refactor: remove stale thread target fallback

lib/app/app_controller_desktop_settings_runtime.dart

@@ -835,28 +835,15 @@ extension AppControllerDesktopSettingsRuntime on AppController {
     final sessionKey = normalizedAssistantSessionKeyInternal(
       sessionsControllerInternal.currentSessionKey,
     );
-    final thread = taskThreadForSessionInternal(sessionKey);
-    final target = thread?.hasExplicitExecutionTargetSelection ?? false
-        ? assistantExecutionTargetForSession(sessionKey)
-        : sanitizePersistedExecutionTargetInternal(
-            snapshot.assistantExecutionTarget,
-          );
-    if (thread?.hasExplicitExecutionTargetSelection ?? false) {
-      upsertTaskThreadInternal(
-        sessionKey,
-        gatewayEntryState: gatewayEntryStateForTargetInternal(target),
-        latestResolvedRuntimeModel: '',
-        updatedAtMs: DateTime.now().millisecondsSinceEpoch.toDouble(),
-      );
-    } else {
-      upsertTaskThreadInternal(
-        sessionKey,
-        executionTarget: target,
-        gatewayEntryState: gatewayEntryStateForTargetInternal(target),
-        latestResolvedRuntimeModel: '',
-        updatedAtMs: DateTime.now().millisecondsSinceEpoch.toDouble(),
-      );
-    }
+    final target = assistantExecutionTargetForSession(sessionKey);
+    upsertTaskThreadInternal(
+      sessionKey,
+      executionTarget: target,
+      executionTargetSource: ThreadSelectionSource.explicit,
+      gatewayEntryState: gatewayEntryStateForTargetInternal(target),
+      latestResolvedRuntimeModel: '',
+      updatedAtMs: DateTime.now().millisecondsSinceEpoch.toDouble(),
+    );
     recomputeTasksInternal();
     notifyIfActiveInternal();
     await applyAssistantExecutionTargetInternal(

lib/app/app_controller_desktop_skill_permissions.dart

@@ -292,7 +292,7 @@ extension AppControllerDesktopSkillPermissions on AppController {
             AssistantExecutionTarget.singleAgent,
           ThreadExecutionMode.gatewayLocal => AssistantExecutionTarget.local,
           ThreadExecutionMode.gatewayRemote => AssistantExecutionTarget.remote,
-          null => settings.assistantExecutionTarget,
+          null => AssistantExecutionTarget.singleAgent,
         };
     final nextImportedSkills =
         importedSkills ??

lib/app/app_controller_desktop_thread_sessions.dart

@@ -48,6 +48,16 @@ import 'app_controller_desktop_thread_sessions_collaboration_impl.dart';
 
 // ignore_for_file: invalid_use_of_visible_for_testing_member, invalid_use_of_protected_member
 extension AppControllerDesktopThreadSessions on AppController {
+  AssistantExecutionTarget resolveAssistantExecutionTargetFromRecordsInternal(
+    TaskThread? primaryRecord, {
+    TaskThread? fallbackRecord,
+  }) {
+    return resolveAssistantExecutionTargetFromRecordsForTest(
+      primaryRecord,
+      fallbackRecord: fallbackRecord,
+    );
+  }
+
   TaskThread? taskThreadForSessionInternal(String sessionKey) {
     final normalizedSessionKey = normalizedAssistantSessionKeyInternal(
       sessionKey,
@@ -544,12 +554,12 @@ extension AppControllerDesktopThreadSessions on AppController {
       sessionKey,
     );
     final record = taskThreadForSessionInternal(normalizedSessionKey);
-    return sanitizePersistedExecutionTargetInternal(
-      record == null
-          ? settings.assistantExecutionTarget
-          : assistantExecutionTargetFromExecutionMode(
-              record.executionBinding.executionMode,
-            ),
+    final mainRecord = matchesSessionKey(normalizedSessionKey, 'main')
+        ? null
+        : taskThreadForSessionInternal('main');
+    return resolveAssistantExecutionTargetFromRecordsInternal(
+      record,
+      fallbackRecord: mainRecord,
     );
   }
 
@@ -621,3 +631,15 @@ extension AppControllerDesktopThreadSessions on AppController {
     return items;
   }
 }
+
+AssistantExecutionTarget resolveAssistantExecutionTargetFromRecordsForTest(
+  TaskThread? primaryRecord, {
+  TaskThread? fallbackRecord,
+}) {
+  final record = primaryRecord ?? fallbackRecord;
+  return record == null
+      ? AssistantExecutionTarget.singleAgent
+      : assistantExecutionTargetFromExecutionMode(
+          record.executionBinding.executionMode,
+        );
+}

lib/app/app_controller_desktop_thread_sessions_collaboration_impl.dart

@@ -323,15 +323,8 @@ List<String> assistantModelChoicesForSessionThreadSessionInternal(
   final normalizedSessionKey = normalizeAssistantSessionKeyThreadInternal(
     sessionKey,
   );
-  final target = controller.sanitizePersistedExecutionTargetInternal(
-    controller.taskThreadForSessionInternal(normalizedSessionKey) == null
-        ? controller.settings.assistantExecutionTarget
-        : assistantExecutionTargetFromExecutionMode(
-            controller
-                .requireTaskThreadForSessionInternal(normalizedSessionKey)
-                .executionBinding
-                .executionMode,
-          ),
+  final target = controller.assistantExecutionTargetForSession(
+    normalizedSessionKey,
   );
   if (target == AssistantExecutionTarget.singleAgent) {
     final singleAgentUsesAiGatewayFallback =

lib/app/app_controller_desktop_workspace_execution.dart

@@ -358,6 +358,7 @@ extension AppControllerDesktopWorkspaceExecution on AppController {
       title: title.trim(),
       ownerScope: initialOwnerScope,
       executionTarget: resolvedTarget,
+      executionTargetSource: ThreadSelectionSource.explicit,
       workspaceBinding: initialWorkspaceBinding,
       messageViewMode:
           messageViewMode ??
@@ -365,6 +366,7 @@ extension AppControllerDesktopWorkspaceExecution on AppController {
       singleAgentProvider:
           singleAgentProvider ??
           singleAgentProviderForSession(currentSessionKey),
+      singleAgentProviderSource: ThreadSelectionSource.explicit,
       updatedAtMs: DateTime.now().millisecondsSinceEpoch.toDouble(),
     );
     // Re-read the current thread target when the async binding sync runs so a
@@ -405,7 +407,9 @@ extension AppControllerDesktopWorkspaceExecution on AppController {
       return;
     }
     final authorizationOverride =
-        await resolveSingleAgentAuthorizationHeaderForProviderInternal(provider);
+        await resolveSingleAgentAuthorizationHeaderForProviderInternal(
+          provider,
+        );
     await replaceSingleAgentThreadSkillsInternal(
       normalizedSessionKey,
       localSkills,

test/app_controller_desktop_thread_target_cleanup_test.dart

@@ -0,0 +1,94 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:xworkmate/app/app_controller_desktop_thread_sessions.dart';
+import 'package:xworkmate/runtime/runtime_models.dart';
+
+void main() {
+  group('resolveAssistantExecutionTargetFromRecordsInternal', () {
+    const owner = ThreadOwnerScope(
+      realm: ThreadRealm.local,
+      subjectType: ThreadSubjectType.user,
+      subjectId: 'u1',
+      displayName: 'User',
+    );
+
+    TaskThread buildThread({
+      required String threadId,
+      required ThreadExecutionMode mode,
+      String providerId = 'auto',
+    }) {
+      return TaskThread(
+        threadId: threadId,
+        ownerScope: owner,
+        workspaceBinding: const WorkspaceBinding(
+          workspaceId: 'ws-1',
+          workspaceKind: WorkspaceKind.localFs,
+          workspacePath: '/tmp/ws',
+          displayPath: '/tmp/ws',
+          writable: true,
+        ),
+        executionBinding: ExecutionBinding(
+          executionMode: mode,
+          executorId: providerId,
+          providerId: providerId,
+          endpointId: '',
+        ),
+      );
+    }
+
+    test('defaults to single-agent when no thread record exists', () {
+      final resolved = _ThreadSessionTargetResolverHarness().resolveTarget(
+        primary: null,
+      );
+
+      expect(resolved, AssistantExecutionTarget.singleAgent);
+    });
+
+    test('prefers the current thread record over the main thread fallback', () {
+      final primary = buildThread(
+        threadId: 'draft:1',
+        mode: ThreadExecutionMode.gatewayRemote,
+      );
+      final fallback = buildThread(
+        threadId: 'main',
+        mode: ThreadExecutionMode.gatewayLocal,
+      );
+
+      final resolved = _ThreadSessionTargetResolverHarness().resolveTarget(
+        primary: primary,
+        fallback: fallback,
+      );
+
+      expect(resolved, AssistantExecutionTarget.remote);
+    });
+
+    test(
+      'uses main thread record instead of settings when current is missing',
+      () {
+        final fallback = buildThread(
+          threadId: 'main',
+          mode: ThreadExecutionMode.localAgent,
+          providerId: SingleAgentProvider.opencode.providerId,
+        );
+
+        final resolved = _ThreadSessionTargetResolverHarness().resolveTarget(
+          primary: null,
+          fallback: fallback,
+        );
+
+        expect(resolved, AssistantExecutionTarget.singleAgent);
+      },
+    );
+  });
+}
+
+class _ThreadSessionTargetResolverHarness {
+  AssistantExecutionTarget resolveTarget({
+    required TaskThread? primary,
+    TaskThread? fallback,
+  }) {
+    return resolveAssistantExecutionTargetFromRecordsForTest(
+      primary,
+      fallbackRecord: fallback,
+    );
+  }
+}