refactor: migrate Gson serialization to kotlinx.serialization (#475)

* refactor: migrate Gson serialization to kotlinx.serialization

* fix(dto): resolve serializing Any? and Map<String, Any?> with JsonElement

* test(refactor): fix unit test issues under kotlinx.serialization and update nullable DTO defaults

* fix(chat): support flexible numeric and string decoding for message timestamps

* fix(chat): sequentialize cache loading to avoid overwriting network state

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatBubble.kt

@@ -82,6 +82,7 @@ import androidx.compose.ui.unit.sp
 import coil.compose.AsyncImage
 import com.m57.hermescontrol.R
 import com.m57.hermescontrol.data.model.Attachment
+import com.m57.hermescontrol.data.remote.OkHttpProvider
 import com.m57.hermescontrol.theme.AssistantBubble
 import com.m57.hermescontrol.theme.AssistantBubbleLight
 import com.m57.hermescontrol.theme.StatusGreen
@@ -92,6 +93,14 @@ import com.m57.hermescontrol.theme.ToolChipColorLight
 import com.m57.hermescontrol.theme.UserBubble
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.launch
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.boolean
+import kotlinx.serialization.json.double
+import kotlinx.serialization.json.int
 import java.time.Instant
 import java.time.ZoneId
 import java.time.format.DateTimeFormatter
@@ -537,9 +546,7 @@ fun parseToolOutput(
     val trimmed = content.trim()
     if (!trimmed.startsWith("{") && !trimmed.startsWith("[")) return null
     return try {
-        val element =
-            com.google.gson.JsonParser
-                .parseString(trimmed)
+        val element = OkHttpProvider.json.parseToJsonElement(trimmed)
         if (!element.isJsonObject) return null
         val obj = element.asJsonObject
 
@@ -2301,3 +2308,23 @@ private fun InlineAttachment(
         }
     }
 }
+
+// Extensions for backward compatibility with Gson API under kotlinx.serialization
+private val JsonElement.isJsonObject: Boolean get() = this is JsonObject
+private val JsonElement.asJsonObject: JsonObject get() = this as JsonObject
+private val JsonElement.isJsonArray: Boolean get() = this is JsonArray
+private val JsonElement.asJsonArray: JsonArray get() = this as JsonArray
+private val JsonElement.isJsonNull: Boolean get() = this is JsonNull
+private val JsonElement.isJsonPrimitive: Boolean get() = this is JsonPrimitive
+private val JsonElement.asString: String get() = (this as JsonPrimitive).content
+private val JsonElement.asDouble: Double get() = (this as JsonPrimitive).double
+private val JsonElement.asInt: Int get() = (this as JsonPrimitive).int
+private val JsonElement.asBoolean: Boolean get() = (this as JsonPrimitive).boolean
+
+private fun JsonElement.has(key: String): Boolean = (this as? JsonObject)?.containsKey(key) == true
+
+private fun JsonObject.entrySet(): Set<Map.Entry<String, JsonElement>> = entries
+
+private fun JsonObject.keySet(): Set<String> = keys
+
+private fun JsonArray.size(): Int = size

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -18,6 +18,7 @@ import com.m57.hermescontrol.data.ws.ConnectionStatus
 import com.m57.hermescontrol.data.ws.HermesWsClient
 import com.m57.hermescontrol.data.ws.WsEvent
 import com.m57.hermescontrol.data.ws.WsMethods
+import com.m57.hermescontrol.data.ws.toJsonElement
 import kotlinx.coroutines.CompletableDeferred
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
@@ -32,6 +33,7 @@ import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import kotlinx.coroutines.withTimeout
+import kotlinx.serialization.json.decodeFromJsonElement
 import java.util.concurrent.ConcurrentHashMap
 
 private const val TAG = "ChatViewModel"
@@ -845,7 +847,9 @@ class ChatViewModel(
                     val body = result.data
                     val platformsStr =
                         body.gateway_platforms?.entries?.joinToString("\n") { (k, v) ->
-                            "  • **$k**: ${v.state ?: "Unknown"}${if (v.error_code != null) " (Error: ${v.error_code})" else ""}"
+                            val status = v?.state ?: "Unknown"
+                            val err = v?.error_code?.let { " (Error: $it)" } ?: ""
+                            "  • **$k**: $status$err"
                         } ?: "No active platforms"
                     addAssistantMessage(
                         """
@@ -983,13 +987,15 @@ class ChatViewModel(
 
     fun refreshCurrentSession() {
         val sessionId = _uiState.value.currentSessionId ?: return
-        loadCachedMessages(sessionId)
-        // Skip the REST call if we already have messages — the WebSocket keeps
-        // the UI current. The REST endpoint can 404 on tab switch-back when the
-        // WS session ID doesn't round-trip through the resolver (issue #366),
-        // showing a misleading error banner while the connection is fine.
-        if (_uiState.value.messages.isNotEmpty()) return
-        loadSessionMessages(sessionId)
+        viewModelScope.launch {
+            loadCachedMessages(sessionId).join()
+            // Skip the REST call if we already have messages — the WebSocket keeps
+            // the UI current. The REST endpoint can 404 on tab switch-back when the
+            // WS session ID doesn't round-trip through the resolver (issue #366),
+            // showing a misleading error banner while the connection is fine.
+            if (_uiState.value.messages.isNotEmpty()) return@launch
+            loadSessionMessages(sessionId)
+        }
     }
 
     fun refreshSettings() {
@@ -1004,8 +1010,8 @@ class ChatViewModel(
     @Suppress("UNCHECKED_CAST")
     private fun parseCommandCatalog(map: Map<*, *>): CommandCatalog? =
         try {
-            val json = OkHttpProvider.gson.toJson(map)
-            OkHttpProvider.gson.fromJson(json, CommandCatalog::class.java)
+            val jsonElement = map.toJsonElement()
+            OkHttpProvider.json.decodeFromJsonElement<CommandCatalog>(jsonElement)
         } catch (e: Exception) {
             Log.e(TAG, "Failed to parse command catalog", e)
             null
@@ -1030,23 +1036,25 @@ class ChatViewModel(
             )
         }
         _streamingState.update { StreamingState() }
-        // Step 1: Load cached messages first — instant display
-        loadCachedMessages(sessionId)
-        // Step 2: Resume session on server
-        viewModelScope.launch(Dispatchers.IO) {
-            wsClient.send(
-                WsMethods.SESSION_RESUME,
-                mapOf("session_id" to sessionId),
-                onSent = { id -> trackRequest(id, WsMethods.SESSION_RESUME) },
-            )
+        viewModelScope.launch {
+            // Step 1: Load cached messages first — instant display
+            loadCachedMessages(sessionId).join()
+            // Step 2: Resume session on server
+            launch(Dispatchers.IO) {
+                wsClient.send(
+                    WsMethods.SESSION_RESUME,
+                    mapOf("session_id" to sessionId),
+                    onSent = { id -> trackRequest(id, WsMethods.SESSION_RESUME) },
+                )
+            }
+            // Step 3: Load fresh messages from REST and merge
+            loadSessionMessages(sessionId)
+            // Step 4: Refresh session list to get latest titles
+            loadSessions()
         }
-        // Step 3: Load fresh messages from REST and merge
-        loadSessionMessages(sessionId)
-        // Step 4: Refresh session list to get latest titles
-        loadSessions()
     }
 
-    private fun loadCachedMessages(sessionId: String) {
+    private fun loadCachedMessages(sessionId: String): Job =
         viewModelScope.launch(Dispatchers.IO) {
             val cachedMessages = repo.loadMessages(sessionId)
             _uiState.update { state ->
@@ -1058,7 +1066,6 @@ class ChatViewModel(
                 }
             }
         }
-    }
 
     private fun loadSessionMessages(sessionId: String) {
         viewModelScope.launch {
@@ -1082,7 +1089,7 @@ class ChatViewModel(
                             // Room duplicates when re-loading the same session.
                             val stableId = "rest-$sessionId-$index"
                             // Preserve original timestamp from the API when available
-                            val ts = msg.timestamp?.toLongOrNull() ?: System.currentTimeMillis()
+                            val ts = msg.timestampText?.toLongOrNull() ?: System.currentTimeMillis()
                             ChatMessage(
                                 id = stableId,
                                 role = role,

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatWsEventReducer.kt

@@ -2,6 +2,8 @@ package com.m57.hermescontrol.ui.chat
 
 import com.m57.hermescontrol.data.remote.OkHttpProvider
 import com.m57.hermescontrol.data.ws.WsEvent
+import com.m57.hermescontrol.data.ws.toJsonElement
+import kotlinx.serialization.encodeToString
 
 /**
  * Pure state reducer for WebSocket events.
@@ -225,7 +227,7 @@ object ChatWsEventReducer {
     ): ReducerResult {
         val contentJson =
             event.data?.let {
-                OkHttpProvider.gson.toJson(it)
+                OkHttpProvider.json.encodeToString(it.toJsonElement())
             } ?: ""
         val toolMessage =
             ChatMessage(
@@ -271,7 +273,7 @@ object ChatWsEventReducer {
     ): ReducerResult {
         val contentJson =
             event.data?.let {
-                OkHttpProvider.gson.toJson(it)
+                OkHttpProvider.json.encodeToString(it.toJsonElement())
             } ?: ""
         val messages = state.messages.toMutableList()
         val toolIdx =

app/src/main/java/com/m57/hermescontrol/ui/config/ConfigScreen.kt

@@ -53,8 +53,6 @@ import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.lifecycle.viewmodel.compose.viewModel
-import com.google.gson.JsonElement
-import com.google.gson.JsonPrimitive
 import com.m57.hermescontrol.R
 import com.m57.hermescontrol.data.model.SchemaField
 import com.m57.hermescontrol.ui.common.ErrorState
@@ -63,6 +61,11 @@ import com.m57.hermescontrol.ui.common.LoadingState
 import com.m57.hermescontrol.ui.common.NavIcon
 import com.m57.hermescontrol.ui.common.SearchBar
 import com.m57.hermescontrol.ui.common.ToastEffect
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.booleanOrNull
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -580,7 +583,7 @@ private fun ConfigField(
                             modifier = Modifier.weight(1f),
                         )
                         Switch(
-                            checked = currentValue?.asBoolean ?: false,
+                            checked = (currentValue as? JsonPrimitive)?.booleanOrNull ?: false,
                             onCheckedChange = { onChange(JsonPrimitive(it)) },
                         )
                     }
@@ -590,7 +593,7 @@ private fun ConfigField(
                     DropdownField(
                         label = label,
                         options = field.options ?: emptyList(),
-                        selectedValue = currentValue?.asString ?: "",
+                        selectedValue = (currentValue as? JsonPrimitive)?.content ?: "",
                         onOptionSelected = { onChange(JsonPrimitive(it)) },
                     )
                     if (description != label) {
@@ -607,7 +610,7 @@ private fun ConfigField(
                     OutlinedTextField(
                         value =
                             currentValue?.let {
-                                if (it.isJsonPrimitive) it.asString else ""
+                                if (it is JsonPrimitive) it.content else ""
                             } ?: "",
                         onValueChange = { text ->
                             text.toDoubleOrNull()?.let { doubleVal ->
@@ -630,10 +633,10 @@ private fun ConfigField(
                     // String or other type
                     val textValue =
                         currentValue?.let {
-                            when {
-                                it.isJsonPrimitive -> it.asString
-                                it.isJsonObject -> it.toString()
-                                it.isJsonArray -> it.toString()
+                            when (it) {
+                                is JsonPrimitive -> it.content
+                                is JsonObject -> it.toString()
+                                is JsonArray -> it.toString()
                                 else -> ""
                             }
                         } ?: ""
@@ -708,20 +711,17 @@ private fun collectUncoveredPaths(
         val dotPath = if (prefix.isEmpty()) key else "$prefix.$key"
         // If this exact path is in the schema, it's covered — skip entirely
         if (dotPath in schemaPaths) continue
-        if (value is com.google.gson.JsonObject) {
-            // Recurse into nested objects to find uncovered leaves
-            val subKeys = value.keySet()
+        if (value is JsonObject) {
             // Before recursing deeper, check if this whole sub-tree has any schema coverage
             val hasSchemaCoverage = schemaPaths.any { it == dotPath || it.startsWith("$dotPath.") }
             if (hasSchemaCoverage) {
                 // Schema covers some sub-paths — recurse to find what's NOT covered
-                val subMap = subKeys.associateWith { value.get(it) }
-                result.addAll(collectUncoveredPaths(subMap, schemaPaths, dotPath))
+                result.addAll(collectUncoveredPaths(value, schemaPaths, dotPath))
             } else {
                 // No schema coverage at all for this subtree — add the whole thing
                 result.add(dotPath)
             }
-        } else if (value is com.google.gson.JsonArray) {
+        } else if (value is JsonArray) {
             // Arrays: check if the path is in schema; if not, add it
             if (dotPath !in schemaPaths) {
                 result.add(dotPath)
@@ -746,7 +746,7 @@ private fun getJsonValue(
         if (i == 0) {
             current = config[key]
         } else {
-            current = current?.asJsonObject?.get(key)
+            current = (current as? JsonObject)?.get(key)
         }
         if (current == null) return null
     }

app/src/main/java/com/m57/hermescontrol/ui/config/ConfigViewModel.kt

@@ -2,8 +2,6 @@ package com.m57.hermescontrol.ui.config
 
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
-import com.google.gson.JsonElement
-import com.google.gson.JsonObject
 import com.m57.hermescontrol.data.model.ConfigSchemaResponse
 import com.m57.hermescontrol.data.model.ConfigUpdateRequest
 import com.m57.hermescontrol.data.model.UpdateRawConfigRequest
@@ -19,6 +17,8 @@ import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonObject
 
 data class ConfigUiState(
     val isLoading: Boolean = false,
@@ -296,15 +296,14 @@ class ConfigViewModel :
         val topKey = parts.first()
         val rest = parts.drop(1).joinToString(".")
         val topValue = config[topKey]
-        val nestedJson = topValue?.asJsonObject ?: JsonObject()
+        val nestedJson = (topValue as? JsonObject) ?: JsonObject(emptyMap())
         val updatedNested =
             applyNestedValue(
-                nestedJson.entrySet().associate { it.key to it.value },
+                nestedJson,
                 rest,
                 value,
             )
-        val nestedObj = JsonObject()
-        updatedNested.forEach { (k, v) -> nestedObj.add(k, v) }
+        val nestedObj = JsonObject(updatedNested)
         val mutable = config.toMutableMap()
         mutable[topKey] = nestedObj
         return mutable
@@ -312,23 +311,38 @@ class ConfigViewModel :
 
     /** Build a nested JSON changeset from dot-path pending changes. */
     private fun buildChangeset(changes: Map<String, JsonElement>): Map<String, JsonElement> {
-        val root = JsonObject()
+        val root = mutableMapOf<String, Any>()
         for ((dotPath, value) in changes) {
             val parts = dotPath.split(".")
             var current = root
             for (i in 0 until parts.size - 1) {
                 val key = parts[i]
-                val existing = current.get(key)
-                if (existing == null || !existing.isJsonObject) {
-                    val newObj = JsonObject()
-                    current.add(key, newObj)
-                    current = newObj
+                val existing = current[key]
+                if (existing !is MutableMap<*, *>) {
+                    val newMap = mutableMapOf<String, Any>()
+                    current[key] = newMap
+                    current = newMap
                 } else {
-                    current = existing.asJsonObject
+                    @Suppress("UNCHECKED_CAST")
+                    current = existing as MutableMap<String, Any>
                 }
             }
-            current.add(parts.last(), value)
+            current[parts.last()] = value
         }
-        return root.entrySet().associate { it.key to it.value as JsonElement }
+
+        fun toJsonObject(map: Map<String, Any>): JsonObject {
+            val content =
+                map.mapValues { (_, v) ->
+                    if (v is Map<*, *>) {
+                        @Suppress("UNCHECKED_CAST")
+                        toJsonObject(v as Map<String, Any>)
+                    } else {
+                        v as JsonElement
+                    }
+                }
+            return JsonObject(content)
+        }
+
+        return toJsonObject(root)
     }
 }

app/src/main/java/com/m57/hermescontrol/ui/connect/ConnectViewModel.kt

@@ -9,6 +9,7 @@ import com.m57.hermescontrol.data.local.AuthManager
 import com.m57.hermescontrol.data.remote.ApiClient
 import com.m57.hermescontrol.data.remote.NetworkError
 import com.m57.hermescontrol.data.remote.NetworkResult
+import com.m57.hermescontrol.data.remote.OkHttpProvider
 import com.m57.hermescontrol.data.remote.safeApiCall
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.MutableStateFlow
@@ -17,6 +18,8 @@ import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
 
 data class ConnectUiState(
     val token: String = "",
@@ -317,13 +320,10 @@ class ConnectViewModel(
                 val decodedBytes = android.util.Base64.decode(trimmed, android.util.Base64.DEFAULT)
                 val decodedString = String(decodedBytes, Charsets.UTF_8)
                 if (decodedString.startsWith("{") && decodedString.endsWith("}")) {
-                    val json =
-                        com.google.gson.JsonParser
-                            .parseString(decodedString)
-                            .asJsonObject
-                    val host = json.get("host")?.asString
-                    val port = json.get("port")?.asInt?.toString() ?: json.get("port")?.asString
-                    val token = json.get("token")?.asString
+                    val json = OkHttpProvider.json.parseToJsonElement(decodedString).jsonObject
+                    val host = json["host"]?.jsonPrimitive?.content
+                    val port = json["port"]?.jsonPrimitive?.content
+                    val token = json["token"]?.jsonPrimitive?.content
                     if (host != null && port != null && token != null) {
                         _uiState.update {
                             it.copy(

app/src/main/java/com/m57/hermescontrol/ui/cron/CronJobsViewModel.kt

@@ -8,6 +8,7 @@ import com.m57.hermescontrol.data.model.UpdateCronJobRequest
 import com.m57.hermescontrol.data.remote.ApiClient
 import com.m57.hermescontrol.data.remote.NetworkResult
 import com.m57.hermescontrol.data.remote.safeApiCall
+import com.m57.hermescontrol.data.ws.toJsonElement
 import com.m57.hermescontrol.ui.common.ToastHost
 import com.m57.hermescontrol.ui.common.safeLaunchLoad
 import kotlinx.coroutines.Dispatchers
@@ -18,6 +19,8 @@ import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
 
 data class CronJobsUiState(
     val isLoading: Boolean = false,
@@ -292,7 +295,7 @@ class CronJobsViewModel :
                         safeApiCall {
                             ApiClient.hermesApi.updateCronJob(
                                 editor.jobId ?: "",
-                                UpdateCronJobRequest(updates = updates),
+                                UpdateCronJobRequest(updates = updates.mapValues { it.value.toJsonElement() }),
                             )
                         }
                     }
@@ -336,8 +339,8 @@ class CronJobsViewModel :
     private fun extractScheduleString(job: CronJob): String {
         val s = job.schedule
         return when (s) {
-            is String -> s
-            is Map<*, *> -> (s["value"] as? String) ?: job.scheduleText
+            is JsonPrimitive -> s.content
+            is JsonObject -> (s["value"] as? JsonPrimitive)?.content ?: job.scheduleText
             else -> job.scheduleText
         }
     }

app/src/main/java/com/m57/hermescontrol/ui/gateway/GatewayScreen.kt

@@ -267,7 +267,7 @@ fun GatewayScreen(
                                                         style = MaterialTheme.typography.bodyLarge,
                                                         fontWeight = FontWeight.Medium,
                                                     )
-                                                    val stateText = pStatus.state?.uppercase() ?: "UNKNOWN"
+                                                    val stateText = pStatus?.state?.uppercase() ?: "UNKNOWN"
                                                     val badgeColor =
                                                         when (stateText) {
                                                             "RUNNING" -> Color(0xFF4CAF50)

app/src/main/java/com/m57/hermescontrol/ui/pairing/PairingCodeEntryViewModel.kt

@@ -6,12 +6,12 @@ import android.util.Log
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.ViewModelProvider
 import androidx.lifecycle.viewModelScope
-import com.google.gson.JsonParser
 import com.m57.hermescontrol.R
 import com.m57.hermescontrol.data.local.AuthManager
 import com.m57.hermescontrol.data.remote.ApiClient
 import com.m57.hermescontrol.data.remote.NetworkError
 import com.m57.hermescontrol.data.remote.NetworkResult
+import com.m57.hermescontrol.data.remote.OkHttpProvider
 import com.m57.hermescontrol.data.remote.safeApiCall
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.MutableStateFlow
@@ -20,6 +20,8 @@ import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
 
 /**
  * UI state for the pairing code entry screen.
@@ -88,12 +90,10 @@ class PairingCodeEntryViewModel(
                 val decodedBytes = Base64.decode(trimmed, Base64.DEFAULT)
                 val decodedString = String(decodedBytes, Charsets.UTF_8)
                 if (decodedString.startsWith("{") && decodedString.endsWith("}")) {
-                    val json = JsonParser.parseString(decodedString).asJsonObject
-                    val host = json.get("host")?.asString
-                    val port =
-                        json.get("port")?.asInt?.toString()
-                            ?: json.get("port")?.asString
-                    val token = json.get("token")?.asString
+                    val json = OkHttpProvider.json.parseToJsonElement(decodedString).jsonObject
+                    val host = json["host"]?.jsonPrimitive?.content
+                    val port = json["port"]?.jsonPrimitive?.content
+                    val token = json["token"]?.jsonPrimitive?.content
                     // Accept raw "host" + "port" + "token", or a composite "pairing" payload
                     if (host != null && port != null && token != null) {
                         connectToDashboard(host, port.toIntOrNull() ?: 9119, token)

app/src/main/java/com/m57/hermescontrol/ui/system/SystemScreen.kt

@@ -786,7 +786,7 @@ private fun LazyListScope.gatewaySection(
                                         style = MaterialTheme.typography.bodyMedium,
                                         modifier = Modifier.weight(1f),
                                     )
-                                    platform.state?.let { platformState ->
+                                    platform?.state?.let { platformState ->
                                         StatusBadge(
                                             text = platformState,
                                             status =