# Fix Step-Back Duplicating Steps in `steps list`

**Status:** Ready for Implementation  
**Date:** January 2026  
**Related:** [dap-step-backwards.md](./dap-step-backwards.md)

## Problem Summary

When stepping backwards during DAP debugging, the step that should be re-run appears **twice** in `steps list`:
1. Once as a **completed** step (from `StepManipulator._completedSteps`)
2. Once as a **pending** step (from the re-queued `JobSteps`)

### Reproduction Steps

1. Run a workflow with DAP debugging enabled
2. Let steps 1-4 execute (checkpoints created)
3. Pause at step 5
4. Step backward
5. Run `steps list`

**Expected:**
```
✓ 1. hello
✓ 2. Run actions/checkout@v6
✓ 3. Run echo "foo=bar" >> "$GITHUB_OUTPUT"
▶ 4. Run cat doesnotexist
5. Run a one-line script
6. Run a multi-line script
```

**Actual (bug):**
```
✓ 1. hello
✓ 2. Run actions/checkout@v6
✓ 3. Run echo "foo=bar" >> "$GITHUB_OUTPUT"
✓ 4. Run cat doesnotexist        ← Still in _completedSteps!
▶ 5. Run cat doesnotexist        ← Re-queued as current
6. Run a one-line script
7. Run a multi-line script
```

## Root Cause

In `DapDebugSession.RestoreCheckpoint()` (line 1713), the session's `_completedSteps` and `_completedStepsTracker` lists are trimmed to match the checkpoint index:

```csharp
// Clear completed steps list for frames after this checkpoint
while (_completedSteps.Count > checkpointIndex)
{
    _completedSteps.RemoveAt(_completedSteps.Count - 1);
}

// Also clear the step tracker for manipulator sync
while (_completedStepsTracker.Count > checkpointIndex)
{
    _completedStepsTracker.RemoveAt(_completedStepsTracker.Count - 1);
}
```

However, **`StepManipulator` has its own separate `_completedSteps` list** that is **not** being synced. When `ClearChanges()` is called (line 1774), it only clears change tracking data (`_changes`, `_modifiedStepIds`, `_addedStepIds`, `_originalSteps`), not the `_completedSteps` list.

### Data Flow During Step-Back

1. Steps 1-4 complete → `StepManipulator._completedSteps` = [step1, step2, step3, step4]
2. Paused at step 5
3. Step back → `RestoreCheckpoint(3, ...)` is called (checkpoint index 3 = before step 4)
4. `DapDebugSession._completedSteps` trimmed to 3 items ✓
5. `DapDebugSession._completedStepsTracker` trimmed to 3 items ✓
6. `StepManipulator._completedSteps` **NOT trimmed** ✗ (still has 4 items)
7. Step 4 and remaining steps re-queued to `JobSteps`
8. `steps list` called → `GetAllSteps()` combines:
   - `_completedSteps` (4 items) + queue (step4, step5, step6, step7)
   - Result: step4 appears twice

## Solution

Add a method to `IStepManipulator` interface to trim completed steps to a specific count, then call it from `RestoreCheckpoint`.

## Files to Modify

| File | Changes |
|------|---------|
| `src/Runner.Worker/Dap/StepCommands/StepManipulator.cs` | Add `TrimCompletedSteps(int count)` method to interface and implementation |
| `src/Runner.Worker/Dap/DapDebugSession.cs` | Call `TrimCompletedSteps` in `RestoreCheckpoint()` |

## Detailed Changes

### 1. `StepManipulator.cs` - Add Interface Method and Implementation

**Add to `IStepManipulator` interface (around line 115, after `ClearChanges`):**

```csharp
/// <summary>
/// Trims the completed steps list to the specified count.
/// Used when restoring a checkpoint to sync state with the debug session.
/// </summary>
/// <param name="count">The number of completed steps to keep.</param>
void TrimCompletedSteps(int count);
```

**Add implementation to `StepManipulator` class (after `ClearChanges()` method, around line 577):**

```csharp
/// <inheritdoc/>
public void TrimCompletedSteps(int count)
{
    var originalCount = _completedSteps.Count;
    while (_completedSteps.Count > count)
    {
        _completedSteps.RemoveAt(_completedSteps.Count - 1);
    }
    Trace.Info($"Trimmed completed steps from {originalCount} to {_completedSteps.Count}");
}
```

### 2. `DapDebugSession.cs` - Call TrimCompletedSteps in RestoreCheckpoint

**Modify `RestoreCheckpoint()` (around line 1774), change:**

```csharp
// Reset the step manipulator to match the restored state
// It will be re-initialized when the restored step starts
_stepManipulator?.ClearChanges();
```

**To:**

```csharp
// Reset the step manipulator to match the restored state
_stepManipulator?.ClearChanges();
_stepManipulator?.TrimCompletedSteps(checkpointIndex);
```

## Why This Approach

| Approach | Pros | Cons |
|----------|------|------|
| **Add `TrimCompletedSteps()` (chosen)** | Minimal change, explicit, mirrors existing DapDebugSession pattern | Requires new interface method |
| Re-initialize StepManipulator completely | Clean slate | Would lose all state, more disruptive, harder to reason about |
| Share completed steps list between DapDebugSession and StepManipulator | Single source of truth | Major refactoring, tight coupling between components |

The chosen approach is the least invasive and follows the existing pattern used by `DapDebugSession` for its own `_completedSteps` list.

## Test Scenarios

After implementing this fix, verify:

1. **Basic step-back:**
   - Run steps 1, 2, 3, 4
   - Pause at step 5
   - Step back
   - `steps list` should show: ✓ 1-3 completed, ▶ 4 current, 5-6 pending (no duplicates)

2. **Multiple step-backs:**
   - Run steps 1, 2, 3
   - Step back twice (back to step 1)
   - `steps list` should show: ▶ 1 current, 2-N pending (no completed steps)

3. **Step back then forward:**
   - Run steps 1, 2
   - Step back (to step 2)
   - Step forward, let step 2 re-run and complete
   - Step forward again
   - `steps list` should show correct state without duplicates at any point

4. **Reverse continue:**
   - Run steps 1, 2, 3, 4
   - Reverse continue (back to step 1)
   - `steps list` should show: ▶ 1 current, 2-N pending (no completed steps)

## Implementation Checklist

- [ ] Add `TrimCompletedSteps(int count)` to `IStepManipulator` interface
- [ ] Implement `TrimCompletedSteps` in `StepManipulator` class
- [ ] Call `TrimCompletedSteps(checkpointIndex)` in `DapDebugSession.RestoreCheckpoint()`
- [ ] Test basic step-back scenario
- [ ] Test multiple step-backs
- [ ] Test step back then forward
- [ ] Test reverse continue