fix: streaming performance, error handling, and API diagnostics

- Eliminate recomposition hell: isolate auto-scroll via derivedStateOf + snapshotFlow
- Preserve partial response on connection drop instead of replacing with error
- O(1) incremental stream parser (StreamConsumer.Incremental) replacing O(N²) re-scan
- Parallel conversation creation via async/await to reduce TTFT
- TCP/TLS prewarm on app resume via LifecycleResumeEffect
- Surface API failures with Log.w and UI error banner instead of silent emptyList()
- Fix nullable KaizenModel.name crash in ModelSheet

app/build.gradle.kts

@@ -50,6 +50,7 @@ dependencies {
     implementation(libs.androidx.compose.ui.tooling.preview)
     implementation(libs.androidx.core.ktx)
     implementation(libs.androidx.lifecycle.runtime.ktx)
+    implementation(libs.androidx.lifecycle.runtime.compose)
     // Networking + JSON for the real backend (POST /api/v1/auth/token, /chat, /me)
     implementation(libs.okhttp)
     implementation(libs.kotlinx.serialization.json)

app/src/main/java/dev/kaizen/app/chat/ChatScreen.kt

@@ -34,12 +34,14 @@ import androidx.compose.material3.rememberDrawerState
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateListOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
@@ -47,7 +49,10 @@ import androidx.compose.ui.draw.shadow
 import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.platform.LocalConfiguration
+import androidx.compose.material3.Text
+import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
 import dev.kaizen.app.haptics.rememberHaptics
 import dev.kaizen.app.net.ChatHttpException
 import dev.kaizen.app.net.ChatSpeed
@@ -59,6 +64,8 @@ import dev.kaizen.app.net.SessionViewModel
 import dev.kaizen.app.net.WireMessage
 import dev.kaizen.app.settings.SettingsScreen
 import dev.kaizen.app.settings.SettingsViewModel
+import androidx.lifecycle.compose.LifecycleResumeEffect
+import kotlinx.coroutines.async
 import kotlinx.coroutines.launch
 
 // Screen enumeration for modular state-driven navigation (Zero Technical Debt)
@@ -100,20 +107,40 @@ fun ChatScreen(
     var conversationId by remember { mutableStateOf<String?>(null) }
     var conversations by remember { mutableStateOf<List<ConversationSummary>>(emptyList()) }
 
+    var loadError by remember { mutableStateOf<String?>(null) }
+
     LaunchedEffect(session.config?.baseUrl, session.config?.token) {
         val cfg = session.config ?: return@LaunchedEffect
-        // Fetch fresh models from server and cache them locally if successful
+        loadError = null
         val fetchedModels = KaizenApi.fetchModels(cfg.baseUrl, cfg.token)
-        if (fetchedModels.isNotEmpty()) {
+        if (fetchedModels != null) {
             models = fetchedModels
-            session.store.saveModelsCache(fetchedModels)
+            if (fetchedModels.isNotEmpty()) session.store.saveModelsCache(fetchedModels)
         }
-        conversations = KaizenApi.fetchConversations(cfg.baseUrl, cfg.token)
+        val fetchedConvos = KaizenApi.fetchConversations(cfg.baseUrl, cfg.token)
+        if (fetchedConvos != null) {
+            conversations = fetchedConvos
+        }
+        if (fetchedModels == null || fetchedConvos == null) {
+            loadError = "Verbindung zum Server fehlgeschlagen"
+        }
+    }
+
+    // Warm the HTTP/2 connection pool on every app resume (covers background → foreground
+    // after OkHttp's 5-minute keep-alive has expired). On first composition, the
+    // LaunchedEffect above already warms the pool via fetchModels/fetchConversations.
+    LifecycleResumeEffect(session.config?.baseUrl) {
+        session.config?.let { cfg ->
+            scope.launch { KaizenApi.prewarm(cfg.baseUrl) }
+        }
+        onPauseOrDispose { }
     }
 
     fun refreshConversations() {
         val cfg = session.config ?: return
-        scope.launch { conversations = KaizenApi.fetchConversations(cfg.baseUrl, cfg.token) }
+        scope.launch {
+            KaizenApi.fetchConversations(cfg.baseUrl, cfg.token)?.let { conversations = it }
+        }
     }
 
     /** Start a fresh chat — drops the active conversation, the next send creates a new one. */
@@ -168,17 +195,19 @@ fun ChatScreen(
             haptics.thinkingStart()
             var sawContent = false
             try {
-                // Ensure a server conversation exists before the exchange is persisted.
-                val convId = conversationId ?: KaizenApi.createConversation(cfg.baseUrl, cfg.token)?.also {
-                    conversationId = it
-                }
+                // Launch conversation creation in parallel — the chat stream does not
+                // need a conversation id, only the post-stream persistence does. This
+                // eliminates a full network RTT from the TTFT on new chats.
+                val convIdDeferred = if (conversationId == null) {
+                    async { KaizenApi.createConversation(cfg.baseUrl, cfg.token) }
+                } else null
 
                 KaizenApi.chat(cfg.baseUrl, cfg.token, cfg.model, history, cfg.speed).collect { visible ->
                     val idx = messages.indexOfLast { it.id == assistantId }
                     if (idx < 0) return@collect
                     if (!sawContent && visible.isNotEmpty()) {
                         sawContent = true
-                        haptics.responseStart() // the answer begins (reasoning may have streamed first)
+                        haptics.responseStart()
                     }
                     messages[idx] = messages[idx].copy(
                         thinking = if (sawContent) false else messages[idx].thinking,
@@ -189,6 +218,9 @@ fun ChatScreen(
                 val finalContent = if (idx >= 0) messages[idx].content else ""
                 if (idx >= 0) messages[idx] = messages[idx].copy(streaming = false, thinking = false)
 
+                // Await the conversation id — the create ran in parallel with the stream
+                val convId = convIdDeferred?.await()?.also { conversationId = it } ?: conversationId
+
                 // Persist the exchange (best-effort); auto-title + sidebar refresh on a new conversation.
                 if (convId != null && finalContent.isNotEmpty()) {
                     val saved = KaizenApi.saveMessages(
@@ -203,11 +235,15 @@ fun ChatScreen(
                 }
             } catch (e: Exception) {
                 val idx = messages.indexOfLast { it.id == assistantId }
-                if (idx >= 0) messages[idx] = messages[idx].copy(
-                    streaming = false,
-                    thinking = false,
-                    content = chatErrorText(e),
-                )
+                if (idx >= 0) {
+                    val existing = messages[idx].content
+                    val errorSuffix = "\n\n⚠ ${chatErrorText(e)}"
+                    messages[idx] = messages[idx].copy(
+                        streaming = false,
+                        thinking = false,
+                        content = if (existing.isNotBlank()) existing + errorSuffix else chatErrorText(e),
+                    )
+                }
             } finally {
                 isStreaming = false
                 haptics.responseEnd()
@@ -215,10 +251,17 @@ fun ChatScreen(
         }
     }
 
-    // Keep pinned to the bottom as the stream grows
-    val lastLength = messages.lastOrNull()?.content?.length ?: 0
-    LaunchedEffect(messages.size, lastLength) {
-        if (messages.isNotEmpty()) listState.scrollToItem(messages.lastIndex)
+    // Auto-scroll: isolated from the main composition via derivedStateOf + snapshotFlow.
+    // Only the snapshot read triggers work — the root composable does NOT recompose on
+    // every streamed character, eliminating the recomposition storm that previously
+    // re-drew the entire screen (top bar, input, background) at 60+ fps.
+    val autoScrollKey by remember {
+        derivedStateOf { messages.size to (messages.lastOrNull()?.content?.length ?: 0) }
+    }
+    LaunchedEffect(Unit) {
+        snapshotFlow { autoScrollKey }.collect {
+            if (messages.isNotEmpty()) listState.scrollToItem(messages.lastIndex)
+        }
     }
 
     // Single source of truth Screen routing
@@ -341,6 +384,27 @@ fun ChatScreen(
                             )
                         }
 
+                        // Error banner when initial data load fails
+                        if (loadError != null) {
+                            Box(
+                                modifier = Modifier
+                                    .align(Alignment.TopCenter)
+                                    .statusBarsPadding()
+                                    .padding(top = 62.dp, start = 24.dp, end = 24.dp)
+                                    .clip(RoundedCornerShape(12.dp))
+                                    .background(Color(0xFFDC2626).copy(alpha = 0.15f))
+                                    .border(1.dp, Color(0xFFDC2626).copy(alpha = 0.3f), RoundedCornerShape(12.dp))
+                                    .padding(horizontal = 14.dp, vertical = 8.dp),
+                            ) {
+                                Text(
+                                    loadError!!,
+                                    color = Color(0xFFDC2626),
+                                    fontSize = 12.sp,
+                                    fontWeight = FontWeight.Medium,
+                                )
+                            }
+                        }
+
                         // 3. Floating Bottom Control Dock (responsive centering and clear bottom margins)
                         Column(
                             modifier = Modifier

app/src/main/java/dev/kaizen/app/chat/ModelSheet.kt

@@ -69,7 +69,7 @@ private fun groupFor(m: KaizenModel): String = when {
 private fun buildGroups(models: List<KaizenModel>, favorites: Set<String>, query: String): List<ModelGroup> {
     val q = query.trim().lowercase()
     val filtered = if (q.isEmpty()) models
-    else models.filter { it.name.lowercase().contains(q) || it.id.lowercase().contains(q) }
+    else models.filter { it.name?.lowercase()?.contains(q) == true || it.id.lowercase().contains(q) }
 
     val groups = mutableListOf<ModelGroup>()
     val favs = filtered.filter { it.id in favorites }
@@ -267,7 +267,7 @@ private fun ModelRow(
         // Model Name & Clean Technical ID
         Column(modifier = Modifier.weight(1f)) {
             Text(
-                model.name,
+                model.name ?: model.id,
                 color = cs.onBackground,
                 fontSize = 15.sp,
                 fontWeight = if (selected) FontWeight.SemiBold else FontWeight.Medium,

app/src/main/java/dev/kaizen/app/net/KaizenApi.kt

@@ -13,8 +13,9 @@ import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.OkHttpClient
 import okhttp3.Request
 import okhttp3.RequestBody.Companion.toRequestBody
-import java.io.ByteArrayOutputStream
+import android.util.Log
 import java.io.IOException
+import java.io.InputStreamReader
 import java.util.concurrent.TimeUnit
 
 // --- Wire DTOs (additive v1 contract — ignoreUnknownKeys tolerates new fields) ---
@@ -95,6 +96,8 @@ class ChatHttpException(val code: Int) : IOException("chat failed: HTTP $code")
  */
 object KaizenApi {
 
+    private const val TAG = "KaizenApi"
+
     private val client = OkHttpClient.Builder()
         .connectTimeout(15, TimeUnit.SECONDS)
         .readTimeout(0, TimeUnit.SECONDS) // streaming chat — never time out a healthy stream
@@ -142,8 +145,8 @@ object KaizenApi {
             }
         }
 
-    /** GET /api/v1/models — the full model catalog for the picker (best-effort, empty on any failure). */
-    suspend fun fetchModels(baseUrl: String, token: String): List<KaizenModel> =
+    /** GET /api/v1/models — the full model catalog for the picker. Returns null on failure (with logged reason). */
+    suspend fun fetchModels(baseUrl: String, token: String): List<KaizenModel>? =
         withContext(Dispatchers.IO) {
             val req = Request.Builder()
                 .url("$baseUrl/api/v1/models")
@@ -151,16 +154,20 @@ object KaizenApi {
                 .build()
             try {
                 client.newCall(req).execute().use { resp ->
-                    if (!resp.isSuccessful) return@use emptyList()
+                    if (!resp.isSuccessful) {
+                        Log.w(TAG, "fetchModels: HTTP ${resp.code} from $baseUrl")
+                        return@use null
+                    }
                     json.decodeFromString<ModelsResponse>(resp.body!!.string()).models
                 }
             } catch (e: Exception) {
-                emptyList()
+                Log.w(TAG, "fetchModels: ${e.javaClass.simpleName}: ${e.message}")
+                null
             }
         }
 
-    /** GET /api/v1/conversations — the sidebar list (best-effort, empty on any failure). */
-    suspend fun fetchConversations(baseUrl: String, token: String): List<ConversationSummary> =
+    /** GET /api/v1/conversations — the sidebar list. Returns null on failure (with logged reason). */
+    suspend fun fetchConversations(baseUrl: String, token: String): List<ConversationSummary>? =
         withContext(Dispatchers.IO) {
             val req = Request.Builder()
                 .url("$baseUrl/api/v1/conversations")
@@ -168,11 +175,15 @@ object KaizenApi {
                 .build()
             try {
                 client.newCall(req).execute().use { resp ->
-                    if (!resp.isSuccessful) return@use emptyList()
+                    if (!resp.isSuccessful) {
+                        Log.w(TAG, "fetchConversations: HTTP ${resp.code} from $baseUrl")
+                        return@use null
+                    }
                     json.decodeFromString<ConversationsResponse>(resp.body!!.string()).conversations
                 }
             } catch (e: Exception) {
-                emptyList()
+                Log.w(TAG, "fetchConversations: ${e.javaClass.simpleName}: ${e.message}")
+                null
             }
         }
 
@@ -187,10 +198,14 @@ object KaizenApi {
                 .build()
             try {
                 client.newCall(req).execute().use { resp ->
-                    if (!resp.isSuccessful) return@use null
+                    if (!resp.isSuccessful) {
+                        Log.w(TAG, "createConversation: HTTP ${resp.code}")
+                        return@use null
+                    }
                     json.decodeFromString<CreatedConversation>(resp.body!!.string()).id
                 }
             } catch (e: Exception) {
+                Log.w(TAG, "createConversation: ${e.javaClass.simpleName}: ${e.message}")
                 null
             }
         }
@@ -204,10 +219,14 @@ object KaizenApi {
                 .build()
             try {
                 client.newCall(req).execute().use { resp ->
-                    if (!resp.isSuccessful) return@use emptyList()
+                    if (!resp.isSuccessful) {
+                        Log.w(TAG, "fetchMessages($id): HTTP ${resp.code}")
+                        return@use emptyList()
+                    }
                     json.decodeFromString<ConversationDetail>(resp.body!!.string()).messages
                 }
             } catch (e: Exception) {
+                Log.w(TAG, "fetchMessages($id): ${e.javaClass.simpleName}: ${e.message}")
                 emptyList()
             }
         }
@@ -222,8 +241,12 @@ object KaizenApi {
                 .header("Authorization", "Bearer $token")
                 .build()
             try {
-                client.newCall(req).execute().use { resp -> resp.isSuccessful }
+                client.newCall(req).execute().use { resp ->
+                    if (!resp.isSuccessful) Log.w(TAG, "saveMessages($id): HTTP ${resp.code}")
+                    resp.isSuccessful
+                }
             } catch (e: Exception) {
+                Log.w(TAG, "saveMessages($id): ${e.javaClass.simpleName}: ${e.message}")
                 false
             }
         }
@@ -246,8 +269,12 @@ object KaizenApi {
     /**
      * POST /api/v1/chat — streams the assistant reply. Emits the cumulative *visible*
      * text after each network chunk (sentinels stripped), so the caller just assigns
-     * it to the message content. The full byte buffer is re-decoded each chunk to keep
-     * UTF-8 multi-byte sequences that span chunk boundaries intact.
+     * it to the message content.
+     *
+     * Uses [InputStreamReader] for correct UTF-8 multi-byte boundary handling and
+     * [StreamConsumer.Incremental] for O(chunk-size) parsing — each character is
+     * visited exactly once across the entire stream, avoiding the O(N²) cost of
+     * re-decoding and re-scanning the full buffer on every chunk.
      *
      * [speed] maps to the additive `reasoningEffort`/`preferThroughput` v1 fields.
      */
@@ -280,14 +307,29 @@ object KaizenApi {
         }
 
         resp.body!!.byteStream().use { input ->
-            val raw = ByteArrayOutputStream()
-            val buf = ByteArray(4096)
+            val reader = InputStreamReader(input, Charsets.UTF_8)
+            val consumer = StreamConsumer.Incremental()
+            val charBuf = CharArray(4096)
             while (true) {
-                val n = input.read(buf)
+                val n = reader.read(charBuf)
                 if (n == -1) break
-                raw.write(buf, 0, n)
-                emit(StreamConsumer.visibleText(raw.toByteArray().decodeToString()))
+                emit(consumer.append(charBuf, 0, n))
             }
         }
     }.flowOn(Dispatchers.IO)
+
+    /**
+     * Warm the OkHttp HTTP/2 connection pool against [baseUrl]. Fire-and-forget
+     * GET to the unauth `/api/v1/meta` endpoint — eliminates the cold TCP/TLS
+     * handshake (~150-300ms on mobile) from the first real request after app
+     * start or background resume.
+     */
+    suspend fun prewarm(baseUrl: String) = withContext(Dispatchers.IO) {
+        try {
+            val req = Request.Builder().url("$baseUrl/api/v1/meta").build()
+            client.newCall(req).execute().close()
+        } catch (_: Exception) {
+            // best-effort — a failed prewarm just means the first real request pays the cold cost
+        }
+    }
 }

gradle/libs.versions.toml

@@ -21,6 +21,7 @@ junit = { group = "junit", name = "junit", version.ref = "junit" }
 androidx-junit = { group = "androidx.test.ext", name = "junit", version.ref = "junitVersion" }
 androidx-espresso-core = { group = "androidx.test.espresso", name = "espresso-core", version.ref = "espressoCore" }
 androidx-lifecycle-runtime-ktx = { group = "androidx.lifecycle", name = "lifecycle-runtime-ktx", version.ref = "lifecycleRuntimeKtx" }
+androidx-lifecycle-runtime-compose = { group = "androidx.lifecycle", name = "lifecycle-runtime-compose", version.ref = "lifecycleRuntimeKtx" }
 androidx-activity-compose = { group = "androidx.activity", name = "activity-compose", version.ref = "activityCompose" }
 androidx-compose-bom = { group = "androidx.compose", name = "compose-bom", version.ref = "composeBom" }
 androidx-compose-ui = { group = "androidx.compose.ui", name = "ui" }