diff --git a/.github/workflows/e2e-local.yml b/.github/workflows/e2e-local.yml
index 1095c68b..e7fbe1c5 100644
--- a/.github/workflows/e2e-local.yml
+++ b/.github/workflows/e2e-local.yml
@@ -426,7 +426,7 @@ jobs:
               --project=${{ matrix.project }} --shard=${{ matrix.shard }} \
               $WORKERS_FLAG \
               --grep-invert='@hosted' \
-              --reporter=line,json || true
+              --reporter=line,json,./tests/e2e/reporters/assertion-count-reporter.ts || true
         env:
           CI: 'true'
           SKIP_WEBSERVER: 'true'
diff --git a/.github/workflows/e2e.yml b/.github/workflows/e2e.yml
index ed860f5b..3b2d0023 100644
--- a/.github/workflows/e2e.yml
+++ b/.github/workflows/e2e.yml
@@ -314,7 +314,7 @@ jobs:
             tests/e2e/auth/sign-up.spec.ts \
             --project=signup \
             --no-deps \
-            --reporter=list \
+            --reporter=list,./tests/e2e/reporters/assertion-count-reporter.ts \
             --timeout=30000
         env:
           CI: true
@@ -407,7 +407,7 @@ jobs:
           scripts/ci/playwright-in-container.sh test \
             --project=basepath \
             --no-deps \
-            --reporter=list \
+            --reporter=list,./tests/e2e/reporters/assertion-count-reporter.ts \
             --timeout=30000
         env:
           CI: true
@@ -503,7 +503,7 @@ jobs:
           echo "serve is responding on http://localhost:3000"
 
       - name: Run rate-limiting tests (ordered)
-        run: scripts/ci/playwright-in-container.sh test --project=rate-limiting --project=brute-force --project=signup --reporter=list --trace=on-first-retry
+        run: scripts/ci/playwright-in-container.sh test --project=rate-limiting --project=brute-force --project=signup --reporter=list,./tests/e2e/reporters/assertion-count-reporter.ts --trace=on-first-retry
         env:
           CI: true
           PLAYWRIGHT_BROWSER: chromium
@@ -591,7 +591,7 @@ jobs:
           echo "serve is responding on http://localhost:3000"
 
       - name: Run auth setup
-        run: scripts/ci/playwright-in-container.sh test --project=setup --reporter=list --timeout=180000
+        run: scripts/ci/playwright-in-container.sh test --project=setup --reporter=list,./tests/e2e/reporters/assertion-count-reporter.ts --timeout=180000
         env:
           CI: true
           AUTH_SETUP_JOB: true
@@ -800,7 +800,7 @@ jobs:
           scripts/ci/playwright-in-container.sh test \
             --project="$PROJECT" \
             --shard="$SHARD" \
-            --reporter=blob \
+            --reporter=blob,./tests/e2e/reporters/assertion-count-reporter.ts \
             --trace=on-first-retry \
             --no-deps \
             $WORKERS_FLAG
@@ -977,7 +977,7 @@ jobs:
           scripts/ci/playwright-in-container.sh test \
             --project="$PROJECT" \
             --shard="$SHARD" \
-            --reporter=blob \
+            --reporter=blob,./tests/e2e/reporters/assertion-count-reporter.ts \
             --trace=on-first-retry \
             --no-deps \
             $WORKERS_FLAG
@@ -1134,7 +1134,7 @@ jobs:
           scripts/ci/playwright-in-container.sh test \
             --project="$PROJECT" \
             --shard="$SHARD" \
-            --reporter=blob \
+            --reporter=blob,./tests/e2e/reporters/assertion-count-reporter.ts \
             --trace=on-first-retry \
             --no-deps \
             $WORKERS_FLAG
diff --git a/scripts/__tests__/reporter-is-not-inert.test.js b/scripts/__tests__/reporter-is-not-inert.test.js
new file mode 100644
index 00000000..c7124e34
--- /dev/null
+++ b/scripts/__tests__/reporter-is-not-inert.test.js
@@ -0,0 +1,121 @@
+/**
+ * A reporter in `playwright.config.ts` does nothing if a lane passes `--reporter=` (#396).
+ *
+ * WHAT HAPPENED, and it is worth recording because the tool built to detect this family
+ * committed the family's own defect. PR #846 added a reporter that names any test
+ * finishing with zero assertions, and wired it into `playwright.config.ts`. It was
+ * verified locally, mutation-tested three ways, and merged.
+ *
+ * It printed NOTHING in CI.
+ *
+ * `playwright test --reporter=X` REPLACES the config's `reporter` array — it does not
+ * append. Every lane passes one (`line,json` in e2e-local, `list` and `blob` in e2e.yml),
+ * so the reporter ran in exactly the two lanes that do not override (smoke,
+ * signup-mailer) and in none of the 25 shards on the REQUIRED lane. Found by grepping
+ * the run logs for its own output and getting nothing back — the check that a control
+ * has an EFFECT, not merely a presence.
+ *
+ * WHY A TEST AND NOT JUST THE FIX. The config and the workflows are different files with
+ * nothing relating them, and the failure is silent in the worst way: the reporter is
+ * present, correct, unit-tested, and mute. Nothing goes red. This relates them.
+ *
+ * WHAT THIS CANNOT CHECK: that the reporter produces useful output, or that a lane's
+ * output is ever read. It asserts the wiring reaches every lane that runs the suite.
+ */
+'use strict';
+
+const { describe, it } = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const ROOT = path.join(__dirname, '..', '..');
+const WORKFLOWS = path.join(ROOT, '.github', 'workflows');
+const REPORTER = 'assertion-count-reporter';
+
+/** Lines that actually run the suite, ignoring comments and report merging. */
+function invocations() {
+  const out = [];
+  for (const file of fs
+    .readdirSync(WORKFLOWS)
+    .filter((f) => /\.ya?ml$/.test(f))) {
+    const src = fs.readFileSync(path.join(WORKFLOWS, file), 'utf8');
+    src.split('\n').forEach((line, i) => {
+      if (/^\s*#/.test(line)) return;
+      if (/merge-reports/.test(line)) return;
+      if (!/--reporter=/.test(line)) return;
+      out.push({ file, line: i + 1, text: line.trim() });
+    });
+  }
+  return out;
+}
+
+describe('the zero-assertion reporter actually runs (#396)', () => {
+  it('the reporter file exists and the config references it', () => {
+    // Non-vacuity: if the reporter were deleted, every assertion below about
+    // `--reporter=` lists would be checking a string nothing produces.
+    const rp = path.join(
+      ROOT,
+      'tests',
+      'e2e',
+      'reporters',
+      'assertion-count-reporter.ts'
+    );
+    assert.ok(fs.existsSync(rp), 'the reporter file is gone');
+    const cfg = fs.readFileSync(
+      path.join(ROOT, 'playwright.config.ts'),
+      'utf8'
+    );
+    assert.match(
+      cfg,
+      new RegExp(REPORTER),
+      'playwright.config.ts no longer lists it'
+    );
+  });
+
+  it('finds the lanes that override the reporter list', () => {
+    // The other half of non-vacuity. "Every override includes it" is trivially true
+    // of zero overrides — which is exactly the state this test would have to
+    // distinguish from the real one.
+    assert.ok(
+      invocations().length >= 5,
+      `expected several --reporter= invocations across the lanes, found ${invocations().length}`
+    );
+  });
+
+  it('every lane that overrides --reporter still includes it', () => {
+    const missing = invocations().filter((v) => !v.text.includes(REPORTER));
+    assert.deepEqual(
+      missing.map((v) => `${v.file}:${v.line}`),
+      [],
+      "`playwright test --reporter=X` REPLACES the config's reporter array rather " +
+        'than appending to it, so these lanes run without the zero-assertion ' +
+        'reporter entirely. That is how #846 shipped a reporter that printed nothing ' +
+        'in the required lane. Append it to the list: ' +
+        `--reporter=<existing>,./tests/e2e/reporters/${REPORTER}.ts`
+    );
+  });
+
+  it('the detector can actually fail', () => {
+    const has = (line) =>
+      !/^\s*#/.test(line) &&
+      /--reporter=/.test(line) &&
+      !/merge-reports/.test(line);
+
+    assert.equal(has('  --reporter=line,json \\'), true);
+    assert.equal(
+      has('  # --reporter=line'),
+      false,
+      'a comment is not an invocation'
+    );
+    assert.equal(
+      has('  pnpm exec playwright merge-reports --reporter=json ./blobs'),
+      false,
+      'merging reports is not running the suite'
+    );
+
+    // The exact bug: an override that omits the reporter must be reported.
+    const bad = { file: 'x.yml', line: 1, text: '--reporter=line,json' };
+    assert.equal(bad.text.includes(REPORTER), false);
+  });
+});