diff --git a/docs/explore-notes/review-merge.md b/docs/explore-notes/review-merge.md
index 4e18763a..a73574e7 100644
--- a/docs/explore-notes/review-merge.md
+++ b/docs/explore-notes/review-merge.md
@@ -138,6 +138,23 @@ mid-merge conflicts **without re-starting the merge** and routes continue/abort
 `ContinuePlanningMerge` / `AbortPlanningMerge`, so a unit-merge conflict re-opens the editor
 per subtask via the `PlanningMergeConflict` broadcast.
 
+**Except when an MCP session is driving the merge.** `PlanningMergeOrchestrator.StartAsync` takes
+an `externallyDriven` bool (default `false`; `ExternalMcpService.review_task`'s parent-with-children
+path passes `true`, since a running Claude session — not the UI — will resolve conflicts via
+`continue_merge`/`abort_merge`). The flag rides along on the `PlanningMergeConflict` broadcast
+(4th arg); `IslandsShellViewModel.OnPlanningMergeConflict` only auto-opens the resolver when it's
+`false` — otherwise it shows a persistent, non-auto-dismissing banner
+(`IsExternalMergeBannerVisible`) with a manual "Open resolver" button, cleared on
+`PlanningMergeAborted`/`PlanningCompleted`. This exists because two parties (a human and the
+driving session) could otherwise end up editing the same shared checkout at once.
+`PlanningMergeOrchestrator.GetActiveExternalConflictsAsync` (hub: `GetActiveExternalPlanningMergeConflicts`)
+re-derives this state by checking `GitService.IsMidMergeAsync` rather than trusting the in-memory
+flag alone, so a UI restart mid-merge (or a stale entry left behind if something resolved the
+repo outside the normal Continue/Abort path) can't show a phantom banner — the Ui calls it on
+`ConnectionRestoredEvent`. The childless single-task conflict path
+(`TaskMergeService.ApproveAndMergeAsync`) has no equivalent broadcast — it only sends the generic
+`TaskUpdated` — so it never auto-opened the resolver and needed no change.
+
 ### View
 
 Three **AvaloniaEdit** panes showing the whole file: MAIN/ours (read-only) | editable Result |
diff --git a/src/ClaudeDo.Localization/locales/de.json b/src/ClaudeDo.Localization/locales/de.json
index 1693ad07..3c73a3da 100644
--- a/src/ClaudeDo.Localization/locales/de.json
+++ b/src/ClaudeDo.Localization/locales/de.json
@@ -632,6 +632,10 @@
     },
     "staleWorker": {
       "message": "Der Worker läuft auf einem älteren Stand als der gemergte main-Branch dieses Repos — Neustart nötig, damit gemergte Änderungen wirken."
+    },
+    "externalMerge": {
+      "banner": "Eine Claude-Session löst gerade einen Merge-Konflikt auf — bitte keine Dateien im Repository bearbeiten.",
+      "open": "Resolver öffnen"
     }
   },
   "vm": {
diff --git a/src/ClaudeDo.Localization/locales/en.json b/src/ClaudeDo.Localization/locales/en.json
index 903ed98b..e2e0a6d2 100644
--- a/src/ClaudeDo.Localization/locales/en.json
+++ b/src/ClaudeDo.Localization/locales/en.json
@@ -632,6 +632,10 @@
     },
     "staleWorker": {
       "message": "The worker is running an older build than this repo's merged main — restart it so your merged changes take effect."
+    },
+    "externalMerge": {
+      "banner": "A Claude session is resolving a merge conflict — don't edit files in the repository.",
+      "open": "Open resolver"
     }
   },
   "vm": {
diff --git a/src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs b/src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs
index 0a848c81..a807b6b3 100644
--- a/src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs
+++ b/src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs
@@ -35,7 +35,10 @@ public interface IWorkerClient : INotifyPropertyChanged
 
     event Action<string, string>? PlanningMergeStartedEvent;
     event Action<string, string>? PlanningSubtaskMergedEvent;
-    event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
+    /// <summary>(planningTaskId, subtaskId, conflictedFiles, externallyDriven). externallyDriven
+    /// is true when an MCP session (not the UI) started the unit merge — the resolver must not
+    /// auto-open in that case.</summary>
+    event Action<string, string, IReadOnlyList<string>, bool>? PlanningMergeConflictEvent;
     event Action<string>? PlanningMergeAbortedEvent;
     event Action<string>? PlanningCompletedEvent;
 
@@ -117,6 +120,9 @@ public interface IWorkerClient : INotifyPropertyChanged
     Task<CombinedDiffResultDto?> BuildPlanningIntegrationBranchAsync(string planningTaskId, string targetBranch);
     Task ContinuePlanningMergeAsync(string planningTaskId);
     Task AbortPlanningMergeAsync(string planningTaskId);
+    /// <summary>Unit merges currently paused on a conflict that an MCP session started. Called
+    /// on (re)connect to recover the "don't auto-open" banner state after a UI restart.</summary>
+    Task<IReadOnlyList<PlanningMergeConflictStateDto>> GetActiveExternalPlanningMergeConflictsAsync();
     Task QueuePlanningSubtasksAsync(string parentTaskId, CancellationToken ct = default);
     Task<string?> GetWeekReportAsync(DateOnly start, DateOnly end);
     Task<string> GenerateWeekReportAsync(DateOnly start, DateOnly end);
diff --git a/src/ClaudeDo.Ui/Services/WorkerClient.cs b/src/ClaudeDo.Ui/Services/WorkerClient.cs
index f4201a7c..3523f584 100644
--- a/src/ClaudeDo.Ui/Services/WorkerClient.cs
+++ b/src/ClaudeDo.Ui/Services/WorkerClient.cs
@@ -66,7 +66,7 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
 
     public event Action<string, string>? PlanningMergeStartedEvent;
     public event Action<string, string>? PlanningSubtaskMergedEvent;
-    public event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
+    public event Action<string, string, IReadOnlyList<string>, bool>? PlanningMergeConflictEvent;
     public event Action<string>? PlanningMergeAbortedEvent;
     public event Action<string>? PlanningCompletedEvent;
 
@@ -182,9 +182,9 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
             Dispatcher.UIThread.Post(() => PlanningSubtaskMergedEvent?.Invoke(planningTaskId, subtaskId));
         });
 
-        _hub.On<string, string, IReadOnlyList<string>>("PlanningMergeConflict", (planningTaskId, subtaskId, conflictedFiles) =>
+        _hub.On<string, string, IReadOnlyList<string>, bool>("PlanningMergeConflict", (planningTaskId, subtaskId, conflictedFiles, externallyDriven) =>
         {
-            Dispatcher.UIThread.Post(() => PlanningMergeConflictEvent?.Invoke(planningTaskId, subtaskId, conflictedFiles));
+            Dispatcher.UIThread.Post(() => PlanningMergeConflictEvent?.Invoke(planningTaskId, subtaskId, conflictedFiles, externallyDriven));
         });
 
         _hub.On<string>("PlanningMergeAborted", planningTaskId =>
@@ -581,6 +581,10 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         await _hub.InvokeAsync("AbortPlanningMerge", planningTaskId);
     }
 
+    public async Task<IReadOnlyList<PlanningMergeConflictStateDto>> GetActiveExternalPlanningMergeConflictsAsync()
+        => await TryInvokeAsync<List<PlanningMergeConflictStateDto>>("GetActiveExternalPlanningMergeConflicts")
+           ?? [];
+
     public async Task QueuePlanningSubtasksAsync(string parentTaskId, CancellationToken ct = default)
     {
         await _hub.InvokeAsync("QueuePlanningSubtasksAsync", parentTaskId, ct);
@@ -695,6 +699,7 @@ public sealed record LaunchSpec(
     IReadOnlyDictionary<string, string> Env);
 
 public sealed record ForceRemoveResultDto(bool Removed, string? Reason);
+public sealed record PlanningMergeConflictStateDto(string PlanningTaskId, string SubtaskId);
 public sealed record PendingQuestionDto(string TaskId, string QuestionId, string Question);
 public sealed record WorkerBuildInfoDto(string? BuildSha);
 
diff --git a/src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs b/src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs
index 9057ce1e..6886dc27 100644
--- a/src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs
+++ b/src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs
@@ -109,6 +109,13 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
     // only — see RefreshStaleWorkerCheckAsync.
     [ObservableProperty] private bool _isStaleWorkerBannerVisible;
 
+    // planningTaskId -> subtaskId, for unit-merge conflicts an MCP session (not the UI) started.
+    // Kept as a dictionary so a later conflict on the same planning task updates in place instead
+    // of piling up duplicate entries.
+    private readonly Dictionary<string, string> _externalMergeConflicts = new();
+    [ObservableProperty] private bool _isExternalMergeBannerVisible;
+
+
     [ObservableProperty]
     private double _windowWidth = 1280;
 
@@ -185,13 +192,59 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         _primeStatusTimer.Start();
     }
 
-    private void OnPlanningMergeConflict(string planningTaskId, string subtaskId, IReadOnlyList<string> conflictedFiles)
+    public void OnPlanningMergeConflict(
+        string planningTaskId, string subtaskId, IReadOnlyList<string> conflictedFiles, bool externallyDriven)
     {
         // Already on UI thread (WorkerClient dispatches via Dispatcher.UIThread.Post).
+        if (externallyDriven)
+        {
+            // An MCP session (review_task/continue_merge) is driving this merge — it owns
+            // resolution. Auto-opening the resolver here raced with the session's own writes
+            // (two parties editing the same shared checkout at once); show a banner instead and
+            // leave the resolver reachable only via a deliberate click.
+            _externalMergeConflicts[planningTaskId] = subtaskId;
+            IsExternalMergeBannerVisible = true;
+            return;
+        }
+
         // A unit-merge conflict resolves in the same in-app 3-way editor as a single-task merge.
         _ = OpenPlanningConflictAsync(planningTaskId, subtaskId);
     }
 
+    public void OnPlanningMergeAborted(string planningTaskId) => ClearExternalMergeConflict(planningTaskId);
+    public void OnPlanningMergeCompleted(string planningTaskId) => ClearExternalMergeConflict(planningTaskId);
+
+    private void ClearExternalMergeConflict(string planningTaskId)
+    {
+        _externalMergeConflicts.Remove(planningTaskId);
+        IsExternalMergeBannerVisible = _externalMergeConflicts.Count > 0;
+    }
+
+    /// <summary>Re-syncs the external-merge banner from the worker on (re)connect — the
+    /// one-shot PlanningMergeConflict broadcast isn't replayed after a UI restart, so this is
+    /// the recovery path. The worker checks MERGE_HEAD before reporting a conflict as active,
+    /// so a session that died mid-merge without cleaning up doesn't leave a stale banner up.</summary>
+    private async Task RefreshExternalMergeConflictsAsync()
+    {
+        if (Worker is null) return;
+        IReadOnlyList<PlanningMergeConflictStateDto> active;
+        try { active = await Worker.GetActiveExternalPlanningMergeConflictsAsync(); }
+        catch { return; }
+
+        _externalMergeConflicts.Clear();
+        foreach (var c in active)
+            _externalMergeConflicts[c.PlanningTaskId] = c.SubtaskId;
+        IsExternalMergeBannerVisible = _externalMergeConflicts.Count > 0;
+    }
+
+    [RelayCommand]
+    private Task OpenExternalMergeConflictAsync()
+    {
+        if (_externalMergeConflicts.Count == 0) return Task.CompletedTask;
+        var (planningTaskId, subtaskId) = _externalMergeConflicts.First();
+        return OpenPlanningConflictAsync(planningTaskId, subtaskId);
+    }
+
     private async Task OpenPlanningConflictAsync(string planningTaskId, string subtaskId)
     {
         if (ConflictResolverFactory is null || Dialogs is null) return;
@@ -298,6 +351,9 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         Worker.WorkerLogReceivedEvent += OnWorkerLogReceived;
         Worker.ConnectionRestoredEvent += () => _ = RefreshStaleWorkerCheckAsync();
         Worker.PlanningMergeConflictEvent += OnPlanningMergeConflict;
+        Worker.PlanningMergeAbortedEvent += OnPlanningMergeAborted;
+        Worker.PlanningCompletedEvent += OnPlanningMergeCompleted;
+        Worker.ConnectionRestoredEvent += () => _ = RefreshExternalMergeConflictsAsync();
         Worker.PrimeFired += OnPrimeFired;
         _clearTimer.Elapsed += (_, _) =>
         {
diff --git a/src/ClaudeDo.Ui/Views/MainWindow.axaml b/src/ClaudeDo.Ui/Views/MainWindow.axaml
index 3331d241..a5dd9fee 100644
--- a/src/ClaudeDo.Ui/Views/MainWindow.axaml
+++ b/src/ClaudeDo.Ui/Views/MainWindow.axaml
@@ -281,5 +281,27 @@
       </StackPanel>
     </Border>
 
+    <!-- External merge popup card — bottom-left corner, while an MCP session owns an
+         unresolved unit-merge conflict. Does not auto-dismiss; stays until the merge ends. -->
+    <Border Grid.Row="0" Grid.RowSpan="3"
+            VerticalAlignment="Bottom" HorizontalAlignment="Left"
+            Margin="16,0,0,30"
+            ZIndex="100"
+            IsVisible="{Binding IsExternalMergeBannerVisible}"
+            Background="{DynamicResource DeepBrush}"
+            BorderBrush="{DynamicResource PeatBrush}"
+            BorderThickness="1"
+            CornerRadius="6"
+            Padding="14,10">
+      <StackPanel Spacing="8" Width="300">
+        <TextBlock Classes="body" Text="{loc:Tr shell.externalMerge.banner}" TextWrapping="Wrap"/>
+        <StackPanel Orientation="Horizontal" Spacing="8" HorizontalAlignment="Right">
+          <Button Classes="btn"
+                  Content="{loc:Tr shell.externalMerge.open}"
+                  Command="{Binding OpenExternalMergeConflictCommand}"/>
+        </StackPanel>
+      </StackPanel>
+    </Border>
+
   </Grid>
 </Window>
diff --git a/src/ClaudeDo.Worker/External/ExternalMcpService.cs b/src/ClaudeDo.Worker/External/ExternalMcpService.cs
index 91fd0afa..4055ebf7 100644
--- a/src/ClaudeDo.Worker/External/ExternalMcpService.cs
+++ b/src/ClaudeDo.Worker/External/ExternalMcpService.cs
@@ -529,7 +529,10 @@ public sealed class ExternalMcpService
                 // pre-merge worktree/commit range at this point, so "did it contribute anything"
                 // reflects the review range the reviewer is about to approve.
                 emptyChildren = await GetEmptyDoneChildrenAsync(taskId, cancellationToken);
-                await _planningMerge.StartAsync(taskId, targetBranch ?? "", cancellationToken);
+                // externallyDriven: true — this call came from an MCP session, not the UI's
+                // Approve button. A unit-merge conflict must not auto-open the in-app resolver;
+                // the driving session resolves it via continue_merge/abort_merge instead.
+                await _planningMerge.StartAsync(taskId, targetBranch ?? "", cancellationToken, externallyDriven: true);
                 var parentDone = (await _tasks.GetByIdAsync(taskId, cancellationToken))!.Status == TaskStatus.Done;
                 mergeStatus = parentDone ? TaskMergeService.StatusMerged : TaskMergeService.StatusConflict;
                 if (!parentDone)
diff --git a/src/ClaudeDo.Worker/Hub/HubBroadcaster.cs b/src/ClaudeDo.Worker/Hub/HubBroadcaster.cs
index 919a1465..f46dabdf 100644
--- a/src/ClaudeDo.Worker/Hub/HubBroadcaster.cs
+++ b/src/ClaudeDo.Worker/Hub/HubBroadcaster.cs
@@ -55,8 +55,9 @@ public sealed class HubBroadcaster : IPrimeBroadcaster, IRefineBroadcaster
     public Task PlanningSubtaskMerged(string planningTaskId, string subtaskId) =>
         _hub.Clients.All.SendAsync("PlanningSubtaskMerged", planningTaskId, subtaskId);
 
-    public Task PlanningMergeConflict(string planningTaskId, string subtaskId, IReadOnlyList<string> files) =>
-        _hub.Clients.All.SendAsync("PlanningMergeConflict", planningTaskId, subtaskId, files);
+    public Task PlanningMergeConflict(
+        string planningTaskId, string subtaskId, IReadOnlyList<string> files, bool externallyDriven) =>
+        _hub.Clients.All.SendAsync("PlanningMergeConflict", planningTaskId, subtaskId, files, externallyDriven);
 
     public Task PlanningMergeAborted(string planningTaskId) =>
         _hub.Clients.All.SendAsync("PlanningMergeAborted", planningTaskId);
diff --git a/src/ClaudeDo.Worker/Hub/WorkerHub.cs b/src/ClaudeDo.Worker/Hub/WorkerHub.cs
index ad40fb56..02e80436 100644
--- a/src/ClaudeDo.Worker/Hub/WorkerHub.cs
+++ b/src/ClaudeDo.Worker/Hub/WorkerHub.cs
@@ -80,6 +80,7 @@ public record WorktreeOverviewDto(
     bool PathExistsOnDisk);
 
 public record ForceRemoveResultDto(bool Removed, string? Reason);
+public record PlanningMergeConflictStateDto(string PlanningTaskId, string SubtaskId);
 public record PendingQuestionDto(string TaskId, string QuestionId, string Question);
 public record MergeResultDto(string Status, IReadOnlyList<string> ConflictFiles, string? ErrorMessage);
 public record MergePreviewDto(string Status, IReadOnlyList<string> ConflictFiles, int ChangedFileCount);
@@ -939,6 +940,17 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         catch (InvalidOperationException ex) { throw new HubException(ex.Message); }
     }
 
+    /// <summary>Unit merges currently paused on a conflict that an MCP session (not the UI)
+    /// started. The UI calls this on (re)connect to recover the "don't auto-open the resolver"
+    /// banner after a restart, since the one-shot PlanningMergeConflict broadcast isn't replayed.</summary>
+    public async Task<List<PlanningMergeConflictStateDto>> GetActiveExternalPlanningMergeConflicts()
+    {
+        var conflicts = await _planningMergeOrchestrator.GetActiveExternalConflictsAsync(CancellationToken.None);
+        return conflicts
+            .Select(c => new PlanningMergeConflictStateDto(c.PlanningTaskId, c.SubtaskId))
+            .ToList();
+    }
+
     public async Task<List<PrimeScheduleDto>> ListPrimeSchedules()
     {
         using var ctx = _dbFactory.CreateDbContext();
diff --git a/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs b/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs
index 50f6d75d..0e49aeb8 100644
--- a/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs
+++ b/src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs
@@ -10,6 +10,9 @@ using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 namespace ClaudeDo.Worker.Planning;
 
+/// <summary>A unit-merge conflict currently paused, driven by an MCP session rather than the UI.</summary>
+public sealed record ExternalPlanningMergeConflict(string PlanningTaskId, string SubtaskId);
+
 public sealed class PlanningMergeOrchestrator
 {
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
@@ -25,6 +28,11 @@ public sealed class PlanningMergeOrchestrator
         public required string TargetBranch { get; init; }
         public required Queue<string> RemainingSubtaskIds { get; init; }
         public required bool IsPlanning { get; init; }
+        public required string WorkingDir { get; init; }
+        /// <summary>True when this unit merge was started by an MCP tool call (e.g. review_task
+        /// from a running Claude session) rather than a direct UI action. Conflicts on such a
+        /// merge must not auto-pop the in-app resolver — the driving session owns resolution.</summary>
+        public required bool ExternallyDriven { get; init; }
         public string? CurrentSubtaskId { get; set; }
     }
 
@@ -48,7 +56,8 @@ public sealed class PlanningMergeOrchestrator
         _logger = logger;
     }
 
-    public async Task StartAsync(string parentTaskId, string targetBranch, CancellationToken ct)
+    public async Task StartAsync(
+        string parentTaskId, string targetBranch, CancellationToken ct, bool externallyDriven = false)
     {
         string workingDir;
         List<TaskEntity> children;
@@ -100,7 +109,14 @@ public sealed class PlanningMergeOrchestrator
 
         var queue = new Queue<string>(idsToMerge);
 
-        var state = new State { TargetBranch = targetBranch, RemainingSubtaskIds = queue, IsPlanning = isPlanning };
+        var state = new State
+        {
+            TargetBranch = targetBranch,
+            RemainingSubtaskIds = queue,
+            IsPlanning = isPlanning,
+            WorkingDir = workingDir,
+            ExternallyDriven = externallyDriven,
+        };
         if (!_states.TryAdd(parentTaskId, state))
             throw new InvalidOperationException($"Merge already in progress for {parentTaskId}.");
 
@@ -112,6 +128,23 @@ public sealed class PlanningMergeOrchestrator
     public bool HasActiveMerge(string parentTaskId) =>
         _states.TryGetValue(parentTaskId, out var s) && s.CurrentSubtaskId is not null;
 
+    /// <summary>Externally-driven unit merges currently paused on a conflict, so the UI can
+    /// recover this on reconnect instead of relying solely on the one-shot broadcast. Checked
+    /// against <see cref="GitService.IsMidMergeAsync"/> rather than trusting the in-memory flag
+    /// alone, so a stale entry (e.g. the repo was already resolved through another path) never
+    /// reports an external merge that no longer exists.</summary>
+    public async Task<IReadOnlyList<ExternalPlanningMergeConflict>> GetActiveExternalConflictsAsync(CancellationToken ct)
+    {
+        var result = new List<ExternalPlanningMergeConflict>();
+        foreach (var (planningTaskId, state) in _states)
+        {
+            if (!state.ExternallyDriven || state.CurrentSubtaskId is null) continue;
+            if (!await _git.IsMidMergeAsync(state.WorkingDir, ct)) continue;
+            result.Add(new ExternalPlanningMergeConflict(planningTaskId, state.CurrentSubtaskId));
+        }
+        return result;
+    }
+
     public async Task ContinueAsync(string planningTaskId, CancellationToken ct)
     {
         if (!_states.TryGetValue(planningTaskId, out var state) || state.CurrentSubtaskId is null)
@@ -123,7 +156,7 @@ public sealed class PlanningMergeOrchestrator
 
         if (result.Status == TaskMergeService.StatusConflict)
         {
-            await _broadcaster.PlanningMergeConflict(planningTaskId, current, result.ConflictFiles);
+            await _broadcaster.PlanningMergeConflict(planningTaskId, current, result.ConflictFiles, state.ExternallyDriven);
             return;
         }
 
@@ -200,7 +233,8 @@ public sealed class PlanningMergeOrchestrator
 
                 if (result.Status == TaskMergeService.StatusConflict)
                 {
-                    await _broadcaster.PlanningMergeConflict(planningTaskId, subtaskId, result.ConflictFiles);
+                    await _broadcaster.PlanningMergeConflict(
+                        planningTaskId, subtaskId, result.ConflictFiles, state.ExternallyDriven);
                     keepState = true;
                     return;
                 }
diff --git a/tests/ClaudeDo.Ui.Tests/IslandsShellViewModelExternalMergeTests.cs b/tests/ClaudeDo.Ui.Tests/IslandsShellViewModelExternalMergeTests.cs
new file mode 100644
index 00000000..c3166e9a
--- /dev/null
+++ b/tests/ClaudeDo.Ui.Tests/IslandsShellViewModelExternalMergeTests.cs
@@ -0,0 +1,101 @@
+using System;
+using System.Threading.Tasks;
+using ClaudeDo.Ui.ViewModels;
+using Xunit;
+
+namespace ClaudeDo.Ui.Tests;
+
+// Covers the MCP-driven-merge popup fix: a unit-merge conflict started by an MCP session
+// (review_task/continue_merge) must not auto-open the in-app resolver — two parties editing the
+// same shared checkout at once is how the original bug manifested. The UI-driven path (a human
+// clicking Approve) keeps opening the resolver as before.
+public class IslandsShellViewModelExternalMergeTests
+{
+    [Fact]
+    public void ExternallyDriven_ShowsBanner()
+    {
+        var vm = new IslandsShellViewModel();
+
+        vm.OnPlanningMergeConflict("plan1", "sub1", new[] { "file.txt" }, externallyDriven: true);
+
+        Assert.True(vm.IsExternalMergeBannerVisible);
+    }
+
+    [Fact]
+    public void UiDriven_DoesNotShowExternalBanner()
+    {
+        var vm = new IslandsShellViewModel();
+
+        vm.OnPlanningMergeConflict("plan1", "sub1", new[] { "file.txt" }, externallyDriven: false);
+
+        Assert.False(vm.IsExternalMergeBannerVisible);
+    }
+
+    [Fact]
+    public void Aborted_ClearsBanner()
+    {
+        var vm = new IslandsShellViewModel();
+        vm.OnPlanningMergeConflict("plan1", "sub1", Array.Empty<string>(), externallyDriven: true);
+        Assert.True(vm.IsExternalMergeBannerVisible);
+
+        vm.OnPlanningMergeAborted("plan1");
+
+        Assert.False(vm.IsExternalMergeBannerVisible);
+    }
+
+    [Fact]
+    public void Completed_ClearsBanner()
+    {
+        var vm = new IslandsShellViewModel();
+        vm.OnPlanningMergeConflict("plan1", "sub1", Array.Empty<string>(), externallyDriven: true);
+
+        vm.OnPlanningMergeCompleted("plan1");
+
+        Assert.False(vm.IsExternalMergeBannerVisible);
+    }
+
+    [Fact]
+    public void UnrelatedPlanningTaskEnding_LeavesBannerVisible()
+    {
+        var vm = new IslandsShellViewModel();
+        vm.OnPlanningMergeConflict("plan1", "sub1", Array.Empty<string>(), externallyDriven: true);
+
+        vm.OnPlanningMergeCompleted("some-other-plan");
+
+        Assert.True(vm.IsExternalMergeBannerVisible);
+    }
+
+    [Fact]
+    public void LaterConflictOnSamePlanningTask_UpdatesInPlaceWithoutDuplicating()
+    {
+        var vm = new IslandsShellViewModel();
+        vm.OnPlanningMergeConflict("plan1", "subA", Array.Empty<string>(), externallyDriven: true);
+        vm.OnPlanningMergeConflict("plan1", "subB", Array.Empty<string>(), externallyDriven: true);
+        Assert.True(vm.IsExternalMergeBannerVisible);
+
+        // A single terminal event for that planning task clears it entirely — proves the second
+        // conflict updated the same entry rather than piling up a second one.
+        vm.OnPlanningMergeCompleted("plan1");
+
+        Assert.False(vm.IsExternalMergeBannerVisible);
+    }
+
+    [Fact]
+    public async Task OpenExternalMergeConflictCommand_NoPendingConflict_DoesNotThrow()
+    {
+        var vm = new IslandsShellViewModel();
+
+        await vm.OpenExternalMergeConflictCommand.ExecuteAsync(null);
+    }
+
+    [Fact]
+    public async Task OpenExternalMergeConflictCommand_NoDialogsWired_DoesNotThrow()
+    {
+        var vm = new IslandsShellViewModel();
+        vm.OnPlanningMergeConflict("plan1", "sub1", Array.Empty<string>(), externallyDriven: true);
+
+        // Dialogs/ConflictResolverFactory aren't wired in the test-only ctor — a deliberate
+        // click must degrade gracefully rather than throw, same as the existing UI-driven path.
+        await vm.OpenExternalMergeConflictCommand.ExecuteAsync(null);
+    }
+}
diff --git a/tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs b/tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs
index 511b78ae..75a79abd 100644
--- a/tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs
+++ b/tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs
@@ -33,7 +33,7 @@ public abstract class StubWorkerClient : IWorkerClient
     public event Action<string, bool, string?>? RefineFinishedEvent;
     public event Action<string, string>? PlanningMergeStartedEvent;
     public event Action<string, string>? PlanningSubtaskMergedEvent;
-    public event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
+    public event Action<string, string, IReadOnlyList<string>, bool>? PlanningMergeConflictEvent;
     public event Action<string>? PlanningMergeAbortedEvent;
     public event Action<string>? PlanningCompletedEvent;
     public event Action<PrimeFiredEvent>? PrimeFired;
@@ -53,6 +53,10 @@ public abstract class StubWorkerClient : IWorkerClient
     public void RaiseTaskQuestionAsked(string taskId, string questionId, string question) => TaskQuestionAskedEvent?.Invoke(taskId, questionId, question);
     public void RaiseTaskQuestionResolved(string taskId, string questionId) => TaskQuestionResolvedEvent?.Invoke(taskId, questionId);
     public void RaiseHandoffRequested(string taskId, IReadOnlyList<string> survivingTaskIds) => HandoffRequestedEvent?.Invoke(taskId, survivingTaskIds);
+    public void RaisePlanningMergeConflict(string planningTaskId, string subtaskId, IReadOnlyList<string> files, bool externallyDriven)
+        => PlanningMergeConflictEvent?.Invoke(planningTaskId, subtaskId, files, externallyDriven);
+    public void RaisePlanningMergeAborted(string planningTaskId) => PlanningMergeAbortedEvent?.Invoke(planningTaskId);
+    public void RaisePlanningCompleted(string planningTaskId) => PlanningCompletedEvent?.Invoke(planningTaskId);
 
     public void RaisePrepStarted() => PrepStartedEvent?.Invoke();
     public void RaisePrepLine(string line) => PrepLineEvent?.Invoke(line);
@@ -129,6 +133,8 @@ public abstract class StubWorkerClient : IWorkerClient
         => Task.FromResult<CombinedDiffResultDto?>(null);
     public virtual Task ContinuePlanningMergeAsync(string planningTaskId) => Task.CompletedTask;
     public virtual Task AbortPlanningMergeAsync(string planningTaskId) => Task.CompletedTask;
+    public virtual Task<IReadOnlyList<PlanningMergeConflictStateDto>> GetActiveExternalPlanningMergeConflictsAsync()
+        => Task.FromResult<IReadOnlyList<PlanningMergeConflictStateDto>>(Array.Empty<PlanningMergeConflictStateDto>());
     public virtual Task QueuePlanningSubtasksAsync(string parentTaskId, CancellationToken ct = default) => Task.CompletedTask;
     public virtual Task<string?> GetWeekReportAsync(DateOnly start, DateOnly end) => Task.FromResult<string?>(null);
     public virtual Task<string> GenerateWeekReportAsync(DateOnly start, DateOnly end) => Task.FromResult("");
diff --git a/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs b/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs
index 5e8707fa..b100eb6f 100644
--- a/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs
+++ b/tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs
@@ -245,6 +245,98 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         Assert.False(await git.IsMidMergeAsync(repo.RepoDir, CancellationToken.None));
     }
 
+    // ─── externallyDriven (MCP-driven merges must not auto-open the UI resolver) ────────────
+
+    [Fact]
+    public async Task StartAsync_ExternallyDriven_ConflictBroadcastFlagsExternallyDriven()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, _, _, _) = await SeedPlanningThreeChildrenMiddleConflictsAsync(db, repo);
+
+        var (orch, spy) = BuildOrchestrator(db);
+        await orch.StartAsync(parentId, "main", CancellationToken.None, externallyDriven: true);
+
+        var conflictCall = Assert.Single(spy, c => c.Method == "PlanningMergeConflict");
+        Assert.True((bool)conflictCall.Args[3]!);
+    }
+
+    [Fact]
+    public async Task StartAsync_DefaultNotExternallyDriven_ConflictBroadcastFlagsFalse()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, _, _, _) = await SeedPlanningThreeChildrenMiddleConflictsAsync(db, repo);
+
+        var (orch, spy) = BuildOrchestrator(db);
+        await orch.StartAsync(parentId, "main", CancellationToken.None);
+
+        var conflictCall = Assert.Single(spy, c => c.Method == "PlanningMergeConflict");
+        Assert.False((bool)conflictCall.Args[3]!);
+    }
+
+    [Fact]
+    public async Task GetActiveExternalConflictsAsync_ExternallyDrivenAndMidMerge_ReturnsEntry()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, _, subB, _) = await SeedPlanningThreeChildrenMiddleConflictsAsync(db, repo);
+
+        var (orch, _) = BuildOrchestrator(db);
+        await orch.StartAsync(parentId, "main", CancellationToken.None, externallyDriven: true);
+
+        var active = await orch.GetActiveExternalConflictsAsync(CancellationToken.None);
+        var entry = Assert.Single(active);
+        Assert.Equal(parentId, entry.PlanningTaskId);
+        Assert.Equal(subB, entry.SubtaskId);
+    }
+
+    [Fact]
+    public async Task GetActiveExternalConflictsAsync_UiDriven_ReturnsEmpty()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, _, _, _) = await SeedPlanningThreeChildrenMiddleConflictsAsync(db, repo);
+
+        var (orch, _) = BuildOrchestrator(db);
+        await orch.StartAsync(parentId, "main", CancellationToken.None); // externallyDriven defaults false
+
+        var active = await orch.GetActiveExternalConflictsAsync(CancellationToken.None);
+        Assert.Empty(active);
+    }
+
+    /// <summary>
+    /// The in-memory ExternallyDriven flag alone must never be trusted: if the repo was already
+    /// taken out of mid-merge through some other path (e.g. the driving Claude session died and
+    /// something else ran `git merge --abort` directly, without going through AbortAsync), the
+    /// query must not report a phantom external conflict. Proves the MERGE_HEAD coupling.
+    /// </summary>
+    [Fact]
+    public async Task GetActiveExternalConflictsAsync_StaleStateRepoNoLongerMidMerge_SelfHeals()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, _, _, _) = await SeedPlanningThreeChildrenMiddleConflictsAsync(db, repo);
+
+        var (orch, _) = BuildOrchestrator(db);
+        await orch.StartAsync(parentId, "main", CancellationToken.None, externallyDriven: true);
+
+        GitRepoFixture.RunGit(repo.RepoDir, "merge", "--abort");
+
+        var active = await orch.GetActiveExternalConflictsAsync(CancellationToken.None);
+        Assert.Empty(active);
+    }
+
     // ─── Stateless abort (post-restart recovery) ───────────────────────────
 
     /// <summary>
diff --git a/tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs b/tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs
index be3a7c1f..735ea2d4 100644
--- a/tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs
+++ b/tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs
@@ -109,7 +109,7 @@ sealed class FakeWorkerClient : IWorkerClient
     public event Action<string, bool, string?>? RefineFinishedEvent;
     public event Action<string, string>? PlanningMergeStartedEvent;
     public event Action<string, string>? PlanningSubtaskMergedEvent;
-    public event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
+    public event Action<string, string, IReadOnlyList<string>, bool>? PlanningMergeConflictEvent;
     public event Action<string>? PlanningMergeAbortedEvent;
     public event Action<string>? PlanningCompletedEvent;
     public event Action<PrimeFiredEvent>? PrimeFired;
@@ -121,6 +121,8 @@ sealed class FakeWorkerClient : IWorkerClient
     public Task<CombinedDiffResultDto?> BuildPlanningIntegrationBranchAsync(string planningTaskId, string targetBranch) => Task.FromResult<CombinedDiffResultDto?>(null);
     public Task ContinuePlanningMergeAsync(string planningTaskId) => Task.CompletedTask;
     public Task AbortPlanningMergeAsync(string planningTaskId) => Task.CompletedTask;
+    public Task<IReadOnlyList<PlanningMergeConflictStateDto>> GetActiveExternalPlanningMergeConflictsAsync()
+        => Task.FromResult<IReadOnlyList<PlanningMergeConflictStateDto>>(Array.Empty<PlanningMergeConflictStateDto>());
 
     public Task<AppSettingsDto?> GetAppSettingsAsync() => Task.FromResult<AppSettingsDto?>(null);
     public Task UpdateAppSettingsAsync(AppSettingsDto dto) => Task.CompletedTask;