diff --git a/AnkiDroid/src/androidTest/java/com/ichi2/anki/ReviewerTest.kt b/AnkiDroid/src/androidTest/java/com/ichi2/anki/ReviewerTest.kt
index 0e90e8c8338f..1245949bf7ef 100755
--- a/AnkiDroid/src/androidTest/java/com/ichi2/anki/ReviewerTest.kt
+++ b/AnkiDroid/src/androidTest/java/com/ichi2/anki/ReviewerTest.kt
@@ -58,7 +58,7 @@ class ReviewerTest : InstrumentedTest() {
 
     @Test
     @Flaky(os = OS.ALL, "Fails on CI with timing issues frequently")
-    fun testCustomSchedulerWithCustomData() {
+    fun testCustomSchedulerWithCustomData() { // replicar
         col.cardStateCustomizer =
             """
             states.good.normal.review.easeFactor = 3.0;
diff --git a/AnkiDroid/src/main/java/com/ichi2/anki/previewer/CardViewerFragment.kt b/AnkiDroid/src/main/java/com/ichi2/anki/previewer/CardViewerFragment.kt
index c021e5a91203..6b52e1f4a49d 100644
--- a/AnkiDroid/src/main/java/com/ichi2/anki/previewer/CardViewerFragment.kt
+++ b/AnkiDroid/src/main/java/com/ichi2/anki/previewer/CardViewerFragment.kt
@@ -20,6 +20,7 @@ import android.net.Uri
 import android.os.Bundle
 import android.view.View
 import android.webkit.CookieManager
+import android.webkit.JsResult
 import android.webkit.WebChromeClient
 import android.webkit.WebResourceError
 import android.webkit.WebResourceRequest
@@ -49,6 +50,7 @@ import timber.log.Timber
 abstract class CardViewerFragment(@LayoutRes layout: Int) : Fragment(layout) {
     protected abstract val viewModel: CardViewerViewModel
     protected abstract val webView: WebView
+    protected open val baseUrl = "http://${AnkiServer.LOCALHOST}/"
 
     @CallSuper
     override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
@@ -83,7 +85,7 @@ abstract class CardViewerFragment(@LayoutRes layout: Int) : Fragment(layout) {
                 domStorageEnabled = true
             }
             loadDataWithBaseURL(
-                "http://${AnkiServer.LOCALHOST}/",
+                baseUrl,
                 stdHtml(requireContext(), Themes.currentTheme.isNightMode),
                 "text/html",
                 null,
@@ -133,22 +135,7 @@ abstract class CardViewerFragment(@LayoutRes layout: Int) : Fragment(layout) {
             }
 
             override fun shouldOverrideUrlLoading(view: WebView, request: WebResourceRequest): Boolean {
-                val urlString = request.url.toString()
-                if (urlString.startsWith("playsound:")) {
-                    viewModel.playSoundFromUrl(urlString)
-                    return true
-                }
-                if (urlString.startsWith("tts-voices:")) {
-                    TtsVoicesDialogFragment().show(childFragmentManager, null)
-                    return true
-                }
-                try {
-                    openUrl(request.url)
-                    return true
-                } catch (_: Exception) {
-                    Timber.w("Could not open url")
-                }
-                return false
+                return this@CardViewerFragment.shouldOverrideUrlLoading(request)
             }
 
             override fun onReceivedError(
@@ -163,8 +150,38 @@ abstract class CardViewerFragment(@LayoutRes layout: Int) : Fragment(layout) {
         }
     }
 
+    protected open fun shouldOverrideUrlLoading(request: WebResourceRequest): Boolean {
+        val urlString = request.url.toString()
+        if (urlString.startsWith("playsound:")) {
+            viewModel.playSoundFromUrl(urlString)
+            return true
+        }
+        if (urlString.startsWith("tts-voices:")) {
+            TtsVoicesDialogFragment().show(childFragmentManager, null)
+            return true
+        }
+        try {
+            openUrl(request.url)
+            return true
+        } catch (_: Exception) {
+            Timber.w("Could not open url")
+        }
+        return false
+    }
+
     private fun onCreateWebChromeClient(): WebChromeClient {
         return object : WebChromeClient() {
+
+            override fun onJsAlert(
+                view: WebView?,
+                url: String?,
+                message: String?,
+                result: JsResult?
+            ): Boolean {
+                requireActivity().showSnackbar(message!!)
+                return super.onJsAlert(view, url, message, result)
+            }
+
             private lateinit var customView: View
 
             // used for displaying `<video>` in fullscreen.
diff --git a/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt b/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt
index 5b6c01d77b79..adcb76bde8d1 100644
--- a/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt
+++ b/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt
@@ -20,6 +20,7 @@ import android.content.Intent
 import android.os.Bundle
 import android.view.MenuItem
 import android.view.View
+import android.webkit.WebResourceRequest
 import android.webkit.WebView
 import androidx.annotation.StringRes
 import androidx.appcompat.view.menu.MenuBuilder
@@ -54,6 +55,8 @@ class ReviewerFragment :
 
     override val webView: WebView
         get() = requireView().findViewById(R.id.webview)
+    override val baseUrl: String
+        get() = viewModel.baseUrl()
 
     override val baseSnackbarBuilder: SnackbarBuilder = {
         anchorView = this@ReviewerFragment.view?.findViewById(R.id.buttons_area)
@@ -88,6 +91,12 @@ class ReviewerFragment :
                 }
             }
         }
+
+        viewModel.statesMutationEval.launchCollect(lifecycleScope) { eval ->
+            webView.evaluateJavascript(eval) { result ->
+                viewModel.onStateMutationCallback(result)
+            }
+        }
     }
 
     // TODO
@@ -172,6 +181,17 @@ class ReviewerFragment :
         }
     }
 
+    override fun shouldOverrideUrlLoading(request: WebResourceRequest): Boolean {
+        return if (super.shouldOverrideUrlLoading(request)) {
+            true
+        } else if (request.url.toString().startsWith("state-mutation-error:")) {
+            viewModel.onStateMutationError()
+            true
+        } else {
+            false
+        }
+    }
+
     companion object {
         fun getIntent(context: Context): Intent {
             return CardViewerActivity.getIntent(context, ReviewerFragment::class)
diff --git a/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerViewModel.kt b/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerViewModel.kt
index bdc1649768dd..5fa3beaef473 100644
--- a/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerViewModel.kt
+++ b/AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerViewModel.kt
@@ -21,6 +21,7 @@ import androidx.core.text.inSpans
 import androidx.lifecycle.ViewModelProvider
 import androidx.lifecycle.viewmodel.initializer
 import androidx.lifecycle.viewmodel.viewModelFactory
+import anki.frontend.SetSchedulingStatesRequest
 import com.ichi2.anki.AGAIN
 import com.ichi2.anki.CollectionManager.withCol
 import com.ichi2.anki.EASY
@@ -30,17 +31,23 @@ import com.ichi2.anki.HARD
 import com.ichi2.anki.asyncIO
 import com.ichi2.anki.cardviewer.SoundPlayer
 import com.ichi2.anki.launchCatchingIO
+import com.ichi2.anki.pages.AnkiServer
+import com.ichi2.anki.pages.PostRequestHandler
 import com.ichi2.anki.previewer.CardViewerViewModel
 import com.ichi2.anki.reviewer.CardSide
 import com.ichi2.libanki.sched.CurrentQueueState
 import com.ichi2.libanki.undoableOp
+import com.ichi2.libanki.utils.TimeManager
 import kotlinx.coroutines.Deferred
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
 
-class ReviewerViewModel(soundPlayer: SoundPlayer) : CardViewerViewModel(soundPlayer) {
+class ReviewerViewModel(soundPlayer: SoundPlayer) :
+    CardViewerViewModel(soundPlayer),
+    PostRequestHandler {
 
     private var queueState: CurrentQueueState? = null
+    private val stateMutationKey = TimeManager.time.intTimeMS().toString()
     var isQueueFinishedFlow = MutableSharedFlow<Boolean>()
 
     val againNextTime: MutableStateFlow<String?> = MutableStateFlow(null)
@@ -52,6 +59,24 @@ class ReviewerViewModel(soundPlayer: SoundPlayer) : CardViewerViewModel(soundPla
         withCol { config.get("estTimes") ?: true }
     }
 
+    private val server = AnkiServer(this).also { it.start() }
+    val statesMutationEval = MutableSharedFlow<String>()
+
+    /**
+     * A flag that determines if the SchedulingStates in CurrentQueueState are
+     * safe to persist in the database when answering a card. This is used to
+     * ensure that the custom JS scheduler has persisted its SchedulingStates
+     * back to the Reviewer before we save it to the database. If the custom
+     * scheduler has not been configured, then it is safe to immediately set
+     * this to true.
+     *
+     * This flag should be set to false when we show the front of the card
+     * and only set to true once we know the custom scheduler has finished its
+     * execution, or set to true immediately if the custom scheduler has not
+     * been configured.
+     */
+    private var statesMutated = true
+
     /* *********************************************************************************************
     ************************ Public methods: meant to be used by the View **************************
     ********************************************************************************************* */
@@ -68,6 +93,8 @@ class ReviewerViewModel(soundPlayer: SoundPlayer) : CardViewerViewModel(soundPla
         }
     }
 
+    fun baseUrl() = server.baseUrl()
+
     fun showAnswer() {
         launchCatchingIO {
             showAnswerInternal()
@@ -81,6 +108,18 @@ class ReviewerViewModel(soundPlayer: SoundPlayer) : CardViewerViewModel(soundPla
     fun answerGood() = answerCard(GOOD)
     fun answerEasy() = answerCard(EASY)
 
+    fun onStateMutationCallback(result: String?) {
+        // eval failed, usually a syntax error
+        // Note, we get "null" (string) and not null
+        if (result == "null") {
+            statesMutated = true
+        }
+    }
+
+    fun onStateMutationError() {
+        statesMutated = true
+    }
+
     /* *********************************************************************************************
     *************************************** Internal methods ***************************************
     ********************************************************************************************* */
@@ -99,6 +138,11 @@ class ReviewerViewModel(soundPlayer: SoundPlayer) : CardViewerViewModel(soundPla
         soundPlayer.playAllSoundsForSide(side)
     }
 
+    override suspend fun showQuestion() {
+        super.showQuestion()
+        runStateMutationHook()
+    }
+
     private suspend fun updateCurrentCard() {
         queueState = withCol {
             sched.currentQueueState()
@@ -127,6 +171,54 @@ class ReviewerViewModel(soundPlayer: SoundPlayer) : CardViewerViewModel(soundPla
         easyNextTime.emit(easy)
     }
 
+    override suspend fun handlePostRequest(uri: String, bytes: ByteArray): ByteArray {
+        return if (uri.startsWith(AnkiServer.ANKI_PREFIX)) {
+            when (uri.substring(AnkiServer.ANKI_PREFIX.length)) {
+                "getSchedulingStatesWithContext" -> getSchedulingStatesWithContext()
+                "setSchedulingStates" -> setSchedulingStates(bytes)
+                else -> byteArrayOf()
+            }
+        } else {
+            byteArrayOf()
+        }
+    }
+
+    private suspend fun runStateMutationHook() {
+        val state = queueState ?: return
+        val js = state.customSchedulingJs
+        if (js.isEmpty()) {
+            statesMutated = true
+            return
+        }
+        statesMutated = false
+        statesMutationEval.emit(
+            """anki.mutateNextCardStates('$stateMutationKey', async (states, customData, ctx) => {{ $js }})
+            .catch(err => console.log(err));"""
+        )
+    }
+
+    private fun getSchedulingStatesWithContext(): ByteArray {
+        val state = queueState ?: return ByteArray(0)
+        return state.schedulingStatesWithContext().toBuilder()
+            .mergeStates(
+                state.states.toBuilder().mergeCurrent(
+                    state.states.current.toBuilder()
+                        .setCustomData(state.topCard.toBackendCard().customData).build()
+                ).build()
+            )
+            .build()
+            .toByteArray()
+    }
+
+    private fun setSchedulingStates(bytes: ByteArray): ByteArray {
+        val state = queueState ?: return ByteArray(0)
+        val req = SetSchedulingStatesRequest.parseFrom(bytes)
+        if (req.key == stateMutationKey) {
+            state.states = req.states
+        }
+        return ByteArray(0)
+    }
+
     companion object {
         fun factory(soundPlayer: SoundPlayer): ViewModelProvider.Factory {
             return viewModelFactory {