src/ClaudeDo.Worker/Planning/PlanningSessionManager.cs

@@ -53,6 +53,25 @@ public sealed class PlanningSessionManager
         return new PlanningSessionStartContext(taskId, list.WorkingDir, files);
     }
 
+    public async Task<PlanningSessionResumeContext> ResumeAsync(string taskId, CancellationToken ct)
+    {
+        var task = await _tasks.GetByIdAsync(taskId, ct)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status != TaskStatus.Planning)
+            throw new InvalidOperationException($"Task is in status {task.Status}; resume requires Planning.");
+        if (string.IsNullOrEmpty(task.PlanningSessionId))
+            throw new InvalidOperationException("No Claude session ID captured yet; cannot resume.");
+
+        var sessionDir = Path.Combine(_rootDirectory, taskId);
+        var mcpConfigPath = Path.Combine(sessionDir, "mcp.json");
+        if (!File.Exists(mcpConfigPath))
+            throw new InvalidOperationException($"Session directory missing: {sessionDir}");
+
+        var list = await _lists.GetByIdAsync(task.ListId, ct)
+            ?? throw new InvalidOperationException($"List {task.ListId} not found.");
+        return new PlanningSessionResumeContext(taskId, list.WorkingDir, task.PlanningSessionId, mcpConfigPath);
+    }
+
     private static string GenerateToken()
     {
         var bytes = RandomNumberGenerator.GetBytes(32);

tests/ClaudeDo.Worker.Tests/Planning/PlanningSessionManagerTests.cs

@@ -120,4 +120,43 @@ public sealed class PlanningSessionManagerTests : IDisposable
         await Assert.ThrowsAsync<InvalidOperationException>(() =>
             _sut.StartAsync(child.Id, CancellationToken.None));
     }
+
+    [Fact]
+    public async Task ResumeAsync_ReturnsExistingSessionDetails()
+    {
+        var (listId, wd) = await SeedListAsync();
+        var parent = await SeedManualTaskAsync(listId);
+        var startCtx = await _sut.StartAsync(parent.Id, CancellationToken.None);
+        await _tasks.UpdatePlanningSessionIdAsync(parent.Id, "claude-session-42");
+
+        var resumeCtx = await _sut.ResumeAsync(parent.Id, CancellationToken.None);
+
+        Assert.Equal(parent.Id, resumeCtx.ParentTaskId);
+        Assert.Equal(wd, resumeCtx.WorkingDir);
+        Assert.Equal("claude-session-42", resumeCtx.ClaudeSessionId);
+        Assert.Equal(startCtx.Files.McpConfigPath, resumeCtx.McpConfigPath);
+        Assert.True(File.Exists(resumeCtx.McpConfigPath));
+    }
+
+    [Fact]
+    public async Task ResumeAsync_NotPlanning_Throws()
+    {
+        var (listId, _) = await SeedListAsync();
+        var parent = await SeedManualTaskAsync(listId);
+        // did not start
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            _sut.ResumeAsync(parent.Id, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task ResumeAsync_NoClaudeSessionId_Throws()
+    {
+        var (listId, _) = await SeedListAsync();
+        var parent = await SeedManualTaskAsync(listId);
+        await _sut.StartAsync(parent.Id, CancellationToken.None);
+        // UpdatePlanningSessionIdAsync not called
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            _sut.ResumeAsync(parent.Id, CancellationToken.None));
+    }
 }