fix(mnnchat): reuse loaded runtime session for API start

apps/Android/MnnLlmChat/README.md

@@ -67,6 +67,7 @@ This is our full multimodal language model (LLM) Android app
 + Bugfix:
   + Fix the Android chat regression that could stop after a single token or fail to continue on the second turn with the prebuilt runtime.
   + Harden dumpapp and smoke coverage for terminal callback delivery, single-token regression, and thinking-mode divergence checks.
+  + Reuse the loaded runtime session when starting API service for the same model to avoid extra reloads that could freeze or crash API startup.
 
 ## Version 0.8.2
 + Click here to [download](https://meta.alicdn.com/data/mnn/apks/mnn_chat_0_8_2.apk)

apps/Android/MnnLlmChat/README_CN.md

@@ -63,6 +63,7 @@
 + 问题修复：
   + 修复 Android 聊天在预编译 runtime 下可能只输出单个 token，或第二轮无法继续生成的问题。
   + 加强 dumpapp 与 smoke 对终止回调、单 token 回归和思考模式差异判定的覆盖。
+  + 对同模型启动 API 服务时复用已加载 runtime session，避免额外重载导致的卡死或崩溃。
 
 ## Version 0.8.2
 + 点击这里 [下载](https://meta.alicdn.com/data/mnn/apks/mnn_chat_0_8_2.apk)

apps/Android/MnnLlmChat/app/src/main/java/com/alibaba/mnnllm/api/openai/service/ApiRuntimeSessionStartPolicy.kt

@@ -0,0 +1,14 @@
+package com.alibaba.mnnllm.api.openai.service
+
+internal object ApiRuntimeSessionStartPolicy {
+    fun shouldReuseLoadedSession(
+        requestedModelId: String?,
+        activeModelId: String?,
+        hasLoadedActiveSession: Boolean
+    ): Boolean {
+        if (!hasLoadedActiveSession) {
+            return false
+        }
+        return requestedModelId.isNullOrBlank() || requestedModelId == activeModelId
+    }
+}

apps/Android/MnnLlmChat/app/src/main/java/com/alibaba/mnnllm/api/openai/service/ApiServiceCoordinator.kt

@@ -57,7 +57,7 @@ class ApiServiceCoordinator(private val context: Context) {
 
             if (application != null && _isServerRunning) {
                 if (!modelId.isNullOrBlank()) {
-                    val switchedSession = ensureRuntimeSessionForModel(modelId)
+                    val switchedSession = resolveRuntimeSessionForStart(modelId)
                     if (switchedSession == null) {
                         notificationManager?.updateNotification(
                             context.getString(R.string.api_service_not_started),
@@ -71,21 +71,7 @@ class ApiServiceCoordinator(private val context: Context) {
             }
 
             return runCatching {
-                val runtimeSession = if (!modelId.isNullOrBlank()) {
-                    val session = ensureRuntimeSessionForModel(modelId)
-                    if (session == null) {
-                        notificationManager?.updateNotification(
-                            context.getString(R.string.api_service_not_started),
-                            context.getString(R.string.no_active_session)
-                        )
-                        return false
-                    }
-                    session
-                } else {
-                    ServiceLocator.getLlmRuntimeController().getActiveSession()
-                }
-
-                val session = runtimeSession ?: ServiceLocator.getChatSessionProvider().getLlmSession()
+                val session = resolveRuntimeSessionForStart(modelId)
                 if (session == null) {
                     Timber.Forest.tag(TAG).w("No active LlmSession found")
                     notificationManager?.updateNotification(
@@ -125,6 +111,31 @@ class ApiServiceCoordinator(private val context: Context) {
         }
     }
 
+    private fun resolveRuntimeSessionForStart(modelId: String?): LlmSession? {
+        val runtime = ServiceLocator.getLlmRuntimeController()
+        val chatSessionProvider = ServiceLocator.getChatSessionProvider()
+        val activeModelId = runtime.getActiveModelId()
+        val hasLoadedActiveSession = chatSessionProvider.hasActiveSession()
+
+        if (ApiRuntimeSessionStartPolicy.shouldReuseLoadedSession(modelId, activeModelId, hasLoadedActiveSession)) {
+            val activeSession = runtime.getActiveSession() ?: chatSessionProvider.getLlmSession()
+            if (activeSession != null) {
+                Timber.Forest.tag(TAG).i(
+                    "Reusing loaded runtime session for API start, requestedModelId=%s activeModelId=%s",
+                    modelId,
+                    activeModelId
+                )
+                return activeSession
+            }
+        }
+
+        if (!modelId.isNullOrBlank()) {
+            return ensureRuntimeSessionForModel(modelId)
+        }
+
+        return runtime.getActiveSession() ?: chatSessionProvider.getLlmSession()
+    }
+
     private fun ensureRuntimeSessionForModel(modelId: String): LlmSession? {
         val ensureResult = ServiceLocator.getLlmRuntimeController().ensureSession(modelId)
         if (!ensureResult.success || ensureResult.session == null) {

apps/Android/MnnLlmChat/app/src/test/java/com/alibaba/mnnllm/api/openai/service/ApiRuntimeSessionStartPolicyTest.kt

@@ -0,0 +1,41 @@
+package com.alibaba.mnnllm.api.openai.service
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class ApiRuntimeSessionStartPolicyTest {
+
+    @Test
+    fun `same model with loaded active session should reuse runtime session`() {
+        assertTrue(
+            ApiRuntimeSessionStartPolicy.shouldReuseLoadedSession(
+                requestedModelId = "ModelScope/MNN/Qwen3.5-0.8B-MNN",
+                activeModelId = "ModelScope/MNN/Qwen3.5-0.8B-MNN",
+                hasLoadedActiveSession = true
+            )
+        )
+    }
+
+    @Test
+    fun `different model should not reuse runtime session`() {
+        assertFalse(
+            ApiRuntimeSessionStartPolicy.shouldReuseLoadedSession(
+                requestedModelId = "ModelScope/MNN/Qwen3.5-2B-MNN",
+                activeModelId = "ModelScope/MNN/Qwen3.5-0.8B-MNN",
+                hasLoadedActiveSession = true
+            )
+        )
+    }
+
+    @Test
+    fun `missing loaded session should not reuse runtime session`() {
+        assertFalse(
+            ApiRuntimeSessionStartPolicy.shouldReuseLoadedSession(
+                requestedModelId = "ModelScope/MNN/Qwen3.5-0.8B-MNN",
+                activeModelId = "ModelScope/MNN/Qwen3.5-0.8B-MNN",
+                hasLoadedActiveSession = false
+            )
+        )
+    }
+}