Merge b640-640: driver diagnostics surface on /hosts (#640)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

CLAUDE.md

@@ -1120,8 +1120,9 @@ host logger factory (wired in `DriverFactoryBootstrap`; the FOCAS and TwinCAT fa
 - **S7 (#632/#638):** STRING/WSTRING/CHAR/WCHAR and DTL/DATE/TOD/TIME/S5TIME codecs (`S7ValueCodecs`; TOD/TIME/S5TIME
   surface as Int32 ms); an undecodable buffer or unrepresentable write is `BadOutOfRange` with no health impact.
   Optional `localTsap`/`remoteTsap`/`connectionType` override the slot TSAP (`remoteTsap` ⊕ `connectionType`). Test
-  Connect reads 2 bytes of `MB0` so a PUT/GET-disabled CPU fails the probe. The negotiated PDU size is logged and on
+  Connect reads 2 bytes of `MB0` so a PUT/GET-disabled CPU fails the probe. The negotiated PDU size is logged, on
-  `S7Driver.GetDiagnostics()` only — there is no driver-diagnostics surface on `/hosts`.
+  `S7Driver.GetDiagnostics()`, and (#640) on `/hosts` via the optional `IDriverDiagnostics` bag (string map on
+  `DriverHealthChanged.Diagnostics` / proto `has_diagnostics`+`diagnostics`; its digest is in the publish fingerprint).
 - **TwinCAT (#637):** TIME/TOD publish Float64 ms, DATE/DT a UTC DateTime; a tag authored `UDInt` keeps the raw counter
   (and keeps historizing — DateTime is not historized).
 - **AbLegacy (#636):** `AbLegacyAddress` (in `AbLegacy.Contracts`) is family-aware: PLC-5 `I:`/`O:` indices are OCTAL

docs/Telemetry.md

@@ -112,6 +112,23 @@ it — but it does mean the panel cannot answer "which node saw this?". Adding t
 widening the wire record, not changing the page. The one sub-instance breakdown the channel *does*
 carry is per-host connectivity, below.
 
+### Driver diagnostics ride `driver-health` (Gitea #640)
+
+A driver may implement the optional `IDriverDiagnostics` (Core.Abstractions): a pure in-memory
+`IReadOnlyDictionary<string,string> GetDiagnostics()` of operator-facing facts that are not driver
+*state* (S7's negotiated PDU size is the first; others opt in later). `DriverInstanceActor` pulls it on
+every health publish (try/caught, unguarded by the invoker like `GetHostStatuses`), bounds it
+(`DriverDiagnostics.Bound`: 16 entries, text truncated to 200 chars, ordinal key order), and carries it as
+`DriverHealthChanged.Diagnostics` -> proto `DriverHealth.has_diagnostics = 13` / `map<string,string>
+diagnostics = 14` (additive; old nodes never set them, old central ignores them) -> `HostsDriverRow.DiagnosticItems`,
+rendered as a small key/value list under the driver name on `/hosts` (nothing when empty).
+
+- **null ≠ empty**: null = driver lacks the capability; the presence flag preserves an empty bag.
+- **The diagnostics digest is part of the publish fingerprint, and must stay there** - a diagnostic fact
+  (e.g. PDU size changing on a reconnect) moves no other component, so without it the dedup swallows the
+  publish. Pinned by `DriverInstanceActorDiagnosticsTests.A_diagnostics_change_republishes_despite_otherwise_unchanged_health`
+  (stable-health stub - the shared `StubDriver` returns `UtcNow` and would pass vacuously).
+
 ### Per-host connectivity rides `driver-health` (Gitea #521)
 
 `DriverHealthChanged.HostStatuses` carries each driver's `IHostConnectivityProbe.GetHostStatuses()`

docs/drivers/S7.md

@@ -223,8 +223,10 @@ The PDU size the CPU negotiates during the connection handshake (typically 240 b
 extended PDU) is snapshotted at every (re)connect and exposed as `S7Driver.NegotiatedPduSize` /
 `S7Driver.GetDiagnostics()["S7.NegotiatedPduSize"]` (0 before the first connect and after shutdown). It is
 also written to the `S7Driver connected` / `reconnected` log lines and to the AdminUI Test Connect
-message (`S7 connected (CPU S71500, PDU 240 bytes)`). `DriverHealth` has no diagnostics bag in this
+message (`S7 connected (CPU S71500, PDU 240 bytes)`). S7 also implements the optional
-codebase, so the value is not yet on the `/hosts` health snapshot — adding one is a Core change.
+`IDriverDiagnostics` capability (explicitly, string-valued: `S7.NegotiatedPduSize` = `"480"`, omitted while 0 =
+not negotiated yet), so the value rides the driver-health snapshot and shows under the driver name on the
+AdminUI `/hosts` page (Gitea #640). See `docs/Telemetry.md` ("Driver diagnostics ride `driver-health`").
 
 ## Capability surface
 

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Messages/Drivers/DriverHealthChanged.cs

@@ -59,6 +59,12 @@ namespace ZB.MOM.WW.OtOpcUa.Commons.Messages.Drivers;
 ///     <para><b>Value-equality caveat</b> — the same one <paramref name="HostStatuses"/> carries: the
 ///     record's generated <c>Equals</c> compares the sample list by REFERENCE.</para>
 /// </param>
+/// <param name="Diagnostics">
+///     Operator-facing driver diagnostics from <c>IDriverDiagnostics.GetDiagnostics()</c> (e.g. S7's
+///     negotiated PDU size), bounded and in ordinal key order; null when the driver does not implement the
+///     capability (Gitea #640). Additive, never a state change. Same by-reference value-equality caveat as
+///     <paramref name="HostStatuses"/>.
+/// </param>
 public sealed record DriverHealthChanged(
     string ClusterId,
     string DriverInstanceId,
@@ -70,7 +76,8 @@ public sealed record DriverHealthChanged(
     DateTime? RediscoveryNeededUtc = null,
     string? RediscoveryReason = null,
     IReadOnlyList<HostConnectivityStatus>? HostStatuses = null,
-    SkippedTagReport? SkippedTags = null)
+    SkippedTagReport? SkippedTags = null,
+    IReadOnlyDictionary<string, string>? Diagnostics = null)
 {
     /// <summary>
     ///     DPS topic name. Both the runtime <c>AkkaDriverHealthPublisher</c> and the AdminUI

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Protos/telemetry.proto

@@ -82,6 +82,12 @@ message DriverHealth {
   // proto3, which a repeated field does not. So no companion has_ flag (and one would collide: protoc
   // already generates a HasSkippedTags property for this field).
   SkippedTags skipped_tags = 12;
+  // Driver diagnostics bag (Gitea #640), e.g. S7's negotiated PDU size. Additive: an old node never sets
+  // these and an old central ignores them. A map cannot distinguish absent from empty, and the two differ
+  // ("driver has no IDriverDiagnostics" vs "it has one and currently has nothing to say"), so the presence
+  // flag carries that bit - the same shape as has_host_statuses.
+  bool has_diagnostics = 13;
+  map<string, string> diagnostics = 14;
 }
 
 // Mirrors ZB.MOM.WW.OtOpcUa.Core.Abstractions.SkippedTagReport.

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverDiagnostics.cs

@@ -0,0 +1,58 @@
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+/// <summary>
+///     Optional driver capability: expose a small bag of operator-facing diagnostic key/value pairs
+///     (e.g. S7's negotiated PDU size) that ride the driver-health snapshot to the AdminUI <c>/hosts</c>
+///     page (Gitea #640).
+/// </summary>
+/// <remarks>
+///     <para>
+///         <b>Why a bag rather than fields on <see cref="DriverHealth"/>.</b> The facts worth showing are
+///         driver-specific and do not change the driver's <i>state</i>; a typed field per driver would
+///         widen the core record and the telemetry proto for every driver. Same additive-never-a-state-change
+///         rule as <see cref="ISkippedTagReporter"/>.
+///     </para>
+///     <para>
+///         Values are strings so the telemetry proto stays a plain <c>map&lt;string,string&gt;</c>. Keep the
+///         bag small (<see cref="DriverDiagnostics.MaxEntries"/> entries, text truncated to
+///         <see cref="DriverDiagnostics.MaxValueLength"/>): it is published on every state change.
+///     </para>
+/// </remarks>
+public interface IDriverDiagnostics
+{
+    /// <summary>
+    ///     Snapshot of the driver's diagnostics. Must be a pure in-memory read with no I/O — it is called on
+    ///     the health-publish path, deliberately NOT through the resilience pipeline.
+    /// </summary>
+    /// <returns>The key/value pairs; empty when there is nothing to show yet.</returns>
+    IReadOnlyDictionary<string, string> GetDiagnostics();
+}
+
+/// <summary>Bounds and helpers shared by every consumer of <see cref="IDriverDiagnostics"/>.</summary>
+public static class DriverDiagnostics
+{
+    /// <summary>Maximum number of entries carried on a health snapshot; extras are dropped (ordinal key order).</summary>
+    public const int MaxEntries = 16;
+
+    /// <summary>Maximum length of a key or value carried on a health snapshot; longer text is truncated.</summary>
+    public const int MaxValueLength = 200;
+
+    /// <summary>
+    ///     Normalises a driver's bag for publishing: ordinal key order, capped entry count, truncated text.
+    ///     Deterministic so the same facts always produce the same fingerprint digest.
+    /// </summary>
+    /// <param name="raw">The bag the driver returned.</param>
+    /// <returns>A bounded, ordered copy.</returns>
+    public static IReadOnlyDictionary<string, string> Bound(IReadOnlyDictionary<string, string> raw)
+    {
+        var result = new SortedDictionary<string, string>(StringComparer.Ordinal);
+        foreach (var kv in raw.OrderBy(k => k.Key, StringComparer.Ordinal))
+        {
+            if (result.Count >= MaxEntries) break;
+            result[Truncate(kv.Key ?? "")] = Truncate(kv.Value ?? "");
+        }
+        return result;
+    }
+
+    private static string Truncate(string s) => s.Length <= MaxValueLength ? s : s[..MaxValueLength];
+}

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverHealthPublisher.cs

@@ -34,6 +34,10 @@ public interface IDriverHealthPublisher
     ///     <see cref="ISkippedTagReporter"/>. Contradicts the "Healthy, 0 errors, reads advancing" reading
     ///     of a driver that is in fact ignoring part or all of its configuration (Gitea #541).
     /// </param>
+    /// <param name="diagnostics">
+    ///     Bounded driver diagnostics from <see cref="IDriverDiagnostics.GetDiagnostics"/>; null when the
+    ///     driver is not an <see cref="IDriverDiagnostics"/> (Gitea #640).
+    /// </param>
     void Publish(
         string clusterId,
         string driverInstanceId,
@@ -42,7 +46,8 @@ public interface IDriverHealthPublisher
         DateTime? rediscoveryNeededUtc = null,
         string? rediscoveryReason = null,
         IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-        SkippedTagReport? skippedTags = null);
+        SkippedTagReport? skippedTags = null,
+        IReadOnlyDictionary<string, string>? diagnostics = null);
 }
 
 /// <summary>
@@ -65,6 +70,7 @@ public sealed class NullDriverHealthPublisher : IDriverHealthPublisher
         DateTime? rediscoveryNeededUtc = null,
         string? rediscoveryReason = null,
         IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-        SkippedTagReport? skippedTags = null)
+        SkippedTagReport? skippedTags = null,
+        IReadOnlyDictionary<string, string>? diagnostics = null)
     { /* no-op */ }
 }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -32,7 +32,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
 ///     </para>
 /// </remarks>
 public sealed class S7Driver
-    : IDriver, ITagDiscovery, IReadable, IWritable, IBatchSubscribable, IHostConnectivityProbe, ISkippedTagReporter, IDisposable, IAsyncDisposable
+    : IDriver, ITagDiscovery, IReadable, IWritable, IBatchSubscribable, IHostConnectivityProbe, ISkippedTagReporter, IDriverDiagnostics, IDisposable, IAsyncDisposable
 {
     /// <summary>Authored tags this driver could not build into its poll table (Gitea #541).</summary>
     private readonly SkippedTagLedger _skippedTags = new();
@@ -463,6 +463,19 @@ public sealed class S7Driver
         ["S7.NegotiatedPduSize"] = NegotiatedPduSize,
     };
 
+    /// <summary>
+    ///     <see cref="IDriverDiagnostics"/> view of the same facts, as strings, for the <c>/hosts</c> page
+    ///     (Gitea #640). Empty before the first connect (PDU size 0 is "not negotiated yet", not a size).
+    /// </summary>
+    /// <returns>The diagnostics bag.</returns>
+    IReadOnlyDictionary<string, string> IDriverDiagnostics.GetDiagnostics()
+    {
+        var pdu = NegotiatedPduSize;
+        return pdu > 0
+            ? new Dictionary<string, string> { ["S7.NegotiatedPduSize"] = pdu.ToString(System.Globalization.CultureInfo.InvariantCulture) }
+            : new Dictionary<string, string>();
+    }
+
     private static readonly HashSet<S7DataType> ByteAnchoredDataTypes = new()
     {
         S7DataType.Int64,

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Hosts.razor

@@ -189,6 +189,17 @@ else
                                                 @* Own line — inline after the name the two identifiers ran together. *@
                                                 <div class="mono small text-muted">@d.DriverInstanceId</div>
                                             }
+                                            @if (d.DiagnosticItems.Count > 0)
+                                            {
+                                                @* Driver-reported diagnostics (Gitea #640), e.g. S7 negotiated PDU size.
+                                                   Renders nothing for a driver with none. *@
+                                                <div class="small text-muted" data-testid="driver-diagnostics">
+                                                    @foreach (var kv in d.DiagnosticItems)
+                                                    {
+                                                        <div class="mono">@kv.Key: @kv.Value</div>
+                                                    }
+                                                </div>
+                                            }
                                         </td>
                                         <td>@(d.DriverType ?? "—")</td>
                                         <td><span class="chip @DriverChipClass(d.State)">@d.State</span></td>

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Hosts/HostsDriverView.cs

@@ -52,6 +52,8 @@ public sealed record HostsDriverInstanceInfo(
 ///     is no answer — the driver cannot report skips, or it faulted before building a tag table. Lets
 ///     "ignoring 20 of 22 tags" show even while every other column on the row — state, last read, error
 ///     count, last error — reads perfectly healthy (Gitea #541).</param>
+/// <param name="Diagnostics">Driver-reported diagnostics (e.g. S7 negotiated PDU size); null or empty when
+///     the driver has none to show (Gitea #640).</param>
 /// <param name="Enabled">Whether the matching configured instance is authored enabled; <see langword="true"/>
 ///     when no instance matched. Fed to the row's expandable <c>DriverStatusPanel</c> (Gitea #546). An
 ///     unmatched row defaults to enabled on purpose: the row only exists because a live snapshot arrived,
@@ -62,8 +64,16 @@ public sealed record HostsDriverRow(
     DateTime? RediscoveryNeededUtc = null, string? RediscoveryReason = null,
     IReadOnlyList<HostConnectivityStatus>? HostStatuses = null,
     SkippedTagReport? SkippedTags = null,
-    bool Enabled = true)
+    bool Enabled = true,
+    IReadOnlyDictionary<string, string>? Diagnostics = null)
 {
+    /// <summary>Diagnostics as ordered key/value pairs for rendering; empty when there are none, in which
+    /// case the page renders nothing.</summary>
+    public IReadOnlyList<KeyValuePair<string, string>> DiagnosticItems { get; } =
+        (Diagnostics ?? new Dictionary<string, string>())
+            .OrderBy(k => k.Key, StringComparer.Ordinal)
+            .ToList();
+
     /// <summary>Whether this driver is ignoring at least one authored tag — the condition the whole
     /// column exists to make visible.</summary>
     public bool HasSkippedTags => SkippedTags is { SkippedCount: > 0 };
@@ -177,7 +187,8 @@ public static class HostsDriverView
                         s.RediscoveryReason,
                         s.HostStatuses,
                         s.SkippedTags,
-                        instance?.Enabled ?? true);
+                        instance?.Enabled ?? true,
+                        s.Diagnostics);
                 })
                 .OrderBy(d => d.Name ?? d.DriverInstanceId, StringComparer.OrdinalIgnoreCase)
                 .ThenBy(d => d.DriverInstanceId, StringComparer.OrdinalIgnoreCase)

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/Telemetry/TelemetryProtoMapCentral.cs

@@ -130,7 +130,8 @@ public static class TelemetryProtoMapCentral
             RediscoveryNeededUtc: msg.RediscoveryNeededUtc?.ToDateTime(),
             RediscoveryReason: msg.HasRediscoveryReason ? msg.RediscoveryReason : null,
             HostStatuses: ToHostStatuses(msg),
-            SkippedTags: ToSkippedTags(msg));
+            SkippedTags: ToSkippedTags(msg),
+            Diagnostics: msg.HasDiagnostics ? new Dictionary<string, string>(msg.Diagnostics) : null);
     }
 
     /// <summary>

src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryProtoMapNode.cs

@@ -164,6 +164,14 @@ public static class TelemetryProtoMapNode
             msg.SkippedTags = payload;
         }
 
+        // Gitea #640. Presence flag first - an empty map cannot say whether the driver has the capability.
+        if (e.Diagnostics is not null)
+        {
+            msg.HasDiagnostics = true;
+            foreach (var kv in e.Diagnostics)
+                msg.Diagnostics[kv.Key ?? ""] = kv.Value ?? "";
+        }
+
         return msg;
     }
 

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/AkkaDriverHealthPublisher.cs

@@ -38,7 +38,8 @@ public sealed class AkkaDriverHealthPublisher : IDriverHealthPublisher
         DateTime? rediscoveryNeededUtc = null,
         string? rediscoveryReason = null,
         IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-        SkippedTagReport? skippedTags = null)
+        SkippedTagReport? skippedTags = null,
+        IReadOnlyDictionary<string, string>? diagnostics = null)
     {
         var msg = new DriverHealthChanged(
             clusterId,
@@ -51,7 +52,8 @@ public sealed class AkkaDriverHealthPublisher : IDriverHealthPublisher
             rediscoveryNeededUtc,
             rediscoveryReason,
             hostStatuses,
-            skippedTags);
+            skippedTags,
+            diagnostics);
         DistributedPubSub.Get(_system).Mediator.Tell(new Publish(TopicName, msg));
         // Phase 5: fan the same snapshot into the node-local live-telemetry hub (no-op until a gRPC
         // client subscribes). The DPS publish above is unchanged — the hub is a strictly additive tap.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverInstanceActor.cs

@@ -1153,6 +1153,28 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
         }
     }
 
+    /// <summary>Bounded driver diagnostics, or null when the driver is not an <see cref="IDriverDiagnostics"/>
+    /// (Gitea #640). Pulled fresh and called UNGUARDED for the same reasons as
+    /// <see cref="CurrentSkippedTags"/>; a throwing implementation loses only the diagnostics.</summary>
+    private IReadOnlyDictionary<string, string>? CurrentDiagnostics()
+    {
+        if (_driver is not IDriverDiagnostics diag) return null;
+        try
+        {
+            return DriverDiagnostics.Bound(diag.GetDiagnostics());
+        }
+        catch (Exception ex)
+        {
+            _log.Warning(ex, "DriverInstance {Id}: GetDiagnostics threw during health publish; omitting diagnostics", _driverInstanceId);
+            return null;
+        }
+    }
+
+    /// <summary>Value digest of the diagnostics bag for the publish fingerprint. Null in, null out. The bag is
+    /// already in ordinal key order (<see cref="DriverDiagnostics.Bound"/>), so equal facts digest equally.</summary>
+    private static string? DiagnosticsDigest(IReadOnlyDictionary<string, string>? d) =>
+        d is null ? null : string.Join('|', d.Select(kv => $"{kv.Key}={kv.Value}"));
+
     /// <summary>Records the driver's rediscovery raise and re-publishes health so the signal reaches the
     /// AdminUI promptly rather than waiting for the next 30 s heartbeat.
     /// <para><b>Advisory only.</b> The served address space is deliberately NOT rebuilt: v3 authors raw tags
@@ -1357,6 +1379,7 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
             var errorCount = ErrorCount5Min();
             var hostStatuses = CurrentHostStatuses();
             var skippedTags = CurrentSkippedTags();
+            var diagnostics = CurrentDiagnostics();
             // _rediscoveryNeededUtc is PART OF THE FINGERPRINT on purpose. A rediscovery raise on an
             // otherwise-unchanged Healthy driver leaves (state, lastSuccess, lastError, errorCount)
             // identical, so without it the dedup below would swallow the very publish that carries the
@@ -1375,14 +1398,20 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
             // is identical to a clean driver's and the publish that carries the signal is exactly the one
             // dedup would swallow. Flattened to a STRING for the same reason as the host digest — the
             // report holds an IReadOnlyList, which a tuple compares by reference.
+            //
+            // The diagnostics digest (Gitea #640) is the fourth instance: a diagnostic fact (S7's PDU size
+            // changing across a reconnect) moves none of the other components, so without it the publish
+            // that carries it is deduped away. Flattened to a STRING - the bag is a dictionary, which a
+            // tuple compares by reference.
             var fingerprint = (health.State, health.LastSuccessfulRead, health.LastError, errorCount,
-                _rediscoveryNeededUtc, HostStatusDigest(hostStatuses), SkippedTagDigest(skippedTags));
+                _rediscoveryNeededUtc, HostStatusDigest(hostStatuses), SkippedTagDigest(skippedTags),
+                DiagnosticsDigest(diagnostics));
             if (_lastPublishedFingerprint is { } prev && prev.Equals(fingerprint))
                 return observed;
             _lastPublishedFingerprint = fingerprint;
             _healthPublisher.Publish(
                 _clusterId, _driverInstanceId, health, errorCount, _rediscoveryNeededUtc, _rediscoveryReason,
-                hostStatuses, skippedTags);
+                hostStatuses, skippedTags, diagnostics);
         }
         catch (Exception ex)
         {
@@ -1418,7 +1447,8 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
 
     /// <summary>Fingerprint of the last <see cref="PublishHealthSnapshot"/> call; null until first publish.</summary>
     private (DriverState State, DateTime? LastSuccess, string? LastError, int ErrorCount,
-        DateTime? RediscoveryNeededUtc, string? HostStatusDigest, string? SkippedTagDigest)? _lastPublishedFingerprint;
+        DateTime? RediscoveryNeededUtc, string? HostStatusDigest, string? SkippedTagDigest,
+        string? DiagnosticsDigest)? _lastPublishedFingerprint;
 
     /// <inheritdoc />
     protected override void PostStop()

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7TsapAndPduTests.cs

@@ -140,13 +140,16 @@ public sealed class S7TsapAndPduTests
         using var drv = new S7Driver(opts, "s7-pdu", factory);
         drv.NegotiatedPduSize.ShouldBe(0);
         drv.GetDiagnostics()["S7.NegotiatedPduSize"].ShouldBe(0);
+        ((IDriverDiagnostics)drv).GetDiagnostics().ShouldBeEmpty("0 is 'not negotiated yet', not a size (#640)");
 
         await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
         drv.NegotiatedPduSize.ShouldBe(480);
         drv.GetDiagnostics()["S7.NegotiatedPduSize"].ShouldBe(480);
+        ((IDriverDiagnostics)drv).GetDiagnostics()["S7.NegotiatedPduSize"].ShouldBe("480");
 
         await drv.ShutdownAsync(TestContext.Current.CancellationToken);
         drv.NegotiatedPduSize.ShouldBe(0);
+        ((IDriverDiagnostics)drv).GetDiagnostics().ShouldBeEmpty();
     }
 
     [Fact]

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Hosts/HostsDriverViewTests.cs

@@ -272,6 +272,24 @@ public sealed class HostsDriverViewTests
         row.SkippedTags!.SkippedCount.ShouldBe(20);
     }
 
+    /// <summary>#640: diagnostics ride the snapshot onto the row, ordered by key; none renders nothing.</summary>
+    [Fact]
+    public void Diagnostics_reach_the_row_ordered_and_absent_diagnostics_yield_no_items()
+    {
+        var with = Snap("MAIN", "s7-a") with
+        {
+            Diagnostics = new Dictionary<string, string> { ["Z"] = "2", ["S7.NegotiatedPduSize"] = "480" },
+        };
+        var without = Snap("MAIN", "s7-b");
+
+        var rows = HostsDriverView.Build([with, without], nodes: null, instances: null).Single().Drivers;
+
+        var a = rows.Single(r => r.DriverInstanceId == "s7-a");
+        a.DiagnosticItems.Select(k => k.Key).ShouldBe(["S7.NegotiatedPduSize", "Z"]);
+        a.DiagnosticItems[0].Value.ShouldBe("480");
+        rows.Single(r => r.DriverInstanceId == "s7-b").DiagnosticItems.ShouldBeEmpty();
+    }
+
     /// <summary>
     ///     A truncated sample must SAY it is truncated. Without the note a capped list reads as the whole
     ///     list, and an operator who fixes the twenty named tags believes the job is done.

tests/Server/ZB.MOM.WW.OtOpcUa.ControlPlane.Tests/Telemetry/TelemetryProtoMapCentralTests.cs

@@ -276,6 +276,43 @@ public sealed class TelemetryProtoMapCentralTests
         TelemetryProtoMapCentral.ToHealth(absentSkips).SkippedTags.ShouldBeNull();
     }
 
+    /// <summary>
+    ///     #640: the diagnostics bag round-trips with the null vs empty distinction intact, and a pre-#640
+    ///     node (field never set) decodes to null.
+    /// </summary>
+    [Fact]
+    public void ToHealth_diagnostics_round_trip_with_the_null_vs_empty_distinction_intact()
+    {
+        var populated = new DriverHealth
+        {
+            ClusterId = "c1", DriverInstanceId = "d1", State = "Healthy",
+            PublishedUtc = Timestamp.FromDateTime(SampleUtc),
+            HasDiagnostics = true,
+            Diagnostics = { ["S7.NegotiatedPduSize"] = "480", ["Other"] = "x" },
+        };
+        var mapped = TelemetryProtoMapCentral.ToHealth(populated).Diagnostics;
+        mapped.ShouldNotBeNull();
+        mapped!.Count.ShouldBe(2);
+        mapped["S7.NegotiatedPduSize"].ShouldBe("480");
+
+        var empty = new DriverHealth
+        {
+            ClusterId = "c1", DriverInstanceId = "d1", State = "Healthy",
+            PublishedUtc = Timestamp.FromDateTime(SampleUtc),
+            HasDiagnostics = true,
+        };
+        var mappedEmpty = TelemetryProtoMapCentral.ToHealth(empty).Diagnostics;
+        mappedEmpty.ShouldNotBeNull();
+        mappedEmpty!.ShouldBeEmpty();
+
+        var absent = new DriverHealth
+        {
+            ClusterId = "c1", DriverInstanceId = "d1", State = "Healthy",
+            PublishedUtc = Timestamp.FromDateTime(SampleUtc),
+        };
+        TelemetryProtoMapCentral.ToHealth(absent).Diagnostics.ShouldBeNull();
+    }
+
     /// <summary>
     ///     An unparseable host state degrades to <see cref="HostState.Unknown"/> rather than throwing. A node
     ///     running a newer build that added an enum member must not be able to kill central's telemetry

tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Grpc/TelemetryStreamGrpcServiceTests.cs

@@ -248,6 +248,33 @@ public sealed class TelemetryStreamGrpcServiceTests
             .DriverHealth.HasHostStatuses.ShouldBeFalse();
     }
 
+    /// <summary>
+    ///     Node side of the #640 diagnostics carry: presence flag + entries written; a capability-less driver
+    ///     (null) leaves the flag clear, and an empty bag still sets it.
+    /// </summary>
+    [Fact]
+    public void ToProto_health_writes_the_diagnostics_presence_flag_and_entries()
+    {
+        var withDiag = new DriverHealthChanged(
+            "cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow,
+            Diagnostics: new Dictionary<string, string> { ["S7.NegotiatedPduSize"] = "480" });
+        var evt = TelemetryProtoMapNode.ToProto(new TelemetryItem.Health(withDiag), "c");
+        evt.DriverHealth.HasDiagnostics.ShouldBeTrue();
+        evt.DriverHealth.Diagnostics["S7.NegotiatedPduSize"].ShouldBe("480");
+
+        var empty = new DriverHealthChanged(
+            "cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow,
+            Diagnostics: new Dictionary<string, string>());
+        var emptyEvt = TelemetryProtoMapNode.ToProto(new TelemetryItem.Health(empty), "c");
+        emptyEvt.DriverHealth.HasDiagnostics.ShouldBeTrue();
+        emptyEvt.DriverHealth.Diagnostics.ShouldBeEmpty();
+
+        var none = new DriverHealthChanged("cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow);
+        var noneEvt = TelemetryProtoMapNode.ToProto(new TelemetryItem.Health(none), "c");
+        noneEvt.DriverHealth.HasDiagnostics.ShouldBeFalse();
+        noneEvt.DriverHealth.Diagnostics.ShouldBeEmpty();
+    }
+
     [Fact]
     public async Task Client_disconnect_mid_stream_ends_cleanly_without_leaking_a_slot()
     {

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverInstanceActorDiagnosticsTests.cs

@@ -0,0 +1,187 @@
+using Akka.Actor;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Runtime.Drivers;
+using ZB.MOM.WW.OtOpcUa.Runtime.Tests.Harness;
+
+namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.Drivers;
+
+/// <summary>
+///     Covers the <see cref="IDriverDiagnostics"/> consumer (Gitea #640). A diagnostic fact (S7's negotiated
+///     PDU size) moves no other fingerprint component, so unless its digest is in
+///     <c>PublishHealthSnapshot</c>'s fingerprint the dedup swallows the very publish that carries it.
+///     <para>The stub's health is deliberately STABLE: the shared <c>StubDriver.GetHealth()</c> returns
+///     <c>UtcNow</c>, so the dedup never engages and a test on it passes vacuously.</para>
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class DriverInstanceActorDiagnosticsTests : RuntimeActorTestBase
+{
+    /// <summary>Base case: the bag reaches the publisher.</summary>
+    [Fact]
+    public void Diagnostics_reach_the_health_publisher()
+    {
+        var driver = new DiagStubDriver();
+        driver.Set(("S7.NegotiatedPduSize", "480"));
+        var publisher = new RecordingHealthPublisher();
+        var actor = SpawnDriverActor(driver, publisher);
+
+        actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
+
+        AwaitAssert(() =>
+        {
+            var latest = publisher.Published.LastOrDefault();
+            latest.ShouldNotBeNull();
+            latest!["S7.NegotiatedPduSize"].ShouldBe("480");
+        }, TimeSpan.FromSeconds(3));
+    }
+
+    /// <summary>
+    ///     The load-bearing case. Falsifiability: publish count must STRICTLY increase across the change;
+    ///     drop <c>DiagnosticsDigest</c> from the fingerprint and this goes red.
+    /// </summary>
+    [Fact]
+    public void A_diagnostics_change_republishes_despite_otherwise_unchanged_health()
+    {
+        var driver = new DiagStubDriver();
+        driver.Set(("S7.NegotiatedPduSize", "240"));
+        var publisher = new RecordingHealthPublisher();
+        var actor = SpawnDriverActor(driver, publisher);
+
+        actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
+        AwaitAssert(() => publisher.Published.Count.ShouldBeGreaterThan(0), TimeSpan.FromSeconds(3));
+        ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+        var before = publisher.Published.Count;
+
+        driver.Set(("S7.NegotiatedPduSize", "480"));
+
+        AwaitAssert(() => publisher.Published.Count.ShouldBeGreaterThan(before), TimeSpan.FromSeconds(3));
+        publisher.Published[^1]!["S7.NegotiatedPduSize"].ShouldBe("480");
+    }
+
+    /// <summary>Equal-but-distinct bags must be deduped: the digest compares by VALUE, not reference.</summary>
+    [Fact]
+    public void An_unchanged_diagnostics_bag_does_not_defeat_the_dedup()
+    {
+        var driver = new DiagStubDriver();
+        driver.Set(("S7.NegotiatedPduSize", "480"));
+        var publisher = new RecordingHealthPublisher();
+        var actor = SpawnDriverActor(driver, publisher);
+
+        actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
+        AwaitAssert(() => publisher.Published.Count.ShouldBeGreaterThan(0), TimeSpan.FromSeconds(3));
+        ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+        var before = publisher.Published.Count;
+
+        driver.Set(("S7.NegotiatedPduSize", "480"));
+
+        ExpectNoMsg(TimeSpan.FromMilliseconds(600));
+        publisher.Published.Count.ShouldBe(before);
+    }
+
+    /// <summary>A driver without the capability publishes null, not an empty bag.</summary>
+    [Fact]
+    public void A_driver_without_the_capability_publishes_null()
+    {
+        var publisher = new RecordingHealthPublisher();
+        var actor = SpawnDriverActor(new StubDriver(), publisher);
+
+        actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
+        AwaitAssert(() => publisher.Published.Count.ShouldBeGreaterThan(0), TimeSpan.FromSeconds(3));
+
+        publisher.Published.ShouldAllBe(p => p == null);
+    }
+
+    /// <summary>A throwing implementation loses only the diagnostics, not the health publish.</summary>
+    [Fact]
+    public void A_throwing_diagnostics_provider_degrades_to_null_without_killing_the_health_publish()
+    {
+        var driver = new DiagStubDriver { Throw = true };
+        var publisher = new RecordingHealthPublisher();
+        var actor = SpawnDriverActor(driver, publisher);
+
+        actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
+
+        AwaitAssert(() => publisher.Published.Count.ShouldBeGreaterThan(0), TimeSpan.FromSeconds(3));
+        publisher.Published[^1].ShouldBeNull();
+    }
+
+    /// <summary>The bag is capped and truncated before it is published.</summary>
+    [Fact]
+    public void Oversized_bags_are_bounded()
+    {
+        var driver = new DiagStubDriver();
+        driver.Set([.. Enumerable.Range(0, 40).Select(i => ($"k{i:D2}", new string('x', 500)))]);
+        var publisher = new RecordingHealthPublisher();
+        var actor = SpawnDriverActor(driver, publisher);
+
+        actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
+
+        AwaitAssert(() =>
+        {
+            var latest = publisher.Published.LastOrDefault();
+            latest.ShouldNotBeNull();
+            latest!.Count.ShouldBe(DriverDiagnostics.MaxEntries);
+            latest.Values.ShouldAllBe(v => v.Length == DriverDiagnostics.MaxValueLength);
+        }, TimeSpan.FromSeconds(3));
+    }
+
+    private IActorRef SpawnDriverActor(IDriver driver, IDriverHealthPublisher publisher)
+    {
+        var parent = CreateTestProbe();
+        parent.IgnoreMessages(_ => true);
+        return parent.ChildActorOf(DriverInstanceActor.Props(
+            driver, healthPublisher: publisher, healthPollInterval: TimeSpan.FromMilliseconds(50)));
+    }
+
+    private sealed class RecordingHealthPublisher : IDriverHealthPublisher
+    {
+        private readonly List<IReadOnlyDictionary<string, string>?> _published = [];
+
+        public IReadOnlyList<IReadOnlyDictionary<string, string>?> Published
+        {
+            get { lock (_published) return _published.ToArray(); }
+        }
+
+        public void Publish(
+            string clusterId,
+            string driverInstanceId,
+            DriverHealth health,
+            int errorCount5Min,
+            DateTime? rediscoveryNeededUtc = null,
+            string? rediscoveryReason = null,
+            IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
+            SkippedTagReport? skippedTags = null,
+            IReadOnlyDictionary<string, string>? diagnostics = null)
+        {
+            lock (_published) _published.Add(diagnostics);
+        }
+    }
+
+    /// <summary>Stable-health driver exposing <see cref="IDriverDiagnostics"/>.</summary>
+    private sealed class DiagStubDriver : IDriver, IDriverDiagnostics
+    {
+        private static readonly DateTime FixedLastRead = new(2026, 10, 8, 12, 0, 0, DateTimeKind.Utc);
+        private volatile IReadOnlyDictionary<string, string> _bag = new Dictionary<string, string>();
+
+        public bool Throw { get; init; }
+
+        public string DriverInstanceId => "diag-stub-1";
+
+        public string DriverType => "Stub";
+
+        public void Set(params (string Key, string Value)[] items) =>
+            _bag = items.ToDictionary(i => i.Key, i => i.Value);
+
+        public IReadOnlyDictionary<string, string> GetDiagnostics() =>
+            Throw ? throw new InvalidOperationException("broken") : _bag;
+
+        public Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken) => Task.CompletedTask;
+
+        public Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken) => Task.CompletedTask;
+
+        public Task ShutdownAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+        public DriverHealth GetHealth() => new(DriverState.Healthy, FixedLastRead, null);
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverInstanceActorHostStatusTests.cs

@@ -211,7 +211,8 @@ public sealed class DriverInstanceActorHostStatusTests : RuntimeActorTestBase
             DateTime? rediscoveryNeededUtc = null,
             string? rediscoveryReason = null,
             IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-            SkippedTagReport? skippedTags = null)
+            SkippedTagReport? skippedTags = null,
+            IReadOnlyDictionary<string, string>? diagnostics = null)
         {
             lock (_published) _published.Add(new HealthPublish(health, hostStatuses));
         }

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverInstanceActorRediscoverySignalTests.cs

@@ -95,19 +95,20 @@ public sealed class DriverInstanceActorRediscoverySignalTests : RuntimeActorTest
         var publisher = new RecordingHealthPublisher();
         var parent = CreateTestProbe();
         parent.IgnoreMessages(_ => true);
-        var actor = parent.ChildActorOf(DriverInstanceActor.Props(driver, healthPublisher: publisher));
+        var actor = parent.ChildActorOf(DriverInstanceActor.Props(driver, healthPublisher: publisher,
+            healthPollInterval: TimeSpan.FromMilliseconds(200)));
 
         actor.Tell(new DriverInstanceActor.InitializeRequested("{}"));
         AwaitAssert(() => publisher.Published.Count.ShouldBeGreaterThan(0), TimeSpan.FromSeconds(3));
         driver.RaiseRediscovery("agent-instance-changed");
         AwaitAssert(
-            () => publisher.Published.Any(p => p.RediscoveryNeededUtc is not null),
+            () => publisher.Published.Any(p => p.RediscoveryNeededUtc is not null).ShouldBeTrue(),
             TimeSpan.FromSeconds(3));
 
         // Force a further publish by changing something else about the driver's health.
         driver.SetLastError("transient read failure");
         AwaitAssert(
-            () => publisher.Published.Any(p => p.LastError == "transient read failure"),
+            () => publisher.Published.Any(p => p.LastError == "transient read failure").ShouldBeTrue(),
             TimeSpan.FromSeconds(3));
 
         var latest = publisher.Published[^1];
@@ -187,7 +188,8 @@ public sealed class DriverInstanceActorRediscoverySignalTests : RuntimeActorTest
             DateTime? rediscoveryNeededUtc = null,
             string? rediscoveryReason = null,
             IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-            SkippedTagReport? skippedTags = null)
+            SkippedTagReport? skippedTags = null,
+            IReadOnlyDictionary<string, string>? diagnostics = null)
         {
             lock (_published)
             {

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverInstanceActorSkippedTagTests.cs

@@ -209,7 +209,8 @@ public sealed class DriverInstanceActorSkippedTagTests : RuntimeActorTestBase
             DateTime? rediscoveryNeededUtc = null,
             string? rediscoveryReason = null,
             IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-            SkippedTagReport? skippedTags = null)
+            SkippedTagReport? skippedTags = null,
+            IReadOnlyDictionary<string, string>? diagnostics = null)
         {
             lock (_published) _published.Add(new HealthPublish(health, skippedTags));
         }

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverInstanceActorSubscriptionReconcileTests.cs

@@ -124,7 +124,8 @@ public sealed class DriverInstanceActorSubscriptionReconcileTests : RuntimeActor
             DateTime? rediscoveryNeededUtc = null,
             string? rediscoveryReason = null,
             IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-            SkippedTagReport? skippedTags = null)
+            SkippedTagReport? skippedTags = null,
+            IReadOnlyDictionary<string, string>? diagnostics = null)
             => Interlocked.Increment(ref _count);
     }
 }