fix(merge): refuse a merge that would overwrite an untracked file in the target

Before merging (or staging a conflict resolution's git add -A), compare the
branch's newly-added paths against what's currently untracked in the target
working directory. A collision aborts with a new untracked_collision status
naming the path and size, surfaced through merge_task/review_task,
preview_merge/preview_merge_set (which merge-tree alone can't see), and the
UI merge paths via FlashFooterError/ShowErrorAsync instead of a silent
catch{}. git's own preflight already refuses this while the path stays
untracked at merge time; this closes the gap once a path becomes trackable
in between (e.g. an unrelated conflict resolution's git add -A) or across
the continue_merge TOCTOU window.

src/ClaudeDo.Data/Git/GitService.cs

@@ -42,6 +42,16 @@ public sealed class GitService
         };
     }
 
+    /// <summary>
+    /// The merge base of two refs, or null when git can't find one (e.g. an unresolvable ref) —
+    /// callers treat that as "can't evaluate", not "no common history".
+    /// </summary>
+    public async Task<string?> MergeBaseAsync(string repoDir, string refA, string refB, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir, ["merge-base", refA, refB], ct);
+        return exitCode == 0 ? stdout.Trim() : null;
+    }
+
     public async Task WorktreeAddAsync(string repoDir, string branchName, string worktreePath, string baseCommit, CancellationToken ct = default)
     {
         await WorktreeAddGate.WaitAsync(ct);
@@ -70,9 +80,12 @@ public sealed class GitService
         }
     }
 
+    // --untracked-files=all: without it, a brand-new untracked directory collapses into a single
+    // "?? dir/" entry instead of listing the files inside it — callers matching against specific
+    // paths (e.g. the untracked-collision guard) need the individual files.
     public async Task<string> GetStatusPorcelainAsync(string workingDirectory, CancellationToken ct = default)
     {
-        var (exitCode, stdout, stderr) = await RunGitAsync(workingDirectory, ["status", "--porcelain"], ct);
+        var (exitCode, stdout, stderr) = await RunGitAsync(workingDirectory, ["status", "--porcelain", "--untracked-files=all"], ct);
         if (exitCode != 0)
             throw new InvalidOperationException($"git status --porcelain failed (exit {exitCode}): {stderr}");
         return stdout;

src/ClaudeDo.Localization/locales/de.json

@@ -662,7 +662,7 @@
     "tasksIsland": { "completedHeader": "ABGESCHLOSSEN", "completedHeaderCount": "ABGESCHLOSSEN · {0}", "planningOpenFailed": "Planungssitzung konnte nicht geöffnet werden: {0}", "planningResumeFailed": "Planungssitzung konnte nicht fortgesetzt werden: {0}", "approveFailed": "Genehmigen & Mergen fehlgeschlagen: {0}", "cancelReviewFailed": "Prüfung abbrechen fehlgeschlagen: {0}", "sendToQueueFailed": "In die Warteschlange stellen fehlgeschlagen: {0}", "queuePlanBlockedInteractive": "Plan kann nicht in die Warteschlange gestellt werden — {0} hat eine offene interaktive Sitzung und muss zuerst geschlossen werden.", "moveRunningRejected": "Ein laufender Task kann nicht in eine andere Liste verschoben werden.", "moveWorktreeRejected": "Verschieben nicht möglich — dieser Task hat einen aktiven Worktree, der auf sein aktuelles Repo zeigt.", "moveRepoConfirm": "Unterschiedliche Repos — {0} → {1}. Task trotzdem verschieben?", "moveConfirmUnavailable": "Verschieben nicht möglich — der Bestätigungsdialog ist nicht verfügbar.", "quickClaudeNoWorkingDir": "Für diese Liste ist kein Arbeitsverzeichnis konfiguriert.", "quickClaudeDirMissing": "Arbeitsverzeichnis existiert nicht mehr: {0}" },
     "diff": { "loadFailed": "Diff konnte nicht geladen werden: {0}", "noChanges": "Keine Änderungen anzuzeigen.", "unavailable": "Diff nicht mehr verfügbar — Commit-Bereich unvollständig." },
     "planningDiff": { "hubError": "Kombinierte Vorschau konnte nicht erstellt werden (Hub-Fehler).", "conflict": "Kombinierte Vorschau nicht möglich: Teilaufgabe {0} steht im Konflikt mit einer früheren Teilaufgabe ({1} Dateien).", "buildFailed": "Kombinierte Vorschau konnte nicht erstellt werden: {0}" },
-    "merge": { "commitMessage": "Merge-Aufgabe: {0}", "workerOfflineBranches": "Worker offline — Branches können nicht aufgelistet werden.", "loadBranchesFailed": "Branches konnten nicht geladen werden: {0}", "merged": "Zusammengeführt.", "conflict": "Merge-Konflikt — Ziel-Branch wiederhergestellt. Manuell oder über Fortsetzen lösen, dann erneut versuchen.", "blocked": "Blockiert: {0}", "verifyFailed": "Merge ist gelandet, aber das Verify-Kommando der Liste ist fehlgeschlagen — die Aufgabe wurde nicht auf Erledigt gesetzt.", "unknownStatus": "Unbekannter Status: {0}", "mergeFailed": "Merge fehlgeschlagen: {0}" },
+    "merge": { "commitMessage": "Merge-Aufgabe: {0}", "workerOfflineBranches": "Worker offline — Branches können nicht aufgelistet werden.", "loadBranchesFailed": "Branches konnten nicht geladen werden: {0}", "merged": "Zusammengeführt.", "conflict": "Merge-Konflikt — Ziel-Branch wiederhergestellt. Manuell oder über Fortsetzen lösen, dann erneut versuchen.", "blocked": "Blockiert: {0}", "verifyFailed": "Merge ist gelandet, aber das Verify-Kommando der Liste ist fehlgeschlagen — die Aufgabe wurde nicht auf Erledigt gesetzt.", "untrackedCollision": "Merge abgelehnt — er würde eine unversionierte Datei im Ziel-Arbeitsverzeichnis überschreiben.", "unknownStatus": "Unbekannter Status: {0}", "mergeFailed": "Merge fehlgeschlagen: {0}" },
     "conflictResolution": { "vsCodeError": "VS Code konnte nicht gestartet werden: {0}. Die Pfade sind oben aufgeführt — kopiere sie manuell.", "subtaskPrefix": "Konflikte in Teilaufgabe: {0}", "targetPrefix": "Zusammenführen in: {0}" },
     "settingsModal": { "workerOffline": "Worker offline — Einstellungen schreibgeschützt.", "saveFailed": "Speichern fehlgeschlagen: {0}" },
     "onlineInbox": { "workerOffline": "Worker offline — Konfiguration kann nicht geladen werden.", "saved": "Konfiguration gespeichert.", "saveFailed": "Speichern fehlgeschlagen: {0}", "signedIn": "Erfolgreich angemeldet.", "signedInNoRole": "Angemeldet, aber diesem Konto fehlt die Rolle 'user' in Zitadel — die Online-Synchronisierung wird abgelehnt, bis die Rolle im ClaudeDo-Projekt zugewiesen wird.", "signInFailed": "Anmeldung fehlgeschlagen: {0}", "signedOut": "Abgemeldet.", "signOutFailed": "Abmeldung fehlgeschlagen: {0}" },
@@ -677,7 +677,7 @@
     "worktreesTab": { "workerOffline": "Worker offline.", "removed": "{0} Worktree(s) entfernt.", "blocked": "Zwangsentfernung nicht möglich: {0} Aufgabe(n) laufen noch. Brich sie zuerst ab.", "removedFrom": "{0} Worktree(s) von {1} Aufgabe(n) entfernt.", "cleanupFailed": "Aufräumen fehlgeschlagen: {0}", "resetFailed": "Zurücksetzen fehlgeschlagen: {0}" },
     "worktreesOverview": { "titleAll": "Worktrees", "titleList": "Worktrees — {0}", "listFallback": "Liste", "cleanupFailed": "Aufräumen fehlgeschlagen.", "cleanupFailedDetailed": "Aufräumen fehlgeschlagen: {0}", "removed": "{0} Worktree(s) entfernt.", "discardFailed": "Worktree konnte nicht verworfen werden.", "keepFailed": "Worktree konnte nicht behalten werden.", "cannotForceRunning": "Eine laufende Aufgabe kann nicht zwangsweise entfernt werden.", "forceRemoveFailed": "Zwangsentfernung fehlgeschlagen.", "forceRemoveFailedDetailed": "Zwangsentfernung fehlgeschlagen: {0}", "batchProgress": "Merge {0}/{1}…", "batchDone": "{0} gemergt, {1} zu lösen." },
     "listSettings": { "untitled": "Unbenannt" },
-    "detailsIsland": { "verifyFailed": "Merge ist erfolgt, aber das Verifikationskommando der Liste ist fehlgeschlagen — die Aufgabe wurde nicht auf 'Erledigt' gesetzt." },
+    "detailsIsland": { "verifyFailed": "Merge ist erfolgt, aber das Verifikationskommando der Liste ist fehlgeschlagen — die Aufgabe wurde nicht auf 'Erledigt' gesetzt.", "untrackedCollision": "Merge abgelehnt — er würde eine unversionierte Datei im Ziel-Arbeitsverzeichnis überschreiben." },
     "lists": { "localSuffix": "{0} / lokal", "smartMyDay": "Mein Tag", "smartImportant": "Wichtig", "smartPlanned": "Geplant", "virtualQueue": "Warteschlange", "virtualRunning": "Läuft", "virtualReview": "Prüfung", "newList": "Neue Liste" },
     "repoImport": { "loadFailed": "Gespeicherte Ordner konnten nicht geladen werden: {0}", "saveFailed": "Ordner konnten nicht gespeichert werden: {0}" }
   },

src/ClaudeDo.Localization/locales/en.json

@@ -662,7 +662,7 @@
     "tasksIsland": { "completedHeader": "COMPLETED", "completedHeaderCount": "COMPLETED · {0}", "planningOpenFailed": "Couldn't open planning session: {0}", "planningResumeFailed": "Couldn't resume planning session: {0}", "approveFailed": "Approve & merge failed: {0}", "cancelReviewFailed": "Cancel review failed: {0}", "sendToQueueFailed": "Send to queue failed: {0}", "queuePlanBlockedInteractive": "Can't queue the plan — {0} has an open interactive session and must be closed first.", "moveRunningRejected": "Can't move a running task to another list.", "moveWorktreeRejected": "Can't move — this task has an active worktree pointing at its current repo.", "moveRepoConfirm": "Different repos — {0} → {1}. Move the task anyway?", "moveConfirmUnavailable": "Can't move — the confirmation dialog isn't available.", "quickClaudeNoWorkingDir": "This list has no working directory configured.", "quickClaudeDirMissing": "Working directory no longer exists: {0}" },
     "diff": { "loadFailed": "Failed to load diff: {0}", "noChanges": "No changes to show.", "unavailable": "Diff no longer available — commit range incomplete." },
     "planningDiff": { "hubError": "Could not build combined preview (hub error).", "conflict": "Cannot build combined preview: subtask {0} conflicts with an earlier subtask ({1} files).", "buildFailed": "Could not build combined preview: {0}" },
-    "merge": { "commitMessage": "Merge task: {0}", "workerOfflineBranches": "Worker offline — cannot list branches.", "loadBranchesFailed": "Failed to load branches: {0}", "merged": "Merged.", "conflict": "Merge conflict — target branch restored. Resolve manually or via Continue, then retry.", "blocked": "Blocked: {0}", "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "unknownStatus": "Unknown status: {0}", "mergeFailed": "Merge failed: {0}" },
+    "merge": { "commitMessage": "Merge task: {0}", "workerOfflineBranches": "Worker offline — cannot list branches.", "loadBranchesFailed": "Failed to load branches: {0}", "merged": "Merged.", "conflict": "Merge conflict — target branch restored. Resolve manually or via Continue, then retry.", "blocked": "Blocked: {0}", "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "untrackedCollision": "Merge refused — it would overwrite an untracked file in the target working directory.", "unknownStatus": "Unknown status: {0}", "mergeFailed": "Merge failed: {0}" },
     "conflictResolution": { "vsCodeError": "Could not launch VS Code: {0}. Paths are listed above — copy them manually.", "subtaskPrefix": "Conflicts in subtask: {0}", "targetPrefix": "Merging into: {0}" },
     "settingsModal": { "workerOffline": "Worker offline — settings read-only.", "saveFailed": "Save failed: {0}" },
     "onlineInbox": { "workerOffline": "Worker offline — cannot load config.", "saved": "Config saved.", "saveFailed": "Save failed: {0}", "signedIn": "Signed in successfully.", "signedInNoRole": "Signed in, but this account is missing the 'user' role in Zitadel — online sync will be rejected until the role is granted in the ClaudeDo project.", "signInFailed": "Sign-in failed: {0}", "signedOut": "Signed out.", "signOutFailed": "Sign-out failed: {0}" },
@@ -677,7 +677,7 @@
     "worktreesTab": { "workerOffline": "Worker offline.", "removed": "Removed {0} worktree(s).", "blocked": "Cannot force-remove: {0} task(s) still running. Cancel them first.", "removedFrom": "Removed {0} worktree(s) from {1} task(s).", "cleanupFailed": "Cleanup failed: {0}", "resetFailed": "Reset failed: {0}" },
     "worktreesOverview": { "titleAll": "Worktrees", "titleList": "Worktrees — {0}", "listFallback": "list", "cleanupFailed": "Cleanup failed.", "cleanupFailedDetailed": "Cleanup failed: {0}", "removed": "Removed {0} worktree(s).", "discardFailed": "Failed to discard worktree.", "keepFailed": "Failed to keep worktree.", "cannotForceRunning": "Cannot force-remove a running task.", "forceRemoveFailed": "Force remove failed.", "forceRemoveFailedDetailed": "Force remove failed: {0}", "batchProgress": "Merging {0}/{1}…", "batchDone": "Merged {0}, {1} need resolution." },
     "listSettings": { "untitled": "Untitled" },
-    "detailsIsland": { "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done." },
+    "detailsIsland": { "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "untrackedCollision": "Merge refused — it would overwrite an untracked file in the target working directory." },
     "lists": { "localSuffix": "{0} / local", "smartMyDay": "My Day", "smartImportant": "Important", "smartPlanned": "Planned", "virtualQueue": "Queue", "virtualRunning": "Running", "virtualReview": "Review", "newList": "New list" },
     "repoImport": { "loadFailed": "Couldn't load remembered folders: {0}", "saveFailed": "Couldn't save folders: {0}" }
   },

src/ClaudeDo.Ui/ViewModels/Conflicts/ConflictResolverViewModel.cs

@@ -143,7 +143,9 @@ public sealed partial class ConflictResolverViewModel : ObservableObject
             var start = await _worker.StartConflictMergeAsync(_taskId, targetBranch);
             if (!string.Equals(start.Status, "conflict", StringComparison.Ordinal))
             {
-                if (string.Equals(start.Status, "blocked", StringComparison.Ordinal))
+                // Any non-conflict outcome here means nothing was left mid-merge for this dialog
+                // to resolve -- surface why instead of quietly closing with no explanation.
+                if (start.Status is "blocked" or "untracked_collision")
                     Error = start.ErrorMessage;
                 return false;
             }

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -1207,6 +1207,10 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
             // out of Done. Surface that instead of silently looking like nothing happened.
             else if (!hasChildren && result?.Status == "verify_failed" && ShowErrorAsync != null)
                 await ShowErrorAsync(result.ErrorMessage ?? Loc.T("vm.detailsIsland.verifyFailed"));
+            // Nothing landed -- the merge was refused before touching the repo because the
+            // branch would have overwritten an untracked file in the target working directory.
+            else if (!hasChildren && result?.Status == "untracked_collision" && ShowErrorAsync != null)
+                await ShowErrorAsync(result.ErrorMessage ?? Loc.T("vm.detailsIsland.untrackedCollision"));
         }
         catch (Exception ex)
         {

src/ClaudeDo.Ui/ViewModels/Modals/MergeModalViewModel.cs

@@ -116,6 +116,12 @@ public sealed partial class MergeModalViewModel : ViewModelBase
                     // point of the gate.
                     ErrorMessage = result.ErrorMessage ?? Loc.T("vm.merge.verifyFailed");
                     break;
+                case "untracked_collision":
+                    // Nothing landed -- the merge was refused before touching the repo. Show the
+                    // real message (it names the colliding path and size); the locale string is
+                    // only a fallback.
+                    ErrorMessage = result.ErrorMessage ?? Loc.T("vm.merge.untrackedCollision");
+                    break;
                 default:
                     ErrorMessage = Loc.T("vm.merge.unknownStatus", result.Status);
                     break;

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -929,13 +929,16 @@ public sealed class ExternalMcpService
 
     [McpServerTool, Description(
         "Check whether a task would merge cleanly before touching anything — `git merge-tree --write-tree`, so the " +
-        "working tree, index and HEAD are untouched. status is 'clean' or 'conflict' (conflictFiles lists where git " +
-        "would stop); behind counts commits on targetBranch not yet on this branch, which flags a stale branch even " +
-        "when the preview is clean. IMPORTANT: a clean preview says nothing about whether the result compiles or " +
-        "passes tests — git can merge two changes cleanly (one file deleting a symbol another still references) and " +
-        "still break the build. isEmpty=true means the task's review range contributed nothing; check that flag " +
-        "rather than reading a small changedFileCount as empty. Throws if the task has neither an active worktree " +
-        "nor a handler commit range, or the list's working directory is missing from disk.")]
+        "working tree, index and HEAD are untouched. status is 'clean', 'conflict' (conflictFiles lists where git " +
+        "would stop), or 'untracked_collision' (conflictFiles lists a path the branch adds that already exists, " +
+        "untracked, in the target working directory — merge-tree can't see the working tree at all, so this is the " +
+        "only way to catch it before a real merge either refuses or, if that path became tracked in the meantime, " +
+        "silently overwrites it); behind counts commits on targetBranch not yet on this branch, which flags a stale " +
+        "branch even when the preview is clean. IMPORTANT: a clean preview says nothing about whether the result " +
+        "compiles or passes tests — git can merge two changes cleanly (one file deleting a symbol another still " +
+        "references) and still break the build. isEmpty=true means the task's review range contributed nothing; " +
+        "check that flag rather than reading a small changedFileCount as empty. Throws if the task has neither an " +
+        "active worktree nor a handler commit range, or the list's working directory is missing from disk.")]
     public async Task<MergePreviewToolDto> PreviewMerge(
         string taskId,
         [Description("Branch to preview against; defaults to the repo's current branch.")]

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -46,6 +46,7 @@ public sealed class TaskMergeService
     public const string StatusBlocked     = "blocked";
     public const string StatusAborted     = "aborted";
     public const string StatusVerifyFailed = "verify_failed";
+    public const string StatusUntrackedCollision = "untracked_collision";
 
     public const string StatusReverted       = "reverted";
     public const string StatusConflictAborted = "conflict_aborted";
@@ -53,6 +54,7 @@ public sealed class TaskMergeService
     public const string PreviewClean       = "clean";
     public const string PreviewConflict    = "conflict";
     public const string PreviewUnavailable = "unavailable";
+    public const string PreviewUntrackedCollision = "untracked_collision";
 
     // The verify command is a trusted, list-owner-configured build/test invocation (not
     // per-request user input), so a generous fixed timeout is enough — no need for a
@@ -143,6 +145,82 @@ public sealed class TaskMergeService
         return trimmed.Length <= maxChars ? trimmed : trimmed[^maxChars..];
     }
 
+    /// <summary>
+    /// Guards against a merge silently overwriting a file that sits untracked in the target
+    /// working directory: git already refuses that itself while the path is still untracked at
+    /// merge time, but a path that only became trackable in between (e.g. an unrelated conflict
+    /// resolution's `git add -A` sweeping it up) loses that protection. Compares the paths
+    /// <paramref name="branchName"/> newly added since its merge base with <paramref name="targetRef"/>
+    /// against what's currently untracked in <paramref name="workingDir"/> — reusing
+    /// <see cref="GitService.GetCommittedFilesAsync"/> (run against the branch's own worktree, where
+    /// HEAD is the branch tip) and <see cref="GitService.GetStatusPorcelainAsync"/> rather than adding
+    /// new git plumbing. Returns null when there's nothing to flag.
+    /// </summary>
+    private async Task<MergeResult?> CheckUntrackedCollisionsAsync(
+        string workingDir, string targetRef, string worktreePath, string branchName, CancellationToken ct)
+    {
+        var mergeBase = await _git.MergeBaseAsync(workingDir, targetRef, branchName, ct);
+        if (mergeBase is null) return null;
+
+        List<string> addedByBranch;
+        try
+        {
+            addedByBranch = ParseAddedPaths(await _git.GetCommittedFilesAsync(worktreePath, mergeBase, ct));
+        }
+        catch { return null; } // worktree missing — nothing to compare, let the merge report its own outcome
+
+        if (addedByBranch.Count == 0) return null;
+
+        var untracked = ParseUntrackedPaths(await _git.GetStatusPorcelainAsync(workingDir, ct));
+        if (untracked.Count == 0) return null;
+
+        var untrackedSet = new HashSet<string>(untracked, StringComparer.OrdinalIgnoreCase);
+        var collisions = addedByBranch.Where(untrackedSet.Contains).ToList();
+        if (collisions.Count == 0) return null;
+
+        var details = collisions.Select(p => $"{p} ({DescribeSize(workingDir, p)})");
+        return new MergeResult(StatusUntrackedCollision, collisions,
+            "merge would overwrite untracked file(s) that exist in the target working directory: " +
+            string.Join(", ", details));
+    }
+
+    private static string DescribeSize(string workingDir, string relativePath)
+    {
+        try
+        {
+            var full = Path.Combine(workingDir, relativePath.Replace('/', Path.DirectorySeparatorChar));
+            return $"{new FileInfo(full).Length} bytes";
+        }
+        catch { return "size unknown"; }
+    }
+
+    // Only "A" (pure add) entries — a path the branch modifies but that already exists on the
+    // target's own history is a normal merge, not a collision with something untracked.
+    private static List<string> ParseAddedPaths(string nameStatus)
+    {
+        var result = new List<string>();
+        foreach (var raw in nameStatus.Split('\n', StringSplitOptions.RemoveEmptyEntries))
+        {
+            var line = raw.TrimEnd('\r');
+            if (line.Length < 2 || line[0] != 'A') continue;
+            var tab = line.IndexOf('\t');
+            if (tab >= 0) result.Add(line[(tab + 1)..].Trim());
+        }
+        return result;
+    }
+
+    private static List<string> ParseUntrackedPaths(string porcelain)
+    {
+        var result = new List<string>();
+        foreach (var raw in porcelain.Split('\n', StringSplitOptions.RemoveEmptyEntries))
+        {
+            var line = raw.TrimEnd('\r');
+            if (line.StartsWith("?? ", StringComparison.Ordinal))
+                result.Add(line[3..].Trim().Trim('"'));
+        }
+        return result;
+    }
+
     private async Task MarkWorktreeMergedAsync(string taskId, string mergeCommitSha, CancellationToken ct)
     {
         using (var ctx = _dbFactory.CreateDbContext())
@@ -198,6 +276,9 @@ public sealed class TaskMergeService
                 catch (Exception ex) { return Blocked($"failed to switch target branch: {ex.Message}"); }
             }
 
+            var collision = await CheckUntrackedCollisionsAsync(list.WorkingDir, "HEAD", wt.Path, wt.BranchName, ct);
+            if (collision is not null) return collision;
+
             var (exitCode, stderr) = await _git.MergeNoFfAsync(list.WorkingDir, wt.BranchName, commitMessage, ct);
             if (exitCode != 0)
             {
@@ -315,6 +396,13 @@ public sealed class TaskMergeService
             if (stillConflicted.Count > 0)
                 return new MergeResult(StatusConflict, stillConflicted, "conflicts not fully resolved");
 
+            // Closes the window between the original `git merge` (which already refused any
+            // untracked collision at that point) and this call: an untracked file matching one
+            // of the branch's own added paths could have appeared in the meantime, and `add -A`
+            // below stages the whole working directory, not just the conflicted paths.
+            var collision = await CheckUntrackedCollisionsAsync(list.WorkingDir, "HEAD", wt.Path, wt.BranchName, ct);
+            if (collision is not null) return collision;
+
             await _git.AddAllAsync(list.WorkingDir, ct);
 
             var remaining = await _git.ListConflictedFilesAsync(list.WorkingDir, ct);
@@ -513,6 +601,12 @@ public sealed class TaskMergeService
             ? await _git.GetCurrentBranchAsync(list.WorkingDir, ct)
             : targetBranch;
 
+        // merge-tree (below) is a pure object-level check — it has no idea the working directory
+        // has an untracked file that would block (or, worse, silently be lost by) the real merge.
+        var collision = await CheckUntrackedCollisionsAsync(list.WorkingDir, target, wt.Path, wt.BranchName, ct);
+        if (collision is not null)
+            return new MergePreviewResult(PreviewUntrackedCollision, collision.ConflictFiles, 0);
+
         var preview = await _git.PreviewMergeAsync(list.WorkingDir, target, wt.BranchName, ct);
         if (!preview.Supported)
             return new MergePreviewResult(PreviewUnavailable, Array.Empty<string>(), 0);

tests/ClaudeDo.Worker.Tests/Services/TaskMergeServiceTests.cs

@@ -349,6 +349,53 @@ public class TaskMergeServiceTests : IDisposable
         Assert.Equal("merged", result.Status);
     }
 
+    [Fact]
+    public async Task MergeAsync_BranchAddsPathUntrackedInTarget_RefusesAndLeavesLocalContentIntact()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) return;
+        var repo = NewRepo();
+        var db = NewDb();
+        var (list, task) = await SeedListAndTask(db, workingDir: repo.RepoDir, status: TaskStatus.Done);
+
+        var wtMgr = BuildWorktreeManager(db);
+        var wtCtx = await wtMgr.CreateAsync(task, list, CancellationToken.None);
+        _wtCleanups.Add((repo.RepoDir, wtCtx.WorktreePath));
+
+        // The task branch creates a brand new file at a path it has never seen tracked.
+        Directory.CreateDirectory(Path.Combine(wtCtx.WorktreePath, "docs"));
+        File.WriteAllText(Path.Combine(wtCtx.WorktreePath, "docs", "PLUGIN.md"), "agent-written replacement\n");
+        await wtMgr.CommitIfChangedAsync(wtCtx, task, list, CancellationToken.None);
+
+        // The same path already exists, untracked, in the target working directory — never
+        // committed, not ignored — carrying content unrelated to the task branch.
+        Directory.CreateDirectory(Path.Combine(repo.RepoDir, "docs"));
+        var localPath = Path.Combine(repo.RepoDir, "docs", "PLUGIN.md");
+        const string localContent = "local plugin docs — must survive\n";
+        File.WriteAllText(localPath, localContent);
+
+        var (svc, proxy) = BuildService(db);
+        var currentBranch = await new GitService().GetCurrentBranchAsync(repo.RepoDir);
+        var headBefore = GitRepoFixture.RunGit(repo.RepoDir, "rev-parse", "HEAD").Trim();
+
+        var result = await svc.MergeAsync(task.Id, currentBranch, removeWorktree: false,
+            commitMessage: "Merge task", ct: CancellationToken.None);
+
+        Assert.Equal(TaskMergeService.StatusUntrackedCollision, result.Status);
+        Assert.Contains("docs/PLUGIN.md", result.ConflictFiles);
+        Assert.Contains("docs/PLUGIN.md", result.ErrorMessage ?? "");
+        Assert.Contains($"{System.Text.Encoding.UTF8.GetByteCount(localContent)} bytes", result.ErrorMessage ?? "");
+
+        // Nothing landed: no merge was attempted, the local file is untouched, worktree unaffected.
+        Assert.Equal(localContent, File.ReadAllText(localPath));
+        Assert.Equal(headBefore, GitRepoFixture.RunGit(repo.RepoDir, "rev-parse", "HEAD").Trim());
+        Assert.False(await new GitService().IsMidMergeAsync(repo.RepoDir));
+        Assert.DoesNotContain(proxy.Calls, c => c.Method == "WorktreeUpdated");
+
+        using var ctx = db.CreateContext();
+        var wt = await new WorktreeRepository(ctx).GetByTaskIdAsync(task.Id);
+        Assert.Equal(WorktreeState.Active, wt!.State);
+    }
+
     [Fact]
     public async Task MergeAsync_TargetBranchDifferentFromHead_ChecksOutBeforeMerging()
     {
@@ -493,6 +540,66 @@ public class TaskMergeServiceTests : IDisposable
         Assert.True(string.IsNullOrWhiteSpace(GitRepoFixture.RunGit(repo.RepoDir, "status", "--porcelain")));
     }
 
+    [Fact]
+    public async Task ContinueMergeAsync_PathBecomesUntrackedDuringResolution_RefusesBeforeAddAll()
+    {
+        // git's own preflight already refuses a merge whose incoming branch collides with a path
+        // that is untracked in the target *at merge-start* (verified against real git while
+        // investigating this guard). The only path left uncovered is one that becomes untracked
+        // again *during* conflict resolution, after the initial merge already staged it cleanly —
+        // this reproduces exactly that narrow window, right before `git add -A` would sweep it
+        // back in unnoticed.
+        if (!GitRepoFixture.IsGitAvailable()) return;
+
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "# main change\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-am", "main change");
+
+        var wtPath = Path.Combine(Path.GetTempPath(), $"wt_{Guid.NewGuid():N}");
+        _wtCleanups.Add((repo.RepoDir, wtPath));
+        GitRepoFixture.RunGit(repo.RepoDir, "worktree", "add", "-b", "claudedo/t-untracked", wtPath, repo.BaseCommit);
+        File.WriteAllText(Path.Combine(wtPath, "README.md"), "# branch change\n");
+        File.WriteAllText(Path.Combine(wtPath, "extra.md"), "agent content\n");
+        GitRepoFixture.RunGit(wtPath, "add", "-A");
+        GitRepoFixture.RunGit(wtPath, "commit", "-m", "branch change + add extra.md");
+
+        var (_, task) = await SeedListAndTask(db, workingDir: repo.RepoDir, status: TaskStatus.Done);
+        await SeedWorktree(db, task.Id, wtPath, "claudedo/t-untracked", repo.BaseCommit);
+
+        var (svc, _) = BuildService(db);
+
+        var first = await svc.MergeAsync(task.Id, "main", false, "msg",
+            leaveConflictsInTree: true, CancellationToken.None);
+        Assert.Equal(TaskMergeService.StatusConflict, first.Status);
+
+        // extra.md landed clean already (git stages any non-conflicting file the moment the
+        // merge starts, even while README.md is left conflicted) -- confirm that, then unstage
+        // it and drop in different, untracked local content, reproducing "became untracked
+        // again mid-resolution".
+        Assert.Equal("A", GitRepoFixture.RunGit(repo.RepoDir, "diff", "--name-status", "--cached", "--", "extra.md").Trim()[..1]);
+        GitRepoFixture.RunGit(repo.RepoDir, "rm", "--cached", "extra.md");
+        const string localContent = "important local content that must survive\n";
+        File.WriteAllText(Path.Combine(repo.RepoDir, "extra.md"), localContent);
+
+        // Resolve the real (unrelated) conflict.
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "# resolved\n");
+
+        var result = await svc.ContinueMergeAsync(task.Id, CancellationToken.None);
+
+        Assert.Equal(TaskMergeService.StatusUntrackedCollision, result.Status);
+        Assert.Contains("extra.md", result.ConflictFiles);
+        Assert.Equal(localContent, File.ReadAllText(Path.Combine(repo.RepoDir, "extra.md")));
+
+        // Nothing committed; still mid-merge so abort/continue can still resolve it.
+        Assert.True(await new GitService().IsMidMergeAsync(repo.RepoDir));
+        using var ctx = db.CreateContext();
+        var wt = ctx.Worktrees.Single(w => w.TaskId == task.Id);
+        Assert.Equal(WorktreeState.Active, wt.State);
+    }
+
     [Fact]
     public async Task AbortMergeAsync_AfterConflict_RestoresCleanStateAndLeavesWorktreeActive()
     {
@@ -577,6 +684,33 @@ public class TaskMergeServiceTests : IDisposable
         Assert.Contains("README.md", preview.ConflictFiles);
     }
 
+    [Fact]
+    public async Task PreviewAsync_BranchAddsPathUntrackedInTarget_ReturnsUntrackedCollision()
+    {
+        // merge-tree (the real preview mechanism) never touches the working directory, so
+        // without this check the preview would say "clean" for a merge that can't actually land.
+        if (!GitRepoFixture.IsGitAvailable()) return;
+        var repo = NewRepo();
+        var db = NewDb();
+        var (list, task) = await SeedListAndTask(db, repo.RepoDir, TaskStatus.WaitingForReview);
+
+        var wtMgr = BuildWorktreeManager(db);
+        var wtCtx = await wtMgr.CreateAsync(task, list, CancellationToken.None);
+        _wtCleanups.Add((repo.RepoDir, wtCtx.WorktreePath));
+        File.WriteAllText(Path.Combine(wtCtx.WorktreePath, "PLUGIN.md"), "agent content\n");
+        await wtMgr.CommitIfChangedAsync(wtCtx, task, list, CancellationToken.None);
+
+        File.WriteAllText(Path.Combine(repo.RepoDir, "PLUGIN.md"), "local content\n");
+
+        var (svc, _) = BuildService(db);
+        var target = await new GitService().GetCurrentBranchAsync(repo.RepoDir);
+
+        var preview = await svc.PreviewAsync(task.Id, target, CancellationToken.None);
+
+        Assert.Equal(TaskMergeService.PreviewUntrackedCollision, preview.Status);
+        Assert.Contains("PLUGIN.md", preview.ConflictFiles);
+    }
+
     [Fact]
     public async Task PreviewAsync_NoActiveWorktree_ReturnsUnavailable()
     {