diff --git a/src/ClaudeDo.Worker/External/ExternalMcpService.cs b/src/ClaudeDo.Worker/External/ExternalMcpService.cs
index bbe13ed..b1a188d 100644
--- a/src/ClaudeDo.Worker/External/ExternalMcpService.cs
+++ b/src/ClaudeDo.Worker/External/ExternalMcpService.cs
@@ -10,6 +10,8 @@ namespace ClaudeDo.Worker.External;
 
 public sealed record TaskListDto(string Id, string Name, string? WorkingDir);
 
+public sealed record TagDto(long Id, string Name);
+
 public sealed record TaskDto(
     string Id,
     string ListId,
@@ -29,17 +31,20 @@ public sealed class ExternalMcpService
     private readonly ListRepository _lists;
     private readonly QueueService _queue;
     private readonly HubBroadcaster _broadcaster;
+    private readonly TagRepository _tags;
 
     public ExternalMcpService(
         TaskRepository tasks,
         ListRepository lists,
         QueueService queue,
-        HubBroadcaster broadcaster)
+        HubBroadcaster broadcaster,
+        TagRepository tags)
     {
         _tasks = tasks;
         _lists = lists;
         _queue = queue;
         _broadcaster = broadcaster;
+        _tags = tags;
     }
 
     [McpServerTool, Description("List all task lists available in ClaudeDo.")]
@@ -183,6 +188,13 @@ public sealed class ExternalMcpService
         return cancelled;
     }
 
+    [McpServerTool, Description("List all known tags. Useful for discovering existing tag names (including 'agent' which marks tasks for auto-execution) before tagging.")]
+    public async Task<IReadOnlyList<TagDto>> ListTags(CancellationToken cancellationToken)
+    {
+        var tags = await _tags.GetAllAsync(cancellationToken);
+        return tags.Select(t => new TagDto(t.Id, t.Name)).ToList();
+    }
+
     private static TaskDto ToDto(TaskEntity t) => new(
         t.Id,
         t.ListId,
diff --git a/src/ClaudeDo.Worker/Program.cs b/src/ClaudeDo.Worker/Program.cs
index cfb768e..2e19c36 100644
--- a/src/ClaudeDo.Worker/Program.cs
+++ b/src/ClaudeDo.Worker/Program.cs
@@ -127,6 +127,7 @@ if (cfg.ExternalMcpPort > 0)
         sp.GetRequiredService<IDbContextFactory<ClaudeDoDbContext>>().CreateDbContext());
     externalBuilder.Services.AddScoped<TaskRepository>();
     externalBuilder.Services.AddScoped<ListRepository>();
+    externalBuilder.Services.AddScoped<TagRepository>();
     externalBuilder.Services.AddScoped<ExternalMcpService>();
     externalBuilder.Services.AddMcpServer()
         .WithHttpTransport()
diff --git a/tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs b/tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs
index a05c3a9..5617859 100644
--- a/tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs
+++ b/tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs
@@ -1,11 +1,16 @@
 using ClaudeDo.Data;
+using ClaudeDo.Data.Git;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Config;
 using ClaudeDo.Worker.External;
 using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Runner;
 using ClaudeDo.Worker.Services;
 using ClaudeDo.Worker.Tests.Infrastructure;
+using ClaudeDo.Worker.Tests.Services;
 using Microsoft.AspNetCore.SignalR;
+using Microsoft.Extensions.Logging.Abstractions;
 using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 namespace ClaudeDo.Worker.Tests.External;
@@ -91,6 +96,27 @@ public sealed class ExternalMcpServiceTests : IDisposable
     private ExternalMcpService BuildSut(QueueService queue) =>
         new(_tasks, _lists, queue, _broadcaster, _tags);
 
+    private QueueService CreateQueue()
+    {
+        var tempDir = Path.Combine(Path.GetTempPath(), $"claudedo_ext_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempDir);
+        var cfg = new WorkerConfig
+        {
+            SandboxRoot = Path.Combine(tempDir, "sandbox"),
+            LogRoot = Path.Combine(tempDir, "logs"),
+            QueueBackstopIntervalMs = 50,
+        };
+        var fake = new FakeClaudeProcess();
+        var hubCtx = new FakeHubContext();
+        var broadcaster = new HubBroadcaster(hubCtx);
+        var dbFactory = _db.CreateFactory();
+        var wtManager = new WorktreeManager(new GitService(), dbFactory, cfg, NullLogger<WorktreeManager>.Instance);
+        var argsBuilder = new ClaudeArgsBuilder();
+        var runner = new TaskRunner(fake, dbFactory, broadcaster, wtManager, argsBuilder, cfg,
+            NullLogger<TaskRunner>.Instance);
+        return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance);
+    }
+
     [Fact]
     public async Task SeededListAndTask_AreRetrievable()
     {
@@ -98,4 +124,20 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var task = await SeedTaskAsync(listId);
         Assert.NotNull(await _tasks.GetByIdAsync(task.Id));
     }
+
+    [Fact]
+    public async Task ListTags_ReturnsSeededAndCustomTags()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId);
+        await _tasks.SetTagsAsync(task.Id, new[] { "agent", "custom-tag" });
+
+        var queue = CreateQueue();
+        var sut = BuildSut(queue);
+
+        var tags = await sut.ListTags(CancellationToken.None);
+
+        Assert.Contains(tags, t => t.Name == "agent");
+        Assert.Contains(tags, t => t.Name == "custom-tag");
+    }
 }