diff --git a/src/ClaudeDo.Worker/Planning/PlanningAggregator.cs b/src/ClaudeDo.Worker/Planning/PlanningAggregator.cs
new file mode 100644
index 0000000..121da6e
--- /dev/null
+++ b/src/ClaudeDo.Worker/Planning/PlanningAggregator.cs
@@ -0,0 +1,73 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Git;
+using ClaudeDo.Data.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Worker.Planning;
+
+public sealed record SubtaskDiff(
+    string SubtaskId,
+    string Title,
+    string BranchName,
+    string BaseCommit,
+    string HeadCommit,
+    string? DiffStat,
+    string UnifiedDiff);
+
+public sealed record CombinedDiffSuccess(string IntegrationBranch, string UnifiedDiff);
+public sealed record CombinedDiffFailure(string FirstConflictSubtaskId, IReadOnlyList<string> ConflictedFiles);
+
+public abstract record CombinedDiffResult
+{
+    public sealed record Ok(CombinedDiffSuccess Value) : CombinedDiffResult;
+    public sealed record Failed(CombinedDiffFailure Value) : CombinedDiffResult;
+}
+
+public sealed class PlanningAggregator
+{
+    private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
+    private readonly GitService _git;
+    private readonly ILogger<PlanningAggregator> _logger;
+
+    public PlanningAggregator(
+        IDbContextFactory<ClaudeDoDbContext> dbFactory,
+        GitService git,
+        ILogger<PlanningAggregator> logger)
+    {
+        _dbFactory = dbFactory;
+        _git = git;
+        _logger = logger;
+    }
+
+    public async Task<IReadOnlyList<SubtaskDiff>> GetAggregatedDiffAsync(
+        string planningTaskId, CancellationToken ct)
+    {
+        using var ctx = _dbFactory.CreateDbContext();
+        var children = await ctx.Tasks
+            .Include(t => t.Worktree)
+            .Where(t => t.ParentTaskId == planningTaskId)
+            .OrderBy(t => t.SortOrder)
+            .ToListAsync(ct);
+
+        var result = new List<SubtaskDiff>();
+        foreach (var child in children)
+        {
+            if (child.Worktree is null) continue;
+            var wt = child.Worktree;
+            var head = wt.HeadCommit ?? await _git.RevParseHeadAsync(wt.Path, ct);
+            string unified;
+            try
+            {
+                unified = await _git.GetBranchDiffAsync(wt.Path, wt.BaseCommit, ct);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "diff failed for subtask {Id}", child.Id);
+                unified = "";
+            }
+            result.Add(new SubtaskDiff(
+                child.Id, child.Title, wt.BranchName, wt.BaseCommit, head, wt.DiffStat, unified));
+        }
+        return result;
+    }
+}
diff --git a/tests/ClaudeDo.Worker.Tests/Planning/PlanningAggregatorTests.cs b/tests/ClaudeDo.Worker.Tests/Planning/PlanningAggregatorTests.cs
new file mode 100644
index 0000000..75e5655
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Planning/PlanningAggregatorTests.cs
@@ -0,0 +1,119 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Git;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Planning;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.Tests.Planning;
+
+public class PlanningAggregatorTests : IDisposable
+{
+    private readonly List<DbFixture> _dbs = new();
+    private readonly List<GitRepoFixture> _repos = new();
+    private readonly List<(string repoDir, string wtPath)> _wtCleanups = new();
+
+    private DbFixture NewDb() { var d = new DbFixture(); _dbs.Add(d); return d; }
+    private GitRepoFixture NewRepo() { var r = new GitRepoFixture(); _repos.Add(r); return r; }
+
+    public void Dispose()
+    {
+        foreach (var (repo, wt) in _wtCleanups)
+            try { GitRepoFixture.RunGit(repo, "worktree", "remove", "--force", wt); } catch { }
+        foreach (var d in _dbs) try { d.Dispose(); } catch { }
+        foreach (var r in _repos) try { r.Dispose(); } catch { }
+    }
+
+    [Fact]
+    public async Task GetAggregatedDiffAsync_ReturnsOneEntryPerSubtaskInSortOrder()
+    {
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        var (parentId, subAId, subBId) = await SeedPlanningWithTwoChildrenAsync(db, repo);
+
+        var svc = new PlanningAggregator(
+            db.CreateFactory(),
+            new GitService(),
+            NullLogger<PlanningAggregator>.Instance);
+
+        var result = await svc.GetAggregatedDiffAsync(parentId, CancellationToken.None);
+
+        Assert.Equal(2, result.Count);
+        Assert.Equal(subAId, result[0].SubtaskId);
+        Assert.Equal(subBId, result[1].SubtaskId);
+        Assert.Contains("diff --git", result[0].UnifiedDiff);
+        Assert.Contains("diff --git", result[1].UnifiedDiff);
+    }
+
+    private async Task<(string parentId, string subA, string subB)> SeedPlanningWithTwoChildrenAsync(
+        DbFixture db, GitRepoFixture repo)
+    {
+        using var ctx = db.CreateContext();
+
+        // List with WorkingDir set to the repo.
+        var listId = Guid.NewGuid().ToString();
+        ctx.Lists.Add(new ListEntity
+        {
+            Id = listId, Name = "test", CreatedAt = DateTime.UtcNow,
+            WorkingDir = repo.RepoDir,
+        });
+
+        // Planning parent.
+        var parentId = Guid.NewGuid().ToString();
+        ctx.Tasks.Add(new TaskEntity
+        {
+            Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
+            Status = TaskStatus.Planning, SortOrder = 0,
+        });
+
+        // Two children (sorted A then B).
+        var subA = Guid.NewGuid().ToString();
+        var subB = Guid.NewGuid().ToString();
+        ctx.Tasks.Add(new TaskEntity
+        {
+            Id = subA, ListId = listId, Title = "child A", CreatedAt = DateTime.UtcNow,
+            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1,
+        });
+        ctx.Tasks.Add(new TaskEntity
+        {
+            Id = subB, ListId = listId, Title = "child B", CreatedAt = DateTime.UtcNow,
+            ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2,
+        });
+        await ctx.SaveChangesAsync();
+
+        // Create real worktrees for each child with a distinct commit.
+        SeedWorktree(ctx, repo, subA, "fileA.txt", "content A");
+        SeedWorktree(ctx, repo, subB, "fileB.txt", "content B");
+        await ctx.SaveChangesAsync();
+
+        return (parentId, subA, subB);
+    }
+
+    private void SeedWorktree(ClaudeDoDbContext ctx, GitRepoFixture repo, string taskId, string filename, string content)
+    {
+        var wtPath = Path.Combine(Path.GetTempPath(), $"wt_{Guid.NewGuid():N}");
+        _wtCleanups.Add((repo.RepoDir, wtPath));
+        var branch = $"claudedo/{taskId[..8]}";
+        GitRepoFixture.RunGit(repo.RepoDir, "worktree", "add", "-b", branch, wtPath, repo.BaseCommit);
+        File.WriteAllText(Path.Combine(wtPath, filename), content);
+        GitRepoFixture.RunGit(wtPath, "add", filename);
+        GitRepoFixture.RunGit(wtPath, "commit", "-m", $"add {filename}");
+        var head = GitRepoFixture.RunGit(wtPath, "rev-parse", "HEAD").Trim();
+
+        ctx.Worktrees.Add(new WorktreeEntity
+        {
+            TaskId = taskId,
+            Path = wtPath,
+            BranchName = branch,
+            BaseCommit = repo.BaseCommit,
+            HeadCommit = head,
+            DiffStat = null,
+            State = WorktreeState.Active,
+            CreatedAt = DateTime.UtcNow,
+        });
+    }
+}