diff --git a/tests/ClaudeDo.Ui.Tests/TestSetup.cs b/tests/ClaudeDo.Ui.Tests/TestSetup.cs
index 79ef3e65..90277d81 100644
--- a/tests/ClaudeDo.Ui.Tests/TestSetup.cs
+++ b/tests/ClaudeDo.Ui.Tests/TestSetup.cs
@@ -1,5 +1,12 @@
 using System.Runtime.CompilerServices;
+using ClaudeDo.Localization;
+using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
+using Xunit;
+
+// Loc.Current (src/ClaudeDo.Ui/Localization/Loc.cs) is a process-wide mutable static that many
+// ViewModel test constructors assign — parallel collections race on which localizer is active.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]
 
 namespace ClaudeDo.Ui.Tests;
 
@@ -17,4 +24,17 @@ internal static class TestSetup
         var path = Path.Combine(Path.GetTempPath(), "claudedo-ui-tests", $"operation-timing-{Environment.ProcessId}.ndjson");
         OperationTiming.Shared = new OperationTiming(path);
     }
+
+    // Installs the real localizer before any test runs, so a class that reads Loc.T/Loc.Current
+    // without setting it up itself (e.g. via a ViewModel error message) doesn't fall back to
+    // Loc's key-echo default depending on which test class happens to run first.
+    [ModuleInitializer]
+    internal static void InstallRealLocalizer()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !Directory.Exists(Path.Combine(dir, "src", "ClaudeDo.Localization", "locales")))
+            dir = Path.GetDirectoryName(dir);
+        Loc.Current = new Localizer(
+            LocaleStore.Load(Path.Combine(dir!, "src", "ClaudeDo.Localization", "locales")), "en");
+    }
 }
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/AgentConfigEditorViewModelTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/AgentConfigEditorViewModelTests.cs
index 12101a51..b6cdbdc6 100644
--- a/tests/ClaudeDo.Ui.Tests/ViewModels/AgentConfigEditorViewModelTests.cs
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/AgentConfigEditorViewModelTests.cs
@@ -1,4 +1,5 @@
 using ClaudeDo.Data.Models;
+using ClaudeDo.Localization;
 using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels.Agent;
@@ -9,6 +10,18 @@ namespace ClaudeDo.Ui.Tests.ViewModels;
 
 public class AgentConfigEditorViewModelTests
 {
+    public AgentConfigEditorViewModelTests()
+    {
+        // Loc.T(...) is read below without installing a localizer via a ViewModel constructor
+        // (unlike its sibling test classes) — install one directly so this class doesn't depend
+        // on whichever neighbour test happened to run first and set Loc.Current.
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !Directory.Exists(Path.Combine(dir, "src", "ClaudeDo.Localization", "locales")))
+            dir = Path.GetDirectoryName(dir);
+        Loc.Current = new Localizer(
+            LocaleStore.Load(Path.Combine(dir!, "src", "ClaudeDo.Localization", "locales")), "en");
+    }
+
     private sealed class FakeWorker : StubWorkerClient
     {
         public AppSettingsDto? App;
diff --git a/tests/ClaudeDo.Worker.Tests/TestSetup.cs b/tests/ClaudeDo.Worker.Tests/TestSetup.cs
index ef2daba8..64564ee7 100644
--- a/tests/ClaudeDo.Worker.Tests/TestSetup.cs
+++ b/tests/ClaudeDo.Worker.Tests/TestSetup.cs
@@ -1,4 +1,6 @@
 using System.Runtime.CompilerServices;
+using ClaudeDo.Localization;
+using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
 
 namespace ClaudeDo.Worker.Tests;
@@ -16,4 +18,20 @@ internal static class TestSetup
         var path = Path.Combine(Path.GetTempPath(), "claudedo-worker-tests", $"operation-timing-{Environment.ProcessId}.ndjson");
         OperationTiming.Shared = new OperationTiming(path);
     }
+
+    // Installs the real localizer before any test runs, mirroring ClaudeDo.Ui.Tests' TestSetup,
+    // so a UiVm test that reads Loc.T without setting up its own localizer doesn't depend on
+    // ambient state left by whichever test class happened to run first. Deliberately does NOT
+    // also disable test parallelization here (unlike Ui.Tests) — measured 14m49s -> 18m46s
+    // (+27%) on this project's real-SQLite/git suite, too costly for a test-only ordering issue
+    // that the module initializer alone already closes.
+    [ModuleInitializer]
+    internal static void InstallRealLocalizer()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !Directory.Exists(Path.Combine(dir, "src", "ClaudeDo.Localization", "locales")))
+            dir = Path.GetDirectoryName(dir);
+        Loc.Current = new Localizer(
+            LocaleStore.Load(Path.Combine(dir!, "src", "ClaudeDo.Localization", "locales")), "en");
+    }
 }