diff --git a/src/ClaudeDo.Worker/Runner/WorktreeManager.cs b/src/ClaudeDo.Worker/Runner/WorktreeManager.cs
index 9f1fa1c..1836887 100644
--- a/src/ClaudeDo.Worker/Runner/WorktreeManager.cs
+++ b/src/ClaudeDo.Worker/Runner/WorktreeManager.cs
@@ -32,7 +32,7 @@ public sealed class WorktreeManager
         if (!await _git.IsGitRepoAsync(workingDir, ct))
             throw new InvalidOperationException($"working_dir is not a git repository: {workingDir}");
 
-        var baseCommit = await _git.RevParseHeadAsync(workingDir, ct);
+        var baseCommit = await ResolveBaseCommitAsync(task, workingDir, ct);
         // Use the full task id (dashes stripped) in the branch name so
         // two GUIDs sharing an 8-char prefix cannot collide on the same branch.
         var idForBranch = task.Id.Replace("-", "");
@@ -184,4 +184,25 @@ public sealed class WorktreeManager
 
         _logger.LogInformation("Discarded worktree for task {TaskId} (branch {Branch})", wt.TaskId, wt.BranchName);
     }
+
+    // Improvement children (parent is a non-planning task with its own worktree) branch
+    // from the parent's recorded HEAD so they build on the parent's not-yet-merged work.
+    // Planning children and standalone tasks base off the list's current HEAD.
+    private async Task<string> ResolveBaseCommitAsync(TaskEntity task, string workingDir, CancellationToken ct)
+    {
+        if (task.ParentTaskId is not null)
+        {
+            using var ctx = _dbFactory.CreateDbContext();
+            var parent = await ctx.Tasks.AsNoTracking()
+                .FirstOrDefaultAsync(t => t.Id == task.ParentTaskId, ct);
+            if (parent is not null && parent.PlanningPhase == PlanningPhase.None)
+            {
+                var parentWt = await new WorktreeRepository(ctx).GetByTaskIdAsync(task.ParentTaskId, ct);
+                var parentHead = parentWt?.HeadCommit ?? parentWt?.BaseCommit;
+                if (parentHead is not null)
+                    return parentHead;
+            }
+        }
+        return await _git.RevParseHeadAsync(workingDir, ct);
+    }
 }
diff --git a/tests/ClaudeDo.Worker.Tests/Runner/ChildWorktreeBaseTests.cs b/tests/ClaudeDo.Worker.Tests/Runner/ChildWorktreeBaseTests.cs
new file mode 100644
index 0000000..c6bcce9
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Runner/ChildWorktreeBaseTests.cs
@@ -0,0 +1,68 @@
+using ClaudeDo.Data.Git;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Runner;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+using Xunit;
+
+namespace ClaudeDo.Worker.Tests.Runner;
+
+public sealed class ChildWorktreeBaseTests : IDisposable
+{
+    private readonly List<GitRepoFixture> _repos = new();
+    private readonly List<DbFixture> _dbs = new();
+    private readonly List<(string repoDir, string wtPath)> _cleanups = new();
+
+    [Fact]
+    public async Task ImprovementChild_basesOff_parentWorktreeHead()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) { Assert.True(true, "git not available -- skipping"); return; }
+        var repo = new GitRepoFixture(); _repos.Add(repo);
+        var db = new DbFixture(); _dbs.Add(db);
+        var listId = Guid.NewGuid().ToString();
+        var parentId = Guid.NewGuid().ToString();
+        var childId = Guid.NewGuid().ToString();
+        var list = new ListEntity { Id = listId, Name = "L", WorkingDir = repo.RepoDir, CreatedAt = DateTime.UtcNow };
+        var parent = new TaskEntity { Id = parentId, ListId = listId, Title = "Parent",
+            CommitType = "chore", PlanningPhase = PlanningPhase.None, CreatedAt = DateTime.UtcNow };
+        var child = new TaskEntity { Id = childId, ListId = listId, Title = "Improve",
+            CommitType = "chore", ParentTaskId = parentId, CreatedBy = parentId, CreatedAt = DateTime.UtcNow };
+
+        using (var seed = db.CreateContext())
+        {
+            await new ListRepository(seed).AddAsync(list);
+            await new TaskRepository(seed).AddAsync(parent);
+            await new TaskRepository(seed).AddAsync(child);
+        }
+
+        var cfg = new WorkerConfig { WorktreeRootStrategy = "sibling" };
+        var mgr = new WorktreeManager(new GitService(), db.CreateFactory(), cfg, NullLogger<WorktreeManager>.Instance);
+
+        var parentCtx = await mgr.CreateAsync(parent, list, CancellationToken.None);
+        _cleanups.Add((repo.RepoDir, parentCtx.WorktreePath));
+        File.WriteAllText(Path.Combine(parentCtx.WorktreePath, "parent.txt"), "parent work");
+        await mgr.CommitIfChangedAsync(parentCtx, parent, list, CancellationToken.None);
+
+        string parentHead;
+        using (var read = db.CreateContext())
+            parentHead = (await new WorktreeRepository(read).GetByTaskIdAsync(parentId))!.HeadCommit!;
+
+        var childCtx = await mgr.CreateAsync(child, list, CancellationToken.None);
+        _cleanups.Add((repo.RepoDir, childCtx.WorktreePath));
+
+        Assert.Equal(parentHead, childCtx.BaseCommit);
+        Assert.NotEqual(repo.BaseCommit, childCtx.BaseCommit);
+        Assert.True(File.Exists(Path.Combine(childCtx.WorktreePath, "parent.txt")));
+    }
+
+    public void Dispose()
+    {
+        foreach (var (repoDir, wtPath) in _cleanups)
+            try { GitRepoFixture.RunGit(repoDir, "worktree", "remove", "--force", wtPath); } catch { }
+        foreach (var r in _repos) r.Dispose();
+        foreach (var d in _dbs) d.Dispose();
+    }
+}