analytics-tracking-testing

Tracking that "looks fine" in GA4 DebugView still drops events silently after a refactor, sends the wrong currency, or double-counts a purchase. Reading `window.dataLayer` or asserting the button is `toBeVisible()` proves nothing — the beacon may never leave the browser. This skill makes you intercept the real network beacon (`google-analytics.com/g/collect`, `facebook.com/tr`, `analytics.tiktok.com`, `px.ads.linkedin.com`), parse the event name and parameters out of the request, and assert them against a tracking plan that is a typed contract — then gate that contract in CI so a dropped event fails the build.

Quick Route

Situation	Go to
Assert one GA4 event fired with correct params	Intercepting GA4 beacons
Treat the tracking plan as a validated contract	The Tracking Plan Is the Contract
Verify dataLayer.push shape (not the beacon)	Asserting dataLayer.push
Pixel + server CAPI deduplication (event_id)	Pixels and Server-Side Deduplication
Events must/mustn't fire by consent state	Consent Mode v2 Gating
Capture every pixel in one reusable fixture	references/capture-fixture.md
News-media: article-view, scroll-depth, paywall	News-Media Events
Fail the build on a tracking regression	Regression-Gating in CI
Hundreds of events, many domains, small team	Buy vs Build

Discovery Questions

First: check .agents/qa-project-context.md in the project root and skip anything it already answers (stack, tag manager, consent platform, target environments).

• Which tracking destinations are live? GA4 (/g/collect), Meta Pixel (facebook.com/tr), TikTok (analytics.tiktok.com), LinkedIn (px.ads.linkedin.com), ad-tech tags — each has a different endpoint grammar, so the capture helper must know them all.
• GTM or hardcoded gtag? GTM means the truth flows through window.dataLayer.push first, then GTM fires the beacon. You may assert at the push layer (input contract) AND the beacon (output contract); they are different tests.
• Is there a tracking plan? If not, build one first — it is the contract everything else validates against. No plan means no objective pass/fail.
• Consent platform and default consent state? Consent Mode v2 changes which beacons are even allowed to fire before consent. You need the CMP's accept/reject selectors to drive the test.
• Client + server (CAPI) dedup in play? If purchases fire both client Pixel and server Conversions API, the event_id must match or you double-count. This is the correctness property, not "did fbq run."
• News-media surface? Article pages add article-view, scroll-depth thresholds (25/50/75/100), and paywall-meter events that generic e-commerce plans miss.

---

Core Principles

1. Intercept the beacon, never trust the DOM or dataLayer alone. A button click that updates the DOM, or a dataLayer.push that GTM silently drops, leaves no GA4 hit. The only proof an event was sent is the outbound request to the collect endpoint. Assert on the network beacon's URL params; reading window.dataLayer via page.evaluate only proves the push happened, not that anything left the browser.

2. The tracking plan is a typed contract, not a comment. Every expected event lives in an external schema (JSON/YAML/TS interface) with required params and their types. Tests validate captured events against it and report missing params and type violations. Hardcoding one expected value inline asserts nothing about the other twenty params and rots on the first plan change.

3. De-duplication is the real correctness property for purchases. Checking that fbevents.js loaded or counting that fbq('track','Purchase') ran misses double-counting entirely. The property that matters: the browser Pixel and the server CAPI send the same event_id/eventID so Meta collapses them into one conversion.

4. Consent state is an input dimension. Test the approved regional policy in basic or advanced mode. Observe the application's actual four-signal default and updates; do not supply a default from the test. Basic mode blocks tags before consent, while advanced mode may emit permitted measurements without cookies. Assert the planned post-accept events and relevant storage independently.

5. Drive real user actions and wait on requests, never the clock. Scroll depth fires from actual scrolling (mouse.wheel, scrollIntoView, evaluate(scrollTo)), and you wait for the beacon with waitForRequest, not waitForTimeout. A fixed sleep is flaky and hides the very timing bug you should catch.

6. A tracking test that can't fail the build is theater. "Check it in GA4 DebugView" and "monitor production" never block a regression. The contract must run in CI and exit non-zero when an event drops or a required param goes missing.

---

Intercepting GA4 Beacons

GA4 web tags use /g/collect requests, sometimes with several events in a POST body and common fields in the query. The following parser handles an observed web-tag format, not the public JSON Measurement Protocol. Retain captures from the deployed tag version; first-party proxy and server-side collection need explicit adapters. A request proves browser emission, not successful downstream ingestion.

GA4 /g/collect URL grammar you assert on:

Param	Meaning	Example
v=2	Version field in the observed web-tag transport	v=2
tid=G-XXXXXXX	Measurement ID	tid=G-ABC123
en=	Event name	en=add_to_cart
ep.<name>=	Custom event parameter, string type	ep.campaign_label=spring
epn.<name>=	Event parameter, number type	epn.value=49.99
gcs= / gcd=	Consent state (see Consent Mode v2 section)	gcs=G111

Preserve wire type information. Custom parameters can use ep. or epn., but reserved ecommerce fields and items have their own web-tag encodings. Do not assume standard currency is ep.currency or an item ID is a flat ep.item_id. Google's logical ecommerce contract uses event-level currency and nested items[]; decode the observed transport through a capture-tested adapter before checking that contract.

Intercept with page.waitForRequest (single expected event) or page.on('request', ...) (collect many). Attach before the action. Match the exact /g/collect path and an approved hostname. Merge each POST event's parameters over the URL query with parseGA4 from references/ga4-interception.md; URL-only matching misses batched events. Use a test property or a local collector so tests do not pollute production analytics.

Pattern using the parser below plus your application's verified ecommerce decoder and logical tracking plan. decodeEcommerceWebEvent is a required project adapter, not a package supplied by this skill; unsupported or missing wire fields must fail. See references/ga4-interception.md and references/tracking-plan.md.

test('add_to_cart fires with correct params', async ({ page }) => {
  await page.goto('/product/42');
  const [request] = await Promise.all([
    page.waitForRequest(r => parseGA4(r).some(event => event.name === 'add_to_cart')),
    page.getByRole('button', { name: 'Add to cart' }).click(),
  ]);
  const events = parseGA4(request).filter(event => event.name === 'add_to_cart');
  expect(events).toHaveLength(1);
  const params = events[0].params;
  expect(params.tid).toBe('G-TEST'); // use the test property's exact measurement ID
  // Application adapter, checked against redacted captures from the deployed tag.
  const payload = decodeEcommerceWebEvent(params);
  expect(validateEvent(plan, 'add_to_cart', payload)).toEqual([]);
  expect(payload).toMatchObject({
    currency: 'USD', value: 49.99,
    items: [{ item_id: 'SKU-42', price: 49.99, quantity: 1 }],
  });
});

Never substitute page.evaluate(() => window.dataLayer) as the only assertion, toBeVisible() on the button, or waitForTimeout() to "let the beacon send." See references/ga4-interception.md for batched-event parsing (GA4 can pack multiple events into one POST body) and region-endpoint handling.

---

The Tracking Plan Is the Contract

A tracking plan is the source of truth: for every event, its name, required params, and each param's type. Keep it as a versioned file (tracking-plan.json / .yaml, or a TS interface/Zod schema) that both the app team and the tests import. Tests read the captured event and validate it against the plan — they check the schema and the known scenario values.

Validate the normalized logical payload and return every missing field and type violation. The helper in references/tracking-plan.md checks finite numbers, nonempty strings and nested nonempty item arrays. It rejects unsupported or empty schemas. Zero is a valid number; an absent field, empty string, null, Infinity or a numeric string is not a typed zero. Add scenario-specific expected values after schema checks.

const payload = decodeEcommerceWebEvent(capturedGA4Event.params); // capture-tested project adapter
expect(validateEvent(plan, 'add_to_cart', payload)).toEqual([]);
expect(payload.currency).toBe('USD');
expect(payload.value).toBe(49.99);
expect(payload.items).toEqual([{ item_id: 'SKU-42', price: 49.99, quantity: 1 }]);

A matching schema cannot prove that a price is correct. Compare the payload with the known test cart, then assert event counts at the application's tracking completion boundary. Keep raw input events and raw emitted records so a decoder cannot hide a missing field, a tag transformation or a duplicate purchase.

---

Asserting dataLayer.push

When the question is specifically the GTM input — "is the right object pushed to dataLayer when the page loads?" — assert the push, not the GA4 beacon. This is the inverse of beacon interception: here the dataLayer push IS the target.

Install the passive observer from references/datalayer-capture.md before navigation. Leave an absent data layer absent, observe the application's first assignment, preserve push behavior and flag later replacement or lost observation. Do not initialize or repair the application queue in a test intended to measure the application's setup. Use a completion signal from the application before inspecting the captured objects.

For GTM ecommerce object events, assert ecommerce.currency, ecommerce.value, and ecommerce.items[] with each item's expected ID, price and quantity. Currency is at event level in Google's ecommerce contract. Check counts as well as shape; selecting the first matching event conceals duplicates. Direct gtag command arrays need the corresponding command adapter rather than an object-event parser.

The reference contains the observer and full example. Use both observed input and network output when the question concerns the whole tracking path, while recording what each observation establishes.

---

Pixels and Server-Side Deduplication

Marketing pixels send their own beacons. Endpoints to intercept:

Destination	Endpoint	Event param	Dedup key
Meta Pixel	facebook.com/tr (also /tr?)	ev=PageView, ev=Purchase	eid / event_id
TikTok	analytics.tiktok.com	event in body/params	event_id
LinkedIn	px.ads.linkedin.com	conversion id	—

For Meta, asserting that connect.facebook.net/en_US/fbevents.js loaded, or that fbq('track','Purchase') ran, is a load/count check — it does not prove correctness. The correctness property for a Purchase that fires both client-side (Pixel) and server-side (Conversions API / CAPI) is deduplication: both must carry the same event_id so Meta merges them into one conversion instead of double-counting.

Test it: capture the browser facebook.com/tr beacon for ev=Purchase, read its event_id, and assert it equals the event_id your server sent to CAPI (from a mocked/captured server call or a known fixture value). Skeleton:

const [pixel] = await Promise.all([
  page.waitForRequest(r => r.url().includes('facebook.com/tr') && r.url().includes('ev=Purchase')),
  completeCheckout(page),
]);
const clientEventId = new URL(pixel.url()).searchParams.get('eid'); // event_id on the wire
expect(clientEventId).toBe(serverCapiEventId); // deduplicates against the server CAPI event

See references/pixels-and-dedup.md for parsing TikTok/LinkedIn payloads and capturing the server CAPI call.

---

Consent Mode v2 Gating

Choose the approved regional defaults and basic or advanced implementation before writing a gate. The examples in the reference use an all-denied default policy. See Google's consent-mode overview.

Observe all four signals against that policy:

Signal	Governs
ad_storage	Advertising cookies
analytics_storage	Analytics cookies
ad_user_data	Sending user data to Google for ads
ad_personalization	Personalized ads / remarketing

Install the passive observer before navigation in every test. It must not create gtag, send a consent command, or repair a replaced data layer. A missing default, an early measurement command, or an incorrect intermediate state must fail.

Before consent: basic mode blocks the configured tag requests. Advanced mode can send the event plan's denied-state pings. Observe a real application completion signal and any declared observation interval; an empty collector or networkidle is not proof that delayed tracking is correct. Basic defaults can be initialized after acceptance.

After accept: checkpoint existing commands and events, click the CMP button, and require a new matching update plus the newly expected beacon. Repeat independently for rejection, partial consent, withdrawal, and persistence.

In the captured web-tag transport, gcs=G100 and gcs=G111 distinguish the two storage signals. gcd presence is a diagnostic, not proof that all four signals or payload identifiers are correct. Keep versioned transport captures and use the four observed states plus the approved payload/storage inventory. Do not rely on an undocumented gcd prefix. See references/consent-mode.md for the executable helpers and recipes.

Whether the law permits a beacon under a given consent state is compliance-testing. This skill asserts that when a beacon fires, its data and consent params are correct.

---

News-Media Events

News and publisher sites have a tracking surface generic e-commerce plans miss. Cover all three:

• article_view (or article-view) on article load — assert the beacon fires once with article metadata (id, section, author).
• scroll-depth at the 25 / 50 / 75 / 100 percent thresholds — one event per bucket, driven by real scrolling.
• paywall / meter — a paywall_hit (or meter) event when the free-article meter is exhausted.

Drive scroll with actual actions — mouse.wheel, element.scrollIntoView, or page.evaluate(() => window.scrollTo(...)) / scrollBy — and wait on the beacon (waitForRequest or your captured-events list), never waitForTimeout. Scrolling in fixed sleeps both flakes and masks threshold-timing bugs.

const buckets = [25, 50, 75, 100];
for (const pct of buckets) {
  const [req] = await Promise.all([
    page.waitForRequest(r => r.url().includes('/g/collect') && r.url().includes('en=scroll')),
    page.evaluate(p => window.scrollTo(0, document.body.scrollHeight * (p / 100)), pct),
  ]);
  expect(new URL(req.url()).searchParams.get('epn.percent_scrolled')).toBe(String(pct));
}

Full suite (article_view metadata, the four scroll buckets de-duplicated, and the paywall-meter event) in references/news-media.md.

---

Multi-Pixel Capture Fixture

Don't re-implement beacon capture in every test. Build one reusable Playwright fixture (test.extend) that listens with page.on('request', ...), matches all destination endpoints (GA4 /g/collect, facebook.com/tr, analytics.tiktok.com, px.ads.linkedin.com), parses each request's new URL(...).searchParams and postData(), and pushes a normalized { destination, eventName, params } onto a collected array tests assert against.

Critical trap: a capture helper must observe, so use page.on('request') (or page.route followed by route.continue()), never page.route(...).abort() — aborting blocks the very beacons you are trying to see. And it must capture pixels too, not GA4 only. Full fixture in references/capture-fixture.md.

---

Regression-Gating in CI

The gate diffs captured events against the tracking-plan baseline and fails the build (non-zero exit) when a previously-firing event stops firing or a required param goes missing after a release. Structure it as a Playwright project that runs the journeys, captures every beacon via the fixture, and validates each against the plan; on any missing/dropped event the test expect fails, Playwright exits non-zero, and the GitHub Actions (or any CI) job goes red.

Do not make the gate "check GA4 DebugView manually," "only run against production traffic," or "warn but pass anyway" — none of those block a regression. The diff-against-baseline run belongs in PR CI, before merge. See references/ci-gating.md for the workflow YAML, the baseline-diff script, and how to surface missing-param failures in the job summary.

---

Buy vs Build

DIY Playwright interception is the right call for a bounded set of events on a few critical journeys gated in CI. It stops paying off at scale: hundreds of events across many domains, with a small team, and a need for continuous production / drift monitoring (catching a tag a marketer breaks in GTM at 2am, which a pre-merge CI gate never sees).

Dimension	Build (Playwright)	Buy
Few events, key journeys, pre-merge gate	Best fit	Overkill
Hundreds of events, many domains	Maintenance crushes you	Buy
Continuous 24/7 production drift monitoring	Out of scope for CI	Buy
Small team, high coverage demand	Build cost too high	Buy

Current live paid options worth naming:

• Trackingplan — always-on/continuous monitoring of live traffic across web, mobile, and server-side; strongest when you need real-time drift detection rather than scheduled checks.
• ObservePoint — scheduled scans/audits of journeys against a tracking plan; mature for periodic governance.

Don't recommend Segment Protocols as the only validation (it governs data flowing through Segment, not arbitrary client beacons), and never call Google Tag Assistant a CI gate — it is an interactive debug tool, not an automated pass/fail. Tie the decision to scale, many domains, maintenance burden, and continuous monitoring — the axes where DIY stops paying off.

---

Anti-Patterns

1. Asserting window.dataLayer (or the DOM) instead of the beacon

page.evaluate(() => window.dataLayer) proves a push happened, not that GA4 sent anything; toBeVisible() proves nothing about tracking. Intercept the /g/collect request and assert its en= and ep./epn. params. (The one exception: when the push itself is the contract — see dataLayer.push — but then never reach for the GA4 beacon instead.)

2. waitForTimeout to "wait for the event to fire"

A fixed sleep flakes and hides timing bugs. Wait on the request: page.waitForRequest(r => r.url().includes('/g/collect')).

3. Asserting only the event name

Name-only assertions pass while currency, value, and item_id are wrong. Validate every required param and its type against the tracking plan.

4. Hardcoding expected values inline with no plan file

There is no source of truth, so nothing catches a renamed param across the suite. Keep the plan external and validate against it.

5. Load/count checks for pixels (fbevents.js loaded, fbq ran)

Neither proves the conversion is correct or de-duplicated. For Purchase, assert the client and server event_id match.

6. Testing a default supplied by the test

Seeding denied consent can hide an application that omits its own default. Observe all four real signals against the selected policy, and distinguish basic from advanced.

7. page.route(...).abort() inside a capture helper

Aborting blocks the beacons you meant to observe. Use page.on('request') (or route.continue()).

8. A "gate" that can't fail the build

GA4 DebugView, production-only monitoring, or warn-but-pass do not stop a regression. Make CI exit non-zero on a dropped event or missing required param.

---

Verification

Smallest check first — prove the suite actually catches a regression, don't just trust a green run:

• One event, one beacon: npx playwright test -g add_to_cart passes, and its trace (--trace on) shows the captured /g/collect request with en=add_to_cart. A green test with no matching request in the trace means you asserted on the wrong thing.
• The gate can fail: rename a required param in a branch (e.g. value → amount in the app) and re-run the CI project — the build must exit non-zero with a missing/violation message. If it still passes, the gate is theater.
• Consent gate detects the wrong app state: the real default matches the selected policy; missing or granted defaults fail an all-denied-policy fixture. A new action requires a new update and planned event. Both healthy and deliberately broken basic and advanced fixtures produce the expected result.
• Dedup holds: the client eid from facebook.com/tr?ev=Purchase equals the server CAPI event_id for the same order. Different values mean Meta will double-count.

Done When

• Each tracked event has a test that intercepts the real collect-endpoint beacon (/g/collect, facebook.com/tr, etc.) and parses en=/ev= plus params from searchParams/postData — no DOM-only or dataLayer-only assertions.
• A versioned tracking plan file exists (JSON/YAML/TS) with required params and types; tests validate captured events against it and report missing/type violations, not just the event name.
• Purchase (or any client+server event) has a deduplication assertion: client beacon event_id equals the server CAPI event_id.
• Consent tests observe all four actual signals, require the configured events, and check storage against the approved regional/basic/advanced policy. They never seed the consent state they are meant to verify. Transport diagnostics have stated limits.
• A reusable capture fixture (test.extend) collects GA4 + Meta + TikTok + LinkedIn beacons; no test re-implements capture and no capture helper calls route.abort().
• News-media surfaces (if present) cover article_view, scroll-depth at 25/50/75/100 driven by real scroll actions, and a paywall/meter event — all using waitForRequest, no waitForTimeout.
• A CI job runs the suite, diffs captured events against the tracking-plan baseline, and exits non-zero on a dropped event or missing required param — verified by a deliberately-broken tag turning the build red.

Related Skills

• compliance-testing — Whether a beacon is allowed to fire under GDPR/CMP consent law, cookie-consent UI, and data-subject rights. This skill assumes the beacon is permitted and checks its data is correct; go there for the legality question.
• playwright-automation — Page Object Model, fixtures, config, and the general E2E patterns this skill builds its interception on.
• api-testing — Validating the server-side Conversions API / Measurement Protocol calls directly (request body, auth, response) when you need to assert the server half of deduplication.
• qa-project-context — Stack, tag manager, consent platform, and target environments that this skill's discovery questions read first.

Reference Files (in references/)

• ga4-interception.md — Full GA4 /g/collect interception helper, batched-event POST-body parsing, region-endpoint handling.
• tracking-plan.md — A JSON ecommerce contract, the reusable validateEvent helper, and separate assertions for schema and scenario values.
• datalayer-capture.md — addInitScript dataLayer.push wrapper and ecommerce items[] shape assertions.
• pixels-and-dedup.md — Meta/TikTok/LinkedIn payload parsing and client-vs-server CAPI event_id dedup capture.
• consent-mode.md — Passive four-signal observation, basic/advanced recipes, action checkpoints, completion signals, and bounded gcs=/gcd= diagnostics.
• news-media.md — article_view, de-duplicated 25/50/75/100 scroll buckets, and paywall-meter event tests.
• capture-fixture.md — The reusable multi-pixel test.extend capture fixture.
• ci-gating.md — GitHub Actions workflow, baseline-diff script, and surfacing missing-param failures.