diff --git a/src/ClaudeDo.Data/TaskNumberAllocator.cs b/src/ClaudeDo.Data/TaskNumberAllocator.cs
index b51edf7c..2e54f940 100644
--- a/src/ClaudeDo.Data/TaskNumberAllocator.cs
+++ b/src/ClaudeDo.Data/TaskNumberAllocator.cs
@@ -1,4 +1,5 @@
 using ClaudeDo.Data.Models;
+using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;
 
 namespace ClaudeDo.Data;
@@ -29,6 +30,16 @@ public static class TaskNumberAllocator
                 RETURNING *
                 """, AppSettingsEntity.SingletonId).AsNoTracking().ToListAsync(ct);
 
+            if (settings.Count == 0)
+            {
+                // The singleton row is missing (e.g. a corrupted/hand-edited DB) -- recreate it,
+                // as its own separate insert/commit, and retry the UPDATE above rather than
+                // indexing into an empty result. Doesn't count against MaxAttempts: it's not a
+                // number collision.
+                await EnsureSingletonRowAsync(context, ct);
+                continue;
+            }
+
             entity.Number = settings[0].NextTaskNumber - 1;
             context.Tasks.Add(entity);
 
@@ -37,10 +48,40 @@ public static class TaskNumberAllocator
                 await context.SaveChangesAsync(ct);
                 return;
             }
-            catch (DbUpdateException) when (attempt < MaxAttempts)
+            catch (DbUpdateException ex)
             {
                 context.Entry(entity).State = EntityState.Detached;
+                if (attempt < MaxAttempts && IsNumberCollision(ex))
+                    continue;
+                throw;
             }
         }
     }
+
+    private static async Task EnsureSingletonRowAsync(ClaudeDoDbContext context, CancellationToken ct)
+    {
+        var row = new AppSettingsEntity { Id = AppSettingsEntity.SingletonId };
+        context.AppSettings.Add(row);
+        try
+        {
+            await context.SaveChangesAsync(ct);
+        }
+        catch (DbUpdateException)
+        {
+            // A concurrent process already inserted the singleton -- fine, the next UPDATE
+            // attempt above will find it.
+        }
+        finally
+        {
+            context.Entry(row).State = EntityState.Detached;
+        }
+    }
+
+    // Distinguishes the expected unique-index collision on idx_tasks_number (worth retrying with
+    // a fresh number) from any other DbUpdateException -- an FK violation, a NOT NULL violation,
+    // etc. -- which must surface immediately instead of silently burning up to MaxAttempts numbers
+    // on a failure a retry can never fix.
+    private static bool IsNumberCollision(DbUpdateException ex) =>
+        ex.InnerException is SqliteException { SqliteErrorCode: 19 } sqliteEx
+        && sqliteEx.Message.Contains("tasks.number", StringComparison.Ordinal);
 }
diff --git a/src/ClaudeDo.Worker/External/TaskWaitMcpTools.cs b/src/ClaudeDo.Worker/External/TaskWaitMcpTools.cs
index 7079672a..4f2b4ce7 100644
--- a/src/ClaudeDo.Worker/External/TaskWaitMcpTools.cs
+++ b/src/ClaudeDo.Worker/External/TaskWaitMcpTools.cs
@@ -109,7 +109,19 @@ public sealed class TaskWaitMcpTools
         var tasks = new TaskRepository(ctx);
         var resolved = new string[ids.Length];
         for (var i = 0; i < ids.Length; i++)
-            resolved[i] = await TaskIdResolver.ResolveAsync(tasks, ids[i], ct);
+        {
+            try
+            {
+                resolved[i] = await TaskIdResolver.ResolveAsync(tasks, ids[i], ct);
+            }
+            catch (InvalidOperationException)
+            {
+                // A #<number>/bare-number id with no matching task -- keep the original,
+                // unresolvable id so CheckOnceAsync's lookup misses and reports "NotFound",
+                // per this tool's documented contract, instead of failing the whole call.
+                resolved[i] = ids[i];
+            }
+        }
         return resolved;
     }
 
diff --git a/tests/ClaudeDo.Data.Tests/TaskNumberAllocatorTests.cs b/tests/ClaudeDo.Data.Tests/TaskNumberAllocatorTests.cs
index 93be3071..fc341443 100644
--- a/tests/ClaudeDo.Data.Tests/TaskNumberAllocatorTests.cs
+++ b/tests/ClaudeDo.Data.Tests/TaskNumberAllocatorTests.cs
@@ -131,6 +131,35 @@ public sealed class TaskNumberAllocatorTests : IDisposable
         Assert.All(numbers, n => Assert.True(n > 0));
     }
 
+    [Fact]
+    public async Task AddAsync_recreates_a_missing_app_settings_row_instead_of_crashing()
+    {
+        await SeedListAsync();
+        await _ctx.Database.ExecuteSqlRawAsync("DELETE FROM app_settings");
+
+        var task = NewTask("l1");
+        await new TaskRepository(_ctx).AddAsync(task);
+
+        Assert.True(task.Number > 0);
+    }
+
+    [Fact]
+    public async Task AddAsync_noncollision_DbUpdateException_surfaces_immediately_without_burning_numbers()
+    {
+        await SeedListAsync();
+        var badTask = NewTask("no-such-list");
+
+        await Assert.ThrowsAsync<DbUpdateException>(() => new TaskRepository(_ctx).AddAsync(badTask));
+
+        // The failed attempt above should have burned exactly one number (attempt #1, no
+        // collision-driven retries) -- confirm the next successful insert isn't several numbers
+        // further along.
+        var goodTask = NewTask("l1");
+        await new TaskRepository(_ctx).AddAsync(goodTask);
+
+        Assert.Equal(2, goodTask.Number);
+    }
+
     [Fact]
     public async Task GetByNumberAsync_finds_the_task_and_returns_null_for_unknown_numbers()
     {
diff --git a/tests/ClaudeDo.Worker.Tests/External/TaskWaitMcpToolsTests.cs b/tests/ClaudeDo.Worker.Tests/External/TaskWaitMcpToolsTests.cs
index 80d400f2..846d7955 100644
--- a/tests/ClaudeDo.Worker.Tests/External/TaskWaitMcpToolsTests.cs
+++ b/tests/ClaudeDo.Worker.Tests/External/TaskWaitMcpToolsTests.cs
@@ -253,6 +253,52 @@ public sealed class TaskWaitMcpToolsTests : IDisposable
         Assert.True(sw.Elapsed >= TimeSpan.FromMilliseconds(900), $"took {sw.Elapsed}");
     }
 
+    [Fact]
+    public async Task WaitForTaskChange_UnknownNumberHashForm_ReturnsImmediatelyAsNotFound()
+    {
+        var sut = BuildSut();
+        var sw = Stopwatch.StartNew();
+
+        var result = await sut.WaitForTaskChange(["#999999"], timeoutSeconds: 30, cancellationToken: CancellationToken.None);
+
+        sw.Stop();
+        Assert.False(result.TimedOut);
+        Assert.Equal("NotFound", Assert.Single(result.Changed).Status);
+        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(2), $"took {sw.Elapsed}");
+    }
+
+    [Fact]
+    public async Task WaitForTaskChange_UnknownBareNumber_ReturnsImmediatelyAsNotFound()
+    {
+        var sut = BuildSut();
+        var sw = Stopwatch.StartNew();
+
+        var result = await sut.WaitForTaskChange(["999999"], timeoutSeconds: 30, cancellationToken: CancellationToken.None);
+
+        sw.Stop();
+        Assert.False(result.TimedOut);
+        Assert.Equal("NotFound", Assert.Single(result.Changed).Status);
+        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(2), $"took {sw.Elapsed}");
+    }
+
+    [Fact]
+    public async Task WaitForTaskChange_MixOfValidAndUnknownNumber_ReportsBothCorrectly()
+    {
+        var task = await SeedTaskAsync(TaskStatus.WaitingForReview);
+        var sut = BuildSut();
+        var sw = Stopwatch.StartNew();
+
+        var result = await sut.WaitForTaskChange(
+            [$"#{task.Number}", "#999999"], timeoutSeconds: 30, cancellationToken: CancellationToken.None);
+
+        sw.Stop();
+        Assert.False(result.TimedOut);
+        Assert.Equal(2, result.Changed.Count);
+        Assert.Contains(result.Changed, c => c.TaskId == task.Id && c.Status == "WaitingForReview");
+        Assert.Contains(result.Changed, c => c.TaskId == "#999999" && c.Status == "NotFound");
+        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(2), $"took {sw.Elapsed}");
+    }
+
     [Fact]
     public void MaxTimeoutSeconds_StaysComfortablyUnderMcpToolTimeout()
     {