diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
index 0a11cdb6..72196c08 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
@@ -229,7 +229,7 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                 {
                     state.ProbeCts = new CancellationTokenSource();
                     var ct = state.ProbeCts.Token;
-                    _ = Task.Run(() => ProbeLoopAsync(state, ct), ct);
+                    state.ProbeTask = Task.Run(() => ProbeLoopAsync(state, ct), ct);
                 }
             }
             _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
@@ -245,10 +245,13 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
             // handles alive. Mirrors the body of ShutdownAsync without awaiting the poll engine
             // (nothing has been subscribed yet at init time).
             foreach (var state in _devices.Values)
-            {
                 try { state.ProbeCts?.Cancel(); } catch { }
+            JoinProbeLoopsSync();
+            foreach (var state in _devices.Values)
+            {
                 state.ProbeCts?.Dispose();
                 state.ProbeCts = null;
+                state.ProbeTask = null;
                 state.DisposeRuntimes();
             }
             _devices.Clear();
@@ -266,15 +269,45 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
         await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
     }
 
+    /// <summary>Upper bound on how long shutdown waits for the probe loops to observe cancellation.</summary>
+    internal static readonly TimeSpan LoopJoinTimeout = TimeSpan.FromSeconds(5);
+
+    /// <summary>
+    ///     Synchronous bounded join of the (already-cancelled) probe loops for the sync teardown paths.
+    ///     The loop body uses <c>ConfigureAwait(false)</c> throughout, so blocking here cannot deadlock.
+    /// </summary>
+    private void JoinProbeLoopsSync()
+    {
+        var loops = _devices.Values.Select(d => d.ProbeTask).OfType<Task>().ToArray();
+        if (loops.Length == 0) return;
+        try { Task.WaitAll(loops, LoopJoinTimeout); }
+        catch (AggregateException) { /* loops observe cancellation; a fault is not actionable at teardown */ }
+    }
+
     /// <inheritdoc />
     public async Task ShutdownAsync(CancellationToken cancellationToken)
     {
         await _poll.DisposeAsync().ConfigureAwait(false);
+        // STAB-6: cancel the probe loops, JOIN them (bounded), then dispose the CTSs and runtimes.
+        // The loop used to be fire-and-forget, so one parked in a probe-runtime create/read could
+        // outlive shutdown and keep a libplctag handle alive past DisposeRuntimes.
+        foreach (var state in _devices.Values)
+            try { state.ProbeCts?.Cancel(); } catch { }
+        var loops = _devices.Values.Select(d => d.ProbeTask).OfType<Task>().ToList();
+        if (loops.Count > 0)
+        {
+            var all = Task.WhenAll(loops);
+            var finished = await Task.WhenAny(all, Task.Delay(LoopJoinTimeout, CancellationToken.None)).ConfigureAwait(false);
+            if (finished != all)
+                _logger.LogWarning(
+                    "AbLegacy shutdown: {Count} probe loop(s) did not stop within {Timeout}; proceeding with teardown. Driver={DriverInstanceId}",
+                    loops.Count, LoopJoinTimeout, _driverInstanceId);
+        }
         foreach (var state in _devices.Values)
         {
-            try { state.ProbeCts?.Cancel(); } catch { }
             state.ProbeCts?.Dispose();
             state.ProbeCts = null;
+            state.ProbeTask = null;
             state.DisposeRuntimes();
         }
         _devices.Clear();
@@ -892,10 +925,13 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
         // can't poison the rest of the disposal chain.
         try { _poll.DisposeAsync().ConfigureAwait(false).GetAwaiter().GetResult(); } catch { }
         foreach (var state in _devices.Values)
-        {
             try { state.ProbeCts?.Cancel(); } catch { }
+        JoinProbeLoopsSync();
+        foreach (var state in _devices.Values)
+        {
             state.ProbeCts?.Dispose();
             state.ProbeCts = null;
+            state.ProbeTask = null;
             state.DisposeRuntimes();
         }
         _devices.Clear();
@@ -1021,6 +1057,11 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
         /// </summary>
         public CancellationTokenSource? ProbeCts { get; set; }
 
+        /// <summary>
+        ///     Gets or sets the probe loop task, retained so shutdown can join it (STAB-6).
+        /// </summary>
+        public Task? ProbeTask { get; set; }
+
         /// <summary>
         ///     Gets or sets a value indicating whether the probe has been initialized.
         /// </summary>
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs
index 9fb93432..07fc913c 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs
@@ -277,7 +277,7 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                 {
                     state.ProbeCts = new CancellationTokenSource();
                     var ct = state.ProbeCts.Token;
-                    _ = Task.Run(() => ProbeLoopAsync(state, ct), ct);
+                    state.ProbeTask = Task.Run(() => ProbeLoopAsync(state, ct), ct);
                 }
             }
 
@@ -287,7 +287,7 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                 {
                     state.RecycleCts = new CancellationTokenSource();
                     var ct = state.RecycleCts.Token;
-                    _ = Task.Run(() => RecycleLoopAsync(state, ct), ct);
+                    state.RecycleTask = Task.Run(() => RecycleLoopAsync(state, ct), ct);
                 }
             }
 
@@ -305,7 +305,7 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                 {
                     state.FixedTreeCts = new CancellationTokenSource();
                     var ct = state.FixedTreeCts.Token;
-                    _ = Task.Run(() => FixedTreeLoopAsync(state, ct), ct);
+                    state.FixedTreeTask = Task.Run(() => FixedTreeLoopAsync(state, ct), ct);
                 }
             }
 
@@ -326,6 +326,9 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
     }
 
+    /// <summary>Upper bound on how long shutdown waits for the background loops to observe cancellation.</summary>
+    internal static readonly TimeSpan LoopJoinTimeout = TimeSpan.FromSeconds(5);
+
     /// <inheritdoc />
     public async Task ShutdownAsync(CancellationToken cancellationToken)
     {
@@ -335,22 +338,43 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             await proj.DisposeAsync().ConfigureAwait(false);
             _alarmProjection = null;
         }
+        // STAB-23: cancel every loop first, JOIN them (bounded), and only then dispose the CTSs and the
+        // client. Fire-and-forget loops used to outlive shutdown: one parked in EnsureConnectedAsync could
+        // finish its connect AFTER DisposeClient and leave a live socket on a shut-down driver.
+        var loops = new List<Task>();
         foreach (var state in _devices.Values)
         {
             // Cancel-then-dispose can race in tight shutdown loops; swallowing is intentional
             // but we now log the cause so a noisy shutdown leaves a Debug trace.
             try { state.ProbeCts?.Cancel(); }
             catch (Exception ex) { _logger.LogDebug(ex, "Cancelling probe CTS for {Host} failed", state.Options.HostAddress); }
-            state.ProbeCts?.Dispose();
-            state.ProbeCts = null;
             try { state.RecycleCts?.Cancel(); }
             catch (Exception ex) { _logger.LogDebug(ex, "Cancelling recycle CTS for {Host} failed", state.Options.HostAddress); }
-            state.RecycleCts?.Dispose();
-            state.RecycleCts = null;
             try { state.FixedTreeCts?.Cancel(); }
             catch (Exception ex) { _logger.LogDebug(ex, "Cancelling fixed-tree CTS for {Host} failed", state.Options.HostAddress); }
+            if (state.ProbeTask is { } pt) loops.Add(pt);
+            if (state.RecycleTask is { } rt) loops.Add(rt);
+            if (state.FixedTreeTask is { } ft) loops.Add(ft);
+        }
+        if (loops.Count > 0)
+        {
+            var all = Task.WhenAll(loops);
+            var finished = await Task.WhenAny(all, Task.Delay(LoopJoinTimeout, CancellationToken.None)).ConfigureAwait(false);
+            if (finished != all)
+                _logger.LogWarning("FOCAS shutdown: {Count} background loop(s) did not stop within {Timeout}; proceeding with teardown",
+                    loops.Count, LoopJoinTimeout);
+        }
+        foreach (var state in _devices.Values)
+        {
+            state.ProbeCts?.Dispose();
+            state.ProbeCts = null;
+            state.ProbeTask = null;
+            state.RecycleCts?.Dispose();
+            state.RecycleCts = null;
+            state.RecycleTask = null;
             state.FixedTreeCts?.Dispose();
             state.FixedTreeCts = null;
+            state.FixedTreeTask = null;
             state.DisposeClient();
         }
         _devices.Clear();
@@ -1575,6 +1599,12 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         public CancellationTokenSource? RecycleCts { get; set; }
         /// <summary>Gets or sets the cancellation token source for the fixed-tree loop.</summary>
         public CancellationTokenSource? FixedTreeCts { get; set; }
+        /// <summary>The probe loop task, retained so shutdown can join it (STAB-23).</summary>
+        public Task? ProbeTask { get; set; }
+        /// <summary>The recycle loop task, retained so shutdown can join it (STAB-23).</summary>
+        public Task? RecycleTask { get; set; }
+        /// <summary>The fixed-tree loop task, retained so shutdown can join it (STAB-23).</summary>
+        public Task? FixedTreeTask { get; set; }
         /// <summary>Gets or sets the fixed-tree cache for this device.</summary>
         public FocasFixedTreeCache? FixedTreeCache { get; set; }
 
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyShutdownJoinTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyShutdownJoinTests.cs
new file mode 100644
index 00000000..7b610135
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyShutdownJoinTests.cs
@@ -0,0 +1,54 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.PlcFamilies;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+/// <summary>
+///     STAB-6 / #602 — the probe loop was fire-and-forget, so shutdown cancelled and disposed while the
+///     loop could still be parked in a probe-runtime call that ignores cancellation, leaving its
+///     libplctag handle alive past teardown. Shutdown now joins the loop (bounded) first.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbLegacyShutdownJoinTests
+{
+    private sealed class ParkedInitTag(
+        AbLegacyTagCreateParams p, TaskCompletionSource entered, TaskCompletionSource release) : FakeAbLegacyTag(p)
+    {
+        public override async Task InitializeAsync(CancellationToken ct)
+        {
+            entered.TrySetResult();
+            await release.Task.ConfigureAwait(false); // deliberately ignores ct
+        }
+    }
+
+    /// <summary>Shutdown blocks on a probe loop parked in runtime init, then the loop disposes its probe runtime.</summary>
+    [Fact]
+    public async Task Shutdown_WaitsForProbeLoopParkedInInit_ThenProbeRuntimeIsDisposed()
+    {
+        var entered = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var release = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new ParkedInitTag(p, entered, release),
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0", AbLegacyPlcFamily.Slc500)],
+            Probe = new AbLegacyProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50), ProbeAddress = "S:0" },
+        }, "drv-join", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await entered.Task.WaitAsync(TimeSpan.FromSeconds(5));
+
+        var shutdown = drv.ShutdownAsync(CancellationToken.None);
+
+        await Task.Delay(300);
+        shutdown.IsCompleted.ShouldBeFalse("shutdown must join the probe loop parked in runtime init");
+
+        // Positive control: releasing the parked call lets shutdown finish, with the probe runtime released.
+        release.SetResult();
+        await shutdown.WaitAsync(TimeSpan.FromSeconds(5));
+
+        factory.Tags["S:0"].Disposed.ShouldBeTrue();
+    }
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasShutdownJoinTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasShutdownJoinTests.cs
new file mode 100644
index 00000000..6e07ae92
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasShutdownJoinTests.cs
@@ -0,0 +1,57 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+/// <summary>
+///     STAB-23 / #602 — the probe / recycle / fixed-tree loops were fire-and-forget, so shutdown
+///     cancelled and disposed while a loop could still be parked inside a connect that ignores
+///     cancellation, and that connect then completed against a shut-down driver. Shutdown now joins
+///     the loops (bounded) before disposing the client.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class FocasShutdownJoinTests
+{
+    /// <summary>A client whose connect parks until released and deliberately ignores the cancellation token.</summary>
+    private sealed class ParkedConnectClient(TaskCompletionSource entered, TaskCompletionSource release) : FakeFocasClient
+    {
+        public override async Task ConnectAsync(FocasHostAddress address, TimeSpan timeout, CancellationToken ct)
+        {
+            entered.TrySetResult();
+            await release.Task.ConfigureAwait(false);
+            await base.ConnectAsync(address, timeout, CancellationToken.None).ConfigureAwait(false);
+        }
+    }
+
+    /// <summary>Shutdown blocks on a probe loop parked in connect, then disposes the client that connect produced.</summary>
+    [Fact]
+    public async Task Shutdown_WaitsForLoopParkedInConnect_ThenDisposesItsClient()
+    {
+        var entered = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var release = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var factory = new FakeFocasClientFactory { Customise = () => new ParkedConnectClient(entered, release) };
+        var options = new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions("focas://10.0.0.5:8193")],
+            RawTags = [FocasRawTags.Entry(new FocasTagDefinition("R", "focas://10.0.0.5:8193", "R100", FocasDataType.Byte))],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50), Timeout = TimeSpan.FromSeconds(10) },
+        };
+        var drv = new FocasDriver(options, "focas-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await entered.Task.WaitAsync(TimeSpan.FromSeconds(5));
+
+        var shutdown = drv.ShutdownAsync(CancellationToken.None);
+
+        await Task.Delay(300);
+        shutdown.IsCompleted.ShouldBeFalse("shutdown must join the probe loop parked in connect");
+
+        // Positive control: releasing the parked connect lets shutdown finish.
+        release.SetResult();
+        await shutdown.WaitAsync(TimeSpan.FromSeconds(5));
+
+        factory.Clients.ShouldNotBeEmpty();
+        factory.Clients.ShouldAllBe(c => c.DisposeCount >= 1);
+        factory.Clients.ShouldAllBe(c => !c.IsConnected);
+    }
+}