diff --git a/apps/electron-backend-e2e/src/performance/journey-renderer-probe.spec.ts b/apps/electron-backend-e2e/src/performance/journey-renderer-probe.spec.ts
index 7dcc107d3..c5559d871 100644
--- a/apps/electron-backend-e2e/src/performance/journey-renderer-probe.spec.ts
+++ b/apps/electron-backend-e2e/src/performance/journey-renderer-probe.spec.ts
@@ -378,6 +378,26 @@ test('rejects a probe that was installed after the document started', async () =
     );
 });
 
+test('rejects a probe whose performance observers were unavailable instead of reporting zeros', async () => {
+    const fixture = createFixture();
+    const state = fixture.rawState() as {
+        capabilities: { layoutShift: boolean; longTask: boolean };
+    };
+    state.capabilities.longTask = false;
+    renderFirstCard(fixture);
+    await settle();
+    assert.equal(fixture.state().counters.longTasks, 0);
+    assert.throws(
+        () => assertJourneyRendererProbeState(fixture.state()),
+        /observer-unavailable: longTask/
+    );
+    state.capabilities.layoutShift = false;
+    assert.throws(
+        () => assertJourneyRendererProbeState(fixture.state()),
+        /observer-unavailable: layoutShift, longTask/
+    );
+});
+
 test('launch options target the workspace source cards and the shared sentinel', () => {
     const options = createLaunchJourneyProbeOptions();
     assert.equal(options.stateKey, JOURNEY_PROBE_STATE_KEY);
diff --git a/apps/electron-backend-e2e/src/performance/journey-renderer-probe.ts b/apps/electron-backend-e2e/src/performance/journey-renderer-probe.ts
index a790ac665..1beef3a41 100644
--- a/apps/electron-backend-e2e/src/performance/journey-renderer-probe.ts
+++ b/apps/electron-backend-e2e/src/performance/journey-renderer-probe.ts
@@ -363,5 +363,16 @@ export function assertJourneyRendererProbeState(
             `journey-renderer-probe-sentinel-${state.sentinel.status}`
         );
     }
+    // A zero from an observer that never ran is not a measurement; a build
+    // without these entry types must fail the iteration, never lower a
+    // baseline.
+    const missing = (['layoutShift', 'longTask'] as const).filter(
+        (capability) => !state.capabilities[capability]
+    );
+    if (missing.length > 0) {
+        throw new Error(
+            `journey-renderer-probe-observer-unavailable: ${missing.join(', ')}`
+        );
+    }
     return state;
 }