diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbHistorianDataUpdater.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbHistorianDataUpdater.cs
new file mode 100644
index 00000000..c818e8b0
--- /dev/null
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbHistorianDataUpdater.cs
@@ -0,0 +1,258 @@
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Npgsql;
+using NpgsqlTypes;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Sql.Common;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb;
+
+/// <summary>
+///     OPC UA HistoryUpdate against the QuestDB sample table (spec §2.3). Every operation is an <c>INSERT</c>: the
+///     table's <c>DEDUP UPSERT KEYS(ts, tag)</c> performs the replace, and a delete writes a tombstone
+///     (<c>deleted = true</c>, NULL values, <c>BadNoData</c>). QuestDB <c>UPDATE</c>/<c>DELETE</c> are never issued.
+///     <para>Every status comes from <see cref="HistoryUpdatePlanner"/>, so codes equal the SQL Server backend's.
+///     "Exists" means present and not deleted, so Insert over a tombstone succeeds (<c>GoodEntryInserted</c>) while
+///     Replace and DeleteAtTime treat a tombstone as no entry. Existence is read with one range query and filtered
+///     client-side (never an IN list).</para>
+///     <para><b>Pre-1970 (RC1):</b> QuestDB cannot store such a designated timestamp. Insert/Replace/Update of such an
+///     item reports <c>BadOutOfRange</c> and sends nothing (a null/<c>MinValue</c> timestamp is still
+///     <c>BadInvalidTimestamp</c>); DeleteAtTime reports <c>BadNoEntryExists</c>; a DeleteRaw window may start before
+///     1970 (read bounds are not clamped).</para>
+///     <para><b>Read-your-writes:</b> after writing, <see cref="QuestDbWalVisibility"/> waits for the rows to become
+///     visible, bounded by <c>VisibilityTimeout</c>; a timeout only logs a Warning, never changes a code.</para>
+/// </summary>
+internal sealed class QuestDbHistorianDataUpdater : IHistorianDataUpdater
+{
+    /// <summary>Rows per INSERT and per DeleteRaw page (T0 Q2: 9,000 parameters, far under the 32,767 limit).</summary>
+    internal const int BatchSize = 1000;
+
+    private const long ResolutionTicks = 10;
+    private readonly QuestDbConnection _conn;
+    private readonly QuestDbTagCatalog _tags;
+    private readonly QuestDbWalVisibility _wal;
+    private readonly ILogger _log;
+
+    /// <summary>Creates the updater.</summary>
+    /// <param name="conn">Connection helper.</param>
+    /// <param name="tags">Tag catalog (R7: the only known-tag authority).</param>
+    /// <param name="wal">WAL visibility wait (R5).</param>
+    /// <param name="log">Optional logger.</param>
+    public QuestDbHistorianDataUpdater(QuestDbConnection conn, QuestDbTagCatalog tags, QuestDbWalVisibility wal,
+        ILogger? log = null)
+    {
+        _conn = conn;
+        _tags = tags;
+        _wal = wal;
+        _log = log ?? NullLogger.Instance;
+    }
+
+    /// <inheritdoc/>
+    public async Task<IReadOnlyList<uint>> UpdateDataAsync(string tag, HistoryUpdateMode mode,
+        IReadOnlyList<HistorizationValue> values, CancellationToken ct)
+    {
+        ArgumentNullException.ThrowIfNull(tag);
+        ArgumentNullException.ThrowIfNull(values);
+
+        // RC1: a valid-timestamp item before 1970 can never be stored; refuse it and plan the rest.
+        var final = new uint[values.Count];
+        var kept = new List<int>(values.Count);
+        for (var i = 0; i < values.Count; i++)
+        {
+            if (values[i].TimestampUtc is { } ts && ts != DateTime.MinValue && !QuestDbConnection.IsStorable(ts))
+                final[i] = HistorianStatusCodes.BadOutOfRange;
+            else
+                kept.Add(i);
+        }
+
+        var plan = HistoryUpdatePlanner.Plan(tag, mode, kept.Select(i => values[i]).ToList(), ResolutionTicks);
+        IReadOnlyList<uint> planned;
+        if (plan.Winners.Count == 0)
+        {
+            planned = HistoryUpdatePlanner.Complete(plan, mode, new Dictionary<long, UpsertOutcome>());
+        }
+        else
+        {
+            var known = mode == HistoryUpdateMode.Replace
+                ? await _tags.ExistsAsync(tag, ct).ConfigureAwait(false)
+                : await EnsureAsync(tag, plan.Winners[0].Row.Value.Kind, ct).ConfigureAwait(false);
+            var outcome = known
+                ? await ApplyAsync(tag, mode, plan.Winners.Select(w => w.Row).ToList(), ct).ConfigureAwait(false)
+                : new Dictionary<long, UpsertOutcome>();
+            planned = HistoryUpdatePlanner.Complete(plan, mode, outcome);
+        }
+        for (var k = 0; k < kept.Count; k++) final[kept[k]] = planned[k];
+        return final;
+    }
+
+    /// <inheritdoc/>
+    public async Task<uint> DeleteRawAsync(string tag, DateTime startUtc, DateTime endUtc, CancellationToken ct)
+    {
+        ArgumentNullException.ThrowIfNull(tag);
+        if (!HistorianTagNames.IsValid(tag)) return HistorianStatusCodes.BadInvalidArgument;
+        if (!await _tags.ExistsAsync(tag, ct).ConfigureAwait(false))
+            return HistoryUpdatePlanner.DeleteRawResult(tagKnown: false, 0);
+
+        var cursor = QuestDbConnection.Round(startUtc);
+        var end = QuestDbConnection.Round(endUtc);
+        long total = 0;
+        while (cursor < end)
+        {
+            var page = new List<DateTime>(BatchSize);
+            await using (var conn = await _conn.OpenAsync(ct).ConfigureAwait(false))
+            await using (var cmd = _conn.CreateCommand(conn,
+                             $"SELECT ts FROM {_conn.SampleTable} WHERE tag = $1 AND ts >= $2 AND ts < $3 AND NOT deleted ORDER BY ts LIMIT {BatchSize}"))
+            {
+                QuestDbConnection.BindText(cmd, tag);
+                QuestDbConnection.BindTimestamp(cmd, cursor);
+                QuestDbConnection.BindTimestamp(cmd, end);
+                await using var rd = await cmd.ExecuteReaderAsync(ct).ConfigureAwait(false);
+                while (await rd.ReadAsync(ct).ConfigureAwait(false)) page.Add(QuestDbConnection.ReadUtc(rd, 0));
+            }
+            if (page.Count == 0) break;
+            await WriteTombstonesAsync(tag, page, ct).ConfigureAwait(false);
+            // The wait between pages is required: without it the next page could re-read rows just tombstoned.
+            await _wal.WaitAsync(_conn.SampleTable, ct).ConfigureAwait(false);
+            total += page.Count;
+            if (page.Count < BatchSize) break;
+            cursor = page[^1].AddTicks(ResolutionTicks);
+        }
+        return HistoryUpdatePlanner.DeleteRawResult(tagKnown: true, total);
+    }
+
+    /// <inheritdoc/>
+    public async Task<IReadOnlyList<uint>> DeleteAtTimeAsync(string tag, IReadOnlyList<DateTime> timesUtc, CancellationToken ct)
+    {
+        ArgumentNullException.ThrowIfNull(tag);
+        ArgumentNullException.ThrowIfNull(timesUtc);
+        if (!HistorianTagNames.IsValid(tag))
+        {
+            var refused = new uint[timesUtc.Count];
+            Array.Fill(refused, HistorianStatusCodes.BadInvalidArgument);
+            return refused;
+        }
+
+        // M-2: round BEFORE both the delete and the comparison, so a sub-µs request time matches the row it deleted.
+        // MinValue stays as is: the planner turns it into BadInvalidTimestamp.
+        var rounded = timesUtc.Select(t => t == DateTime.MinValue ? t : QuestDbConnection.Round(t)).ToList();
+        var candidates = new HashSet<long>();
+        foreach (var t in rounded)
+            if (t != DateTime.MinValue && t >= DateTime.UnixEpoch) candidates.Add(t.Ticks);   // pre-1970 can hold no row
+        var deleted = new HashSet<long>();
+        if (candidates.Count == 0 || !await _tags.ExistsAsync(tag, ct).ConfigureAwait(false))
+            return HistoryUpdatePlanner.DeleteAtTimeResults(rounded, deleted, tagKnown: false);
+
+        var live = await ReadLiveAsync(tag, candidates.Min(), candidates.Max(), ct).ConfigureAwait(false);
+        var toDelete = candidates.Where(live.Contains).OrderBy(x => x).ToList();
+        foreach (var chunk in toDelete.Chunk(BatchSize))
+            await WriteTombstonesAsync(tag, chunk.Select(t => new DateTime(t, DateTimeKind.Utc)).ToList(), ct).ConfigureAwait(false);
+        if (toDelete.Count > 0) await _wal.WaitAsync(_conn.SampleTable, ct).ConfigureAwait(false);
+        deleted.UnionWith(toDelete);
+        return HistoryUpdatePlanner.DeleteAtTimeResults(rounded, deleted, tagKnown: true);
+    }
+
+    private async Task<bool> EnsureAsync(string tag, HistorianValueKind kind, CancellationToken ct)
+    {
+        await _tags.EnsureAsync(tag, HistorianDataTypes.FromKind(kind), ct).ConfigureAwait(false);
+        return true;
+    }
+
+    private async Task<Dictionary<long, UpsertOutcome>> ApplyAsync(string tag, HistoryUpdateMode mode,
+        List<HistorianSampleRow> rows, CancellationToken ct)
+    {
+        // Key = the planner's tick (what Complete looks up); the stored time is Round(of the same row).
+        var live = await ReadLiveAsync(tag, rows.Min(r => QuestDbConnection.Round(r.TimeUtc).Ticks),
+            rows.Max(r => QuestDbConnection.Round(r.TimeUtc).Ticks), ct).ConfigureAwait(false);
+        var outcome = new Dictionary<long, UpsertOutcome>();
+        var toWrite = new List<HistorianSampleRow>();
+        foreach (var r in rows)
+        {
+            var present = live.Contains(QuestDbConnection.Round(r.TimeUtc).Ticks);
+            if (mode == HistoryUpdateMode.Insert && present) continue;
+            if (mode == HistoryUpdateMode.Replace && !present) continue;
+            outcome[r.TimeUtc.Ticks] = present ? UpsertOutcome.Replaced : UpsertOutcome.Inserted;
+            toWrite.Add(r);
+        }
+        foreach (var chunk in toWrite.Chunk(BatchSize))
+            await WriteRowsAsync(tag, chunk, ct).ConfigureAwait(false);
+        if (toWrite.Count > 0) await _wal.WaitAsync(_conn.SampleTable, ct).ConfigureAwait(false);
+        return outcome;
+    }
+
+    /// <summary>Ticks of the live (present, not deleted) rows with <c>ts</c> in <c>[minTicks, maxTicks]</c>: one range query.</summary>
+    private async Task<HashSet<long>> ReadLiveAsync(string tag, long minTicks, long maxTicks, CancellationToken ct)
+    {
+        var live = new HashSet<long>();
+        await using var conn = await _conn.OpenAsync(ct).ConfigureAwait(false);
+        await using var cmd = _conn.CreateCommand(conn,
+            $"SELECT ts FROM {_conn.SampleTable} WHERE tag = $1 AND ts >= $2 AND ts <= $3 AND NOT deleted");
+        QuestDbConnection.BindText(cmd, tag);
+        QuestDbConnection.BindTimestamp(cmd, new DateTime(minTicks, DateTimeKind.Utc));
+        QuestDbConnection.BindTimestamp(cmd, new DateTime(maxTicks, DateTimeKind.Utc));
+        await using var rd = await cmd.ExecuteReaderAsync(ct).ConfigureAwait(false);
+        while (await rd.ReadAsync(ct).ConfigureAwait(false)) live.Add(QuestDbConnection.ReadUtc(rd, 0).Ticks);
+        return live;
+    }
+
+    private Task WriteRowsAsync(string tag, IReadOnlyList<HistorianSampleRow> rows, CancellationToken ct) =>
+        InsertAsync(rows.Count, ct, (cmd, i) =>
+        {
+            var row = rows[i];
+            var c = SampleColumnMapper.ToColumns(row.Value);
+            QuestDbConnection.BindTimestamp(cmd, row.TimeUtc, clampToEpoch: true);
+            QuestDbConnection.BindText(cmd, tag);
+            QuestDbConnection.BindByte(cmd, c.ValueKind);
+            Typed(cmd, NpgsqlDbType.Double, c.NumValue);
+            Typed(cmd, NpgsqlDbType.Bigint, c.IntValue);
+            Typed(cmd, NpgsqlDbType.Varchar, c.StrValue);
+            QuestDbConnection.BindLong(cmd, SampleColumnMapper.ToLongStatus(row.StatusCode));
+            Typed(cmd, NpgsqlDbType.Boolean, false);
+        });
+
+    private Task WriteTombstonesAsync(string tag, IReadOnlyList<DateTime> times, CancellationToken ct) =>
+        InsertAsync(times.Count, ct, (cmd, i) =>
+        {
+            QuestDbConnection.BindTimestamp(cmd, times[i], clampToEpoch: true);
+            QuestDbConnection.BindText(cmd, tag);
+            QuestDbConnection.BindByte(cmd, (byte)HistorianValueKind.Empty);
+            Typed(cmd, NpgsqlDbType.Double, null);
+            Typed(cmd, NpgsqlDbType.Bigint, null);
+            Typed(cmd, NpgsqlDbType.Varchar, null);
+            QuestDbConnection.BindLong(cmd, SampleColumnMapper.ToLongStatus(HistorianStatusCodes.BadNoData));
+            Typed(cmd, NpgsqlDbType.Boolean, true);
+        });
+
+    /// <summary>Multi-row INSERT; <paramref name="bindRow"/> adds the first eight parameters of a row, this adds server_ts.</summary>
+    private async Task InsertAsync(int count, CancellationToken ct, Action<NpgsqlCommand, int> bindRow)
+    {
+        if (count == 0) return;
+        await using var conn = await _conn.OpenAsync(ct).ConfigureAwait(false);
+        await using var cmd = _conn.CreateCommand(conn, "");
+        var now = DateTime.UtcNow;
+        var sb = new System.Text.StringBuilder(
+            $"INSERT INTO {_conn.SampleTable} (ts, tag, kind, num_value, int_value, str_value, status, deleted, server_ts) VALUES ");
+        for (var i = 0; i < count; i++)
+        {
+            bindRow(cmd, i);
+            QuestDbConnection.BindTimestamp(cmd, now);
+            var b = i * 9;
+            if (i > 0) sb.Append(',');
+            sb.Append('(');
+            for (var p = 1; p <= 9; p++) sb.Append(p > 1 ? "," : "").Append('$').Append(b + p);
+            sb.Append(')');
+        }
+        cmd.CommandText = sb.ToString();
+        try
+        {
+            await cmd.ExecuteNonQueryAsync(ct).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (ex is not OperationCanceledException)
+        {
+            _log.LogWarning(ex, "QuestDB HistoryUpdate INSERT of {Count} rows into {Table} failed.", count, _conn.SampleTable);
+            throw;
+        }
+    }
+
+    private static void Typed(NpgsqlCommand cmd, NpgsqlDbType type, object? value) =>
+        cmd.Parameters.Add(new NpgsqlParameter { NpgsqlDbType = type, Value = value ?? DBNull.Value });
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TombstoneTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TombstoneTests.cs
new file mode 100644
index 00000000..2f3f53e3
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TombstoneTests.cs
@@ -0,0 +1,50 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Sql.Common;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests;
+
+/// <summary>Soft-delete behaviour. The writer-redelivery case lives in T7 (<c>TombstoneTests.Redelivery.cs</c>).</summary>
+[Collection(QuestDbCollection.Name)]
+public sealed class TombstoneTests(QuestDbFixture fx)
+{
+    private static readonly DateTime T0 = new(2026, 3, 1, 12, 0, 0, DateTimeKind.Utc);
+
+    [Fact]
+    public async Task Delete_then_raw_read_hides_the_row_and_leaves_a_NULL_valued_tombstone()
+    {
+        fx.SkipIfUnavailable();
+        var rig = UpdateTests.MakeRig(fx);
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert,
+            [UpdateTests.V(T0, 1), UpdateTests.V(T0.AddSeconds(1), 2)], default);
+        (await rig.Updater.DeleteAtTimeAsync("t", [T0], default)).ShouldBe([HistorianStatusCodes.Good]);
+
+        (await UpdateTests.RowsAsync(rig, "t", liveOnly: true)).Select(r => r.Num).ShouldBe([2d]);
+        var all = await UpdateTests.RowsAsync(rig, "t");
+        all.Count.ShouldBe(2);                       // the tombstone replaced the row in place (dedup upsert)
+        var tomb = all[0];
+        tomb.Deleted.ShouldBeTrue();
+        tomb.Num.ShouldBeNull();
+        tomb.Kind.ShouldBe((byte)0);
+        tomb.Status.ShouldBe((long)HistorianStatusCodes.BadNoData);
+    }
+
+    [Fact]
+    public async Task Insert_over_a_tombstone_reports_GoodEntryInserted_and_the_row_is_visible()
+    {
+        fx.SkipIfUnavailable();
+        var rig = UpdateTests.MakeRig(fx);
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [UpdateTests.V(T0, 1)], default);
+        await rig.Updater.DeleteAtTimeAsync("t", [T0], default);
+
+        (await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Replace, [UpdateTests.V(T0, 5)], default))
+            .ShouldBe([HistorianStatusCodes.BadNoEntryExists]);   // a tombstone is no entry for Replace
+        (await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [UpdateTests.V(T0, 9)], default))
+            .ShouldBe([HistorianStatusCodes.GoodEntryInserted]);
+        var live = await UpdateTests.RowsAsync(rig, "t", liveOnly: true);
+        live.Count.ShouldBe(1);
+        live[0].Num.ShouldBe(9d);
+        live[0].Deleted.ShouldBeFalse();
+    }
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/UpdateTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/UpdateTests.cs
new file mode 100644
index 00000000..c77f3d64
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/UpdateTests.cs
@@ -0,0 +1,235 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Sql.Common;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests;
+
+/// <summary>QuestDB-specific checks of <see cref="QuestDbHistorianDataUpdater"/> (the literal codes run through conformance in T7).</summary>
+[Collection(QuestDbCollection.Name)]
+public sealed class UpdateTests(QuestDbFixture fx)
+{
+    private static readonly DateTime T0 = new(2026, 3, 1, 12, 0, 0, DateTimeKind.Utc);   // µs-aligned
+    private const uint Good = HistorianStatusCodes.Good;
+
+    internal sealed record Rig(QuestDbConnection Conn, QuestDbHistorianDataUpdater Updater, QuestDbTagCatalog Tags);
+
+    internal static Rig MakeRig(QuestDbFixture fx, TimeSpan? visibility = null)
+    {
+        var o = fx.Options(fx.UniquePrefix());
+        o.VisibilityTimeout = visibility ?? o.VisibilityTimeout;
+        var c = new QuestDbConnection(o);
+        var wal = new QuestDbWalVisibility(c, o.VisibilityTimeout);
+        var tags = new QuestDbTagCatalog(c, new QuestDbSchemaBootstrapper(c, o), wal);
+        return new Rig(c, new QuestDbHistorianDataUpdater(c, tags, wal), tags);
+    }
+
+    internal static HistorizationValue V(DateTime? t, double v, uint status = Good) =>
+        new(t, HistorianValue.FromDouble(v), status);
+
+    internal sealed record Row(DateTime Ts, byte Kind, double? Num, long? Status, bool Deleted);
+
+    /// <summary>Every stored row for the tag (tombstones included), straight from the table.</summary>
+    internal static async Task<List<Row>> RowsAsync(Rig rig, string tag, bool liveOnly = false)
+    {
+        await using var conn = await rig.Conn.OpenAsync(default);
+        await using var cmd = rig.Conn.CreateCommand(conn,
+            $"SELECT ts, kind, num_value, status, deleted FROM {rig.Conn.SampleTable} WHERE tag = $1 {(liveOnly ? "AND NOT deleted" : "")} ORDER BY ts");
+        QuestDbConnection.BindText(cmd, tag);
+        var rows = new List<Row>();
+        await using var r = await cmd.ExecuteReaderAsync();
+        while (await r.ReadAsync())
+            rows.Add(new Row(QuestDbConnection.ReadUtc(r, 0), QuestDbConnection.ReadKind(r, 1),
+                r.IsDBNull(2) ? null : r.GetDouble(2), r.IsDBNull(3) ? null : r.GetInt64(3), r.GetBoolean(4)));
+        return rows;
+    }
+
+    internal static async Task<long> TagRowsAsync(Rig rig, string tag)
+    {
+        await using var conn = await rig.Conn.OpenAsync(default);
+        await using var cmd = rig.Conn.CreateCommand(conn, $"SELECT count() FROM {rig.Conn.TagTable} WHERE name = $1");
+        QuestDbConnection.BindText(cmd, tag);
+        return Convert.ToInt64(await cmd.ExecuteScalarAsync());
+    }
+
+    [Fact]
+    public async Task Insert_writes_absent_and_refuses_existing_and_is_visible_immediately()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        (await rig.Updater.UpdateDataAsync("t1", HistoryUpdateMode.Insert, [V(T0, 1)], default))
+            .ShouldBe([HistorianStatusCodes.GoodEntryInserted]);
+        var codes = await rig.Updater.UpdateDataAsync("t1", HistoryUpdateMode.Insert,
+            [V(T0, 99), V(T0.AddSeconds(1), 2)], default);
+        codes.ShouldBe([HistorianStatusCodes.BadEntryExists, HistorianStatusCodes.GoodEntryInserted]);
+        var rows = await RowsAsync(rig, "t1");   // read right after return: read-your-writes
+        rows.Select(r => r.Num).ShouldBe([1d, 2d]);
+        (await TagRowsAsync(rig, "t1")).ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Replace_overwrites_present_refuses_missing_and_never_creates_the_tag()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        (await rig.Updater.UpdateDataAsync("nope", HistoryUpdateMode.Replace, [V(T0, 1)], default))
+            .ShouldBe([HistorianStatusCodes.BadNoEntryExists]);
+        (await TagRowsAsync(rig, "nope")).ShouldBe(0);
+        (await RowsAsync(rig, "nope")).ShouldBeEmpty();
+
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T0, 1)], default);
+        var codes = await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Replace,
+            [V(T0, 5), V(T0.AddSeconds(1), 6)], default);
+        codes.ShouldBe([HistorianStatusCodes.GoodEntryReplaced, HistorianStatusCodes.BadNoEntryExists]);
+        (await RowsAsync(rig, "t")).Select(r => r.Num).ShouldBe([5d]);
+    }
+
+    [Fact]
+    public async Task Update_upserts_both_ways_and_last_duplicate_wins()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T0, 1)], default);
+        var codes = await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Update,
+            [V(T0, 2), V(T0.AddSeconds(1), 3), V(T0.AddSeconds(1).AddTicks(3), 4)], default);
+        codes.ShouldBe([HistorianStatusCodes.GoodEntryReplaced, HistorianStatusCodes.GoodEntryInserted,
+            HistorianStatusCodes.GoodEntryInserted]);
+        (await RowsAsync(rig, "t")).Select(r => r.Num).ShouldBe([2d, 4d]);
+    }
+
+    [Fact]
+    public async Task Same_microsecond_duplicates_collapse_before_the_database_sees_them()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        // offsets 0 and 3 ticks sit inside one 10-tick window => the same stored µs
+        var codes = await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert,
+            [V(T0, 1), V(T0.AddTicks(3), 2)], default);
+        codes.ShouldBe([HistorianStatusCodes.GoodEntryInserted, HistorianStatusCodes.BadEntryExists]);
+        (await RowsAsync(rig, "t")).Select(r => r.Num).ShouldBe([1d]);
+    }
+
+    [Fact]
+    public async Task Invalid_items_get_the_planners_codes_and_the_rest_proceed()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        var codes = await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert,
+        [
+            V(null, 1), V(DateTime.MinValue, 1), V(T0, double.NaN),
+            new HistorizationValue(T0.AddSeconds(1), HistorianValue.Empty, Good),
+            V(T0.AddSeconds(2), 7),
+        ], default);
+        codes.ShouldBe([HistorianStatusCodes.BadInvalidTimestamp, HistorianStatusCodes.BadInvalidTimestamp,
+            HistorianStatusCodes.BadInvalidArgument, HistorianStatusCodes.BadInvalidArgument,
+            HistorianStatusCodes.GoodEntryInserted]);
+        (await RowsAsync(rig, "t")).Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Over_long_tag_is_refused_and_nothing_is_written()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        var tag = new string('x', HistorianTagNames.MaxLength + 1);
+        (await rig.Updater.UpdateDataAsync(tag, HistoryUpdateMode.Insert, [V(T0, 1)], default))
+            .ShouldBe([HistorianStatusCodes.BadInvalidArgument]);
+        (await rig.Updater.DeleteAtTimeAsync(tag, [T0], default)).ShouldBe([HistorianStatusCodes.BadInvalidArgument]);
+        (await rig.Updater.DeleteRawAsync(tag, T0, T0.AddHours(1), default)).ShouldBe(HistorianStatusCodes.BadInvalidArgument);
+    }
+
+    [Fact]
+    public async Task Pre_1970_items_are_BadOutOfRange_and_nothing_is_sent()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        var pre = new DateTime(1969, 12, 31, 23, 59, 59, DateTimeKind.Utc);
+        foreach (var mode in new[] { HistoryUpdateMode.Insert, HistoryUpdateMode.Replace, HistoryUpdateMode.Update })
+        {
+            var codes = await rig.Updater.UpdateDataAsync("t", mode, [V(pre, 1), V(DateTime.MinValue, 1), V(T0, 2)], default);
+            codes[0].ShouldBe(HistorianStatusCodes.BadOutOfRange);
+            codes[1].ShouldBe(HistorianStatusCodes.BadInvalidTimestamp);
+        }
+        // Pre-1970 only: the whole batch is refused, no tag row is created, no INSERT would succeed anyway.
+        (await rig.Updater.UpdateDataAsync("only-pre", HistoryUpdateMode.Update, [V(pre, 1)], default))
+            .ShouldBe([HistorianStatusCodes.BadOutOfRange]);
+        (await TagRowsAsync(rig, "only-pre")).ShouldBe(0);
+        (await rig.Updater.DeleteAtTimeAsync("t", [pre], default)).ShouldBe([HistorianStatusCodes.BadNoEntryExists]);
+    }
+
+    [Fact]
+    public async Task DeleteAtTime_tombstones_live_rows_and_reports_per_time_codes()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T0, 1), V(T0.AddSeconds(1), 2)], default);
+        var codes = await rig.Updater.DeleteAtTimeAsync("t",
+            [T0, T0.AddSeconds(5), T0, DateTime.MinValue], default);
+        codes.ShouldBe([Good, HistorianStatusCodes.BadNoEntryExists, HistorianStatusCodes.BadNoEntryExists,
+            HistorianStatusCodes.BadInvalidTimestamp]);
+        (await RowsAsync(rig, "t", liveOnly: true)).Select(r => r.Num).ShouldBe([2d]);
+        // a tombstone is not a live row: deleting it again finds nothing
+        (await rig.Updater.DeleteAtTimeAsync("t", [T0], default)).ShouldBe([HistorianStatusCodes.BadNoEntryExists]);
+        (await rig.Updater.DeleteAtTimeAsync("unknown", [T0], default)).ShouldBe([HistorianStatusCodes.BadNoEntryExists]);
+    }
+
+    [Fact]
+    public async Task DeleteAtTime_with_a_sub_microsecond_request_time_still_reports_Good_for_the_row_it_deleted()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T0, 1)], default);
+        // +7 ticks is inside the same µs; the row lives at T0.
+        (await rig.Updater.DeleteAtTimeAsync("t", [T0.AddTicks(7)], default)).ShouldBe([Good]);
+        (await RowsAsync(rig, "t", liveOnly: true)).ShouldBeEmpty();
+    }
+
+    [Fact]
+    public async Task DeleteRaw_is_half_open_pages_through_big_ranges_and_reports_BadNoData_when_empty()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        (await rig.Updater.DeleteRawAsync("unknown", T0, T0.AddHours(1), default)).ShouldBe(HistorianStatusCodes.BadNoData);
+
+        // 2,500 rows => 3 pages of at most 1,000; plus one row exactly at the (exclusive) end.
+        var values = Enumerable.Range(0, 2500).Select(i => V(T0.AddSeconds(i), i)).ToList();
+        values.Add(V(T0.AddSeconds(2500), -1));
+        (await rig.Updater.UpdateDataAsync("big", HistoryUpdateMode.Insert, values, default))
+            .ShouldAllBe(c => c == HistorianStatusCodes.GoodEntryInserted);
+        (await rig.Updater.DeleteRawAsync("big", T0, T0.AddSeconds(2500), default)).ShouldBe(Good);
+        var live = await RowsAsync(rig, "big", liveOnly: true);
+        live.Count.ShouldBe(1);
+        live[0].Num.ShouldBe(-1d);
+        (await rig.Updater.DeleteRawAsync("big", T0, T0.AddSeconds(2500), default)).ShouldBe(HistorianStatusCodes.BadNoData);
+    }
+
+    [Fact]
+    public async Task DeleteRaw_with_a_pre_1970_start_reads_from_the_bound_unclamped()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(T0, 1), V(T0.AddSeconds(1), 2)], default);
+        (await rig.Updater.DeleteRawAsync("t", new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc), T0.AddSeconds(1), default))
+            .ShouldBe(Good);
+        (await RowsAsync(rig, "t", liveOnly: true)).Select(r => r.Num).ShouldBe([2d]);
+        // DateTime.MinValue as the start is nudged inside, not bound as -infinity
+        (await rig.Updater.DeleteRawAsync("t", DateTime.MinValue, DateTime.MaxValue, default)).ShouldBe(Good);
+        (await RowsAsync(rig, "t", liveOnly: true)).ShouldBeEmpty();
+    }
+
+    [Fact]
+    public async Task Read_after_write_is_visible_across_many_immediate_calls()
+    {
+        fx.SkipIfUnavailable();
+        var rig = MakeRig(fx);
+        // ~15% of un-waited inserts are not yet visible (spike Q4), so 40 round trips expose a missing wait.
+        for (var i = 0; i < 40; i++)
+        {
+            var t = T0.AddSeconds(i);
+            (await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Insert, [V(t, i)], default))
+                .ShouldBe([HistorianStatusCodes.GoodEntryInserted]);
+            (await rig.Updater.UpdateDataAsync("t", HistoryUpdateMode.Replace, [V(t, i + 1000)], default))
+                .ShouldBe([HistorianStatusCodes.GoodEntryReplaced], $"iteration {i}: the insert was not visible");
+        }
+    }
+}