diff --git a/src/commonMain/kotlin/baaahs/api/ws/WebSocketClient.kt b/src/commonMain/kotlin/baaahs/api/ws/WebSocketClient.kt
index e104fd773b..0f9f8e6856 100644
--- a/src/commonMain/kotlin/baaahs/api/ws/WebSocketClient.kt
+++ b/src/commonMain/kotlin/baaahs/api/ws/WebSocketClient.kt
@@ -80,7 +80,7 @@ class WebSocketClient(
             "loadSession",
             json.encodeToJsonElement(String.serializer(), name)
         )
-        return mappingSessionStore.decode(response)
+        return mappingSessionStore.decode(response, name)
     }
 
     private suspend fun sendCommand(command: String, vararg args: JsonElement): JsonElement {
diff --git a/src/commonMain/kotlin/baaahs/client/document/DataStore.kt b/src/commonMain/kotlin/baaahs/client/document/DataStore.kt
index 2823b3257c..b7f682d476 100644
--- a/src/commonMain/kotlin/baaahs/client/document/DataStore.kt
+++ b/src/commonMain/kotlin/baaahs/client/document/DataStore.kt
@@ -16,20 +16,20 @@ class DataStore<T : Any>(
     private val plugins: Plugins,
     private val migrator: DataMigrator<T>
 ) {
-    fun decode(content: String) =
-        plugins.json.decodeFromString(migrator, content)
+    fun decode(content: String, fileName: String? = null) =
+        plugins.json.decodeFromString(migrator.Migrate(fileName), content)
 
-    fun decode(content: JsonElement) =
-        plugins.json.decodeFromJsonElement(migrator, content)
+    fun decode(content: JsonElement, fileName: String? = null) =
+        plugins.json.decodeFromJsonElement(migrator.Migrate(fileName), content)
 
     suspend fun load(file: Fs.File): T? =
-        file.read()?.let { decode(it) }
+        file.read()?.let { decode(it, file.toString()) }
 
     fun encode(content: T) =
-        plugins.json.encodeToString(migrator, content)
+        plugins.json.encodeToString(migrator.Migrate(), content)
 
     fun encodeToJsonElement(content: T) =
-        plugins.json.encodeToJsonElement(migrator, content)
+        plugins.json.encodeToJsonElement(migrator.Migrate(), content)
 
     suspend fun save(file: Fs.File, content: T, allowOverwrite: Boolean = false) =
         file.write(
diff --git a/src/commonMain/kotlin/baaahs/migrator/DataMigrator.kt b/src/commonMain/kotlin/baaahs/migrator/DataMigrator.kt
index f5ecc5019b..8a82c12650 100644
--- a/src/commonMain/kotlin/baaahs/migrator/DataMigrator.kt
+++ b/src/commonMain/kotlin/baaahs/migrator/DataMigrator.kt
@@ -5,10 +5,10 @@ import kotlinx.serialization.KSerializer
 import kotlinx.serialization.json.*
 
 open class DataMigrator<T : Any>(
-    tSerializer: KSerializer<T>,
+    private val tSerializer: KSerializer<T>,
     private val migrations: List<Migration> = emptyList(),
     private val versionKey: String = "version"
-) : JsonTransformingSerializer<T>(tSerializer) {
+) {
     init {
         val versionDupes = migrations.groupBy { it.toVersion }
             .filter { (_, matching) -> matching.size > 1 }
@@ -20,39 +20,45 @@ open class DataMigrator<T : Any>(
     }
     private val currentVersion = migrations.maxOfOrNull { it.toVersion } ?: 0
 
-    override fun transformDeserialize(element: JsonElement): JsonElement {
-        if (element !is JsonObject) return element
-        val fromVersion = element[versionKey]?.jsonPrimitive?.int ?: 0
-        if (fromVersion == currentVersion)
-            return element.toMutableMap()
-                .apply { remove(versionKey) }.toJsonObj()
+    inner class Migrate(
+        fileName: String? = null
+    ) : JsonTransformingSerializer<T>(tSerializer) {
+        private val context = fileName?.let { "$it: " } ?: ""
 
-        var newJson = element
-            .toMutableMap().apply {
-                remove(versionKey)
-            }.toJsonObj()
+        override fun transformDeserialize(element: JsonElement): JsonElement {
+            if (element !is JsonObject) return element
+            val fromVersion = element[versionKey]?.jsonPrimitive?.int ?: 0
+            if (fromVersion == currentVersion)
+                return element.toMutableMap()
+                    .apply { remove(versionKey) }.toJsonObj()
+
+            var newJson = element
+                .toMutableMap().apply {
+                    remove(versionKey)
+                }.toJsonObj()
 
-        logger.debug { "Migrating from v$fromVersion:\n$newJson" }
+            logger.debug { "${context}Migrating from v$fromVersion:\n$newJson" }
 
-        migrations.forEach { migration ->
-            if (fromVersion < migration.toVersion) {
-                logger.info {
-                    "Migrating from $fromVersion to ${migration.toVersion} (${migration::class.simpleName})."
+            migrations.forEach { migration ->
+                if (fromVersion < migration.toVersion) {
+                    logger.info {
+                        "${context}Migrating from $fromVersion to ${migration.toVersion} (${migration::class.simpleName})."
+                    }
+                    newJson = migration.migrate(newJson)
                 }
-                newJson = migration.migrate(newJson)
             }
-        }
 
-        logger.debug { "Migrated to v$currentVersion:\n$newJson" }
+            logger.debug { "${context}Migrated to v$currentVersion:\n$newJson" }
 
-        return newJson.toJsonObj()
-    }
+            return newJson.toJsonObj()
+        }
 
-    override fun transformSerialize(element: JsonElement): JsonElement {
-        if (element !is JsonObject) return element
-        return element.toMutableMap().apply {
-            put(versionKey, JsonPrimitive(currentVersion))
-        }.toJsonObj()
+        override fun transformSerialize(element: JsonElement): JsonElement {
+            if (element !is JsonObject) return element
+            return element.toMutableMap().apply {
+                put(versionKey, JsonPrimitive(currentVersion))
+            }.toJsonObj()
+        }
     }
 
     abstract class Migration(val toVersion: Int) {
diff --git a/src/commonMain/kotlin/baaahs/show/migration/V10_RemoveVarPrefixFromXyPad.kt b/src/commonMain/kotlin/baaahs/show/migration/V10_RemoveVarPrefixFromXyPad.kt
index 29913375e4..d29f1b225e 100644
--- a/src/commonMain/kotlin/baaahs/show/migration/V10_RemoveVarPrefixFromXyPad.kt
+++ b/src/commonMain/kotlin/baaahs/show/migration/V10_RemoveVarPrefixFromXyPad.kt
@@ -14,7 +14,6 @@ object V10_RemoveVarPrefixFromXyPad : DataMigrator.Migration(10) {
     override fun migrate(from: JsonObject): JsonObject {
         return from.toMutableMap().apply {
             mapObjsInDict("feeds") { _, control ->
-                println("from = $control")
                 if (control.type == "baaahs.Core:XyPad") {
                     control.remove("varPrefix")
                 }
diff --git a/src/jsMain/kotlin/baaahs/app/ui/UiActions.kt b/src/jsMain/kotlin/baaahs/app/ui/UiActions.kt
index bac98ae286..f77aa24d8c 100644
--- a/src/jsMain/kotlin/baaahs/app/ui/UiActions.kt
+++ b/src/jsMain/kotlin/baaahs/app/ui/UiActions.kt
@@ -20,14 +20,14 @@ actual object UiActions {
         val type = FileType.Show
         val filename = "${show.title}${type.extension}"
         val contentType = "application/json;charset=utf-8;"
-        doDownload(filename, show, ShowMigrator, contentType, plugins)
+        doDownload(filename, show, ShowMigrator.Migrate(filename), contentType, plugins)
     }
 
     actual fun downloadScene(scene: Scene, plugins: Plugins) {
         val type = FileType.Scene
         val filename = "${scene.title}${type.extension}"
         val contentType = "application/json;charset=utf-8;"
-        doDownload(filename, scene, SceneMigrator, contentType, plugins)
+        doDownload(filename, scene, SceneMigrator.Migrate(filename), contentType, plugins)
     }
 
     private fun <T: Any> doDownload(