diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianRetention.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianRetention.cs
index c8bc8e00..24816415 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianRetention.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer/SqlServerHistorianRetention.cs
@@ -42,6 +42,9 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
     /// <summary>Test seam for <see cref="SqlConnection.ClearPool"/>.</summary>
     internal Action<SqlConnection> ClearPool { get; set; } = SqlConnection.ClearPool;
 
+    /// <summary>Test seam: the release statement. Tests point it at a failing statement to exercise the pool-clear fallback.</summary>
+    internal string ReleaseSql { get; set; } = "EXEC sp_releaseapplock @Resource = @res, @LockOwner = 'Session';";
+
     public async Task<IReadOnlyList<RetentionPurgeResult>> PurgeAsync(CancellationToken ct)
     {
         await _bootstrap.EnsureAsync(ct).ConfigureAwait(false);
@@ -137,8 +140,7 @@ internal sealed class SqlServerHistorianRetention : IHistorianRetention
     {
         try
         {
-            await using var cmd = _conn.CreateCommand(conn,
-                "EXEC sp_releaseapplock @Resource = @res, @LockOwner = 'Session';");
+            await using var cmd = _conn.CreateCommand(conn, ReleaseSql);
             cmd.Parameters.Add(new SqlParameter("@res", SqlDbType.NVarChar, 255) { Value = LockResource });
             await cmd.ExecuteNonQueryAsync(CancellationToken.None).ConfigureAwait(false);
         }
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer.IntegrationTests/RetentionTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer.IntegrationTests/RetentionTests.cs
index 046869d6..50870b4a 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer.IntegrationTests/RetentionTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.SqlServer.IntegrationTests/RetentionTests.cs
@@ -178,6 +178,80 @@ public sealed class RetentionTests(SqlHistorianFixture fx)
         (await LockIsFreeAsync(s)).ShouldBeTrue();
     }
 
+    [Fact]
+    public async Task Cancelling_mid_command_while_the_delete_is_blocked_still_releases_the_applock()
+    {
+        fx.SkipIfUnavailable();
+        var (c, b, _, s) = Make();
+        await SeedAsync(b, s);
+
+        // A second session holds an exclusive table lock, so the purger's first DELETE blocks server-side.
+        await using var holder = new SqlConnection(fx.ConnectionString);
+        await holder.OpenAsync();
+        await using var tx = (SqlTransaction)await holder.BeginTransactionAsync();
+        await using (var cmd = holder.CreateCommand())
+        {
+            cmd.Transaction = tx;
+            cmd.CommandText = $"SELECT COUNT(*) FROM [{s}].Sample WITH (TABLOCKX, HOLDLOCK)";
+            await cmd.ExecuteScalarAsync();
+        }
+
+        using var cts = new CancellationTokenSource();
+        var purger = new SqlServerHistorianRetention(c, b, TimeSpan.FromHours(1), 30, 4000, time: new FixedTime(Now));
+        var purge = purger.PurgeAsync(cts.Token);
+        await Task.Delay(500);
+        purge.IsCompleted.ShouldBeFalse("the DELETE must be blocked behind the holder's table lock");
+        cts.Cancel();
+
+        // SqlClient may surface the attention as a SqlException rather than OperationCanceledException.
+        await Should.ThrowAsync<Exception>(() => purge);
+
+        await tx.RollbackAsync();
+        (await LockIsFreeAsync(s)).ShouldBeTrue();
+        (await CountAsync($"SELECT COUNT(*) FROM [{s}].Sample")).ShouldBe(21);
+    }
+
+    [Fact]
+    public async Task A_failing_release_falls_back_to_clearing_the_pool_so_the_applock_is_freed()
+    {
+        fx.SkipIfUnavailable();
+        var (c, b, _, s) = Make();
+        await SeedAsync(b, s);
+        var purger = new SqlServerHistorianRetention(c, b, TimeSpan.FromHours(1), 30, 4000, time: new FixedTime(Now))
+        {
+            ReleaseSql = "EXEC sp_this_procedure_does_not_exist;",
+        };
+
+        await purger.PurgeAsync(CancellationToken.None);
+
+        (await LockIsFreeAsync(s)).ShouldBeTrue();
+    }
+
+    [Fact]
+    public async Task Falsification_control_a_failing_release_with_a_no_op_ClearPool_leaves_the_applock_held()
+    {
+        fx.SkipIfUnavailable();
+        var (c, b, _, s) = Make();
+        await SeedAsync(b, s);
+        var purger = new SqlServerHistorianRetention(c, b, TimeSpan.FromHours(1), 30, 4000, time: new FixedTime(Now))
+        {
+            ReleaseSql = "EXEC sp_this_procedure_does_not_exist;",
+            ClearPool = _ => { },
+        };
+
+        await purger.PurgeAsync(CancellationToken.None);
+
+        try
+        {
+            // The pooled session still owns the Session-scoped lock: proves the fallback test above is not vacuous.
+            (await LockIsFreeAsync(s)).ShouldBeFalse();
+        }
+        finally
+        {
+            SqlConnection.ClearAllPools(); // drop the leaked session so the lock does not outlive the test
+        }
+    }
+
     [Fact]
     public async Task A_throwing_purge_releases_the_applock()
     {