feat(worker): warn on near-duplicate titles in add_task/batch_add_tasks

add_task and batch_add_tasks now report up to 3 open (non-terminal)
tasks in the same list with a strongly overlapping title, so a
parallel agent can notice and mention a likely duplicate instead of
silently creating one. The task is always created regardless. Uses a
cheap normalized-word overlap heuristic (no embeddings/LLM call),
robust to German umlaut/digraph spelling variants. Breaking change:
AddTask now returns AddTaskResult { task, possibleDuplicates } instead
of a bare TaskRefDto; BatchAddTaskResult gained a PossibleDuplicates
field.

docs/explore-notes/external-mcp.md

@@ -1,8 +1,8 @@
 # External MCP tool surface
 
 > **Explore-note — verify before trusting.** Distilled map of a subsystem, not authoritative.
-> Last verified against commit `bdee731` (2026-08-05).
+> Last verified against commit `7cfe280` (2026-08-06).
-> Drift check: `git log --oneline bdee731..HEAD -- src/ClaudeDo.Worker/External`
+> Drift check: `git log --oneline 7cfe280..HEAD -- src/ClaudeDo.Worker/External`
 > Stable structure only (no line numbers). See docs/explore-notes/README.md.
 
 Covers `src/ClaudeDo.Worker/External/` — the always-on MCP tools ClaudeDo exposes to general
@@ -71,6 +71,12 @@ Daily prep: `GetDailyPrepCandidates`, `SetMyDay`.
   UI's "set status freely" affordance uses) but is **refused** for a task with an active
   worktree, since that would skip `review_task`'s merge.
 
+**`AddTask`** — always creates the task; also returns `possibleDuplicates` (up to 3, id/title/status
+only, no descriptions) — open (non-terminal) tasks in the *same list* whose normalized title
+overlaps strongly with the new one. Cheap word-overlap heuristic (`ExternalMcpService`'s
+`FindPossibleDuplicatesAsync`/`NormalizeTitleWords`), no embeddings/LLM call, no blocking —
+the caller just gets a heads-up to relay. `BatchAddTasks` carries the same field per item.
+
 **`ReviewTask`** — `approve` / `reject_rerun` / `reject_park` / `cancel` for a
 `WaitingForReview` task. Approve is review+merge exactly like the hub's `ApproveReview`: unit
 merge for parents, worktree merge into optional `targetBranch` for childless tasks. Conflicts

src/ClaudeDo.Worker/External/BatchMcpTools.cs

@@ -7,7 +7,9 @@ public sealed record BatchAddTaskInput(string Title, string? Description = null,
 public sealed record BatchSetMyDayInput(string TaskId, bool IsMyDay, int? SortOrder = null);
 
 public sealed record BatchGetTaskResult(string Id, bool Found, TaskDto? Task, string? Error);
-public sealed record BatchAddTaskResult(int Index, string Title, bool Ok, TaskRefDto? Task, string? Error);
+public sealed record BatchAddTaskResult(
+    int Index, string Title, bool Ok, TaskRefDto? Task,
+    IReadOnlyList<PossibleDuplicateDto>? PossibleDuplicates, string? Error);
 public sealed record BatchTaskResult(string TaskId, bool Ok, string? Error);
 public sealed record BatchCancelResult(string TaskId, bool Ok, bool Cancelled, string? Error);
 public sealed record BatchCleanupResult(string TaskId, bool Ok, bool Removed, bool BranchDeleted, string? Error);
@@ -62,8 +64,11 @@ public sealed class BatchMcpTools
         "Create many tasks in one list at once. Each item: { title, description?, model? } " +
         "(model: haiku|sonnet|opus, blank = inherit list/global default). " +
         "queueImmediately enqueues every created task. " +
-        "Returns one result per item: { index, title, ok, task, error }; task is a lean reference " +
+        "Returns one result per item: { index, title, ok, task, possibleDuplicates, error }; task is " +
-        "(id, listId, title, status, sortOrder, isMyDay), not the description you just sent. Max 100 items.")]
+        "a lean reference (id, listId, title, status, sortOrder, isMyDay), not the description you just " +
+        "sent. Each item is always created — possibleDuplicates is a non-blocking heads-up (up to 3 open " +
+        "tasks in the same list with a strongly overlapping title, id/title/status only); check it and " +
+        "mention any hit to the caller, but do not treat it as an error. Max 100 items.")]
     public async Task<IReadOnlyList<BatchAddTaskResult>> BatchAddTasks(
         string listId,
         BatchAddTaskInput[] tasks,
@@ -82,12 +87,12 @@ public sealed class BatchMcpTools
                 var created = await _svc.AddTask(
                     listId, item.Title, item.Description, createdBy,
                     queueImmediately, item.Model, cancellationToken);
-                results.Add(new BatchAddTaskResult(i, item.Title, true, created, null));
+                results.Add(new BatchAddTaskResult(i, item.Title, true, created.Task, created.PossibleDuplicates, null));
             }
             catch (OperationCanceledException) { throw; }
             catch (Exception ex)
             {
-                results.Add(new BatchAddTaskResult(i, item.Title, false, null, ex.Message));
+                results.Add(new BatchAddTaskResult(i, item.Title, false, null, null, ex.Message));
             }
         }
         return results;

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -1,5 +1,8 @@
 using System.ComponentModel;
 using System.Diagnostics;
+using System.Globalization;
+using System.Text;
+using System.Text.RegularExpressions;
 using ClaudeDo.Data;
 using ClaudeDo.Data.Git;
 using ClaudeDo.Data.Models;
@@ -47,6 +50,11 @@ public sealed record TaskRefDto(
     int SortOrder,
     bool IsMyDay);
 
+// Deliberately small: no descriptions, capped candidate count (see AddTask's duplicate check).
+public sealed record PossibleDuplicateDto(string TaskId, string Title, string Status);
+
+public sealed record AddTaskResult(TaskRefDto Task, IReadOnlyList<PossibleDuplicateDto> PossibleDuplicates);
+
 public sealed record WorktreeInfoDto(
     string Path, string Branch, string HeadCommit, string BaseCommit,
     int Ahead, int Behind, bool IsDirty, string? MergeCommit = null);
@@ -182,8 +190,12 @@ public sealed class ExternalMcpService
         "Set model to the cheapest model that can do the task well — 'haiku' for trivial/mechanical work, " +
         "'sonnet' for normal coding (the default), 'opus' only for complex or cross-cutting work. " +
         "Leave model null to inherit the list/global default. " +
-        "Returns a lean task reference (id, listId, title, status, sortOrder, isMyDay) — not the description you just sent.")]
+        "Returns { task, possibleDuplicates }: task is a lean reference (id, listId, title, status, " +
-    public async Task<TaskRefDto> AddTask(
+        "sortOrder, isMyDay) — not the description you just sent. The task is always created — " +
+        "possibleDuplicates is a non-blocking heads-up (up to 3 open tasks in the same list with a " +
+        "strongly overlapping title, id/title/status only); check it and mention any hit to the caller, " +
+        "but do not treat it as an error.")]
+    public async Task<AddTaskResult> AddTask(
         string listId,
         string title,
         string? description = null,
@@ -200,6 +212,8 @@ public sealed class ExternalMcpService
         var list = await _lists.GetByIdAsync(listId, cancellationToken)
             ?? throw new InvalidOperationException($"List {listId} not found.");
 
+        var possibleDuplicates = await FindPossibleDuplicatesAsync(listId, title, cancellationToken);
+
         var entity = new TaskEntity
         {
             Id = Guid.NewGuid().ToString(),
@@ -225,7 +239,77 @@ public sealed class ExternalMcpService
         }
 
         await _broadcaster.TaskUpdated(entity.Id);
-        return ToRefDto(entity);
+        return new AddTaskResult(ToRefDto(entity), possibleDuplicates);
+    }
+
+    // Non-terminal: a task still open enough that a new, similarly-titled task might be a duplicate
+    // of it. Done/Failed/Cancelled tasks are retired and never flagged.
+    private static readonly TaskStatus[] NonTerminalStatuses =
+    [
+        TaskStatus.Idle, TaskStatus.Queued, TaskStatus.Running,
+        TaskStatus.WaitingForChildren, TaskStatus.WaitingForReview,
+    ];
+
+    private const int MaxPossibleDuplicates = 3;
+
+    // Cheap, dependency-free "near-duplicate title" heuristic — no embeddings, no LLM call, just a
+    // normalized-word overlap coefficient (shared significant words / smaller word-set size, not
+    // Jaccard). Overlap-coefficient beats Jaccard here because titles in this project vary wildly in
+    // length (a terse one-liner vs. a fully-spelled-out German sentence) — dividing by the union
+    // would wash out a real match against a longer title. Threshold tuned against the pair that
+    // motivated this check: "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal" vs.
+    // "Settings: MaxTurnsCeiling editierbar machen" (>=2 shared significant words, >=50% overlap of
+    // the shorter title's words) without over-firing on titles that merely share one generic word.
+    private async Task<IReadOnlyList<PossibleDuplicateDto>> FindPossibleDuplicatesAsync(
+        string listId, string title, CancellationToken ct)
+    {
+        var newWords = NormalizeTitleWords(title);
+        if (newWords.Count == 0) return [];
+
+        var candidates = await _tasks.GetByListIdAsync(listId, ct);
+
+        return candidates
+            .Where(t => NonTerminalStatuses.Contains(t.Status))
+            .Select(t => (Task: t, Words: NormalizeTitleWords(t.Title)))
+            .Select(x => (x.Task, Shared: newWords.Count(x.Words.Contains), x.Words))
+            .Where(x => x.Shared >= 2 && x.Words.Count > 0
+                        && (double)x.Shared / Math.Min(newWords.Count, x.Words.Count) >= 0.5)
+            .OrderByDescending(x => (double)x.Shared / Math.Min(newWords.Count, x.Words.Count))
+            .ThenByDescending(x => x.Shared)
+            .Take(MaxPossibleDuplicates)
+            .Select(x => new PossibleDuplicateDto(x.Task.Id, x.Task.Title, x.Task.Status.ToString()))
+            .ToList();
+    }
+
+    private static readonly Regex NonWordChars = new(@"[^a-z0-9]+", RegexOptions.Compiled);
+
+    // Common structural filler that would otherwise dominate the overlap score across most titles
+    // in this project's lists without indicating a real duplicate (German articles/prepositions
+    // plus the recurring "mcp"/"task(s)"/"cleanup" nouns called out in the task write-up).
+    private static readonly HashSet<string> TitleStopWords = new(StringComparer.Ordinal)
+    {
+        "der", "die", "das", "und", "von", "auf", "mit", "fur", "ein", "eine",
+        "ist", "sind", "oder", "nicht", "mcp", "task", "tasks", "cleanup",
+    };
+
+    // Lowercases, transliterates German umlauts/ß to their digraph spelling (ä→ae, ß→ss, ...) so
+    // "Prüfung" and "Pruefung" normalize to the same token, strips remaining diacritics/punctuation,
+    // then drops short (<3 char) and stopword tokens.
+    private static HashSet<string> NormalizeTitleWords(string title)
+    {
+        var s = title.ToLowerInvariant()
+            .Replace("ä", "ae").Replace("ö", "oe").Replace("ü", "ue").Replace("ß", "ss");
+        s = s.Normalize(NormalizationForm.FormD);
+
+        var sb = new StringBuilder(s.Length);
+        foreach (var ch in s)
+            if (CharUnicodeInfo.GetUnicodeCategory(ch) != UnicodeCategory.NonSpacingMark)
+                sb.Append(ch);
+
+        var ascii = NonWordChars.Replace(sb.ToString(), " ");
+        return ascii.Split(' ', StringSplitOptions.RemoveEmptyEntries)
+            .Where(w => w.Length >= 3 && !TitleStopWords.Contains(w))
+            .ToHashSet(StringComparer.Ordinal);
     }
 
     [McpServerTool, Description(

tests/ClaudeDo.Worker.Tests/External/BatchMcpToolsTests.cs

@@ -142,6 +142,24 @@ public sealed class BatchMcpToolsTests : IDisposable
         Assert.Equal(2, inList.Count);
     }
 
+    [Fact]
+    public async Task BatchAddTasks_SimilarTitleInSameList_ReportsPossibleDuplicate()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.Idle);
+        var sut = BuildSut();
+
+        var results = await sut.BatchAddTasks(listId, new[]
+        {
+            new BatchAddTaskInput("Settings: MaxTurnsCeiling editierbar machen"),
+        }, cancellationToken: CancellationToken.None);
+
+        var result = Assert.Single(results);
+        Assert.True(result.Ok);
+        Assert.NotNull(result.PossibleDuplicates);
+        Assert.Single(result.PossibleDuplicates!);
+    }
+
     [Fact]
     public async Task BatchGetTasks_MissingId_IsFoundFalseNotError()
     {

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -1518,7 +1518,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var dto = await sut.AddTask(listId, "t", cancellationToken: CancellationToken.None);
 
-        var loaded = await _tasks.GetByIdAsync(dto.Id);
+        var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
         Assert.Null(loaded!.Model);
     }
 
@@ -1530,7 +1530,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var dto = await sut.AddTask(listId, "t", model: "HAIKU", cancellationToken: CancellationToken.None);
 
-        var loaded = await _tasks.GetByIdAsync(dto.Id);
+        var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
         Assert.Equal("haiku", loaded!.Model);
     }
 
@@ -1544,6 +1544,80 @@ public sealed class ExternalMcpServiceTests : IDisposable
             () => sut.AddTask(listId, "t", model: "gpt4", cancellationToken: CancellationToken.None));
     }
 
+    // ── AddTask possible-duplicate check ────────────────────────────────────────
+
+    [Fact]
+    public async Task AddTask_SimilarOpenTitleInSameList_IsReportedAsPossibleDuplicate()
+    {
+        // The exact pair that motivated this check (bf4cd901 vs f3718cd8, see task write-up).
+        var listId = await SeedListAsync();
+        var existing = await SeedTaskAsync(
+            listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.WaitingForReview);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Settings: MaxTurnsCeiling editierbar machen", cancellationToken: CancellationToken.None);
+
+        var dup = Assert.Single(result.PossibleDuplicates);
+        Assert.Equal(existing.Id, dup.TaskId);
+        Assert.Equal(existing.Title, dup.Title);
+        Assert.Equal("WaitingForReview", dup.Status);
+    }
+
+    [Fact]
+    public async Task AddTask_StillCreatesTheTask_EvenWhenAPossibleDuplicateIsFound()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Settings: MaxTurnsCeiling editierbar machen", cancellationToken: CancellationToken.None);
+
+        Assert.NotEmpty(result.PossibleDuplicates);
+        Assert.NotNull(await _tasks.GetByIdAsync(result.Task.Id));
+    }
+
+    [Fact]
+    public async Task AddTask_SimilarTitle_ButExistingTaskIsTerminal_IsNotReported()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "MaxTurnsCeiling ohne Bedienoberflaeche im Settings-Modal", TaskStatus.Done);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Settings: MaxTurnsCeiling editierbar machen", cancellationToken: CancellationToken.None);
+
+        Assert.Empty(result.PossibleDuplicates);
+    }
+
+    [Fact]
+    public async Task AddTask_ClearlyDifferentTitle_IsNotReported()
+    {
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "Refactor GitService worktree cleanup", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Add dark mode toggle to the settings page", cancellationToken: CancellationToken.None);
+
+        Assert.Empty(result.PossibleDuplicates);
+    }
+
+    [Fact]
+    public async Task AddTask_MoreThanThreeMatches_CapsPossibleDuplicatesAtThree()
+    {
+        var listId = await SeedListAsync();
+        for (var i = 0; i < 5; i++)
+            await SeedTaskAsync(listId, $"Fix flaky queue picker test number {i}", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Fix flaky queue picker test number extra", cancellationToken: CancellationToken.None);
+
+        Assert.Equal(3, result.PossibleDuplicates.Count);
+    }
+
     // ── ContinueTask validation ───────────────────────────────────────────────
 
     [Fact]