Merge handoff-refresh-counts: SendEvent over gRPC + 2023 R2 binary-dive verdicts

- SendEvent over gRPC SHIPPED + live-validated (HistorianGrpcEventWriteOrchestrator;
  reuses the WCF "OS" event buffer verbatim on HistoryService.AddStreamValues).
- 2023 R2 stock-client binary dive: sharpened every pending item to evidence-based
  verdicts; DeleteTagExtendedProperties confirmed walled via native capture.
- Event registration (RegisterTags/EnsureTags) golden-tested vs live capture.
- Handoff date/test-count refresh; revision-probe scope comment fix.
331 offline tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

docs/reverse-engineering/handoff.md

@@ -1,6 +1,6 @@
 # AVEVA Historian Managed Driver Handoff
 
-Last updated: 2026-06-22 (roadmap exhausted — no actionable pure-code tasks remain)
+Last updated: 2026-06-23 (event-row parser fix merged; roadmap still exhausted — no actionable pure-code tasks remain)
 
 > **Current status supersedes the historical blocker narrative below.** The
 > sections from "Active Blocker" onward are a preserved reverse-engineering
@@ -76,18 +76,33 @@ reuses the proven 2020 WCF byte serializers/parsers unchanged inside protobuf
    `capture-event` harness (native, returns rows).
 2. **R4.3 active-SF magnitude** — needs an **SF-active server** (D2 storage-engine
    console handle).
-3. **SendEvent over gRPC** — **capture-gated**: no distinct RPC, framing uncaptured.
+3. **SendEvent over gRPC** — ✅ **SHIPPED + LIVE-VALIDATED 2026-06-23.** `SendEventAsync`
+   now routes over `RemoteGrpc` (`HistorianGrpcEventWriteOrchestrator`). Captured the native
+   client live (`capture-send-event` harness scenario): the send rides
+   `HistoryService.AddStreamValues` with the **same "OS" (0x534F) buffer the WCF path uses**
+   (`HistorianEventWriteProtocol` — "no distinct RPC" confirmed true), on a v8 Event session +
+   CM_EVENT registration. The write-enabled Event open is **byte-identical** to the read-only one
+   (diffed live — only per-session crypto differs), so the existing event-open path is reused
+   unchanged. End-to-end: pure-managed SDK send → `BSuccess=true` → event read back from the live
+   server (markers `SdkSendProbe`/`SdkCaptureProbe` confirmed in returned rows). Golden-tested
+   (`GrpcEventSendProtocolTests`) + gated live test (`SendEventAsync_OverGrpc_AcceptsEvent`,
+   opt-in `HISTORIAN_GRPC_EVENT_SEND=1`).
 4. **ExecuteSqlCommand over gRPC** — **server-walled** (`CSrvDbConnection`;
    RegisterTags prime doesn't help). Use WCF for SQL.
 5. **R4.2 revision EDITS** — storage-engine-pipe-only on BOTH transports (the D2 wall).
 6. **ReadBlocks** (`StartBlockRetrievalQuery`) — never captured on either transport.
-7. **DeleteTagExtendedProperties** — server-blocked on BOTH transports. The gRPC
-   multiplexed-channel hypothesis was **PROBED + DISPROVEN 2026-06-22** (merge
-   `c88260c`): GetTgByNm + GetTepByNm primes succeed on one shared write-enabled
-   gRPC channel, yet DelTep is still rejected (native code=1) and the property
-   survives — the working set is native in-process registration state, not the
-   wire session. Pinned by gated negative test
-   `DeleteTagExtendedProperties_OverGrpc_ProbeMultiplexedChannel`.
+7. **DeleteTagExtendedProperties** — server-blocked; **confirmed walled by capturing the NATIVE
+   client 2026-06-23.** The agent's "use `deleteFromServer=true`" angle is moot: the native
+   `HistorianAccess.DeleteTagExtendedPropertiesByName(...,deleteFromServer:true)`, driven with the
+   cross-session sync trick that gets it past the client-side err-229 sync gate
+   (`Capture-DeleteTagExtendedProperties.ps1`), returns **`Success=true` / ErrorCode=Success** —
+   yet across repeated fresh sessions the property is **re-fetchable and re-deletable every time**,
+   i.e. it is **never durably removed**. So the native client itself only performs an optimistic
+   client-side cache delete; the server does not durably honor it (matches the HCAL cache-sync
+   model the decompile shows). Shipping a `DeleteTagExtendedPropertiesAsync` would return a
+   misleading success while the property persists, so it correctly stays **unshipped**. (Earlier
+   gRPC multiplexed-channel hypothesis also PROBED + DISPROVEN 2026-06-22, merge `c88260c`; pinned
+   by `DeleteTagExtendedProperties_OverGrpc_ProbeMultiplexedChannel`.)
 8. **Deferred-by-design** items (`write-commands` D1–D3, non-analog tag create,
    etc.) — bounded out until an explicit customer/user demand signal.
 
@@ -101,6 +116,73 @@ Live-server gRPC probe recipe: set
 quotes — `reference_wonder_sql_vd03_credentials`) and run the gated
 `HistorianGrpcIntegrationTests`.
 
+### 2023 R2 stock-client binary dive (2026-06-23) — sharpened verdicts
+
+Re-read the full decompiled stock 2023 R2 managed client
+(`histsdk-2023r2-analysis/decompiled/`: `Archestra.Historian.GrpcClient`,
+`ArchestrA.HistorianAccess`, `Archestra.Grpc.Contract`, `HistorianEvent`,
+`HistorianAccessUtil`) as the oracle for every still-pending item. **Governing
+fact:** `ArchestrA.HistorianAccess.dll` is a C++/CLI mixed assembly — every
+data/config/write method is a thin shim into native `<Module>.HistorianClient.*`,
+and the managed `Grpc*Client` wrappers are instantiated by **nothing** in the
+decompiled set (`new Grpc*Client(` → zero call sites). So the buffer-building and
+RPC-dispatch sequencing for these items lives in native C++ not present in the
+binaries. That confirms the "gated" calls were not from missing managed steps —
+with these refinements:
+
+- **Item 1 (gRPC event rows)** — **confirmed native/server-side.** Stock event
+  call graph is provably identical to ours (transport, per-service channels,
+  gzip-only metadata, CM_EVENT registration, v8 ECDH Event-open, `StartEventQuery`
+  request bytes). `EventQuery.StartQuery`/`MoveNext` dispatch straight into native
+  `HistorianClient.StartEventQuery`/`GetNextRow`; the query orchestration that
+  would differ is native and not on the wire. One untested low-effort check
+  remains: byte-diff a captured **Event-connection** EnsureTags/RegisterTags
+  against our replay (the 83-vs-86-byte EnsT gap was never actually compared).
+- **Item 3 (SendEvent over gRPC)** — ✅ **SHIPPED + LIVE-VALIDATED 2026-06-23** (was
+  "capturable"). RPC confirmed = `HistoryService.AddStreamValues` (the "no distinct RPC"
+  note is TRUE). The `btValues` VTQ buffer turned out to be already-owned: our M2
+  `HistorianEventWriteProtocol.SerializeAddStreamValuesBuffer` ("OS" buffer, decoded from
+  the WCF event-send) is the transport-independent `PackToVtq` equivalent and the gRPC send
+  uses it **verbatim** (live capture: sig `OS`/0x534F, CM_EVENT GUID, identical framing — NOT
+  the historical write's "ON" buffer). The write-enabled Event open is byte-identical to the
+  read-only one (live diff). So SendEvent-over-gRPC was pure assembly:
+  `HistorianGrpcEventWriteOrchestrator` = existing v8 Event open + existing CM_EVENT
+  registration + `AddStreamValues`(OS buffer). End-to-end live-validated (send → `BSuccess`
+  → read back from the live server). Golden-tested + gated live test.
+- **Item 4 (ExecuteSql over gRPC)** — **confirmed walled + explained.** The stock
+  client gates SQL **out client-side**: `HistorianAccess.ExecuteSqlCommand` returns
+  `OperationNotSupported` when `IsManagedHistorian(node)` or `!IsProcessConnectionRequested()`
+  (decompile ~:6198/:6214) and never sends the RPC. SQL-over-gRPC is unsupported by
+  design on a managed/gRPC historian; our `ProtocolEvidenceMissingException` is correct.
+- **Item 5 (R4.2 revision edits)** — **confirmed HARD.** There is **no Revision RPC
+  in the gRPC contract** (zero "Revision" message types); the stock client reaches a
+  revision edit only via the native `HistorianClient.AddRevisionValuesBegin/AddRevisionValue/
+  AddRevisionValuesEnd` transaction trio over the storage-engine channel. NOTE: this is
+  a **distinct capability** from `AddNonStreamValues` (non-streamed original insert) —
+  `HistorianGrpcRevisionProbe` probes the latter; its doc comment was corrected to say so.
+- **Item 6 (ReadBlocks/LoadBlocks)** — `LoadBlocks` request is a trivial
+  handle+sequence cursor but the `historyBlocks` response is a native blob with no
+  managed decoder, and it needs the D2-blocked `OpenStorageConnection` console handle.
+  Walled.
+- **Item 7 (DeleteTagExtendedProperties)** — **capture done 2026-06-23; CONFIRMED WALLED, don't
+  ship.** RPC + string handle are correct; ADD/DELETE are structurally identical and neither uses
+  `StartJob`. The `deleteFromServer`-flag hypothesis is now tested and moot: the native
+  `DeleteTagExtendedPropertiesByName(...,deleteFromServer:true)`, driven past the err-229 client
+  sync gate with the cross-session trick (`Capture-DeleteTagExtendedProperties.ps1`), returns
+  `Success=true` — yet the property is **re-fetchable + re-deletable across repeated fresh sessions
+  (never durably removed)**. So the native client only does an optimistic client-side cache delete
+  the server doesn't durably honor (the HCAL cache-sync model). Shipping
+  `DeleteTagExtendedPropertiesAsync` would return a misleading success, so it stays unshipped.
+- **SF/snapshot/shard/ForwardSnapshot ops** — only `Get/SetSFParameter` are managed-built
+  (typed strings); all others carry opaque native buffers and need the storage console
+  handle. Walled / tooling-internal.
+
+**Net:** 3 items hard-confirmed walled with real explanations (4, 5, 6 + OpenStorageConnection),
+and **2 moved to a precise, local-box-capturable target**: **SendEvent** (`PackToVtq` output)
+and **DeleteTEP** (`BtInput` with `deleteFromServer=true`). Both need native instrumentation of
+`aahClientManaged.dll` (Frida / IL-rewrite — repo tooling exists under
+`tools/AVEVA.Historian.NativeTraceHarness` + `scripts/frida/`), not a special server.
+
 ## Project Direction
 
 The project goal is still a fully managed .NET 10 C# AVEVA Historian client.
@@ -172,12 +254,14 @@ dotnet build .\Histsdk.slnx --no-restore
 dotnet test .\Histsdk.slnx --no-build --logger "console;verbosity=minimal"
 ```
 
-Current known-good result (2026-06-22):
+Current known-good result (2026-06-23):
 
 - Build succeeds (0 warnings / 0 errors).
-- Offline tests pass: 321/321 (live gRPC/integration tests skip cleanly without
+- Offline tests pass: 328/328 (live gRPC/integration tests skip cleanly without
   their env vars). Gated live tests add to this when `HISTORIAN_*` /
-  `HISTORIAN_GRPC_*` are set.
+  `HISTORIAN_GRPC_*` are set. The +7 over the prior 321 are the event-row parser
+  fix's golden + gated-capture coverage (`HistorianEventRowProtocolTests`:
+  markerless multi-row, the v11 gRPC header, and the 50-event stock-client capture).
 
 The workspace is a Git working tree (origin: gitea.dohertylan.com). Use
 normal git workflow for change tracking; the prior "no working tree, use

src/AVEVA.Historian.Client/Grpc/HistorianGrpcEventWriteOrchestrator.cs

@@ -0,0 +1,136 @@
+using Google.Protobuf;
+using AVEVA.Historian.Client.Models;
+using AVEVA.Historian.Client.Wcf;
+using GrpcHistory = ArchestrA.Grpc.Contract.History;
+using GrpcStatus = ArchestrA.Grpc.Contract.Status;
+
+namespace AVEVA.Historian.Client.Grpc;
+
+/// <summary>
+/// 2023 R2 gRPC orchestrator for the event SEND (<see cref="HistorianClient.SendEventAsync"/>).
+/// Captured live from the native 2023 R2 client (<c>capture-send-event</c> scenario,
+/// 2026-06-23): an event send rides <c>HistoryService.AddStreamValues</c> with the SAME
+/// <c>"OS"</c> (0x534F) storage-sample buffer the WCF AddS2 path uses
+/// (<see cref="HistorianEventWriteProtocol"/>) — NOT a distinct event RPC and NOT the historical
+/// write's <c>"ON"</c> buffer. The native client's write-enabled Event <c>OpenConnection</c>
+/// request is byte-identical to the read-only event open (the ReadOnly arg does not change the v8
+/// open buffer; diffed live — only the per-session client key + credential token differ), so the
+/// existing <see cref="HistorianGrpcHandshake.OpenSession"/> event path is reused unchanged. The
+/// chain on a single Event session:
+/// <list type="number">
+///   <item>OpenConnection (v8 Event, ExchangeKey ECDH auth) → string storage handle</item>
+///   <item>CM_EVENT registration: UpdateClientStatus → RegisterTags → EnsureTags (the same
+///   buffers the gRPC event READ replays — verified byte-identical to the capture)</item>
+///   <item><c>HistoryService.AddStreamValues</c>(strHandle, "OS" event buffer)</item>
+/// </list>
+/// Only original events (<see cref="HistorianEvent.RevisionVersion"/> = 0) with string-valued
+/// properties have a captured encoding; others throw <see cref="ProtocolEvidenceMissingException"/>
+/// from <see cref="HistorianEventWriteProtocol"/>.
+/// </summary>
+internal sealed class HistorianGrpcEventWriteOrchestrator
+{
+    private readonly HistorianClientOptions _options;
+
+    public HistorianGrpcEventWriteOrchestrator(HistorianClientOptions options)
+    {
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+    }
+
+    /// <summary>Diagnostic: type+code description of the most recent AddStreamValues error buffer.</summary>
+    public string LastSendErrorDescription { get; private set; } = string.Empty;
+
+    /// <summary>Diagnostic: outcomes of the CM_EVENT registration RPCs.</summary>
+    public string RegistrationDiag { get; private set; } = string.Empty;
+
+    public Task<bool> SendEventAsync(HistorianEvent evt, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(evt);
+        if (!_options.IntegratedSecurity && string.IsNullOrEmpty(_options.UserName))
+        {
+            throw new ProtocolEvidenceMissingException(
+                "Managed gRPC event send currently requires IntegratedSecurity or an explicit UserName + Password.");
+        }
+
+        if (evt.RevisionVersion != 0)
+        {
+            throw new ProtocolEvidenceMissingException(
+                "Only original events (RevisionVersion = 0) have a captured send encoding; " +
+                "revision/update/delete event sends are not yet supported.");
+        }
+
+        return Task.Run(() => Run(evt, cancellationToken), cancellationToken);
+    }
+
+    private bool Run(HistorianEvent evt, CancellationToken cancellationToken)
+    {
+        using HistorianGrpcConnection connection = HistorianGrpcChannelFactory.Create(_options);
+
+        // The event SEND uses the same v8 Event connection as the event READ. The write-enabled
+        // open buffer is byte-identical to the read-only one (verified live), so OpenSession's
+        // event path is reused unchanged.
+        HistorianGrpcHandshake.Session session = HistorianGrpcHandshake.OpenSession(
+            connection, _options, cancellationToken, eventConnection: true);
+
+        RegisterCmEventTag(connection, session, cancellationToken);
+
+        var historyClient = new GrpcHistory.HistoryService.HistoryServiceClient(connection.Channel);
+        byte[] pBuf = HistorianEventWriteProtocol.SerializeAddStreamValuesBuffer(evt, DateTime.UtcNow);
+
+        GrpcHistory.AddStreamValuesResponse response = historyClient.AddStreamValues(
+            new GrpcHistory.AddStreamValuesRequest
+            {
+                StrHandle = session.StringHandle,
+                BtValues = ByteString.CopyFrom(pBuf),
+            },
+            connection.Metadata,
+            DateTime.UtcNow.Add(_options.RequestTimeout),
+            cancellationToken);
+
+        byte[] error = response.Status?.BtError?.ToByteArray() ?? [];
+        LastSendErrorDescription = HistorianEventRegistrationProtocol.DescribeNativeError(error);
+        return response.Status?.BSuccess ?? false;
+    }
+
+    /// <summary>
+    /// Replays the CM_EVENT registration the native event connection performs before a send:
+    /// UpdateClientStatus → RegisterTags(CM_EVENT) → EnsureTags(CM_EVENT). The buffers are shared
+    /// with the gRPC event READ path (<see cref="HistorianEventRegistrationProtocol"/> +
+    /// <see cref="HistorianAddTagsProtocol.SerializeCmEventEnsureTagsGrpc"/>) and were verified
+    /// byte-identical to the live capture. Best-effort: an individual rejection does not abort the
+    /// send (the server may already hold CM_EVENT registered for the session).
+    /// </summary>
+    private void RegisterCmEventTag(HistorianGrpcConnection connection, HistorianGrpcHandshake.Session session, CancellationToken cancellationToken)
+    {
+        var historyClient = new GrpcHistory.HistoryService.HistoryServiceClient(connection.Channel);
+        DateTime Deadline() => DateTime.UtcNow.Add(_options.RequestTimeout);
+
+        byte[] clientStatus = HistorianEventRegistrationProtocol.BuildUpdateClientStatusBlob();
+        try
+        {
+            historyClient.UpdateClientStatus(
+                new GrpcHistory.UpdateClientStatusRequest { StrHandle = session.StringHandle, BtClientStatus = ByteString.CopyFrom(clientStatus) },
+                connection.Metadata, Deadline(), cancellationToken);
+        }
+        catch { /* best-effort */ }
+
+        byte[] registerBuffer = HistorianEventRegistrationProtocol.BuildRegisterCmEventInputBuffer();
+        try
+        {
+            GrpcHistory.RegisterTagsResponse rt = historyClient.RegisterTags(
+                new GrpcHistory.RegisterTagsRequest { StrHandle = session.StringHandle, BtTagInfos = ByteString.CopyFrom(registerBuffer) },
+                connection.Metadata, Deadline(), cancellationToken);
+            RegistrationDiag += $"RTag={rt.Status?.BSuccess}; ";
+        }
+        catch (Exception ex) { RegistrationDiag += $"RTag=EX:{ex.GetType().Name}; "; }
+
+        byte[] payload = HistorianAddTagsProtocol.SerializeCmEventEnsureTagsGrpc(DateTime.UtcNow);
+        try
+        {
+            GrpcHistory.EnsureTagsResponse et = historyClient.EnsureTags(
+                new GrpcHistory.EnsureTagsRequest { StrHandle = session.StringHandle, BtTagInfos = ByteString.CopyFrom(payload), ElementCount = 1 },
+                connection.Metadata, Deadline(), cancellationToken);
+            RegistrationDiag += $"EnsT={et.Status?.BSuccess}; ";
+        }
+        catch (Exception ex) { RegistrationDiag += $"EnsT=EX:{ex.GetType().Name}; "; }
+    }
+}

src/AVEVA.Historian.Client/Grpc/HistorianGrpcRevisionProbe.cs

@@ -20,6 +20,18 @@ namespace AVEVA.Historian.Client.Grpc;
 /// write-enabled session and calls <c>AddNonStreamValuesBegin</c>, surfacing whatever the server
 /// returns. It writes NO data — if Begin succeeds it immediately calls <c>AddNonStreamValuesEnd</c>
 /// with <c>bCommit=false</c> to discard the transaction.
+///
+/// <para><b>Scope note (corrected 2026-06-23 after a 2023 R2 binary re-read).</b> Despite the type
+/// name, this probes the <i>non-streamed ORIGINAL / backfill insert</i> capability
+/// (<c>AddNonStreamValues</c>), which is a <b>distinct capability from a revision EDIT</b>
+/// (overwriting an existing historized value with a new revision). The stock high-level client
+/// reaches a revision edit via a separate native transaction trio
+/// <c>HistorianClient.AddRevisionValuesBegin/AddRevisionValue/AddRevisionValuesEnd</c>
+/// (<c>ArchestrA.HistorianAccess.AddRevisionValues</c>, REVISION_MODE ∈ InsertLatest/UpdateSingle/
+/// UpdateMultiple). That trio has <b>NO corresponding RPC in the gRPC contract</b> (no "Revision"
+/// message type exists in <c>Archestra.Grpc.Contract</c>) — it rides the native storage-engine
+/// transaction channel only. So R4.2 revision edits remain unreachable over gRPC regardless of this
+/// probe's outcome; this probe neither covers nor unblocks them.</para>
 /// </summary>
 internal sealed class HistorianGrpcRevisionProbe
 {

src/AVEVA.Historian.Client/HistorianClient.cs

@@ -114,18 +114,24 @@ public sealed class HistorianClient : IAsyncDisposable
     }
 
     /// <summary>
-    /// Sends a single <see cref="HistorianEvent"/> to the Historian's built-in CM_EVENT tag
-    /// over the WCF event pipeline (Open2 event mode → CM_EVENT registration → AddS2). The
-    /// event is appended to the historian's event history and is readable back via
-    /// <see cref="ReadEventsAsync"/> / the <c>v_AlarmEventHistory2</c> view. Only original
-    /// events (<see cref="HistorianEvent.RevisionVersion"/> = 0) with string-valued properties
-    /// are supported; other property value types and revision/update/delete events throw
+    /// Sends a single <see cref="HistorianEvent"/> to the Historian's built-in CM_EVENT tag.
+    /// Over WCF this runs Open2 event mode → CM_EVENT registration → AddS2; over the 2023 R2
+    /// <see cref="HistorianTransport.RemoteGrpc"/> transport it runs the captured-equivalent
+    /// v8 Event OpenConnection → CM_EVENT registration → <c>HistoryService.AddStreamValues</c>
+    /// with the same "OS" event buffer (live-captured 2026-06-23 — the send rides the same RPC
+    /// and buffer as the WCF path, not a distinct event RPC). The event is appended to the
+    /// historian's event history and is readable back via <see cref="ReadEventsAsync"/> /
+    /// the <c>v_AlarmEventHistory2</c> view. Only original events
+    /// (<see cref="HistorianEvent.RevisionVersion"/> = 0) with string-valued properties are
+    /// supported; other property value types and revision/update/delete events throw
     /// <see cref="ProtocolEvidenceMissingException"/> until their wire encoding is captured.
     /// </summary>
     public Task<bool> SendEventAsync(HistorianEvent historianEvent, CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(historianEvent);
-        return new HistorianWcfEventOrchestrator(_options).SendEventAsync(historianEvent, cancellationToken);
+        return _options.Transport == HistorianTransport.RemoteGrpc
+            ? new Grpc.HistorianGrpcEventWriteOrchestrator(_options).SendEventAsync(historianEvent, cancellationToken)
+            : new HistorianWcfEventOrchestrator(_options).SendEventAsync(historianEvent, cancellationToken);
     }
 
     /// <summary>

tests/AVEVA.Historian.Client.Tests/GrpcEventSendProtocolTests.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Buffers.Binary;
+using AVEVA.Historian.Client.Wcf;
+using Xunit;
+
+namespace AVEVA.Historian.Client.Tests;
+
+/// <summary>
+/// Golden-byte coverage for the 2023 R2 gRPC event-SEND registration buffers, pinned against a live
+/// native capture (<c>capture-send-event</c> scenario, 2026-06-23). The send itself rides
+/// <c>HistoryService.AddStreamValues</c> with the same "OS" buffer the WCF path uses
+/// (<see cref="HistorianEventWriteProtocol"/>, already golden-tested in
+/// <c>WcfEventWriteProtocolTests</c>); what is gRPC-specific is the CM_EVENT registration the event
+/// connection performs first (RegisterTags + the 86-byte gRPC EnsureTags). These fixtures are the raw
+/// bytes the native client sent on the wire — they carry no identity (CM_EVENT / "AnE Event" /
+/// constant tag + event-type GUIDs / a registration FILETIME).
+/// </summary>
+public class GrpcEventSendProtocolTests
+{
+    // GrpcHistoryClient.RegisterTags.tagInfos captured from the native event connection: the packet
+    // header 50 67 02 00 + count(1) + the 16-byte CM_EVENT tag GUID.
+    private const string CapturedRegisterTagsHex =
+        "506702000100000045813b35f05d464da253871aef49b321";
+
+    // GrpcHistoryClient.EnsureTags.tagInfos captured from the native event connection (86 bytes): the
+    // 8-byte EnsureTags header + CM_EVENT CTagMetadata + a registration FILETIME + the …e01f2f27
+    // event-type GUID.
+    private const string CapturedEnsureTagsHex =
+        "4e670300010000000386000545813b35f05d464da253871aef49b321090800434d5f4556454e54090900416e45204576656e7402020100000001000000004e18d6bd4503dd0142ae595fb63b604791a5ab0be01f2f27";
+
+    [Fact]
+    public void BuildRegisterCmEventInputBuffer_MatchesNativeGrpcCapture()
+    {
+        byte[] expected = Convert.FromHexString(CapturedRegisterTagsHex);
+        byte[] actual = HistorianEventRegistrationProtocol.BuildRegisterCmEventInputBuffer();
+        Assert.Equal(expected, actual);
+    }
+
+    [Fact]
+    public void SerializeCmEventEnsureTagsGrpc_MatchesNativeGrpcCapture()
+    {
+        byte[] expected = Convert.FromHexString(CapturedEnsureTagsHex);
+        Assert.Equal(86, expected.Length);
+
+        // The only run-varying field is the registration FILETIME (the 8 bytes immediately before the
+        // trailing 16-byte event-type GUID). Feed the captured time back so the comparison is exact.
+        long filetime = BinaryPrimitives.ReadInt64LittleEndian(expected.AsSpan(expected.Length - 24, 8));
+        DateTime createdUtc = DateTime.FromFileTimeUtc(filetime);
+
+        byte[] actual = HistorianAddTagsProtocol.SerializeCmEventEnsureTagsGrpc(createdUtc);
+        Assert.Equal(expected, actual);
+    }
+}

tests/AVEVA.Historian.Client.Tests/HistorianGrpcIntegrationTests.cs

@@ -233,6 +233,37 @@ public sealed class HistorianGrpcIntegrationTests
         Assert.Contains(samples, s => s.NumericValue is { } v && Math.Abs(v - expected) < 0.01);
     }
 
+    [Fact]
+    public async Task SendEventAsync_OverGrpc_AcceptsEvent()
+    {
+        string? host = Environment.GetEnvironmentVariable("HISTORIAN_GRPC_HOST");
+        // Gated additionally on a dedicated opt-in so this WRITE test never runs by accident — it
+        // appends a clearly-marked test event to the server's event history. Captured 2026-06-23:
+        // the gRPC event send rides HistoryService.AddStreamValues with the same "OS" buffer the WCF
+        // path uses (HistorianEventWriteProtocol), on a v8 Event session + CM_EVENT registration.
+        if (string.IsNullOrWhiteSpace(host)
+            || string.IsNullOrEmpty(Environment.GetEnvironmentVariable("HISTORIAN_USER"))
+            || Environment.GetEnvironmentVariable("HISTORIAN_GRPC_EVENT_SEND") is null)
+        {
+            return;
+        }
+
+        HistorianClient client = new(BuildOptions(host));
+
+        var evt = new HistorianEvent(
+            Id: Guid.NewGuid(),
+            EventTimeUtc: DateTime.UtcNow,
+            ReceivedTimeUtc: DateTime.UtcNow,
+            Type: "SdkSendProbe",
+            SourceName: "SdkSendProbe",
+            Namespace: "SdkCapture",
+            RevisionVersion: 0,
+            Properties: new Dictionary<string, object?> { ["SdkProbeProp"] = "SdkProbeValue" });
+
+        bool sent = await client.SendEventAsync(evt, CancellationToken.None);
+        Assert.True(sent, "gRPC SendEvent should be accepted by the server (AddStreamValues BSuccess).");
+    }
+
     [Fact]
     public async Task ReadAggregateAsync_OverGrpc_ReturnsTimeWeightedAverageRows()
     {

tools/AVEVA.Historian.Grpc2023CaptureHarness/Program.cs

@@ -92,8 +92,10 @@ namespace AVEVA.Historian.Grpc2023CaptureHarness
                     return DeleteTag(managedDll, args);
                 case "capture-event":
                     return CaptureEvent(managedDll, args);
+                case "capture-send-event":
+                    return CaptureSendEvent(managedDll, args);
                 default:
-                    Console.Error.WriteLine($"Unknown scenario '{scenario}'. Supported: load-check, connect, capture-write, delete-tag, capture-event.");
+                    Console.Error.WriteLine($"Unknown scenario '{scenario}'. Supported: load-check, connect, capture-write, delete-tag, capture-event, capture-send-event.");
                     return 1;
             }
         }
@@ -603,6 +605,132 @@ namespace AVEVA.Historian.Grpc2023CaptureHarness
             }
         }
 
+        /// <summary>
+        /// Drives the native 2023 R2 client through an event SEND so the IL-rewritten GrpcClient dumps
+        /// the AddStreamValues.btValues (the event VTQ storage-sample buffer — resolves whether a gRPC
+        /// event send uses the "OS" or "ON" outer signature) AND the Event-connection EnsureTags.btTagInfos
+        /// (the 83-vs-86-byte CM_EVENT registration byte-diff). Opens a WRITE-ENABLED Event connection,
+        /// builds a clearly-marked test HistorianEvent, calls AddStreamedValue, then CloseConnection to
+        /// flush the queued event onto the wire. WRITES a real test event into the server's event history.
+        /// Run with --grpc-rewrite pointing at the instrumented copy and AVEVA_HISTORIAN_RE_CAPTURE set.
+        /// Usage: capture-send-event [--server <host>] [--port 32565] [--cert <host>]
+        ///        [--event-type SdkCaptureProbe] [--flush-seconds 6]
+        /// </summary>
+        private static int CaptureSendEvent(string managedDll, string[] args)
+        {
+            Assembly asm = Assembly.LoadFrom(managedDll);
+            Type accessType = Req(asm, "ArchestrA.HistorianAccess");
+            Type connArgsType = Req(asm, "ArchestrA.HistorianConnectionArgs");
+            Type connModeType = Req(asm, "ArchestrA.HistorianConnectionMode");
+            Type connTypeType = Req(asm, "ArchestrA.HistorianConnectionType");
+            Type errorType = Req(asm, "ArchestrA.HistorianAccessError");
+            Type eventType = Req(asm, "ArchestrA.HistorianEvent");
+            Type propTypeEnum = Req(asm, "ArchestrA.HistorianEventPropertyType");
+            Type certInfoType = Req(asm, "ArchestrA.CertificateInfo");
+            Type secModeType = Req(asm, "ArchestrA.HistorianSecurityMode");
+
+            string server = GetOption(args, "--server") ?? Environment.GetEnvironmentVariable("HISTORIAN_GRPC_HOST") ?? "localhost";
+            int port = int.TryParse(GetOption(args, "--port"), out int p) ? p : 32565;
+            string certName = GetOption(args, "--cert") ?? server;
+            string evtTypeName = GetOption(args, "--event-type") ?? "SdkCaptureProbe";
+            int flushSeconds = int.TryParse(GetOption(args, "--flush-seconds"), out int fs) ? fs : 6;
+            string? user = Environment.GetEnvironmentVariable("HISTORIAN_USER");
+            string? password = Environment.GetEnvironmentVariable("HISTORIAN_PASSWORD");
+            if (string.IsNullOrEmpty(user))
+            {
+                Console.Error.WriteLine("Set HISTORIAN_USER/HISTORIAN_PASSWORD.");
+                return 1;
+            }
+
+            if (string.IsNullOrWhiteSpace(Environment.GetEnvironmentVariable("AVEVA_HISTORIAN_RE_CAPTURE")))
+            {
+                string defaultCap = Path.GetFullPath(Path.Combine(
+                    "artifacts", "reverse-engineering", "grpc-event-capture", "send-event-capture.ndjson"));
+                Environment.SetEnvironmentVariable("AVEVA_HISTORIAN_RE_CAPTURE", defaultCap);
+            }
+            Console.WriteLine($"Capture sink: {Environment.GetEnvironmentVariable("AVEVA_HISTORIAN_RE_CAPTURE")}");
+
+            object connArgs = Activator.CreateInstance(connArgsType)!;
+            SetProp(connArgs, "ServerName", server);
+            SetProp(connArgs, "TcpPort", checked((ushort)port));
+            SetProp(connArgs, "ConnectionMode", Enum.Parse(connModeType, "Historian"));
+            SetProp(connArgs, "ConnectionType", Enum.Parse(connTypeType, "Event")); // EVENT connection
+            SetProp(connArgs, "ReadOnly", false);                                   // WRITE-enabled
+            SetProp(connArgs, "IntegratedSecurity", false);
+            SetProp(connArgs, "AllowUnTrustedConnection", true);
+            SetProp(connArgs, "UserName", user);
+            SetProp(connArgs, "Password", password ?? string.Empty);
+            object certInfo = Activator.CreateInstance(certInfoType)!;
+            TrySetProp(certInfo, "CertificateName", certName);
+            TrySetProp(certInfo, "SecurityMode", Enum.Parse(secModeType, "TransportCertificate"));
+            TrySetProp(connArgs, "SecurityInfo", certInfo);
+
+            object access = Activator.CreateInstance(accessType)!;
+            object?[] openArgs = { connArgs, Activator.CreateInstance(errorType) };
+            Console.WriteLine($"OpenConnection: server={server} port={port} type=Event readonly=false");
+            bool opened;
+            try
+            {
+                opened = (bool)accessType.GetMethod("OpenConnection", new[] { connArgsType, errorType.MakeByRefType() })!
+                    .Invoke(access, openArgs)!;
+            }
+            catch (TargetInvocationException tie)
+            {
+                Console.Error.WriteLine($"OpenConnection threw: {tie.InnerException?.GetType().Name}: {tie.InnerException?.Message}");
+                return 2;
+            }
+            Console.WriteLine($"OpenConnection returned: {opened} err={DescribeError(openArgs[1])}");
+            if (!opened) { return 2; }
+
+            try
+            {
+                // Build a clearly-marked test event. Required: Type (≤32 chars), Id, EventTime.
+                object evt = Activator.CreateInstance(eventType)!;
+                SetProp(evt, "Type", evtTypeName);
+                TrySetProp(evt, "Id", Guid.NewGuid());
+                TrySetProp(evt, "EventTime", DateTime.SpecifyKind(DateTime.UtcNow, DateTimeKind.Utc));
+                TrySetProp(evt, "Namespace", "SdkCapture");
+                TrySetProp(evt, "Source", "SdkCaptureProbe");
+
+                // One string property to exercise the property-bag framing.
+                MethodInfo? addProp = eventType.GetMethods().FirstOrDefault(m =>
+                    m.Name == "AddProperty" && m.GetParameters().Length == 4);
+                if (addProp != null)
+                {
+                    try
+                    {
+                        object strEnum = Enum.Parse(propTypeEnum, "String", true);
+                        object?[] apArgs = { "SdkProbeProp", "SdkProbeValue", strEnum, Activator.CreateInstance(errorType) };
+                        addProp.Invoke(evt, apArgs);
+                        Console.WriteLine($"AddProperty: err={DescribeError(apArgs[3])}");
+                    }
+                    catch (Exception ex) { Console.WriteLine($"AddProperty skipped: {ex.GetType().Name}"); }
+                }
+
+                MethodInfo addStreamed = accessType.GetMethods().First(m =>
+                    m.Name == "AddStreamedValue" && m.GetParameters().Length == 2
+                    && m.GetParameters()[0].ParameterType == eventType);
+                object?[] asArgs = { evt, Activator.CreateInstance(errorType) };
+                bool sent = (bool)addStreamed.Invoke(access, asArgs)!;
+                Console.WriteLine($"AddStreamedValue({evtTypeName}): {sent} err={DescribeError(asArgs[1])}");
+
+                // Let the native delivery queue flush the event onto the wire (AddStreamValues).
+                System.Threading.Thread.Sleep(flushSeconds * 1000);
+                Console.WriteLine(sent ? "CAPTURE-SEND-EVENT: AddStreamedValue accepted (buffer captured on flush)" : "CAPTURE-SEND-EVENT: AddStreamedValue rejected");
+                return sent ? 0 : 3;
+            }
+            finally
+            {
+                try
+                {
+                    // CloseConnection flushes any remaining queued values before teardown.
+                    MethodInfo? close = accessType.GetMethod("CloseConnection", new[] { errorType.MakeByRefType() });
+                    if (close != null) close.Invoke(access, new object?[] { Activator.CreateInstance(errorType) });
+                }
+                catch { /* best-effort */ }
+            }
+        }
+
         /// <summary>
         /// Read-only gRPC connect probe: opens a 2023 R2 Historian (mode=Historian) connection via the
         /// native client and reports the resulting connection status. Proves the mixed-mode client can