diff --git a/demo-app/src/main/java/com/personalization/demo/MainActivity.kt b/demo-app/src/main/java/com/personalization/demo/MainActivity.kt
index a211f1aa..b5ba72e8 100644
--- a/demo-app/src/main/java/com/personalization/demo/MainActivity.kt
+++ b/demo-app/src/main/java/com/personalization/demo/MainActivity.kt
@@ -1,11 +1,15 @@
 package com.personalization.demo
 
 import android.os.Bundle
+import android.widget.Button
+import android.widget.Toast
 import androidx.appcompat.app.AppCompatActivity
 import com.google.firebase.FirebaseApp
 import com.personalization.SDK
+import com.personalization.api.OnApiCallbackListener
 import com.personalization.demo.BuildConfig
 import com.personalization.sdk.data.models.dto.popUp.Components
+import org.json.JSONObject
 import com.personalization.sdk.data.models.dto.popUp.PopupActions
 import com.personalization.sdk.data.models.dto.popUp.PopupDto
 import com.personalization.sdk.data.models.dto.popUp.Position
@@ -14,6 +18,20 @@ class MainActivity : AppCompatActivity() {
 
     private lateinit var sdk: SDK
 
+    private object DemoTrackEventConstants {
+        /** Same value as SDK client-side validation errors for custom field key collisions. */
+        const val CLIENT_VALIDATION_ERROR_CODE = -1
+        const val EVENT_NAME = "custom_event"
+        const val SAMPLE_UNIX_TIME = 123_456
+        const val CATEGORY = "demo_category"
+        const val LABEL = "demo_label"
+        const val SAMPLE_VALUE = 100
+        const val SAFE_CUSTOM_KEY = "demo_custom_key"
+        const val SAFE_CUSTOM_VALUE = "android_demo_app"
+        const val COLLISION_RESERVED_KEY = "shop_id"
+        const val COLLISION_PLACEHOLDER_VALUE = "collision_demo"
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         setContentView(R.layout.activity_main)
@@ -44,10 +62,88 @@ class MainActivity : AppCompatActivity() {
         // Initialize fragment manager for popups
         sdk.inAppNotificationManager.initFragmentManager(supportFragmentManager)
 
-        // Setup button
-        findViewById<android.widget.Button>(R.id.btnShowTestPopup).setOnClickListener {
+        findViewById<Button>(R.id.btnShowTestPopup).setOnClickListener {
             showTestPopup()
         }
+
+        findViewById<Button>(R.id.btnTrackEventCustomFields).setOnClickListener {
+            trackEventWithCustomFieldsSuccess()
+        }
+
+        findViewById<Button>(R.id.btnTrackEventCollision).setOnClickListener {
+            trackEventWithReservedKeyCollision()
+        }
+    }
+
+    private fun trackEventWithCustomFieldsSuccess() {
+        val customFields = mapOf(DemoTrackEventConstants.SAFE_CUSTOM_KEY to DemoTrackEventConstants.SAFE_CUSTOM_VALUE)
+        sdk.trackEvent(
+            event = DemoTrackEventConstants.EVENT_NAME,
+            time = DemoTrackEventConstants.SAMPLE_UNIX_TIME,
+            category = DemoTrackEventConstants.CATEGORY,
+            label = DemoTrackEventConstants.LABEL,
+            value = DemoTrackEventConstants.SAMPLE_VALUE,
+            customFields = customFields,
+            listener = object : OnApiCallbackListener() {
+                override fun onSuccess(response: JSONObject?) {
+                    runOnUiThread {
+                        Toast.makeText(
+                            this@MainActivity,
+                            getString(R.string.track_event_ok),
+                            Toast.LENGTH_LONG
+                        ).show()
+                    }
+                }
+
+                override fun onError(code: Int, msg: String?) {
+                    runOnUiThread {
+                        Toast.makeText(
+                            this@MainActivity,
+                            "${getString(R.string.track_event_fail)}: $msg",
+                            Toast.LENGTH_LONG
+                        ).show()
+                    }
+                }
+            }
+        )
+    }
+
+    private fun trackEventWithReservedKeyCollision() {
+        val customFields = mapOf(
+            DemoTrackEventConstants.COLLISION_RESERVED_KEY to DemoTrackEventConstants.COLLISION_PLACEHOLDER_VALUE
+        )
+        sdk.trackEvent(
+            event = DemoTrackEventConstants.EVENT_NAME,
+            time = DemoTrackEventConstants.SAMPLE_UNIX_TIME,
+            category = DemoTrackEventConstants.CATEGORY,
+            label = DemoTrackEventConstants.LABEL,
+            value = DemoTrackEventConstants.SAMPLE_VALUE,
+            customFields = customFields,
+            listener = object : OnApiCallbackListener() {
+                override fun onSuccess(response: JSONObject?) {
+                    runOnUiThread {
+                        Toast.makeText(
+                            this@MainActivity,
+                            getString(R.string.track_event_unexpected_success),
+                            Toast.LENGTH_LONG
+                        ).show()
+                    }
+                }
+
+                override fun onError(code: Int, msg: String?) {
+                    runOnUiThread {
+                        val isClientValidation = code == DemoTrackEventConstants.CLIENT_VALIDATION_ERROR_CODE
+                            && msg?.contains("customFields contains reserved keys") == true
+                        val text = if (isClientValidation) {
+                            "${getString(R.string.track_event_collision_ok)}\n$msg"
+                        } else {
+                            "${getString(R.string.track_event_fail)}: $msg"
+                        }
+                        Toast.makeText(this@MainActivity, text, Toast.LENGTH_LONG).show()
+                    }
+                }
+            }
+        )
     }
 
     private fun showTestPopup() {
diff --git a/demo-app/src/main/res/layout/activity_main.xml b/demo-app/src/main/res/layout/activity_main.xml
index 395cd9d2..5e3f94b5 100644
--- a/demo-app/src/main/res/layout/activity_main.xml
+++ b/demo-app/src/main/res/layout/activity_main.xml
@@ -13,6 +13,20 @@
         android:text="@string/show_test_popup"
         android:layout_marginTop="32dp" />
 
+    <Button
+        android:id="@+id/btnTrackEventCustomFields"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:text="@string/track_event_custom_fields"
+        android:layout_marginTop="16dp" />
+
+    <Button
+        android:id="@+id/btnTrackEventCollision"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:text="@string/track_event_reserved_collision"
+        android:layout_marginTop="16dp" />
+
 </LinearLayout>
 
 
diff --git a/demo-app/src/main/res/values/strings.xml b/demo-app/src/main/res/values/strings.xml
index 8132554d..6616ee21 100644
--- a/demo-app/src/main/res/values/strings.xml
+++ b/demo-app/src/main/res/values/strings.xml
@@ -2,6 +2,12 @@
 <resources>
     <string name="app_name">Personalization SDK Demo</string>
     <string name="show_test_popup">Show Test Popup</string>
+    <string name="track_event_custom_fields">Track event (custom fields)</string>
+    <string name="track_event_reserved_collision">Track event (reserved key collision)</string>
+    <string name="track_event_ok">trackEvent: request sent</string>
+    <string name="track_event_collision_ok">trackEvent: validation error (expected)</string>
+    <string name="track_event_fail">trackEvent failed</string>
+    <string name="track_event_unexpected_success">Unexpected: expected client validation failure</string>
 </resources>
 
 
diff --git a/personalization-sdk/build.gradle b/personalization-sdk/build.gradle
index 804f4b26..82c82bae 100644
--- a/personalization-sdk/build.gradle
+++ b/personalization-sdk/build.gradle
@@ -105,6 +105,10 @@ dependencies {
     implementation("com.google.android.gms:play-services-ads-identifier:17.0.0")
 
     kapt 'com.google.dagger:dagger-compiler:2.48'
+
+    testImplementation 'junit:junit:4.13.2'
+    testImplementation 'io.mockk:mockk:1.13.11'
+    testImplementation 'org.robolectric:robolectric:4.12.1'
 }
 
 configurations {
diff --git a/personalization-sdk/src/main/kotlin/com/personalization/SDK.kt b/personalization-sdk/src/main/kotlin/com/personalization/SDK.kt
index b0d018f1..ddbc21de 100644
--- a/personalization-sdk/src/main/kotlin/com/personalization/SDK.kt
+++ b/personalization-sdk/src/main/kotlin/com/personalization/SDK.kt
@@ -494,6 +494,37 @@ open class SDK {
         trackEventManager.track(event, params, listener)
     }
 
+    /**
+     * Custom event tracking (aligned with the iOS SDK).
+     *
+     * @param event Event key
+     * @param time Optional UNIX time in seconds
+     * @param category Event category
+     * @param label Event label
+     * @param value Event value (sent as string in JSON)
+     * @param customFields Optional map merged at top level and under `payload`
+     * @param listener Callback
+     */
+    fun trackEvent(
+        event: String,
+        time: Int? = null,
+        category: String? = null,
+        label: String? = null,
+        value: Int? = null,
+        customFields: Map<String, Any?>? = null,
+        listener: OnApiCallbackListener? = null
+    ) {
+        trackEventManager.trackEvent(
+            event = event,
+            time = time,
+            category = category,
+            label = label,
+            value = value,
+            customFields = customFields,
+            listener = listener
+        )
+    }
+
     /**
      * Tracking custom events
      *
@@ -504,9 +535,12 @@ open class SDK {
      * @param listener Callback
      */
     @Deprecated(
-        "This method will be removed in future versions.",
+        "This method will be removed in future versions. Use trackEvent.",
         level = DeprecationLevel.WARNING,
-        replaceWith = ReplaceWith("trackEventManager.customTrack(event, category, label, value = value, listener = listener)")
+        replaceWith = ReplaceWith(
+            "trackEvent(event = event, category = category, label = label, value = value, listener = listener)",
+            imports = []
+        )
     )
     fun track(
         event: String,
@@ -515,7 +549,15 @@ open class SDK {
         value: Int? = null,
         listener: OnApiCallbackListener? = null
     ) {
-        trackEventManager.customTrack(event, category, label, value = value, listener = listener)
+        trackEventManager.trackEvent(
+            event = event,
+            time = null,
+            category = category,
+            label = label,
+            value = value,
+            customFields = null,
+            listener = listener
+        )
     }
 
     /**
diff --git a/personalization-sdk/src/main/kotlin/com/personalization/api/managers/TrackEventManager.kt b/personalization-sdk/src/main/kotlin/com/personalization/api/managers/TrackEventManager.kt
index f0b02084..fc0d40bd 100644
--- a/personalization-sdk/src/main/kotlin/com/personalization/api/managers/TrackEventManager.kt
+++ b/personalization-sdk/src/main/kotlin/com/personalization/api/managers/TrackEventManager.kt
@@ -28,7 +28,31 @@ interface TrackEventManager {
     )
 
     /**
-     * Tracking custom events
+     * Custom event tracking (aligned with the iOS SDK `trackEvent`).
+     *
+     * @param event Event key
+     * @param time Optional UNIX time (seconds), same as iOS
+     * @param category Optional category
+     * @param label Optional label
+     * @param value Optional value (sent as string in JSON, same as iOS)
+     * @param customFields Optional map merged at the top level and duplicated under `payload` (payload only contains these entries)
+     * @param listener Callback
+     */
+    fun trackEvent(
+        event: String,
+        time: Int? = null,
+        category: String? = null,
+        label: String? = null,
+        value: Int? = null,
+        customFields: Map<String, Any?>? = null,
+        listener: OnApiCallbackListener? = null
+    )
+
+    /**
+     * Tracking custom events (legacy).
+     *
+     * Use [trackEvent] for the standard event shape (`time`, `customFields`).
+     * This method remains for optional identity fields: `email`, `phone`, `loyalty_id`, `external_id`.
      *
      * @param event Event key
      * @param email Email
@@ -40,6 +64,14 @@ interface TrackEventManager {
      * @param value Event value
      * @param listener Callback
      */
+    @Deprecated(
+        message = "Use trackEvent(event, time, category, label, value, customFields, listener) for the iOS-aligned API. " +
+            "Use this method only when you still need email, phone, loyalty_id, or external_id.",
+        replaceWith = ReplaceWith(
+            expression = "trackEvent(event = event, time = null, category = category, label = label, value = value, customFields = null, listener = listener)",
+            imports = []
+        )
+    )
     fun customTrack(
         event: String,
         email: String? = null,
diff --git a/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackCustomEventPayloadHelper.kt b/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackCustomEventPayloadHelper.kt
new file mode 100644
index 00000000..9d0b3161
--- /dev/null
+++ b/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackCustomEventPayloadHelper.kt
@@ -0,0 +1,79 @@
+package com.personalization.features.trackEvent.impl
+
+import com.personalization.sdk.data.models.params.UserBasicParams
+import org.json.JSONArray
+import org.json.JSONException
+import org.json.JSONObject
+
+/**
+ * Shared rules for custom event `customFields` merge and validation (parity with iOS trackEvent).
+ */
+internal object TrackCustomEventPayloadHelper {
+
+    const val CLIENT_VALIDATION_ERROR_CODE: Int = -1
+
+    private const val KEY_EVENT = "event"
+    private const val KEY_TIME = "time"
+    private const val KEY_CATEGORY = "category"
+    private const val KEY_LABEL = "label"
+    private const val KEY_VALUE = "value"
+    private const val KEY_SOURCE = "source"
+    private const val KEY_PAYLOAD = "payload"
+    private const val KEY_FROM = "from"
+    private const val KEY_CODE = "code"
+    private const val KEY_STREAM = "stream"
+
+    val RESERVED_CUSTOM_EVENT_KEYS: Set<String> = setOf(
+        UserBasicParams.SHOP_ID,
+        UserBasicParams.DID,
+        UserBasicParams.SEANCE,
+        UserBasicParams.SID,
+        UserBasicParams.SEGMENT,
+        KEY_STREAM,
+        KEY_EVENT,
+        KEY_TIME,
+        KEY_CATEGORY,
+        KEY_LABEL,
+        KEY_VALUE,
+        KEY_SOURCE,
+        KEY_PAYLOAD,
+        KEY_FROM,
+        KEY_CODE,
+    )
+
+    fun effectiveCustomFields(map: Map<String, Any?>?): Map<String, Any> {
+        if (map.isNullOrEmpty()) return emptyMap()
+        val out = LinkedHashMap<String, Any>()
+        for ((key, value) in map) {
+            if (key.isBlank() || value == null) continue
+            out[key] = value
+        }
+        return out
+    }
+
+    /**
+     * @return error message for [listener.onError], or null if valid
+     */
+    fun validateNoReservedKeyCollisions(customFields: Map<String, Any>): String? {
+        if (customFields.isEmpty()) return null
+        val collisions = customFields.keys.intersect(RESERVED_CUSTOM_EVENT_KEYS)
+        if (collisions.isEmpty()) return null
+        val sorted = collisions.toSortedSet().joinToString(", ")
+        return "trackEvent: customFields contains reserved keys: $sorted"
+    }
+
+    @Throws(JSONException::class)
+    fun putJsonValue(target: JSONObject, key: String, value: Any) {
+        when (value) {
+            is String -> target.put(key, value)
+            is Int -> target.put(key, value)
+            is Long -> target.put(key, value)
+            is Double -> target.put(key, value)
+            is Float -> target.put(key, value.toDouble())
+            is Boolean -> target.put(key, value)
+            is JSONObject -> target.put(key, value)
+            is JSONArray -> target.put(key, value)
+            else -> target.put(key, value.toString())
+        }
+    }
+}
diff --git a/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImpl.kt b/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImpl.kt
index 2e9ef5ea..f7876ec0 100644
--- a/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImpl.kt
+++ b/personalization-sdk/src/main/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImpl.kt
@@ -13,6 +13,7 @@ import com.personalization.sdk.domain.usecases.recommendation.GetRecommendedByUs
 import com.personalization.sdk.domain.usecases.recommendation.SetRecommendedByUseCase
 import com.personalization.sdk.domain.usecases.userSettings.GetUserSettingsValueUseCase
 import com.personalization.sdk.data.models.params.UserBasicParams
+import org.json.JSONException
 import org.json.JSONObject
 import javax.inject.Inject
 
@@ -56,7 +57,58 @@ internal class TrackEventManagerImpl @Inject constructor(
         )
     }
 
+    override fun trackEvent(
+        event: String,
+        time: Int?,
+        category: String?,
+        label: String?,
+        value: Int?,
+        customFields: Map<String, Any?>?,
+        listener: OnApiCallbackListener?
+    ) {
+        val effectiveCustom = TrackCustomEventPayloadHelper.effectiveCustomFields(customFields)
+        TrackCustomEventPayloadHelper.validateNoReservedKeyCollisions(effectiveCustom)?.let { msg ->
+            listener?.onError(TrackCustomEventPayloadHelper.CLIENT_VALIDATION_ERROR_CODE, msg)
+            return
+        }
+
+        val params = Params()
+        params.put(EVENT_PARAMETER, event)
+        if (time != null) {
+            params.put(TIME_PARAMETER, time)
+        }
+        if (category != null) {
+            params.put(CATEGORY_PARAMETER, category)
+        }
+        if (label != null) {
+            params.put(LABEL_PARAMETER, label)
+        }
+        if (value != null) {
+            params.put(VALUE_PARAMETER, value)
+        }
+
+        val body = params.build()
+        if (effectiveCustom.isNotEmpty()) {
+            try {
+                val payload = JSONObject()
+                for ((key, fieldValue) in effectiveCustom) {
+                    TrackCustomEventPayloadHelper.putJsonValue(body, key, fieldValue)
+                    TrackCustomEventPayloadHelper.putJsonValue(payload, key, fieldValue)
+                }
+                body.put(PAYLOAD_PARAMETER, payload)
+            } catch (e: JSONException) {
+                listener?.onError(
+                    TrackCustomEventPayloadHelper.CLIENT_VALIDATION_ERROR_CODE,
+                    "trackEvent: failed to build custom fields payload: ${e.message}"
+                )
+                return
+            }
+        }
 
+        postCustomEvent(body, listener)
+    }
+
+    @Deprecated("Use trackEvent for iOS-aligned custom events.")
     override fun customTrack(
         event: String,
         email: String?,
@@ -92,20 +144,7 @@ internal class TrackEventManagerImpl @Inject constructor(
             params.put(VALUE_PARAMETER, value)
         }
 
-        val internalListener = object : OnApiCallbackListener() {
-            override fun onSuccess(response: JSONObject?) {
-                response?.let {
-                    handlePopup(response)
-                }
-                listener?.onSuccess(response)
-            }
-        }
-
-        sendNetworkMethodUseCase.postAsync(
-            CUSTOM_PUSH_REQUEST,
-            params.build(),
-            internalListener
-        )
+        postCustomEvent(params.build(), listener)
     }
 
     override fun trackPopupShown(popupId: Int, listener: OnApiCallbackListener?) {
@@ -122,6 +161,23 @@ internal class TrackEventManagerImpl @Inject constructor(
         )
     }
 
+    private fun postCustomEvent(json: JSONObject, listener: OnApiCallbackListener?) {
+        val internalListener = object : OnApiCallbackListener() {
+            override fun onSuccess(response: JSONObject?) {
+                response?.let {
+                    handlePopup(response)
+                }
+                listener?.onSuccess(response)
+            }
+        }
+
+        sendNetworkMethodUseCase.postAsync(
+            CUSTOM_PUSH_REQUEST,
+            json,
+            internalListener
+        )
+    }
+
     private fun handlePopup(response: JSONObject) {
         val popUpData = response.optJSONObject(PARAM_POPUP)?.let { PopupDtoMapper.map(it) }
         if (popUpData != null) {
@@ -135,6 +191,8 @@ internal class TrackEventManagerImpl @Inject constructor(
         private const val POPUP_SHOWN_PATH = "popup/showed"
 
         private const val EVENT_PARAMETER = "event"
+        private const val TIME_PARAMETER = "time"
+        private const val PAYLOAD_PARAMETER = "payload"
         private const val EMAIL_PARAMETER = "email"
         private const val PHONE_PARAMETER = "phone"
         private const val LOYALTY_ID_PARAMETER = "loyalty_id"
diff --git a/personalization-sdk/src/test/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImplTrackEventTest.kt b/personalization-sdk/src/test/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImplTrackEventTest.kt
new file mode 100644
index 00000000..53928899
--- /dev/null
+++ b/personalization-sdk/src/test/kotlin/com/personalization/features/trackEvent/impl/TrackEventManagerImplTrackEventTest.kt
@@ -0,0 +1,152 @@
+package com.personalization.features.trackEvent.impl
+
+import com.personalization.api.OnApiCallbackListener
+import com.personalization.api.managers.InAppNotificationManager
+import com.personalization.sdk.domain.usecases.network.SendNetworkMethodUseCase
+import com.personalization.sdk.domain.usecases.recommendation.GetRecommendedByUseCase
+import com.personalization.sdk.domain.usecases.recommendation.SetRecommendedByUseCase
+import com.personalization.sdk.domain.usecases.userSettings.GetUserSettingsValueUseCase
+import io.mockk.Runs
+import io.mockk.every
+import io.mockk.just
+import io.mockk.mockk
+import io.mockk.slot
+import io.mockk.verify
+import org.json.JSONObject
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [28])
+class TrackEventManagerImplTrackEventTest {
+
+    private lateinit var sendNetworkMethodUseCase: SendNetworkMethodUseCase
+    private lateinit var inAppNotificationManager: InAppNotificationManager
+    private lateinit var getRecommendedByUseCase: GetRecommendedByUseCase
+    private lateinit var setRecommendedByUseCase: SetRecommendedByUseCase
+    private lateinit var getUserSettingsValueUseCase: GetUserSettingsValueUseCase
+    private lateinit var impl: TrackEventManagerImpl
+
+    @Before
+    fun setUp() {
+        sendNetworkMethodUseCase = mockk(relaxed = true)
+        inAppNotificationManager = mockk(relaxed = true)
+        getRecommendedByUseCase = mockk(relaxed = true)
+        setRecommendedByUseCase = mockk(relaxed = true)
+        getUserSettingsValueUseCase = mockk(relaxed = true)
+
+        every { getRecommendedByUseCase.invoke() } returns null
+
+        impl = TrackEventManagerImpl(
+            getRecommendedByUseCase,
+            setRecommendedByUseCase,
+            sendNetworkMethodUseCase,
+            inAppNotificationManager,
+            getUserSettingsValueUseCase
+        )
+    }
+
+    @Test
+    fun trackEvent_reservedKeyInCustomFields_doesNotPost_invokesOnError() {
+        var capturedCode = 0
+        var capturedMessage: String? = null
+        val listener = mockk<OnApiCallbackListener>()
+        every { listener.onError(any(), any()) } answers {
+            capturedCode = invocation.args[0] as Int
+            capturedMessage = invocation.args[1] as String?
+        }
+
+        impl.trackEvent(
+            event = "demo_event",
+            time = null,
+            category = null,
+            label = null,
+            value = null,
+            customFields = mapOf("shop_id" to "x"),
+            listener = listener
+        )
+
+        verify(exactly = 0) { sendNetworkMethodUseCase.postAsync(any(), any(), any()) }
+        verify(exactly = 1) { listener.onError(any(), any()) }
+        assertEquals(TrackCustomEventPayloadHelper.CLIENT_VALIDATION_ERROR_CODE, capturedCode)
+        assertTrue(capturedMessage!!.contains("shop_id"))
+        assertTrue(capturedMessage!!.contains("trackEvent:"))
+    }
+
+    @Test
+    fun trackEvent_multipleReservedKeys_errorListsSortedKeys() {
+        var capturedMessage: String? = null
+        val listener = mockk<OnApiCallbackListener>()
+        every { listener.onError(any(), any()) } answers {
+            capturedMessage = invocation.args[1] as String?
+        }
+
+        impl.trackEvent(
+            event = "e",
+            customFields = mapOf("event" to "bad", "payload" to "bad"),
+            listener = listener
+        )
+
+        verify(exactly = 0) { sendNetworkMethodUseCase.postAsync(any(), any(), any()) }
+        assertTrue(capturedMessage!!.contains("event, payload"))
+    }
+
+    @Test
+    fun trackEvent_validCustomFields_postsPushCustom_mergesTopLevelAndPayload() {
+        val listener = mockk<OnApiCallbackListener>(relaxed = true)
+        val paramsSlot = slot<JSONObject>()
+        every {
+            sendNetworkMethodUseCase.postAsync(any(), capture(paramsSlot), any())
+        } just Runs
+
+        impl.trackEvent(
+            event = "my_event",
+            time = 123_456,
+            category = "cat",
+            label = "lab",
+            value = 100,
+            customFields = mapOf("demo_custom_key" to "ios_demo_app", "n" to 42),
+            listener = listener
+        )
+
+        verify(exactly = 1) { sendNetworkMethodUseCase.postAsync("push/custom", any(), any()) }
+
+        val body = paramsSlot.captured
+        assertEquals("my_event", body.getString("event"))
+        assertEquals(123_456, body.getInt("time"))
+        assertEquals("cat", body.getString("category"))
+        assertEquals("lab", body.getString("label"))
+        assertEquals(100, body.getInt("value"))
+        assertEquals("ios_demo_app", body.getString("demo_custom_key"))
+        assertEquals(42, body.getInt("n"))
+
+        val payload = body.getJSONObject("payload")
+        assertEquals(2, payload.length())
+        assertEquals("ios_demo_app", payload.getString("demo_custom_key"))
+        assertEquals(42, payload.getInt("n"))
+    }
+
+    @Test
+    fun trackEvent_onlyNullCustomFieldValues_omitsPayload() {
+        val paramsSlot = slot<JSONObject>()
+        every {
+            sendNetworkMethodUseCase.postAsync(any(), capture(paramsSlot), any())
+        } just Runs
+
+        impl.trackEvent(
+            event = "e",
+            customFields = mapOf("k" to null),
+            listener = null
+        )
+
+        val body = paramsSlot.captured
+        assertEquals("e", body.getString("event"))
+        assertFalse(body.has("payload"))
+    }
+}