diff --git a/src/ClaudeDo.Data/TaskPromptComposer.cs b/src/ClaudeDo.Data/TaskPromptComposer.cs
new file mode 100644
index 0000000..5ca3a82
--- /dev/null
+++ b/src/ClaudeDo.Data/TaskPromptComposer.cs
@@ -0,0 +1,29 @@
+using System.Text;
+
+namespace ClaudeDo.Data;
+
+/// <summary>
+/// Single source of truth for the text handed to Claude as a task prompt:
+/// title + description + the OPEN sub-tasks. Resolved sub-tasks are dropped.
+/// Shared by the Worker (real prompt) and the UI (the card's "what Claude gets" preview).
+/// </summary>
+public static class TaskPromptComposer
+{
+    public static string Compose(string title, string? description, IEnumerable<(string Title, bool Completed)> subtasks)
+    {
+        var sb = new StringBuilder((title ?? "").Trim());
+
+        if (!string.IsNullOrWhiteSpace(description))
+            sb.Append("\n\n").Append(description.Trim());
+
+        var open = subtasks?.Where(s => !s.Completed).ToList() ?? new List<(string, bool)>();
+        if (open.Count > 0)
+        {
+            sb.Append("\n\n## Sub-Tasks\n");
+            foreach (var s in open)
+                sb.Append("- [ ] ").Append(s.Title).Append('\n');
+        }
+
+        return sb.ToString();
+    }
+}
diff --git a/src/ClaudeDo.Worker/Runner/TaskRunner.cs b/src/ClaudeDo.Worker/Runner/TaskRunner.cs
index 53fda84..816d8b4 100644
--- a/src/ClaudeDo.Worker/Runner/TaskRunner.cs
+++ b/src/ClaudeDo.Worker/Runner/TaskRunner.cs
@@ -101,16 +101,10 @@ public sealed class TaskRunner
             await _state.StartRunningAsync(task.Id, now, ct);
             await _broadcaster.TaskStarted(slot, task.Id, now);
 
-            // Build prompt.
-            var sb = new System.Text.StringBuilder(task.Title);
-            if (!string.IsNullOrWhiteSpace(task.Description)) sb.Append("\n\n").Append(task.Description.Trim());
-            if (subtasks.Count > 0)
-            {
-                sb.Append("\n\n## Sub-Tasks\n");
-                foreach (var s in subtasks)
-                    sb.Append(s.Completed ? "- [x] " : "- [ ] ").Append(s.Title).Append('\n');
-            }
-            var prompt = sb.ToString();
+            // Build prompt: title + description + only the OPEN sub-tasks (resolved ones are dropped).
+            var prompt = TaskPromptComposer.Compose(
+                task.Title, task.Description,
+                subtasks.Select(s => (s.Title, s.Completed)));
 
             // Run 1.
             var result = await RunOnceAsync(task.Id, task.Title, slot, runDir, resolvedConfig, 1, false, prompt, ct);
diff --git a/tests/ClaudeDo.Data.Tests/TaskPromptComposerTests.cs b/tests/ClaudeDo.Data.Tests/TaskPromptComposerTests.cs
new file mode 100644
index 0000000..17279b3
--- /dev/null
+++ b/tests/ClaudeDo.Data.Tests/TaskPromptComposerTests.cs
@@ -0,0 +1,45 @@
+using ClaudeDo.Data;
+using Xunit;
+
+namespace ClaudeDo.Data.Tests;
+
+public class TaskPromptComposerTests
+{
+    [Fact]
+    public void Composes_title_description_and_open_steps()
+    {
+        var result = TaskPromptComposer.Compose(
+            "Refactor diff viewer",
+            "Share the row template.",
+            new (string, bool)[] { ("Done step", true), ("Open step", false) });
+
+        Assert.Equal(
+            "Refactor diff viewer\n\nShare the row template.\n\n## Sub-Tasks\n- [ ] Open step\n",
+            result);
+    }
+
+    [Fact]
+    public void Drops_resolved_steps_and_omits_section_when_none_open()
+    {
+        var result = TaskPromptComposer.Compose(
+            "Title",
+            "Desc",
+            new (string, bool)[] { ("a", true), ("b", true) });
+
+        Assert.Equal("Title\n\nDesc", result);
+    }
+
+    [Fact]
+    public void Omits_description_when_blank()
+    {
+        var result = TaskPromptComposer.Compose("Title", "   ", new (string, bool)[] { ("open", false) });
+
+        Assert.Equal("Title\n\n## Sub-Tasks\n- [ ] open\n", result);
+    }
+
+    [Fact]
+    public void Title_only_when_no_description_or_steps()
+    {
+        Assert.Equal("Just a title", TaskPromptComposer.Compose("Just a title", null, System.Array.Empty<(string, bool)>()));
+    }
+}