diff --git a/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json b/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json
index b1f16985..6790b898 100644
--- a/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json
+++ b/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json
@@ -50,7 +50,7 @@
     {
       "id": "B2.4",
       "subject": "B2: failing FocasFixedTreeConcurrencyTests (writer-vs-reader stress on the four DeviceState caches) \u2014 STAB-5 repro",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": []
     },
     {
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasFixedTreeConcurrencyTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasFixedTreeConcurrencyTests.cs
new file mode 100644
index 00000000..d608ba59
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasFixedTreeConcurrencyTests.cs
@@ -0,0 +1,99 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+/// <summary>
+///     Regression coverage for 05/STAB-5 — the four per-device fixed-tree caches
+///     (<c>LastFixedSnapshots</c>, <c>LastTimers</c>, <c>LastServoLoads</c>, <c>LastSpindleLoads</c>)
+///     are written by the background fixed-tree loop while <c>ReadAsync</c> callers read them
+///     concurrently. A plain <see cref="System.Collections.Generic.Dictionary{TKey,TValue}"/>
+///     throws / corrupts when a resize races a read — undefined behaviour. They must be
+///     <see cref="System.Collections.Concurrent.ConcurrentDictionary{TKey,TValue}"/>.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class FocasFixedTreeConcurrencyTests
+{
+    private static FocasDriver.DeviceState NewDeviceState() =>
+        new(new FocasHostAddress("10.0.0.5", FocasHostAddress.DefaultPort),
+            new FocasDeviceOptions("focas://10.0.0.5"));
+
+    /// <summary>
+    ///     Hammers concurrent writers (which force dictionary resizes) against concurrent readers on
+    ///     all four caches. A plain <c>Dictionary</c> is not concurrency-safe: its structural-mutation
+    ///     version check throws "operations that change non-concurrent collections must have exclusive
+    ///     access to the collection", and its lookup collision guard throws "concurrent operations
+    ///     are not supported" — deterministically under this load. A
+    ///     <c>ConcurrentDictionary</c> never throws.
+    /// </summary>
+    /// <remarks>
+    ///     Uses several writers, not one, so the fault is deterministic rather than timing-dependent
+    ///     UB: the fixed-tree caches are shared mutable state with zero synchronization, so ANY
+    ///     concurrent access is the defect — a driver that survives this survives the real
+    ///     writer-loop-vs-reader-callers race the finding cites.
+    /// </remarks>
+    [Fact]
+    public async Task Concurrent_access_to_fixed_tree_caches_never_throws()
+    {
+        var state = NewDeviceState();
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        var faults = new System.Collections.Concurrent.ConcurrentQueue<Exception>();
+        const int rounds = 40;
+        const int growth = 4_000;
+
+        void Writer(int seed)
+        {
+            try
+            {
+                for (var round = 0; round < rounds && !cts.IsCancellationRequested; round++)
+                {
+                    state.LastFixedSnapshots.Clear();
+                    state.LastServoLoads.Clear();
+                    state.LastSpindleLoads.Clear();
+                    for (var i = 0; i < growth; i++)
+                    {
+                        // Distinct keys force repeated dictionary growth (resize = the race window);
+                        // the Clear() each round keeps the caches churning through resizes.
+                        state.LastFixedSnapshots[$"Axes/A{seed}_{i}/AbsolutePosition"] = i;
+                        state.LastServoLoads[$"Servo{seed}_{i}"] = i;
+                        state.LastSpindleLoads[seed * growth + i] = i;
+                        state.LastTimers[(FocasTimerKind)(i % 4)] = new FocasTimer((FocasTimerKind)(i % 4), i, i);
+                    }
+                }
+            }
+            catch (Exception ex) { faults.Enqueue(ex); }
+        }
+
+        void Reader()
+        {
+            try
+            {
+                while (!cts.IsCancellationRequested && faults.IsEmpty)
+                {
+                    for (var i = 0; i < growth; i++)
+                    {
+                        state.LastFixedSnapshots.TryGetValue($"Axes/A0_{i}/AbsolutePosition", out _);
+                        state.LastServoLoads.TryGetValue($"Servo0_{i}", out _);
+                        state.LastSpindleLoads.TryGetValue(i, out _);
+                        state.LastTimers.TryGetValue((FocasTimerKind)(i % 4), out _);
+                    }
+                }
+            }
+            catch (Exception ex) { faults.Enqueue(ex); }
+        }
+
+        var tasks = new List<Task>
+        {
+            Task.Run(() => Writer(0)),
+            Task.Run(() => Writer(1)),
+            Task.Run(() => Writer(2)),
+            Task.Run(Reader),
+            Task.Run(Reader),
+        };
+        await Task.WhenAll(tasks);
+
+        faults.ShouldBeEmpty(
+            faults.TryPeek(out var first) ? $"concurrent cache access threw: {first}" : "");
+    }
+}