diff --git a/.paul/ROADMAP.md b/.paul/ROADMAP.md
index c8b3102..dd66d08 100644
--- a/.paul/ROADMAP.md
+++ b/.paul/ROADMAP.md
@@ -6,7 +6,7 @@
 |---------|------|--------|--------|-----------|
 | v0.1 | Walking Skeleton (MTConnect) | 1 | ✅ Shipped | 2026-07-21 |
 | v0.2 | Machine Management UX | 2, 2.1 | ✅ Shipped (2.1 commit pending) | 2026-07-21 |
-| v0.3 | Data-Item Selection + UX | 3, 3.1 | 📋 Planned | - |
+| v0.3 | Data-Item Selection + UX | 3, 3.1 | 🚧 Phase 3 done, 3.1 (theming) next | - |
 | v0.4 | OPC UA Protocol | 4 | 📋 Planned | - |
 | v0.5 | Fanuc FOCAS Protocol | 5 | 📋 Planned | - |
 | v0.6 | History & Trends | 6 | 📋 Planned | - |
diff --git a/.paul/STATE.md b/.paul/STATE.md
index 293f478..79d2a08 100644
--- a/.paul/STATE.md
+++ b/.paul/STATE.md
@@ -5,27 +5,30 @@
 See: .paul/PROJECT.md (updated 2026-07-21)
 
 **Core value:** Operators see live state of every configured machine across heterogeneous protocols in one place, adding machines/protocols without code.
-**Current focus:** v0.2 + v0.2.1 shipped → starting v0.3 Data-Item Selection
+**Current focus:** v0.3 Phase 3 (item selection) shipped → PAUSED. Next: Phase 3.1 theming/UX.
 
 ## Current Position
 
-Milestone: v0.2 Machine Management UX (+ 2.1 refinements) — SHIPPED
-Phase: 2 ✓, 2.1 ✓ → next Phase 3 (Data-Item Selection)
-Plan: 02-A/B/C ✓, 02.1 R1/R2R3/tablefix ✓
-Status: v0.2.1 committing; v0.3 backend next
-Last activity: 2026-07-22 — v0.2.1 done: shared HttpClient, disconnection UX, delete-confirm overlay, dashboard DataGrid (aligned + Details column). Build 0/0, 127 tests + 2 docker-skip.
+Milestone: v0.3 Data-Item Selection + UX
+Phase: 3 (item selection) ✓ SHIPPED — Phase 3.1 (theming/UX) NOT started
+Plan: v0.3-backend ✓, v0.3-appui ✓
+Status: PAUSED (user requested). Build 0/0, 138 tests + 2 docker-skip.
+Last activity: 2026-07-22 — v0.3 item selection: Machine.MonitoredItemIds (opt-in), IProtocolDriver.ProbeAsync, MTConnect probe+filter, Persistence col+migration, monitor.ProbeAsync, config probe→checklist UI (47 items live), detail empty-state.
 
 Progress:
-- v0.2 (+2.1): [██████████] 100%
-- Next: Phase 3 Data-Item Selection
+- v0.3: Phase 3 done, Phase 3.1 (theming) pending
 
 ## Loop Position
 
 ```
 PLAN ──▶ APPLY ──▶ UNIFY
-  ✓        ✓        ✓     [v0.2.1 complete → PLAN Phase 3]
+  ✓        ✓        ✓     [Phase 3 complete → PAUSED; next PLAN Phase 3.1 theming]
 ```
 
+## NEXT (on resume)
+
+Phase 3.1 theming/UX (task #34): Fluent light/dark + accent, per-protocol icons (mtconnect/opc/fanuc), curated layout across dashboard/detail/config. App-only. Then v0.4 OPC UA plugin.
+
 ## Standing Authorization
 
 Auto-commit + push GREEN chunks (build 0-err + tests pass) to origin/main WITHOUT asking (user-granted 2026-07-22). Never commit red; no force-push; artifact leak-check each time.
diff --git a/.paul/paul.json b/.paul/paul.json
index 2e8ad49..6c7989c 100644
--- a/.paul/paul.json
+++ b/.paul/paul.json
@@ -1,23 +1,23 @@
 {
   "name": "Junction",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "milestone": {
-    "name": "Machine Management UX",
-    "version": "0.2.0",
-    "status": "shipped"
+    "name": "Data-Item Selection + UX",
+    "version": "0.3.0",
+    "status": "in_progress"
   },
   "phase": {
-    "number": 2,
-    "name": "Machine Management UX",
+    "number": 3,
+    "name": "Data-Item Selection",
     "status": "complete"
   },
   "loop": {
-    "plan": "02-C",
-    "position": "UNIFY"
+    "plan": "v0.3-appui",
+    "position": "PAUSED"
   },
   "timestamps": {
     "created_at": "2026-07-21T23:30:00Z",
-    "updated_at": "2026-07-21T23:45:00Z"
+    "updated_at": "2026-07-22T00:40:00Z"
   },
   "satellite": {
     "groom": true
diff --git a/src/Junction.App/ViewModels/MachineConfigViewModel.cs b/src/Junction.App/ViewModels/MachineConfigViewModel.cs
index d679423..5f0c5be 100644
--- a/src/Junction.App/ViewModels/MachineConfigViewModel.cs
+++ b/src/Junction.App/ViewModels/MachineConfigViewModel.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using CommunityToolkit.Mvvm.ComponentModel;
@@ -8,6 +9,7 @@ using CommunityToolkit.Mvvm.Input;
 using Junction.Core.Monitoring;
 using Junction.Domain.Models;
 using Junction.Domain.Persistence;
+using Junction.Domain.Protocols;
 using Microsoft.Extensions.Logging;
 
 namespace Junction.App.ViewModels
@@ -30,6 +32,14 @@ namespace Junction.App.ViewModels
 
         private Guid? _editingId;
 
+        /// <summary>
+        /// MonitoredItemIds the machine had when loaded for EDIT. Used to pre-select the probed
+        /// catalog and to PRESERVE the selection on save when the user never reloaded the catalog
+        /// (an empty/never-probed <see cref="AvailableItems"/> must not wipe an existing selection).
+        /// Empty for ADD.
+        /// </summary>
+        private List<string> _existingMonitoredItemIds = new List<string>();
+
         /// <summary>Shell reference used to return to the dashboard after save/cancel.</summary>
         public MainWindowViewModel? Navigator { get; set; }
 
@@ -41,12 +51,14 @@ namespace Junction.App.ViewModels
 
         [ObservableProperty]
         [NotifyCanExecuteChangedFor(nameof(SaveCommand))]
+        [NotifyCanExecuteChangedFor(nameof(LoadItemsCommand))]
         [NotifyPropertyChangedFor(nameof(ValidationError))]
         [NotifyPropertyChangedFor(nameof(HasValidationError))]
         private string _selectedProtocol = "";
 
         [ObservableProperty]
         [NotifyCanExecuteChangedFor(nameof(SaveCommand))]
+        [NotifyCanExecuteChangedFor(nameof(LoadItemsCommand))]
         [NotifyPropertyChangedFor(nameof(ValidationError))]
         [NotifyPropertyChangedFor(nameof(HasValidationError))]
         private string _agentUrl = "";
@@ -59,9 +71,29 @@ namespace Junction.App.ViewModels
 
         [ObservableProperty] private bool _isEdit;
 
+        /// <summary>True while a probe (LoadItems) is in flight; drives the busy indicator and disables the button.</summary>
+        [ObservableProperty]
+        [NotifyCanExecuteChangedFor(nameof(LoadItemsCommand))]
+        private bool _isProbing;
+
+        /// <summary>User-visible status for the item section (probe failures, hints). Empty when nothing to say.</summary>
+        [ObservableProperty]
+        [NotifyPropertyChangedFor(nameof(HasItemsStatus))]
+        private string _itemsStatus = "";
+
+        public bool HasItemsStatus => !string.IsNullOrEmpty(ItemsStatus);
+
         /// <summary>Protocol ids offered in the dropdown, sourced from the monitor at Initialize.</summary>
         public ObservableCollection<string> AvailableProtocols { get; } = new ObservableCollection<string>();
 
+        /// <summary>
+        /// Catalog of selectable data items for the current machine, populated by <see cref="LoadItems"/>.
+        /// Empty until the user probes (or, in EDIT after a failed probe, seeded from the existing
+        /// selection as a fallback so the user can still deselect items while the agent is offline).
+        /// </summary>
+        public ObservableCollection<SelectableDataItemViewModel> AvailableItems { get; } =
+            new ObservableCollection<SelectableDataItemViewModel>();
+
         public MachineConfigViewModel(
             IMachineRepository repository,
             IMachineMonitor monitor,
@@ -93,12 +125,16 @@ namespace Junction.App.ViewModels
             _editingId = machineId;
             IsEdit = machineId.HasValue;
 
+            _existingMonitoredItemIds = new List<string>();
+            AvailableItems.Clear();
+
             if (machineId is null)
             {
                 Name = "";
                 SelectedProtocol = AvailableProtocols.Count > 0 ? AvailableProtocols[0] : "";
                 AgentUrl = "";
                 PollIntervalSeconds = 2;
+                ItemsStatus = "Load available items to choose what to monitor (requires a reachable agent).";
             }
             else
             {
@@ -137,6 +173,123 @@ namespace Junction.App.ViewModels
             SelectedProtocol = m.ProtocolId;
             AgentUrl = m.ConnectionConfig.TryGetValue("AgentUrl", out var url) ? url : "";
             PollIntervalSeconds = m.PollInterval.TotalSeconds > 0 ? (int)m.PollInterval.TotalSeconds : 1;
+
+            _existingMonitoredItemIds = m.MonitoredItemIds != null
+                ? m.MonitoredItemIds.ToList()
+                : new List<string>();
+
+            ItemsStatus = _existingMonitoredItemIds.Count > 0
+                ? "Currently monitoring " + _existingMonitoredItemIds.Count + " item(s). Load available items to change the selection."
+                : "No items selected yet. Load available items to choose what to monitor.";
+        }
+
+        /// <summary>
+        /// Probes the machine for its full data-item catalog and populates <see cref="AvailableItems"/>.
+        /// Runs off the button click (async): sets <see cref="IsProbing"/> for the duration, marshals
+        /// nothing manually — the awaited continuation resumes on the UI thread. On success each item is
+        /// pre-selected when its id is already in the machine's existing selection. On failure a
+        /// user-visible message is set and (in EDIT) the existing selection is shown as a fallback so
+        /// the user can still deselect items while the agent is offline.
+        /// </summary>
+        [RelayCommand(CanExecute = nameof(CanLoadItems))]
+        private async Task LoadItems()
+        {
+            IsProbing = true;
+            ItemsStatus = "Probing agent…";
+            try
+            {
+                var machine = new Machine(
+                    _editingId ?? Guid.NewGuid(),
+                    string.IsNullOrWhiteSpace(Name) ? "(probe)" : Name.Trim(),
+                    SelectedProtocol,
+                    new Dictionary<string, string> { ["AgentUrl"] = AgentUrl.Trim() },
+                    TimeSpan.FromSeconds(PollIntervalSeconds > 0 ? PollIntervalSeconds : 1));
+
+                var result = await _monitor.ProbeAsync(machine, CancellationToken.None).ConfigureAwait(true);
+
+                if (!result.IsSuccess)
+                {
+                    var detail = result.WasCancelled
+                        ? "cancelled"
+                        : (result.Errors.Count > 0 ? result.Errors[0].Message : "unknown error");
+                    _logger.LogWarning("Probe failed for {MachineId} ({Protocol} @ {Url}): {Detail}",
+                        machine.Id, SelectedProtocol, AgentUrl, detail);
+
+                    ShowFallbackSelection();
+                    ItemsStatus = "Probe failed: " + detail +
+                        ". Check the Agent URL and that the machine is reachable." +
+                        (AvailableItems.Count > 0 ? " Showing the existing selection so you can still deselect items." : "");
+                    return;
+                }
+
+                var existing = new HashSet<string>(_existingMonitoredItemIds, StringComparer.Ordinal);
+                AvailableItems.Clear();
+                var catalog = result.Value ?? (IReadOnlyList<DataItemDescriptor>)Array.Empty<DataItemDescriptor>();
+                foreach (var descriptor in catalog)
+                {
+                    AvailableItems.Add(new SelectableDataItemViewModel(descriptor, existing.Contains(descriptor.Id)));
+                }
+
+                _logger.LogInformation("Probe returned {Count} item(s) for {MachineId}.", AvailableItems.Count, machine.Id);
+
+                ItemsStatus = AvailableItems.Count == 0
+                    ? "Probe succeeded but the machine exposes no data items."
+                    : AvailableItems.Count + " item(s) available. Tick the ones to monitor.";
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "Probe threw for {Protocol} @ {Url}.", SelectedProtocol, AgentUrl);
+                ShowFallbackSelection();
+                ItemsStatus = "Probe failed: " + ex.Message +
+                    ". Check the Agent URL and that the machine is reachable.";
+            }
+            finally
+            {
+                IsProbing = false;
+            }
+        }
+
+        private bool CanLoadItems() =>
+            !IsProbing
+            && !string.IsNullOrWhiteSpace(SelectedProtocol)
+            && !string.IsNullOrWhiteSpace(AgentUrl)
+            && Uri.TryCreate(AgentUrl, UriKind.Absolute, out _);
+
+        /// <summary>
+        /// Seeds <see cref="AvailableItems"/> from the existing selection (EDIT) when a probe could not
+        /// return a catalog, so the user can still deselect items offline. No-op if already populated or
+        /// there is no existing selection.
+        /// </summary>
+        private void ShowFallbackSelection()
+        {
+            if (AvailableItems.Count > 0 || _existingMonitoredItemIds.Count == 0)
+            {
+                return;
+            }
+
+            foreach (var id in _existingMonitoredItemIds)
+            {
+                AvailableItems.Add(new SelectableDataItemViewModel(
+                    new DataItemDescriptor(id, id, "", "", ""), true));
+            }
+        }
+
+        [RelayCommand]
+        private void SelectAll()
+        {
+            foreach (var item in AvailableItems)
+            {
+                item.IsSelected = true;
+            }
+        }
+
+        [RelayCommand]
+        private void SelectNone()
+        {
+            foreach (var item in AvailableItems)
+            {
+                item.IsSelected = false;
+            }
         }
 
         private string? Validate()
@@ -170,12 +323,20 @@ namespace Junction.App.ViewModels
         private async Task Save()
         {
             var id = _editingId ?? Guid.NewGuid();
+
+            // Selection source: if the user probed (AvailableItems populated), take the ticked ids.
+            // Otherwise PRESERVE the existing selection (EDIT with no reload) — empty for ADD (opt-in).
+            IReadOnlyCollection<string> monitoredItemIds = AvailableItems.Count > 0
+                ? AvailableItems.Where(i => i.IsSelected).Select(i => i.Descriptor.Id).ToList()
+                : _existingMonitoredItemIds.ToList();
+
             var machine = new Machine(
                 id,
                 Name.Trim(),
                 SelectedProtocol,
                 new Dictionary<string, string> { ["AgentUrl"] = AgentUrl.Trim() },
-                TimeSpan.FromSeconds(PollIntervalSeconds));
+                TimeSpan.FromSeconds(PollIntervalSeconds),
+                monitoredItemIds);
 
             var upsert = await _repository.UpsertAsync(machine, CancellationToken.None).ConfigureAwait(true);
             if (!upsert.IsSuccess)
diff --git a/src/Junction.App/ViewModels/MachineDetailViewModel.cs b/src/Junction.App/ViewModels/MachineDetailViewModel.cs
index 3095e3c..3ef1c2f 100644
--- a/src/Junction.App/ViewModels/MachineDetailViewModel.cs
+++ b/src/Junction.App/ViewModels/MachineDetailViewModel.cs
@@ -53,7 +53,16 @@ namespace Junction.App.ViewModels
         /// <summary>Timestamp of the last CONNECTED snapshot; "—" until first connect.</summary>
         [ObservableProperty] private string _lastSeen = "—";
 
-        [ObservableProperty] private int _itemCount;
+        [ObservableProperty]
+        [NotifyPropertyChangedFor(nameof(HasItems))]
+        [NotifyPropertyChangedFor(nameof(HasNoItems))]
+        private int _itemCount;
+
+        /// <summary>True when there is at least one monitored data item to display.</summary>
+        public bool HasItems => ItemCount > 0;
+
+        /// <summary>True when nothing is monitored — drives the empty-state hint instead of a blank table.</summary>
+        public bool HasNoItems => ItemCount == 0;
 
         /// <summary>True only when currently connected.</summary>
         public bool IsOnline => ConnectionState == ConnectionState.Connected;
diff --git a/src/Junction.App/ViewModels/SelectableDataItemViewModel.cs b/src/Junction.App/ViewModels/SelectableDataItemViewModel.cs
new file mode 100644
index 0000000..ae7051f
--- /dev/null
+++ b/src/Junction.App/ViewModels/SelectableDataItemViewModel.cs
@@ -0,0 +1,35 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+using Junction.Domain.Protocols;
+
+namespace Junction.App.ViewModels
+{
+    /// <summary>
+    /// One selectable row in the config screen's monitored-item checklist. Wraps a
+    /// <see cref="DataItemDescriptor"/> (catalog entry from a probe) and carries the user's
+    /// <see cref="IsSelected"/> choice. The descriptor is immutable; only the selection mutates.
+    /// </summary>
+    public sealed partial class SelectableDataItemViewModel : ObservableObject
+    {
+        /// <summary>The underlying catalog entry. Its <c>Id</c> is what gets persisted when selected.</summary>
+        public DataItemDescriptor Descriptor { get; }
+
+        public string Id => Descriptor.Id;
+
+        /// <summary>Display name; falls back to the id when the descriptor carries no name.</summary>
+        public string Name => string.IsNullOrWhiteSpace(Descriptor.Name) ? Descriptor.Id : Descriptor.Name;
+
+        public string Type => string.IsNullOrWhiteSpace(Descriptor.Type) ? "—" : Descriptor.Type;
+
+        public string Category => string.IsNullOrWhiteSpace(Descriptor.Category) ? "—" : Descriptor.Category;
+
+        public string Units => string.IsNullOrWhiteSpace(Descriptor.Units) ? "—" : Descriptor.Units;
+
+        [ObservableProperty] private bool _isSelected;
+
+        public SelectableDataItemViewModel(DataItemDescriptor descriptor, bool isSelected)
+        {
+            Descriptor = descriptor;
+            _isSelected = isSelected;
+        }
+    }
+}
diff --git a/src/Junction.App/Views/MachineConfigView.axaml b/src/Junction.App/Views/MachineConfigView.axaml
index b9fa742..1238c30 100644
--- a/src/Junction.App/Views/MachineConfigView.axaml
+++ b/src/Junction.App/Views/MachineConfigView.axaml
@@ -57,5 +57,73 @@
       </StackPanel>
 
     </StackPanel>
+
+    <!-- Data items to monitor (fills remaining space) -->
+    <Border BorderThickness="1"
+            BorderBrush="{DynamicResource SystemControlForegroundBaseMediumLowBrush}"
+            CornerRadius="4"
+            Padding="12" Margin="0,16,0,0" MaxWidth="640" HorizontalAlignment="Left">
+      <DockPanel>
+
+        <TextBlock DockPanel.Dock="Top" Text="Data items to monitor"
+                   FontSize="16" FontWeight="SemiBold" Margin="0,0,0,8" />
+
+        <!-- Controls row -->
+        <StackPanel DockPanel.Dock="Top" Orientation="Horizontal" Spacing="8" Margin="0,0,0,8">
+          <Button Content="Load available items"
+                  Command="{Binding LoadItemsCommand}" />
+          <Button Content="Select all"
+                  Command="{Binding SelectAllCommand}" />
+          <Button Content="Select none"
+                  Command="{Binding SelectNoneCommand}" />
+          <ProgressBar IsIndeterminate="True"
+                       IsVisible="{Binding IsProbing}"
+                       Width="120" VerticalAlignment="Center" />
+        </StackPanel>
+
+        <!-- Status message -->
+        <TextBlock DockPanel.Dock="Top"
+                   Text="{Binding ItemsStatus}"
+                   IsVisible="{Binding HasItemsStatus}"
+                   Opacity="0.85" TextWrapping="Wrap" Margin="0,0,0,8" />
+
+        <!-- Column header -->
+        <Border DockPanel.Dock="Top"
+                BorderThickness="0,0,0,1"
+                BorderBrush="{DynamicResource SystemControlForegroundBaseMediumLowBrush}"
+                Padding="0,0,0,4" Margin="0,0,0,4">
+          <Grid ColumnDefinitions="Auto,2*,1.2*,1.2*,1*" Margin="0,0,8,0">
+            <TextBlock Grid.Column="0" Text="" Width="28" />
+            <TextBlock Grid.Column="1" Text="Name" FontWeight="Bold" />
+            <TextBlock Grid.Column="2" Text="Type" FontWeight="Bold" />
+            <TextBlock Grid.Column="3" Text="Category" FontWeight="Bold" />
+            <TextBlock Grid.Column="4" Text="Units" FontWeight="Bold" />
+          </Grid>
+        </Border>
+
+        <!-- Scrollable checklist -->
+        <ScrollViewer MaxHeight="320">
+          <ItemsControl ItemsSource="{Binding AvailableItems}">
+            <ItemsControl.ItemTemplate>
+              <DataTemplate x:DataType="vm:SelectableDataItemViewModel">
+                <Grid ColumnDefinitions="Auto,2*,1.2*,1.2*,1*" Margin="0,3">
+                  <CheckBox Grid.Column="0" Width="28"
+                            IsChecked="{Binding IsSelected}"
+                            VerticalAlignment="Center" />
+                  <StackPanel Grid.Column="1" VerticalAlignment="Center">
+                    <TextBlock Text="{Binding Name}" FontWeight="SemiBold" TextWrapping="Wrap" />
+                    <TextBlock Text="{Binding Id}" FontSize="11" Opacity="0.6" TextWrapping="Wrap" />
+                  </StackPanel>
+                  <TextBlock Grid.Column="2" Text="{Binding Type}" VerticalAlignment="Center" TextWrapping="Wrap" />
+                  <TextBlock Grid.Column="3" Text="{Binding Category}" VerticalAlignment="Center" TextWrapping="Wrap" />
+                  <TextBlock Grid.Column="4" Text="{Binding Units}" VerticalAlignment="Center" TextWrapping="Wrap" />
+                </Grid>
+              </DataTemplate>
+            </ItemsControl.ItemTemplate>
+          </ItemsControl>
+        </ScrollViewer>
+
+      </DockPanel>
+    </Border>
   </DockPanel>
 </UserControl>
diff --git a/src/Junction.App/Views/MachineDetailView.axaml b/src/Junction.App/Views/MachineDetailView.axaml
index e4ee199..8fe4de9 100644
--- a/src/Junction.App/Views/MachineDetailView.axaml
+++ b/src/Junction.App/Views/MachineDetailView.axaml
@@ -67,7 +67,8 @@
     <Border DockPanel.Dock="Top"
             BorderThickness="0,0,0,1"
             BorderBrush="{DynamicResource SystemControlForegroundBaseMediumLowBrush}"
-            Padding="0,0,0,6" Margin="0,0,0,4">
+            Padding="0,0,0,6" Margin="0,0,0,4"
+            IsVisible="{Binding HasItems}">
       <Grid ColumnDefinitions="2*,2*,1.2*,1.2*">
         <TextBlock Grid.Column="0" Text="Name" FontWeight="Bold" />
         <TextBlock Grid.Column="1" Text="Value" FontWeight="Bold" />
@@ -76,24 +77,37 @@
       </Grid>
     </Border>
 
-    <!-- Full data-item table -->
-    <ScrollViewer>
-      <ItemsControl ItemsSource="{Binding Items}">
-        <ItemsControl.ItemTemplate>
-          <DataTemplate x:DataType="vm:DataItemRowViewModel">
-            <Grid ColumnDefinitions="2*,2*,1.2*,1.2*" Margin="0,5">
-              <StackPanel Grid.Column="0">
-                <TextBlock Text="{Binding Name}" FontWeight="SemiBold" TextWrapping="Wrap" />
-                <TextBlock Text="{Binding Id}" FontSize="11" Opacity="0.6" TextWrapping="Wrap" />
-              </StackPanel>
-              <TextBlock Grid.Column="1" Text="{Binding Value}" VerticalAlignment="Center" TextWrapping="Wrap" />
-              <TextBlock Grid.Column="2" Text="{Binding Category}" VerticalAlignment="Center" />
-              <TextBlock Grid.Column="3" Text="{Binding Timestamp}" VerticalAlignment="Center" />
-            </Grid>
-          </DataTemplate>
-        </ItemsControl.ItemTemplate>
-      </ItemsControl>
-    </ScrollViewer>
+    <!-- Data-item table (fills), or an empty-state hint when nothing is monitored -->
+    <Grid>
+      <ScrollViewer IsVisible="{Binding HasItems}">
+        <ItemsControl ItemsSource="{Binding Items}">
+          <ItemsControl.ItemTemplate>
+            <DataTemplate x:DataType="vm:DataItemRowViewModel">
+              <Grid ColumnDefinitions="2*,2*,1.2*,1.2*" Margin="0,5">
+                <StackPanel Grid.Column="0">
+                  <TextBlock Text="{Binding Name}" FontWeight="SemiBold" TextWrapping="Wrap" />
+                  <TextBlock Text="{Binding Id}" FontSize="11" Opacity="0.6" TextWrapping="Wrap" />
+                </StackPanel>
+                <TextBlock Grid.Column="1" Text="{Binding Value}" VerticalAlignment="Center" TextWrapping="Wrap" />
+                <TextBlock Grid.Column="2" Text="{Binding Category}" VerticalAlignment="Center" />
+                <TextBlock Grid.Column="3" Text="{Binding Timestamp}" VerticalAlignment="Center" />
+              </Grid>
+            </DataTemplate>
+          </ItemsControl.ItemTemplate>
+        </ItemsControl>
+      </ScrollViewer>
+
+      <!-- Empty-state -->
+      <StackPanel IsVisible="{Binding HasNoItems}"
+                  HorizontalAlignment="Center" VerticalAlignment="Center"
+                  Spacing="6" MaxWidth="420">
+        <TextBlock Text="No monitored items"
+                   FontSize="16" FontWeight="SemiBold"
+                   HorizontalAlignment="Center" />
+        <TextBlock Text="Edit this machine to select which data items to monitor."
+                   Opacity="0.75" TextWrapping="Wrap" TextAlignment="Center" />
+      </StackPanel>
+    </Grid>
   </DockPanel>
 
   <!-- Delete-confirmation overlay: dims the screen, names the machine, requires an explicit choice. -->
diff --git a/src/Junction.Core/Monitoring/IMachineMonitor.cs b/src/Junction.Core/Monitoring/IMachineMonitor.cs
index 64fbc3b..b76a5ae 100644
--- a/src/Junction.Core/Monitoring/IMachineMonitor.cs
+++ b/src/Junction.Core/Monitoring/IMachineMonitor.cs
@@ -4,6 +4,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Junction.Domain;
 using Junction.Domain.Models;
+using Junction.Domain.Protocols;
 
 namespace Junction.Core.Monitoring
 {
@@ -84,5 +85,19 @@ namespace Junction.Core.Monitoring
         /// monitor to have been started.
         /// </summary>
         Task<Result> RemoveMachineAsync(Guid machineId, CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Fetches the full (unfiltered) catalog of selectable data items a machine exposes, so the
+        /// config screen can present the monitored-item selection. Resolves the protocol factory by
+        /// <see cref="Machine.ProtocolId"/> from the retained factory map, builds a driver via
+        /// <c>factory.Create(machine)</c>, and delegates to <see cref="IProtocolDriver.ProbeAsync"/>.
+        /// <para>
+        /// Requires <see cref="StartAsync"/> to have completed: returns <see cref="Result{T}.Fail"/>
+        /// ("monitor not started") otherwise. Returns <see cref="Result{T}.Fail"/> when the protocol
+        /// has no loaded plugin or the driver cannot be created, and <see cref="Result{T}.Cancelled"/>
+        /// if the token trips first.
+        /// </para>
+        /// </summary>
+        Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(Machine machine, CancellationToken cancellationToken);
     }
 }
diff --git a/src/Junction.Core/Monitoring/MachineMonitor.cs b/src/Junction.Core/Monitoring/MachineMonitor.cs
index 740304d..b8f765e 100644
--- a/src/Junction.Core/Monitoring/MachineMonitor.cs
+++ b/src/Junction.Core/Monitoring/MachineMonitor.cs
@@ -309,6 +309,44 @@ namespace Junction.Core.Monitoring
             return Result.Ok();
         }
 
+        /// <inheritdoc />
+        public async Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(Machine machine, CancellationToken cancellationToken)
+        {
+            if (machine is null)
+            {
+                return Result<IReadOnlyList<DataItemDescriptor>>.Fail(OperationError.Of(Source, "machine is null."));
+            }
+
+            if (cancellationToken.IsCancellationRequested)
+            {
+                return Result<IReadOnlyList<DataItemDescriptor>>.Cancelled();
+            }
+
+            IProtocolDriverFactory factory;
+            lock (_lifecycleLock)
+            {
+                if (_factories is null)
+                {
+                    return Result<IReadOnlyList<DataItemDescriptor>>.Fail(
+                        OperationError.Of(Source, "Monitor not started; call StartAsync first."));
+                }
+
+                if (!_factories.TryGetValue(machine.ProtocolId, out factory))
+                {
+                    return Result<IReadOnlyList<DataItemDescriptor>>.Fail(
+                        OperationError.Of(Source, $"No plugin loaded for protocol '{machine.ProtocolId}'."));
+                }
+            }
+
+            Result<IProtocolDriver> driverResult = factory.Create(machine);
+            if (!driverResult.IsSuccess)
+            {
+                return Result<IReadOnlyList<DataItemDescriptor>>.Fail(driverResult.Errors);
+            }
+
+            return await driverResult.Value.ProbeAsync(cancellationToken).ConfigureAwait(false);
+        }
+
         /// <summary>
         /// Removes and returns the tracked loop for <paramref name="machineId"/>, or null.
         /// Caller must hold <see cref="_lifecycleLock"/>.
diff --git a/src/Junction.Domain/Models/Machine.cs b/src/Junction.Domain/Models/Machine.cs
index 6135df0..6704bd7 100644
--- a/src/Junction.Domain/Models/Machine.cs
+++ b/src/Junction.Domain/Models/Machine.cs
@@ -13,6 +13,8 @@ namespace Junction.Domain.Models
         private static readonly IReadOnlyDictionary<string, string> EmptyConfig =
             new Dictionary<string, string>(0);
 
+        private static readonly IReadOnlyCollection<string> EmptyItemIds = new string[0];
+
         /// <summary>Stable unique identifier.</summary>
         public Guid Id { get; }
 
@@ -28,18 +30,26 @@ namespace Junction.Domain.Models
         /// <summary>How often to poll the machine.</summary>
         public TimeSpan PollInterval { get; }
 
+        /// <summary>
+        /// DataItem ids the user selected to monitor for this machine. Never null; empty is allowed
+        /// and means opt-in "monitor nothing" (drivers keep/persist only the items listed here).
+        /// </summary>
+        public IReadOnlyCollection<string> MonitoredItemIds { get; }
+
         public Machine(
             Guid id,
             string name,
             string protocolId,
             IReadOnlyDictionary<string, string>? connectionConfig,
-            TimeSpan pollInterval)
+            TimeSpan pollInterval,
+            IReadOnlyCollection<string>? monitoredItemIds = null)
         {
             Id = id;
             Name = name ?? "";
             ProtocolId = protocolId ?? "";
             ConnectionConfig = connectionConfig ?? EmptyConfig;
             PollInterval = pollInterval;
+            MonitoredItemIds = monitoredItemIds ?? EmptyItemIds;
         }
 
         /// <summary>Returns a copy with the given fields overridden; null args keep the current value.</summary>
@@ -47,14 +57,16 @@ namespace Junction.Domain.Models
             string? name = null,
             string? protocolId = null,
             IReadOnlyDictionary<string, string>? connectionConfig = null,
-            TimeSpan? pollInterval = null)
+            TimeSpan? pollInterval = null,
+            IReadOnlyCollection<string>? monitoredItemIds = null)
         {
             return new Machine(
                 Id,
                 name ?? Name,
                 protocolId ?? ProtocolId,
                 connectionConfig ?? ConnectionConfig,
-                pollInterval ?? PollInterval);
+                pollInterval ?? PollInterval,
+                monitoredItemIds ?? MonitoredItemIds);
         }
     }
 }
diff --git a/src/Junction.Domain/Protocols/DataItemDescriptor.cs b/src/Junction.Domain/Protocols/DataItemDescriptor.cs
new file mode 100644
index 0000000..f6f0825
--- /dev/null
+++ b/src/Junction.Domain/Protocols/DataItemDescriptor.cs
@@ -0,0 +1,34 @@
+namespace Junction.Domain.Protocols
+{
+    /// <summary>
+    /// Protocol-agnostic catalog entry describing a single selectable data item a machine exposes.
+    /// Returned (unfiltered) by <see cref="IProtocolDriver.ProbeAsync"/> so the config screen can
+    /// present the full set of items the user may choose to monitor. Immutable; nulls collapse to "".
+    /// </summary>
+    public sealed class DataItemDescriptor
+    {
+        /// <summary>Stable identifier of the data item (e.g. MTConnect dataItemId).</summary>
+        public string Id { get; }
+
+        /// <summary>Human-readable name (may be empty).</summary>
+        public string Name { get; }
+
+        /// <summary>Protocol type (e.g. MTConnect POSITION, EXECUTION, AVAILABILITY).</summary>
+        public string Type { get; }
+
+        /// <summary>Category (e.g. MTConnect SAMPLE, EVENT, CONDITION). Kept as string to stay generic.</summary>
+        public string Category { get; }
+
+        /// <summary>Units (may be empty).</summary>
+        public string Units { get; }
+
+        public DataItemDescriptor(string id, string name, string type, string category, string units)
+        {
+            Id = id ?? "";
+            Name = name ?? "";
+            Type = type ?? "";
+            Category = category ?? "";
+            Units = units ?? "";
+        }
+    }
+}
diff --git a/src/Junction.Domain/Protocols/IProtocolDriver.cs b/src/Junction.Domain/Protocols/IProtocolDriver.cs
index 1c30196..6ba8980 100644
--- a/src/Junction.Domain/Protocols/IProtocolDriver.cs
+++ b/src/Junction.Domain/Protocols/IProtocolDriver.cs
@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using Junction.Domain.Models;
@@ -20,5 +21,14 @@ namespace Junction.Domain.Protocols
         /// </summary>
         /// <param name="cancellationToken">Cancellation token. Mandatory.</param>
         Task<Result<MachineSnapshot>> ReadCurrentAsync(CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Read the FULL (unfiltered) catalog of data items the machine exposes, so the config
+        /// screen can present the selectable set. Independent of the per-machine monitored-item
+        /// selection: selection is applied to <see cref="ReadCurrentAsync"/> output, not here.
+        /// Returns a failed <see cref="Result{T}"/> on error, cancelled when the token trips.
+        /// </summary>
+        /// <param name="cancellationToken">Cancellation token. Mandatory.</param>
+        Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(CancellationToken cancellationToken);
     }
 }
diff --git a/src/Junction.Persistence/SqliteMachineRepository.cs b/src/Junction.Persistence/SqliteMachineRepository.cs
index 0c69716..f23d164 100644
--- a/src/Junction.Persistence/SqliteMachineRepository.cs
+++ b/src/Junction.Persistence/SqliteMachineRepository.cs
@@ -60,7 +60,7 @@ namespace Junction.Persistence
                 using (var connection = OpenConnection())
                 {
                     var command = new CommandDefinition(
-                        "SELECT Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson FROM machines;",
+                        "SELECT Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson, MonitoredItemIdsJson FROM machines;",
                         cancellationToken: cancellationToken);
 
                     var rows = await connection.QueryAsync<MachineRow>(command).ConfigureAwait(false);
@@ -97,7 +97,7 @@ namespace Junction.Persistence
                 using (var connection = OpenConnection())
                 {
                     var command = new CommandDefinition(
-                        "SELECT Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson " +
+                        "SELECT Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson, MonitoredItemIdsJson " +
                         "FROM machines WHERE Id = @Id;",
                         new { Id = GuidText(id) },
                         cancellationToken: cancellationToken);
@@ -141,20 +141,22 @@ namespace Junction.Persistence
                 using (var connection = OpenConnection())
                 {
                     var command = new CommandDefinition(
-                        "INSERT INTO machines (Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson) " +
-                        "VALUES (@Id, @Name, @ProtocolId, @PollIntervalTicks, @ConnectionConfigJson) " +
+                        "INSERT INTO machines (Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson, MonitoredItemIdsJson) " +
+                        "VALUES (@Id, @Name, @ProtocolId, @PollIntervalTicks, @ConnectionConfigJson, @MonitoredItemIdsJson) " +
                         "ON CONFLICT(Id) DO UPDATE SET " +
                         "Name = excluded.Name, " +
                         "ProtocolId = excluded.ProtocolId, " +
                         "PollIntervalTicks = excluded.PollIntervalTicks, " +
-                        "ConnectionConfigJson = excluded.ConnectionConfigJson;",
+                        "ConnectionConfigJson = excluded.ConnectionConfigJson, " +
+                        "MonitoredItemIdsJson = excluded.MonitoredItemIdsJson;",
                         new
                         {
                             Id = GuidText(machine.Id),
                             machine.Name,
                             machine.ProtocolId,
                             PollIntervalTicks = machine.PollInterval.Ticks,
-                            ConnectionConfigJson = SerializeConfig(machine.ConnectionConfig)
+                            ConnectionConfigJson = SerializeConfig(machine.ConnectionConfig),
+                            MonitoredItemIdsJson = SerializeItemIds(machine.MonitoredItemIds)
                         },
                         cancellationToken: cancellationToken);
 
@@ -376,7 +378,8 @@ namespace Junction.Persistence
                 row.Name,
                 row.ProtocolId,
                 DeserializeConfig(row.ConnectionConfigJson),
-                TimeSpan.FromTicks(row.PollIntervalTicks));
+                TimeSpan.FromTicks(row.PollIntervalTicks),
+                DeserializeItemIds(row.MonitoredItemIdsJson));
         }
 
         private static string GuidText(Guid id) => id.ToString("D");
@@ -416,6 +419,31 @@ namespace Junction.Persistence
             return dict ?? new Dictionary<string, string>(0);
         }
 
+        private static string SerializeItemIds(IReadOnlyCollection<string> ids)
+        {
+            var list = new List<string>(ids?.Count ?? 0);
+            if (ids != null)
+            {
+                foreach (var id in ids)
+                {
+                    list.Add(id);
+                }
+            }
+
+            return JsonSerializer.Serialize(list, JsonOptions);
+        }
+
+        private static IReadOnlyCollection<string> DeserializeItemIds(string? json)
+        {
+            if (string.IsNullOrWhiteSpace(json))
+            {
+                return Array.Empty<string>();
+            }
+
+            var list = JsonSerializer.Deserialize<List<string>>(json!, JsonOptions);
+            return list ?? (IReadOnlyCollection<string>)Array.Empty<string>();
+        }
+
         // -- row DTOs (private; never cross the boundary) -----------------------------------
 
         private sealed class MachineRow
@@ -425,6 +453,7 @@ namespace Junction.Persistence
             public string ProtocolId { get; set; } = "";
             public long PollIntervalTicks { get; set; }
             public string? ConnectionConfigJson { get; set; }
+            public string? MonitoredItemIdsJson { get; set; }
         }
 
         private sealed class SnapshotRow
diff --git a/src/Junction.Persistence/SqliteSchema.cs b/src/Junction.Persistence/SqliteSchema.cs
index f141432..71d1627 100644
--- a/src/Junction.Persistence/SqliteSchema.cs
+++ b/src/Junction.Persistence/SqliteSchema.cs
@@ -18,7 +18,8 @@ namespace Junction.Persistence
                 Name                  TEXT    NOT NULL,
                 ProtocolId            TEXT    NOT NULL,
                 PollIntervalTicks     INTEGER NOT NULL,
-                ConnectionConfigJson  TEXT    NOT NULL
+                ConnectionConfigJson  TEXT    NOT NULL,
+                MonitoredItemIdsJson  TEXT
             );";
 
         // latest_snapshots: exactly one row per machine (PK = MachineId). Upsert overwrites.
@@ -63,6 +64,11 @@ namespace Junction.Persistence
                 Execute(connection, CreateLatestSnapshots);
                 Execute(connection, CreateSnapshotItems);
 
+                // Migration for DBs created by an older schema (before MonitoredItemIdsJson existed):
+                // CREATE TABLE IF NOT EXISTS never alters an existing table, so add the column here if
+                // missing. Idempotent and safe on both fresh (already has it) and pre-existing DBs.
+                EnsureColumn(connection, "machines", "MonitoredItemIdsJson", "TEXT");
+
                 return Result.Ok();
             }
             catch (Exception ex)
@@ -100,5 +106,38 @@ namespace Junction.Persistence
                 command.ExecuteNonQuery();
             }
         }
+
+        /// <summary>Adds <paramref name="column"/> to <paramref name="table"/> if not already present. Idempotent.</summary>
+        private static void EnsureColumn(IDbConnection connection, string table, string column, string sqlType)
+        {
+            if (ColumnExists(connection, table, column))
+            {
+                return;
+            }
+
+            Execute(connection, "ALTER TABLE " + table + " ADD COLUMN " + column + " " + sqlType + ";");
+        }
+
+        private static bool ColumnExists(IDbConnection connection, string table, string column)
+        {
+            using (var command = connection.CreateCommand())
+            {
+                command.CommandText = "PRAGMA table_info(" + table + ");";
+                using (var reader = command.ExecuteReader())
+                {
+                    while (reader.Read())
+                    {
+                        // PRAGMA table_info columns: cid(0), name(1), type(2), ...
+                        var name = reader.GetValue(1)?.ToString();
+                        if (string.Equals(name, column, StringComparison.OrdinalIgnoreCase))
+                        {
+                            return true;
+                        }
+                    }
+                }
+            }
+
+            return false;
+        }
     }
 }
diff --git a/src/Junction.Protocols.MTConnect/MtconnectDriver.cs b/src/Junction.Protocols.MTConnect/MtconnectDriver.cs
index e6c3ebc..2d94755 100644
--- a/src/Junction.Protocols.MTConnect/MtconnectDriver.cs
+++ b/src/Junction.Protocols.MTConnect/MtconnectDriver.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Net.Http;
 using System.Threading;
 using System.Threading.Tasks;
@@ -25,12 +26,17 @@ namespace Junction.Protocols.MTConnect
     {
         private const string Source = "MtconnectDriver";
         private const string CurrentPath = "current";
+        private const string ProbePath = "probe";
 
         private readonly Guid _machineId;
         private readonly Uri _currentUri;
+        private readonly Uri _probeUri;
         private readonly HttpClient _http;
         private readonly TimeSpan _requestTimeout;
 
+        // Selected data item ids to keep in ReadCurrentAsync snapshots. Empty => opt-in "monitor nothing".
+        private readonly HashSet<string> _monitoredItemIds;
+
         /// <summary>Protocol identifier this driver serves.</summary>
         public string ProtocolId => "mtconnect";
 
@@ -48,7 +54,16 @@ namespace Junction.Protocols.MTConnect
         /// Per-request timeout enforced via a linked <see cref="CancellationTokenSource"/>. Must be positive
         /// or <see cref="Timeout.InfiniteTimeSpan"/>.
         /// </param>
-        public MtconnectDriver(Guid machineId, string agentUrl, HttpClient httpClient, TimeSpan requestTimeout)
+        /// <param name="monitoredItemIds">
+        /// DataItem ids the user selected to monitor. <see cref="ReadCurrentAsync"/> keeps only these in the
+        /// snapshot (so persistence stores only selected items). Empty/null => opt-in "monitor nothing".
+        /// </param>
+        public MtconnectDriver(
+            Guid machineId,
+            string agentUrl,
+            HttpClient httpClient,
+            TimeSpan requestTimeout,
+            IReadOnlyCollection<string>? monitoredItemIds = null)
         {
             if (agentUrl is null) throw new ArgumentNullException(nameof(agentUrl));
             _http = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
@@ -59,18 +74,33 @@ namespace Junction.Protocols.MTConnect
             _requestTimeout = requestTimeout;
             _machineId = machineId;
 
+            // DataItem ids are case-sensitive; use ordinal set membership for the selection filter.
+            _monitoredItemIds = new HashSet<string>(StringComparer.Ordinal);
+            if (monitoredItemIds != null)
+            {
+                foreach (var id in monitoredItemIds)
+                {
+                    if (!string.IsNullOrEmpty(id))
+                    {
+                        _monitoredItemIds.Add(id);
+                    }
+                }
+            }
+
             if (!Uri.TryCreate(agentUrl, UriKind.Absolute, out var baseUri))
             {
                 throw new ArgumentException("Agent URL must be an absolute URI: '" + agentUrl + "'.", nameof(agentUrl));
             }
 
-            // Combine base + "current" preserving any base path segment.
+            // Combine base + endpoint preserving any base path segment.
             var basePath = baseUri.AbsoluteUri;
             if (!basePath.EndsWith("/", StringComparison.Ordinal))
             {
                 basePath += "/";
             }
-            _currentUri = new Uri(new Uri(basePath, UriKind.Absolute), CurrentPath);
+            var normalizedBase = new Uri(basePath, UriKind.Absolute);
+            _currentUri = new Uri(normalizedBase, CurrentPath);
+            _probeUri = new Uri(normalizedBase, ProbePath);
         }
 
         /// <inheritdoc />
@@ -139,11 +169,124 @@ namespace Junction.Protocols.MTConnect
                 }
 
                 // Delegate parsing (T15). Parser never throws; returns Fail on malformed input.
-                return MtconnectCurrentParser.Parse(body, _machineId);
+                Result<MachineSnapshot> parsed = MtconnectCurrentParser.Parse(body, _machineId);
+                if (!parsed.IsSuccess)
+                {
+                    return parsed;
+                }
+
+                // Apply the per-machine selection: keep only monitored items (empty => opt-in nothing).
+                // ConnectionState/CapturedAt are preserved from the parsed snapshot.
+                return Result<MachineSnapshot>.Ok(FilterToMonitored(parsed.Value));
             }
         }
 
+        /// <inheritdoc />
+        public async Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(CancellationToken cancellationToken)
+        {
+            if (cancellationToken.IsCancellationRequested)
+            {
+                return Result<IReadOnlyList<DataItemDescriptor>>.Cancelled();
+            }
+
+            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            cts.CancelAfter(_requestTimeout);
+
+            HttpResponseMessage response;
+            try
+            {
+                response = await _http.GetAsync(_probeUri, cts.Token).ConfigureAwait(false);
+            }
+            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+            {
+                return Result<IReadOnlyList<DataItemDescriptor>>.Cancelled();
+            }
+            catch (OperationCanceledException ex)
+            {
+                return ProbeFail("PROBE_TIMEOUT", "HTTP request to '" + _probeUri + "' timed out: " + ex.Message);
+            }
+            catch (HttpRequestException ex)
+            {
+                return ProbeFail("PROBE_HTTP_ERROR", "HTTP request to '" + _probeUri + "' failed: " + ex.Message);
+            }
+
+            using (response)
+            {
+                if (!response.IsSuccessStatusCode)
+                {
+                    return ProbeFail(
+                        "PROBE_HTTP_STATUS",
+                        "Agent returned non-success status " + (int)response.StatusCode + " (" + response.StatusCode + ") for '" + _probeUri + "'.");
+                }
+
+                string body;
+                try
+                {
+#if NET5_0_OR_GREATER
+                    body = await response.Content.ReadAsStringAsync(cts.Token).ConfigureAwait(false);
+#else
+                    body = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
+#endif
+                }
+                catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+                {
+                    return Result<IReadOnlyList<DataItemDescriptor>>.Cancelled();
+                }
+                catch (OperationCanceledException ex)
+                {
+                    return ProbeFail("PROBE_TIMEOUT", "Reading response body from '" + _probeUri + "' timed out: " + ex.Message);
+                }
+                catch (HttpRequestException ex)
+                {
+                    return ProbeFail("PROBE_HTTP_ERROR", "Reading response body from '" + _probeUri + "' failed: " + ex.Message);
+                }
+
+                Result<IReadOnlyList<ProbeDataItemDescriptor>> parsed = MtconnectProbeParser.Parse(body);
+                if (!parsed.IsSuccess)
+                {
+                    return Result<IReadOnlyList<DataItemDescriptor>>.Fail(parsed.Errors);
+                }
+
+                var catalog = new List<DataItemDescriptor>(parsed.Value.Count);
+                foreach (ProbeDataItemDescriptor d in parsed.Value)
+                {
+                    catalog.Add(new DataItemDescriptor(d.Id, d.Name, d.Type, d.Category, d.Units));
+                }
+
+                return Result<IReadOnlyList<DataItemDescriptor>>.Ok(catalog);
+            }
+        }
+
+        /// <summary>
+        /// Returns a copy of <paramref name="snapshot"/> keeping only items whose id is in the
+        /// monitored selection. Empty selection yields an empty item set (opt-in). Connection
+        /// state and capture instant are preserved.
+        /// </summary>
+        private MachineSnapshot FilterToMonitored(MachineSnapshot snapshot)
+        {
+            if (_monitoredItemIds.Count == 0)
+            {
+                return new MachineSnapshot(
+                    snapshot.MachineId, snapshot.CapturedAt, snapshot.ConnectionState, Array.Empty<DataItem>());
+            }
+
+            var kept = new List<DataItem>();
+            for (int i = 0; i < snapshot.Items.Count; i++)
+            {
+                DataItem item = snapshot.Items[i];
+                if (_monitoredItemIds.Contains(item.Id))
+                {
+                    kept.Add(item);
+                }
+            }
+
+            return new MachineSnapshot(snapshot.MachineId, snapshot.CapturedAt, snapshot.ConnectionState, kept);
+        }
+
         private static Result<MachineSnapshot> Fail(string code, string message) =>
             Result<MachineSnapshot>.Fail(new OperationError(code, Source, message));
+
+        private static Result<IReadOnlyList<DataItemDescriptor>> ProbeFail(string code, string message) =>
+            Result<IReadOnlyList<DataItemDescriptor>>.Fail(new OperationError(code, Source, message));
     }
 }
diff --git a/src/Junction.Protocols.MTConnect/MtconnectDriverFactory.cs b/src/Junction.Protocols.MTConnect/MtconnectDriverFactory.cs
index d3f63de..a3e1577 100644
--- a/src/Junction.Protocols.MTConnect/MtconnectDriverFactory.cs
+++ b/src/Junction.Protocols.MTConnect/MtconnectDriverFactory.cs
@@ -86,7 +86,7 @@ namespace Junction.Protocols.MTConnect
                 timeout = TimeSpan.FromSeconds(seconds);
             }
 
-            var driver = new MtconnectDriver(machine.Id, agentUrl, SharedHttpClient, timeout);
+            var driver = new MtconnectDriver(machine.Id, agentUrl, SharedHttpClient, timeout, machine.MonitoredItemIds);
             return Result<IProtocolDriver>.Ok(driver);
         }
 
diff --git a/tests/Junction.Tests/Unit/MachineMonitorTests.cs b/tests/Junction.Tests/Unit/MachineMonitorTests.cs
index c9bca86..47f253c 100644
--- a/tests/Junction.Tests/Unit/MachineMonitorTests.cs
+++ b/tests/Junction.Tests/Unit/MachineMonitorTests.cs
@@ -385,6 +385,72 @@ namespace Junction.Tests.Unit
             Assert.Contains(result.Errors, e => e.Message.Contains("not started"));
         }
 
+        [Fact]
+        public async Task ProbeAsync_ReturnsCatalog_FromFactoryDriver()
+        {
+            var catalog = new List<DataItemDescriptor>
+            {
+                new DataItemDescriptor("x1_pos", "X", "POSITION", "SAMPLE", "MILLIMETER"),
+                new DataItemDescriptor("dev1_avail", "avail", "AVAILABILITY", "EVENT", ""),
+            };
+
+            var factory = new FakeFactory("test", m => Result<IProtocolDriver>.Ok(
+                new FakeDriver(
+                    "test",
+                    () => Result<MachineSnapshot>.Ok(Snapshot(m.Id)),
+                    () => Result<IReadOnlyList<DataItemDescriptor>>.Ok(catalog))));
+            var loader = LoaderReturning(factory);
+            var repo = RepoReturning();
+            var monitor = NewMonitor(repo.Object, loader.Object);
+
+            await monitor.StartAsync(PluginsDir, CancellationToken.None);
+
+            var machine = MachineWith("test");
+            var result = await monitor.ProbeAsync(machine, CancellationToken.None);
+
+            Assert.True(result.IsSuccess);
+            Assert.Equal(2, result.Value.Count);
+            Assert.Contains(result.Value, d => d.Id == "x1_pos" && d.Type == "POSITION");
+
+            await monitor.StopAsync();
+        }
+
+        [Fact]
+        public async Task ProbeAsync_UnknownProtocol_ReturnsFail()
+        {
+            var factory = new FakeFactory("test", m => Result<IProtocolDriver>.Ok(
+                new FakeDriver("test", () => Result<MachineSnapshot>.Ok(Snapshot(m.Id)))));
+            var loader = LoaderReturning(factory);
+            var repo = RepoReturning();
+            var monitor = NewMonitor(repo.Object, loader.Object);
+
+            await monitor.StartAsync(PluginsDir, CancellationToken.None);
+
+            var bad = MachineWith("nope");
+            var result = await monitor.ProbeAsync(bad, CancellationToken.None);
+
+            Assert.False(result.IsSuccess);
+            Assert.NotEmpty(result.Errors);
+
+            await monitor.StopAsync();
+        }
+
+        [Fact]
+        public async Task ProbeAsync_BeforeStart_ReturnsFail()
+        {
+            var factory = new FakeFactory("test", m => Result<IProtocolDriver>.Ok(
+                new FakeDriver("test", () => Result<MachineSnapshot>.Ok(Snapshot(m.Id)))));
+            var loader = LoaderReturning(factory);
+            var repo = RepoReturning();
+            var monitor = NewMonitor(repo.Object, loader.Object);
+
+            var machine = MachineWith("test");
+            var result = await monitor.ProbeAsync(machine, CancellationToken.None);
+
+            Assert.False(result.IsSuccess);
+            Assert.Contains(result.Errors, e => e.Message.Contains("not started"));
+        }
+
         /// <summary>Hand-rolled factory; behavior supplied by a delegate.</summary>
         private sealed class FakeFactory : IProtocolDriverFactory
         {
@@ -401,15 +467,20 @@ namespace Junction.Tests.Unit
             public Result<IProtocolDriver> Create(Machine machine) => _create(machine);
         }
 
-        /// <summary>Hand-rolled driver; behavior supplied by a delegate.</summary>
+        /// <summary>Hand-rolled driver; behavior supplied by a delegate. Optional canned probe catalog.</summary>
         private sealed class FakeDriver : IProtocolDriver
         {
             private readonly Func<Result<MachineSnapshot>> _read;
+            private readonly Func<Result<IReadOnlyList<DataItemDescriptor>>>? _probe;
 
-            public FakeDriver(string protocolId, Func<Result<MachineSnapshot>> read)
+            public FakeDriver(
+                string protocolId,
+                Func<Result<MachineSnapshot>> read,
+                Func<Result<IReadOnlyList<DataItemDescriptor>>>? probe = null)
             {
                 ProtocolId = protocolId;
                 _read = read;
+                _probe = probe;
             }
 
             public string ProtocolId { get; }
@@ -419,6 +490,11 @@ namespace Junction.Tests.Unit
                 cancellationToken.ThrowIfCancellationRequested();
                 return Task.FromResult(_read());
             }
+
+            public Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(CancellationToken cancellationToken) =>
+                Task.FromResult(_probe != null
+                    ? _probe()
+                    : Result<IReadOnlyList<DataItemDescriptor>>.Ok(Array.Empty<DataItemDescriptor>()));
         }
 
         /// <summary>
@@ -454,6 +530,9 @@ namespace Junction.Tests.Unit
                     _exit();
                 }
             }
+
+            public Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(CancellationToken cancellationToken) =>
+                Task.FromResult(Result<IReadOnlyList<DataItemDescriptor>>.Ok(Array.Empty<DataItemDescriptor>()));
         }
     }
 }
diff --git a/tests/Junction.Tests/Unit/MtconnectDriverTests.cs b/tests/Junction.Tests/Unit/MtconnectDriverTests.cs
index 5965b8a..f024d99 100644
--- a/tests/Junction.Tests/Unit/MtconnectDriverTests.cs
+++ b/tests/Junction.Tests/Unit/MtconnectDriverTests.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Net;
 using System.Net.Http;
 using System.Threading;
@@ -23,6 +24,22 @@ namespace Junction.Tests.Unit
             return File.ReadAllText(path);
         }
 
+        private static string LoadProbeFixture()
+        {
+            var path = Path.Combine(AppContext.BaseDirectory, "Fixtures", "mtconnect", "probe.xml");
+            return File.ReadAllText(path);
+        }
+
+        // All dataItem ids present in current.xml. Used as the default selection so item-agnostic
+        // tests keep observing a non-empty snapshot despite the opt-in ("monitor nothing") default.
+        private static readonly string[] AllCurrentItemIds =
+        {
+            "dev1_avail", "x1_pos", "x1_pos_cmd", "x1_load", "y1_pos", "z1_pos",
+            "c1_spindle_speed", "c1_spindle_speed_cmd", "c1_load", "c1_rot_mode", "c1_temp_cond",
+            "cn1_mode", "cn1_estop", "cn1_system", "path1_exec", "path1_program", "path1_line",
+            "path1_feed", "path1_logic",
+        };
+
         /// <summary>Stub handler: canned response or thrown exception, per configuration.</summary>
         private sealed class StubHandler : HttpMessageHandler
         {
@@ -53,12 +70,17 @@ namespace Junction.Tests.Unit
         private static readonly TimeSpan GenerousTimeout = TimeSpan.FromSeconds(30);
 
         // Shared client with infinite global timeout mirrors production; per-request timeout enforced in driver.
-        private static MtconnectDriver DriverWith(HttpMessageHandler handler, TimeSpan? requestTimeout = null) =>
+        // Defaults the monitored selection to every current.xml id so item-agnostic tests still see items.
+        private static MtconnectDriver DriverWith(
+            HttpMessageHandler handler,
+            TimeSpan? requestTimeout = null,
+            IReadOnlyCollection<string>? monitoredItemIds = null) =>
             new MtconnectDriver(
                 MachineId,
                 AgentUrl,
                 new HttpClient(handler) { Timeout = Timeout.InfiniteTimeSpan },
-                requestTimeout ?? GenerousTimeout);
+                requestTimeout ?? GenerousTimeout,
+                monitoredItemIds ?? AllCurrentItemIds);
 
         // ---- ReadCurrentAsync: happy path ----
 
@@ -199,6 +221,93 @@ namespace Junction.Tests.Unit
             Assert.True(result.WasCancelled);
         }
 
+        // ---- ProbeAsync: catalog ----
+
+        [Fact]
+        public async Task ProbeAsync_200WithValidProbeXml_ReturnsFullCatalog()
+        {
+            var handler = new StubHandler(HttpStatusCode.OK, LoadProbeFixture());
+            var driver = DriverWith(handler);
+
+            var result = await driver.ProbeAsync(CancellationToken.None);
+
+            Assert.True(result.IsSuccess);
+            Assert.False(result.WasCancelled);
+            Assert.NotNull(result.Value);
+            Assert.True(result.Value.Count > 0);
+
+            // Probe returns the FULL (unfiltered) catalog. Spot-check a known descriptor.
+            var pos = result.Value.Single(d => d.Id == "x1_pos");
+            Assert.Equal("POSITION", pos.Type);
+            Assert.Equal("SAMPLE", pos.Category);
+            Assert.Equal("MILLIMETER", pos.Units);
+
+            // Driver hits the agent's /probe endpoint.
+            Assert.NotNull(handler.LastRequestUri);
+            Assert.EndsWith("/probe", handler.LastRequestUri!.AbsoluteUri);
+        }
+
+        [Fact]
+        public async Task ProbeAsync_NonSuccessStatus_ReturnsFailNoThrow()
+        {
+            var handler = new StubHandler(HttpStatusCode.InternalServerError, "irrelevant");
+            var driver = DriverWith(handler);
+
+            var result = await driver.ProbeAsync(CancellationToken.None);
+
+            Assert.False(result.IsSuccess);
+            Assert.Contains(result.Errors, e => e.Code == "PROBE_HTTP_STATUS");
+        }
+
+        [Fact]
+        public async Task ProbeAsync_CancelledBeforeCall_ReturnsCancelled()
+        {
+            var handler = new StubHandler(HttpStatusCode.OK, LoadProbeFixture());
+            var driver = DriverWith(handler);
+            using var cts = new CancellationTokenSource();
+            cts.Cancel();
+
+            var result = await driver.ProbeAsync(cts.Token);
+
+            Assert.False(result.IsSuccess);
+            Assert.True(result.WasCancelled);
+        }
+
+        // ---- ReadCurrentAsync: per-machine selection filter (opt-in) ----
+
+        [Fact]
+        public async Task ReadCurrentAsync_SelectionSubset_KeepsOnlySelectedItems_PreservesConnectionState()
+        {
+            var handler = new StubHandler(HttpStatusCode.OK, LoadCurrentFixture());
+            var selection = new[] { "x1_pos", "c1_load" };
+            var driver = DriverWith(handler, monitoredItemIds: selection);
+
+            var result = await driver.ReadCurrentAsync(CancellationToken.None);
+
+            Assert.True(result.IsSuccess);
+            Assert.Equal(2, result.Value.Items.Count);
+            Assert.Contains(result.Value.Items, i => i.Id == "x1_pos");
+            Assert.Contains(result.Value.Items, i => i.Id == "c1_load");
+            // dev1_avail is not selected: it is filtered out of Items, but still drove the state.
+            Assert.DoesNotContain(result.Value.Items, i => i.Id == "dev1_avail");
+            Assert.Equal(ConnectionState.Connected, result.Value.ConnectionState);
+            Assert.Equal(MachineId, result.Value.MachineId);
+        }
+
+        [Fact]
+        public async Task ReadCurrentAsync_EmptySelection_ReturnsEmptyItems_PreservesConnectionState()
+        {
+            var handler = new StubHandler(HttpStatusCode.OK, LoadCurrentFixture());
+            var driver = DriverWith(handler, monitoredItemIds: Array.Empty<string>());
+
+            var result = await driver.ReadCurrentAsync(CancellationToken.None);
+
+            Assert.True(result.IsSuccess);
+            Assert.Empty(result.Value.Items);
+            // Opt-in: nothing kept, but availability-derived state is preserved.
+            Assert.Equal(ConnectionState.Connected, result.Value.ConnectionState);
+        }
+
         // ---- Factory: config validation ----
 
         private static Machine MachineWithConfig(IReadOnlyDictionary<string, string>? config) =>
diff --git a/tests/Junction.Tests/Unit/PluginLoaderTests.cs b/tests/Junction.Tests/Unit/PluginLoaderTests.cs
index b4b1013..8a1ee00 100644
--- a/tests/Junction.Tests/Unit/PluginLoaderTests.cs
+++ b/tests/Junction.Tests/Unit/PluginLoaderTests.cs
@@ -45,6 +45,9 @@ namespace Junction.Tests.Unit
                         DateTimeOffset.UtcNow,
                         ConnectionState.Connected,
                         Array.Empty<DataItem>())));
+
+            public Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(CancellationToken cancellationToken) =>
+                Task.FromResult(Result<IReadOnlyList<DataItemDescriptor>>.Ok(Array.Empty<DataItemDescriptor>()));
         }
 
         /// <summary>Type that does NOT implement the factory contract.</summary>
diff --git a/tests/Junction.Tests/Unit/PollingEngineTests.cs b/tests/Junction.Tests/Unit/PollingEngineTests.cs
index 92e4098..1974b2c 100644
--- a/tests/Junction.Tests/Unit/PollingEngineTests.cs
+++ b/tests/Junction.Tests/Unit/PollingEngineTests.cs
@@ -282,6 +282,9 @@ namespace Junction.Tests.Unit
                 cancellationToken.ThrowIfCancellationRequested();
                 return Task.FromResult(_behavior(null!));
             }
+
+            public Task<Result<IReadOnlyList<DataItemDescriptor>>> ProbeAsync(CancellationToken cancellationToken) =>
+                Task.FromResult(Result<IReadOnlyList<DataItemDescriptor>>.Ok(Array.Empty<DataItemDescriptor>()));
         }
     }
 }
diff --git a/tests/Junction.Tests/Unit/SqliteMachineRepositoryTests.cs b/tests/Junction.Tests/Unit/SqliteMachineRepositoryTests.cs
index e2e38f8..5898f0c 100644
--- a/tests/Junction.Tests/Unit/SqliteMachineRepositoryTests.cs
+++ b/tests/Junction.Tests/Unit/SqliteMachineRepositoryTests.cs
@@ -76,6 +76,121 @@ namespace Junction.Tests.Unit
             Assert.Equal("M1", loaded.ConnectionConfig["device"]);
         }
 
+        [Fact]
+        public async Task Upsert_MonitoredItemIds_GetById_RoundTripsSelection()
+        {
+            var id = Guid.NewGuid();
+            var selection = new[] { "x1_pos", "c1_load", "dev1_avail" };
+            var machine = new Machine(
+                id, "Mill 02", "mtconnect", null, TimeSpan.FromSeconds(3), selection);
+
+            Result upsert = await _repo.UpsertAsync(machine, CancellationToken.None);
+            Assert.True(upsert.IsSuccess, Describe(upsert));
+
+            Result<Machine> got = await _repo.GetByIdAsync(id, CancellationToken.None);
+            Assert.True(got.IsSuccess, Describe(got));
+
+            Assert.Equal(3, got.Value.MonitoredItemIds.Count);
+            Assert.Equal(selection.OrderBy(x => x), got.Value.MonitoredItemIds.OrderBy(x => x));
+        }
+
+        [Fact]
+        public async Task Upsert_DefaultMachine_MonitoredItemIds_IsEmpty_NotNull()
+        {
+            var id = Guid.NewGuid();
+            // Backward-compat ctor (no selection) => empty opt-in set, round-trips as empty.
+            var machine = new Machine(id, "Mill 03", "mtconnect", null, TimeSpan.FromSeconds(1));
+            await _repo.UpsertAsync(machine, CancellationToken.None);
+
+            Result<Machine> got = await _repo.GetByIdAsync(id, CancellationToken.None);
+            Assert.True(got.IsSuccess, Describe(got));
+            Assert.NotNull(got.Value.MonitoredItemIds);
+            Assert.Empty(got.Value.MonitoredItemIds);
+        }
+
+        [Fact]
+        public async Task EnsureCreated_OnPreExistingMachinesTable_AddsMonitoredItemIdsColumn_KeepsRows()
+        {
+            // Simulate a DB created by the OLD schema (no MonitoredItemIdsJson column).
+            var oldDbPath = Path.Combine(
+                Path.GetTempPath(), "junction_migration_test_" + Guid.NewGuid().ToString("N") + ".db");
+            var connectionString = "Data Source=" + oldDbPath;
+            var factory = new SqliteConnectionFactory(connectionString);
+
+            try
+            {
+                var rowId = Guid.NewGuid().ToString("D");
+                using (var conn = factory.CreateOpenConnection())
+                {
+                    Exec(conn,
+                        @"CREATE TABLE machines (
+                            Id                    TEXT    NOT NULL PRIMARY KEY,
+                            Name                  TEXT    NOT NULL,
+                            ProtocolId            TEXT    NOT NULL,
+                            PollIntervalTicks     INTEGER NOT NULL,
+                            ConnectionConfigJson  TEXT    NOT NULL
+                        );");
+                    Exec(conn,
+                        "INSERT INTO machines (Id, Name, ProtocolId, PollIntervalTicks, ConnectionConfigJson) " +
+                        "VALUES ('" + rowId + "', 'Legacy', 'mtconnect', 10000000, '{}');");
+
+                    Assert.False(HasColumn(conn, "machines", "MonitoredItemIdsJson"));
+                }
+
+                // Migration path: EnsureCreated must ALTER-add the missing column, idempotently.
+                Result schema = SqliteSchema.EnsureCreated(factory);
+                Assert.True(schema.IsSuccess, Describe(schema));
+
+                using (var conn = factory.CreateOpenConnection())
+                {
+                    Assert.True(HasColumn(conn, "machines", "MonitoredItemIdsJson"));
+                }
+
+                // Existing row survived and reads back (null selection => empty set).
+                var repo = new SqliteMachineRepository(factory);
+                Result<Machine> got = await repo.GetByIdAsync(Guid.Parse(rowId), CancellationToken.None);
+                Assert.True(got.IsSuccess, Describe(got));
+                Assert.Equal("Legacy", got.Value.Name);
+                Assert.Empty(got.Value.MonitoredItemIds);
+
+                // Idempotent: running again does not fail.
+                Assert.True(SqliteSchema.EnsureCreated(factory).IsSuccess);
+            }
+            finally
+            {
+                try { if (File.Exists(oldDbPath)) File.Delete(oldDbPath); } catch { /* best-effort */ }
+            }
+        }
+
+        private static void Exec(System.Data.IDbConnection conn, string sql)
+        {
+            using (var cmd = conn.CreateCommand())
+            {
+                cmd.CommandText = sql;
+                cmd.ExecuteNonQuery();
+            }
+        }
+
+        private static bool HasColumn(System.Data.IDbConnection conn, string table, string column)
+        {
+            using (var cmd = conn.CreateCommand())
+            {
+                cmd.CommandText = "PRAGMA table_info(" + table + ");";
+                using (var reader = cmd.ExecuteReader())
+                {
+                    while (reader.Read())
+                    {
+                        if (string.Equals(reader.GetValue(1)?.ToString(), column, StringComparison.OrdinalIgnoreCase))
+                        {
+                            return true;
+                        }
+                    }
+                }
+            }
+
+            return false;
+        }
+
         [Fact]
         public async Task Upsert_ExistingId_Updates_GetAllCountStable()
         {