diff --git a/src/ZB.MOM.WW.ScadaBridge.Security/LoginThrottle.cs b/src/ZB.MOM.WW.ScadaBridge.Security/LoginThrottle.cs
index 8d2a46bd..164d6b65 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Security/LoginThrottle.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Security/LoginThrottle.cs
@@ -41,6 +41,18 @@ public sealed class LoginThrottle
     /// </summary>
     public const int MaxTrackedKeys = 10_000;
 
+    /// <summary>Amortization cadence: a full prune sweep runs every Nth failure write
+    /// (plus immediately whenever the map exceeds <see cref="MaxTrackedKeys"/>), so a
+    /// spray does not pay an O(n) scan — and a possible full sort — on every failed
+    /// request (arch-review R2 N8). Lockout SEMANTICS are unaffected: IsLockedOut reads
+    /// window/lockout expiry directly and never depends on pruning.</summary>
+    internal const int PruneEveryNFailures = 64;
+
+    /// <summary>Diagnostics/test accessor: number of currently tracked keys.</summary>
+    internal int TrackedKeyCount => _entries.Count;
+
+    private int _failureWrites;
+
     private readonly TimeProvider _clock;
     private readonly IOptions<SecurityOptions> _options;
 
@@ -118,7 +130,13 @@ public sealed class LoginThrottle
                 return Arm(existing with { Count = existing.Count + 1 }, opts, now, lockout);
             });
 
-        Prune(now);
+        // Amortized prune (R2 N8): every Nth failure, or immediately when over the hard
+        // cap — the cap is the memory-safety invariant and must never wait for the cadence.
+        if (Interlocked.Increment(ref _failureWrites) % PruneEveryNFailures == 0
+            || _entries.Count > MaxTrackedKeys)
+        {
+            Prune(now);
+        }
     }
 
     /// <summary>
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Security.Tests/LoginThrottleTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Security.Tests/LoginThrottleTests.cs
index 47f26d09..91ed4b1c 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.Security.Tests/LoginThrottleTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.Security.Tests/LoginThrottleTests.cs
@@ -25,6 +25,12 @@ public class LoginThrottleTests
     private static LoginThrottle Create(TimeProvider clock, SecurityOptions? options = null) =>
         new(clock, Options.Create(options ?? new SecurityOptions()));
 
+    private static (LoginThrottle throttle, TestTimeProvider clock) Create()
+    {
+        var clock = new TestTimeProvider(Start);
+        return (Create(clock), clock);
+    }
+
     [Fact]
     public void FiveFailuresInWindow_LocksOut_ThenWindowExpiryUnlocks()
     {
@@ -110,4 +116,29 @@ public class LoginThrottleTests
 
         Assert.False(throttle.IsLockedOut("alice", "10.0.0.1"));
     }
+
+    // ── arch-review R2 N8: amortized prune ─────────────────────────────────────
+
+    [Fact]
+    public void KeySpray_MapStaysBounded_AtTheHardCap()
+    {
+        var (throttle, _) = Create(); // existing helper: throttle over a fake clock
+        // Spray far past the cap: the over-cap eviction must run on EVERY write once
+        // the cap is exceeded, regardless of the amortized cadence (arch-review R2 N8).
+        for (var i = 0; i < LoginThrottle.MaxTrackedKeys + 500; i++)
+            throttle.RecordFailure($"user{i}", "10.0.0.1");
+        Assert.True(throttle.TrackedKeyCount <= LoginThrottle.MaxTrackedKeys);
+    }
+
+    [Fact]
+    public void ExpiredEntries_AreEventuallySwept_OnTheAmortizedCadence()
+    {
+        var (throttle, clock) = Create();
+        throttle.RecordFailure("stale", "10.0.0.1");
+        clock.Advance(TimeSpan.FromMinutes(10)); // window fully expired
+        for (var i = 0; i < LoginThrottle.PruneEveryNFailures; i++)
+            throttle.RecordFailure($"fresh{i}", "10.0.0.1"); // cadence tick fires within these
+        Assert.False(throttle.IsLockedOut("stale", "10.0.0.1"));
+        Assert.True(throttle.TrackedKeyCount <= LoginThrottle.PruneEveryNFailures); // "stale" swept
+    }
 }