICROSECONDS, not milliseconds. Stay inside the 72h backdate window.
const conversionMs = orderCompletedAt.getTime();        // epoch milliseconds
const timestamp_micros = conversionMs * 1000;           // -> microseconds
const withinWindow = Date.now() - conversionMs < 72 * 3600 * 1000;

curl -s "https://www.google-analytics.com/debug/mp/collect?measurement_id=G-XXXXXXX&api_secret=API_SECRET" \
  -H "Content-Type: application/json" -d @payload.json
# pass = {"validationMessages":[]}
# NOTE: this endpoint does NOT check api_secret or measurement_id.

Related

• Hub: Server-side guides
• GA4 Measurement Protocol v2 reference
• GA4 ecommerce events reference

Sources

Official pages these values were read from:

• Measurement Protocol overview (read 2026-07-20)
• Send Measurement Protocol events (web/gtag) (read 2026-07-20)
• Send Measurement Protocol events (read 2026-07-20)
• Measurement Protocol reference (web/gtag) (read 2026-07-20)
• Measurement Protocol reference (read 2026-07-20)
• Validate Measurement Protocol events (read 2026-07-20)
• Minimize duplicate key events with transaction IDs (read 2026-07-20)

Community references for the _ga cookie format and the session_id join, which the canonical MP pages do not spell out:

• Session attribution with GA4 Measurement Protocol — Simo Ahava (read 2026-07-20)
• Understanding GA4 cookies — the _ga cookie (Optimize Smart) (read 2026-07-20)