feat(cluster): simultaneous-cold-start split-brain guard (opt-in)

Closes the residual Phase-7 gap: when BOTH nodes of a 2-node pair cold-start at the
same instant, each self-first seed runs FirstSeedNodeProcess, times out waiting for
the other, and forms its own 1-node cluster — two Primaries in one pair (the Phase 6
live gate reproduced this reliably on docker). The prior mitigation was operational
(staggered start / compose depends_on), which does not exist on production hardware.

The guard (dark switch Cluster:BootstrapGuard:Enabled, default OFF) prevents the split
without giving up cold-start-alone:
- The lower-address node is the preferred founder: self-first, forms immediately.
- The higher node probes its partner's Akka port (TCP connect) up to PartnerProbeSeconds:
  reachable => peer-first (join the founder, never race it); unreachable => self-first
  (partner is dead, form alone). The order is decided BEFORE the single JoinSeedNodes,
  from an explicit reachability signal — never re-formed mid-handshake (the retired
  SelfFormAfter failure mode). When on, Akka gets no config seeds (BuildClusterOptions)
  and ClusterBootstrapCoordinator drives the join.

Review-driven hardening: case-insensitive tie-break (a hostname-casing mismatch would
reopen the split); fail-fast validation of the timing knobs; the residual "founder dies
in the probe->join window" hang is documented and made operator-visible (warning + a
restart recovers). Real-ActorSystem coordinator tests cover the load-bearing higher-node
cold-start-alone case; 147/147 Cluster tests pass.

Live-gated on docker-dev (site-a = enablement demo, serialization removed, guard on;
site-b keeps depends_on-serialization + guard off as the A/B control): simultaneous
start -> site-a-1 founds, site-a-2 probes-reachable-joins -> 250/240, NO split;
higher-node-alone -> probes dead founder, self-forms -> 250; founder rejoin -> 240.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

docker-dev/docker-compose.yml

@@ -516,6 +516,12 @@ services:
       # FetchAndCache, never gossip; central and site-a share no seed list.
       Cluster__SeedNodes__0: "akka.tcp://otopcua@site-a-1:4053"
       Cluster__SeedNodes__1: "akka.tcp://otopcua@site-a-2:4053"
+      # Simultaneous-cold-start split-brain guard (SITE-A enablement demo). With the pair no longer
+      # startup-serialized, this is what stops both self-first seeds forming their own 1-node cluster:
+      # the lower-address node (site-a-1) founds immediately; the higher (site-a-2) probes site-a-1 and
+      # joins it when reachable, or forms alone only if site-a-1 is truly down. Akka gets NO config
+      # seeds when this is on (BuildClusterOptions); ClusterBootstrapCoordinator issues JoinSeedNodes.
+      Cluster__BootstrapGuard__Enabled: "true"
       Cluster__Roles__0: "driver"
       Cluster__Roles__1: "cluster-SITE-A"
       # Mesh command transport (per-cluster mesh Phase 2, made mandatory by Phase 6) — see
@@ -588,13 +594,11 @@ services:
       sql: { condition: service_healthy }
       central-1: { condition: service_started }
       migrator: { condition: service_completed_successfully }
-      # Serialize the SITE-A pair's startup so site-a-1 forms the mesh first and site-a-2 JOINS it —
+      # SITE-A is the BOOTSTRAP-GUARD enablement demo: NO startup serialization on the pair — both
-      # exactly as central-2 waits on central-1. Both nodes are self-first seeds (seed[0]=self), so BOTH
+      # nodes start simultaneously (the exact race that split-brained in the Phase 6 gate). The
-      # run Akka's FirstSeedNodeProcess and, if they start simultaneously, EACH forms its own single-node
+      # Cluster:BootstrapGuard (enabled in both site-a env blocks) is the ONLY thing preventing the
-      # cluster (split brain — observed in the Phase 6 live gate: both logged "JOINING itself ... forming
+      # split: it makes the lower-address node the founder and the higher-address node probe-then-join.
-      # a new cluster", both elected themselves Primary → 250/250 instead of 250/240). Waiting on site-a-1
+      # site-b keeps the depends_on:service_healthy serialization + guard OFF, as the A/B control.
-      # gives it the head start to be Up before site-a-2's InitJoin, so site-a-2 joins the existing mesh.
-      site-a-1: { condition: service_healthy }
     environment:
       OTOPCUA_ROLES: "driver,cluster-SITE-A"
       # Per-cluster mesh Phase 4: no ConfigDb string on driver-only site nodes (see site-a-1).
@@ -617,6 +621,9 @@ services:
       # second) — it no longer seeds off central-1.
       Cluster__SeedNodes__0: "akka.tcp://otopcua@site-a-2:4053"
       Cluster__SeedNodes__1: "akka.tcp://otopcua@site-a-1:4053"
+      # Bootstrap-guard enablement demo — see site-a-1. site-a-2 is the HIGHER address, so the guard
+      # makes it probe site-a-1 and join it (peer-first) rather than race-form its own cluster.
+      Cluster__BootstrapGuard__Enabled: "true"
       Cluster__Roles__0: "driver"
       Cluster__Roles__1: "cluster-SITE-A"
       # Mesh command transport (per-cluster mesh Phase 2, made mandatory by Phase 6) — see

docs/Redundancy.md

@@ -401,11 +401,61 @@ Comparing against `Cluster:Hostname` would be worse than wrong: in docker-dev it
 no seed URI anywhere, so the rule would silently exempt the entire rig.
 
 Sequential recovery is island-free by construction: a `FirstSeedNodeProcess` node self-joins only when **no**
-seed answered, so a peer booting after the survivor formed simply joins it as the youngest member. Both
+seed answered, so a peer booting after the survivor formed simply joins it as the youngest member.
-nodes cold-starting *simultaneously* converge on one cluster while they are mutually reachable — the
+
-`InitJoin` handshake resolves it before either self-join deadline expires. The residual risk is a boot-time
+**Simultaneous cold-start CAN split — this is the residual gap the bootstrap guard below closes.** When
-**partition** (both cold-starting, mutually unreachable): both form, which is the same dual-active class the
+BOTH nodes start from nothing at the same instant, each runs `FirstSeedNodeProcess`, and if neither has
-`auto-down` strategy already accepts, with the same recovery (restart one side).
+formed before the other's `seed-node-timeout` expires, EACH self-joins and forms its own single-node
+cluster — two Primaries in one pair. On in-process loopback the `InitJoin` handshake usually resolves fast
+enough to converge, but the Phase 6 live gate reproduced the split reliably on the docker-dev rig (real
+container start timing + DNS): both nodes logged `JOINING itself … forming a new cluster` and both advertised
+ServiceLevel 250. Two independent clusters do **not** auto-merge, so the split persists until an operator
+restarts one side.
+
+### Bootstrap guard (`Cluster:BootstrapGuard`, opt-in)
+
+The guard eliminates the simultaneous-start split without giving up cold-start-alone. It is a **dark switch,
+default off** — a node with `Cluster:BootstrapGuard:Enabled=false` (the default) keeps Akka's config-driven
+self-first auto-join exactly as above.
+
+When enabled, the node is given **no config seed nodes** (so Akka does not auto-join), and
+`ClusterBootstrapCoordinator` picks the join order from a deterministic address tie-break plus a reachability
+probe:
+
+- The node with the lexicographically **lower** canonical `host:port` is the **preferred founder**: it joins
+  self-first and forms immediately if no peer answers — no probe, no delay.
+- The **higher** node probes its partner's Akka port (a plain TCP connect; the partner binds its port well
+  before it forms) for up to `PartnerProbeSeconds` (default 25 s): **reachable ⇒ peer-first** (it joins the
+  founder, never races it); **unreachable after the window ⇒ self-first** (the partner is genuinely down, so
+  it forms alone — cold-start-alone preserved for the higher node too).
+
+The order is decided **before** issuing a single `Cluster.JoinSeedNodes`, from an explicit reachability
+signal — it never re-decides mid-handshake, the failure mode that retired the `SelfFormAfter` watchdog.
+
+**Residual trade-off (accepted, operator-visible):** once the higher node commits peer-first it cannot
+self-form (Akka's `JoinSeedNodeProcess` retries forever). If the founder dies in the small probe→join window,
+the higher node hangs unjoined; the coordinator logs a clear WARNING after a bounded grace, and a **restart
+recovers it** (the guard re-runs, finds the founder down, and forms alone). Config keys:
+
+| Key | Default | Notes |
+|---|---|---|
+| `Cluster:BootstrapGuard:Enabled` | `false` | Dark switch. Only meaningful on a node that is one of its own two pair seeds; inert elsewhere. |
+| `Cluster:BootstrapGuard:PartnerProbeSeconds` | `25` | Higher node's probe window. Must comfortably exceed the founder's process-start-to-Akka-bind time, or a slow-but-alive founder is mistaken for dead and the split re-opens. Validated `> 0` at boot. |
+| `Cluster:BootstrapGuard:PartnerProbeIntervalMs` | `500` | Interval between probes. |
+| `Cluster:BootstrapGuard:ProbeConnectTimeoutMs` | `1000` | Per-probe TCP connect timeout. |
+
+On the docker-dev rig the **site-a pair is the enablement demo** (guard on, startup serialization removed so
+both nodes cold-start simultaneously); **site-b keeps the `depends_on: service_healthy` startup serialization
+with the guard off**, as the A/B control. Either mechanism prevents the split; the guard is the one that also
+works on production hardware, where compose `depends_on` does not exist.
+
+The alternative to the guard is **operational**: stagger the two VMs' service-manager start, or bring the
+designated founder VM up first — see the Phase 7 co-located operator runbook
+(`docs/plans/2026-07-24-mesh-phase7-failover-drills.md`).
+
+The residual risk that remains even with the guard is a boot-time **partition** (both cold-starting, mutually
+unreachable from the start): both form, which is the same dual-active class the `auto-down` strategy already
+accepts, with the same recovery (restart one side).
 
 Pinned by `SelfFirstSeedBootstrapTests` (`tests/Core/ZB.MOM.WW.OtOpcUa.Cluster.Tests/`), which starts real
 in-process clusters through the production bootstrap at production failure-detection timings: a lone

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/AkkaClusterOptions.cs

@@ -79,4 +79,46 @@ public sealed class AkkaClusterOptions
     ///     </para>
     /// </remarks>
     public string SplitBrainResolverStrategy { get; set; } = "auto-down";
+
+    /// <summary>
+    /// The simultaneous-cold-start split-brain guard (<c>Cluster:BootstrapGuard</c>). Default OFF — a
+    /// dark switch, so existing deployments and tests keep Akka's config-driven self-first auto-join
+    /// unchanged. See <see cref="ClusterBootstrapGuard"/> for the decision logic and
+    /// <c>ClusterBootstrapCoordinator</c> for the runtime.
+    /// </summary>
+    public ClusterBootstrapGuardOptions BootstrapGuard { get; set; } = new();
+}
+
+/// <summary>
+/// Configuration for the simultaneous-cold-start split-brain guard. When
+/// <see cref="Enabled"/>, the node does NOT auto-join from its config seeds; a coordinator picks the
+/// join order (founder self-first / joiner peer-first) after a reachability probe. See
+/// <see cref="ClusterBootstrapGuard"/>.
+/// </summary>
+public sealed class ClusterBootstrapGuardOptions
+{
+    /// <summary>
+    /// Gets or sets whether the guard is active. Default <see langword="false"/> — Akka auto-joins from
+    /// the config seed list exactly as before. Only meaningful on a node that is one of its own two
+    /// pair seeds; inert everywhere else.
+    /// </summary>
+    public bool Enabled { get; set; }
+
+    /// <summary>
+    /// Gets or sets how long the higher-address node probes its partner's Akka endpoint before
+    /// concluding the partner is dead and forming alone. Must comfortably exceed the partner's
+    /// worst-case process-start-to-Akka-bind time so a slow-but-alive partner is never mistaken for a
+    /// dead one (which would re-open the split). Default 25 s.
+    /// </summary>
+    public int PartnerProbeSeconds { get; set; } = 25;
+
+    /// <summary>
+    /// Gets or sets the interval between partner reachability probes, in milliseconds. Default 500 ms.
+    /// </summary>
+    public int PartnerProbeIntervalMs { get; set; } = 500;
+
+    /// <summary>
+    /// Gets or sets the per-probe TCP connect timeout, in milliseconds. Default 1000 ms.
+    /// </summary>
+    public int ProbeConnectTimeoutMs { get; set; } = 1000;
 }

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/AkkaClusterOptionsValidator.cs

@@ -41,6 +41,8 @@ public sealed class AkkaClusterOptionsValidator : OptionsValidatorBase<AkkaClust
     {
         ArgumentNullException.ThrowIfNull(options);
 
+        ValidateBootstrapGuard(builder, options.BootstrapGuard);
+
         var seeds = options.SeedNodes ?? Array.Empty<string>();
         if (seeds.Length == 0)
         {
@@ -68,6 +70,25 @@ public sealed class AkkaClusterOptionsValidator : OptionsValidatorBase<AkkaClust
             + "the entries — see docs/Redundancy.md → 'Bootstrap: self-first seed ordering'.");
     }
 
+    /// <summary>
+    /// When the bootstrap guard is enabled, its timing knobs must be positive. A zero/negative
+    /// <c>PartnerProbeSeconds</c> in particular silently degrades the guard to "never wait, always
+    /// conclude the partner is dead" — it would form alone immediately and re-open the very split it
+    /// exists to close. Fail-fast at boot rather than producing silence (the same rationale every
+    /// sibling options validator in this project cites).
+    /// </summary>
+    private static void ValidateBootstrapGuard(ValidationBuilder builder, ClusterBootstrapGuardOptions guard)
+    {
+        if (guard is null || !guard.Enabled) return;
+
+        if (guard.PartnerProbeSeconds <= 0)
+            builder.Add($"Cluster:BootstrapGuard:PartnerProbeSeconds must be > 0 when the guard is enabled (was {guard.PartnerProbeSeconds}).");
+        if (guard.PartnerProbeIntervalMs <= 0)
+            builder.Add($"Cluster:BootstrapGuard:PartnerProbeIntervalMs must be > 0 when the guard is enabled (was {guard.PartnerProbeIntervalMs}).");
+        if (guard.ProbeConnectTimeoutMs <= 0)
+            builder.Add($"Cluster:BootstrapGuard:ProbeConnectTimeoutMs must be > 0 when the guard is enabled (was {guard.ProbeConnectTimeoutMs}).");
+    }
+
     /// <summary>
     /// True when <paramref name="seed"/> addresses this node itself — host AND port.
     /// </summary>

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/ClusterBootstrapCoordinator.cs

@@ -0,0 +1,201 @@
+using System.Diagnostics;
+using System.Net.Sockets;
+using Akka.Actor;
+using Akka.Cluster;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace ZB.MOM.WW.OtOpcUa.Cluster;
+
+/// <summary>
+/// Runs the simultaneous-cold-start split-brain guard: when <c>Cluster:BootstrapGuard:Enabled</c>, the
+/// node starts with NO config seed nodes (see <c>BuildClusterOptions</c>), and this coordinator picks
+/// the join order and issues the single <see cref="Akka.Cluster.Cluster.JoinSeedNodes"/> once the
+/// ActorSystem is up. See <see cref="ClusterBootstrapGuard"/> for the decision logic and the
+/// split-brain rationale.
+/// </summary>
+/// <remarks>
+///     <para>
+///         The founder (lower canonical address) joins its self-first order immediately. The higher
+///         node probes its partner's Akka endpoint (a plain TCP connect — the partner has bound its
+///         port well before it forms) up to <see cref="ClusterBootstrapGuardOptions.PartnerProbeSeconds"/>:
+///         reachable ⇒ peer-first (join the founder); unreachable ⇒ self-first (partner is dead, form
+///         alone). The join runs on a background task so it never blocks host startup, and it is issued
+///         exactly once — the coordinator never re-forms a node mid-handshake, the failure mode of the
+///         retired <c>SelfFormAfter</c> watchdog.
+///     </para>
+/// </remarks>
+public sealed class ClusterBootstrapCoordinator : IHostedService
+{
+    private readonly Func<ActorSystem> _system;
+    private readonly AkkaClusterOptions _options;
+    private readonly ILogger<ClusterBootstrapCoordinator> _log;
+    private readonly CancellationTokenSource _cts = new();
+    private Task? _joinTask;
+
+    /// <summary>Creates the coordinator.</summary>
+    /// <param name="system">Lazy accessor for the node's ActorSystem (resolved after Akka's hosted service starts it).</param>
+    /// <param name="options">The bound cluster options (seed list + guard settings).</param>
+    /// <param name="log">Logger for the bootstrap decision.</param>
+    public ClusterBootstrapCoordinator(
+        Func<ActorSystem> system, IOptions<AkkaClusterOptions> options, ILogger<ClusterBootstrapCoordinator> log)
+    {
+        _system = system ?? throw new ArgumentNullException(nameof(system));
+        _options = options?.Value ?? throw new ArgumentNullException(nameof(options));
+        _log = log ?? throw new ArgumentNullException(nameof(log));
+    }
+
+    /// <inheritdoc/>
+    public Task StartAsync(CancellationToken cancellationToken)
+    {
+        if (!_options.BootstrapGuard.Enabled)
+            return Task.CompletedTask; // dark switch off — Akka auto-joined from config seeds already.
+
+        // Fire-and-forget so a higher node with a dead partner (which waits the full probe window)
+        // never blocks host startup. Exceptions are logged; a failed join leaves the node unjoined,
+        // which is visible and recoverable, never a silent split.
+        _joinTask = Task.Run(() => RunAsync(_cts.Token), _cts.Token);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc/>
+    public async Task StopAsync(CancellationToken cancellationToken)
+    {
+        await _cts.CancelAsync().ConfigureAwait(false);
+        if (_joinTask is not null)
+        {
+            try { await _joinTask.ConfigureAwait(false); }
+            catch (OperationCanceledException) { /* expected on shutdown */ }
+        }
+    }
+
+    private async Task RunAsync(CancellationToken ct)
+    {
+        try
+        {
+            var seeds = _options.SeedNodes ?? Array.Empty<string>();
+            var role = ClusterBootstrapGuard.Analyze(seeds, _options.PublicHostname, _options.Port);
+
+            string[] order;
+            var committedPeerFirst = false;
+            if (!role.Applies)
+            {
+                // Not a pair seed (single-seed legacy site node, self absent, malformed, or 3+ seeds):
+                // join the configured seeds unchanged — the guard arbitrates only the 2-node pair race.
+                order = seeds;
+                _log.LogInformation(
+                    "Bootstrap guard: not a pair seed ({SeedCount} seed(s)); joining configured seeds unchanged.",
+                    seeds.Length);
+            }
+            else if (role.IsFounder)
+            {
+                order = role.SelfFirstOrder;
+                _log.LogInformation(
+                    "Bootstrap guard: this node is the preferred founder (lower address); joining self-first, forming immediately if no peer answers.");
+            }
+            else
+            {
+                var reachable = await ProbePartnerAsync(role.PartnerHost!, role.PartnerPort, ct).ConfigureAwait(false);
+                order = ClusterBootstrapGuard.HigherNodeOrder(role, reachable);
+                committedPeerFirst = reachable;
+                _log.LogInformation(
+                    "Bootstrap guard: this node is the higher address; partner {PartnerHost}:{PartnerPort} was {Reachability} within the probe window; joining {Order}.",
+                    role.PartnerHost, role.PartnerPort, reachable ? "REACHABLE" : "UNREACHABLE",
+                    reachable ? "peer-first (join the founder)" : "self-first (partner down, forming alone)");
+            }
+
+            if (order.Length == 0)
+            {
+                _log.LogWarning("Bootstrap guard: no seed nodes to join; node stays unjoined until a seed is configured.");
+                return;
+            }
+
+            var cluster = Akka.Cluster.Cluster.Get(_system());
+            var addresses = order.Select(Address.Parse).ToArray();
+            cluster.JoinSeedNodes(addresses);
+
+            // Peer-first is a one-way commitment: JoinSeedNodeProcess never self-forms. If the founder
+            // died in the probe→join window, this node hangs unjoined forever. We do NOT re-form here
+            // (the retired SelfFormAfter mid-handshake failure mode) — we make the hang operator-visible
+            // so a restart, which re-runs the guard against the now-dead founder and self-forms, recovers
+            // it. Nothing is done on the founder / self-first paths, which always self-form on their own.
+            if (committedPeerFirst)
+                await WarnIfNotUpAsync(cluster, ct).ConfigureAwait(false);
+        }
+        catch (OperationCanceledException)
+        {
+            // Host shutting down before the join completed — nothing to do.
+        }
+        catch (Exception ex)
+        {
+            _log.LogError(ex, "Bootstrap guard: failed to issue JoinSeedNodes; node remains unjoined.");
+        }
+    }
+
+    /// <summary>
+    /// After committing peer-first, waits a bounded time for this node to reach <c>Up</c> and logs a
+    /// clear warning if it does not — the founder must have died in the probe→join window, leaving this
+    /// node hung in <c>JoinSeedNodeProcess</c>. Read-only: it never re-forms or re-joins (that is the
+    /// retired mid-handshake failure mode); it only surfaces the condition so an operator restarts the node.
+    /// </summary>
+    private async Task WarnIfNotUpAsync(Akka.Cluster.Cluster cluster, CancellationToken ct)
+    {
+        // Give the join a generous grace: the founder's own self-form (seed-node-timeout) plus this
+        // node's join round-trip. Two probe windows is comfortably beyond both.
+        var grace = TimeSpan.FromSeconds(Math.Max(10, _options.BootstrapGuard.PartnerProbeSeconds * 2));
+        var sw = Stopwatch.StartNew();
+        while (!ct.IsCancellationRequested && sw.Elapsed < grace)
+        {
+            if (cluster.SelfMember.Status == MemberStatus.Up) return; // joined — all good
+            try { await Task.Delay(TimeSpan.FromSeconds(1), ct).ConfigureAwait(false); }
+            catch (OperationCanceledException) { return; }
+        }
+
+        if (!ct.IsCancellationRequested && cluster.SelfMember.Status != MemberStatus.Up)
+            _log.LogWarning(
+                "Bootstrap guard: committed peer-first but this node is still {Status} after {Grace}s — its founder likely died in the probe→join window. This node will NOT self-form on its own (peer-first never does). RESTART it to recover: the guard will re-run, find the founder down, and form alone.",
+                cluster.SelfMember.Status, (int)grace.TotalSeconds);
+    }
+
+    /// <summary>
+    /// Polls the partner's Akka endpoint with a plain TCP connect until it is reachable or the probe
+    /// window elapses. Reachable-at-TCP is a sound proxy for "the partner is coming up": a node binds
+    /// its Akka port near the start of startup, well before it forms or joins a cluster.
+    /// </summary>
+    private async Task<bool> ProbePartnerAsync(string host, int port, CancellationToken ct)
+    {
+        var window = TimeSpan.FromSeconds(Math.Max(0, _options.BootstrapGuard.PartnerProbeSeconds));
+        var interval = TimeSpan.FromMilliseconds(Math.Max(50, _options.BootstrapGuard.PartnerProbeIntervalMs));
+        var sw = Stopwatch.StartNew();
+
+        while (!ct.IsCancellationRequested)
+        {
+            if (await TryConnectAsync(host, port, ct).ConfigureAwait(false)) return true;
+            if (sw.Elapsed >= window) return false;
+            try { await Task.Delay(interval, ct).ConfigureAwait(false); }
+            catch (OperationCanceledException) { return false; }
+        }
+        return false;
+    }
+
+    private async Task<bool> TryConnectAsync(string host, int port, CancellationToken ct)
+    {
+        try
+        {
+            using var client = new TcpClient();
+            using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+            cts.CancelAfter(Math.Max(100, _options.BootstrapGuard.ProbeConnectTimeoutMs));
+            await client.ConnectAsync(host, port, cts.Token).ConfigureAwait(false);
+            return client.Connected;
+        }
+        catch (OperationCanceledException) when (ct.IsCancellationRequested)
+        {
+            throw; // host shutdown — propagate
+        }
+        catch
+        {
+            return false; // connect refused / timed out / DNS not resolvable yet — partner not up
+        }
+    }
+}

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/ClusterBootstrapGuard.cs

@@ -0,0 +1,153 @@
+using System.Text.RegularExpressions;
+
+namespace ZB.MOM.WW.OtOpcUa.Cluster;
+
+/// <summary>
+/// Pure decision logic for the simultaneous-cold-start split-brain guard.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Both nodes of a 2-node pair are self-first seeds so <b>either</b> can cold-start alone when
+///         its partner is dead (see <see cref="AkkaClusterOptions.SeedNodes"/>). The cost is that when
+///         BOTH cold-start at the same instant, each runs Akka's <c>FirstSeedNodeProcess</c>, times out
+///         waiting for the other, and forms its OWN single-node cluster — a split brain (two Primaries
+///         in one pair). Two independent clusters do not auto-merge, so the split persists until an
+///         operator intervenes.
+///     </para>
+///     <para>
+///         This guard breaks the symmetry deterministically without giving up cold-start-alone. The
+///         node with the lexicographically <b>lower</b> canonical address is the <i>preferred
+///         founder</i>: it always uses self-first order and forms immediately. The <b>higher</b> node
+///         first probes whether its partner's Akka endpoint is reachable (see
+///         <see cref="ClusterBootstrapCoordinator"/>): reachable ⇒ peer-first order so it JOINS the
+///         founder rather than racing it; unreachable after the probe window ⇒ the partner is genuinely
+///         down, so it falls back to self-first and forms alone. Exactly one node founds when both are
+///         present; the lower node always founds when it starts alone.
+///     </para>
+///     <para>
+///         <b>Residual trade-off (accepted).</b> Once the higher node observes the partner reachable it
+///         commits to peer-first — Akka's <c>JoinSeedNodeProcess</c> retries <c>InitJoin</c> forever and
+///         never self-forms. If the founder dies in the small window between the probe succeeding and
+///         the join completing, the higher node hangs unjoined until it is restarted (a restart re-runs
+///         the guard, finds the founder unreachable, and self-forms). We deliberately do NOT re-decide
+///         mid-handshake — that is exactly the retired <c>SelfFormAfter</c> failure mode. Instead
+///         <see cref="ClusterBootstrapCoordinator"/> makes the hang operator-visible with a warning if
+///         the node has not come Up within a bounded time after committing peer-first.
+///     </para>
+///     <para>
+///         This decides the join order BEFORE issuing a single <c>JoinSeedNodes</c>, from an explicit
+///         reachability signal — unlike the retired <c>SelfFormAfter</c> watchdog, which fired a
+///         <c>Join(self)</c> mid-handshake on a bare timeout and could not tell "no seed answered" from
+///         "a join is in flight", islanding a node a failover had just bounced.
+///     </para>
+/// </remarks>
+public static class ClusterBootstrapGuard
+{
+    private static readonly Regex SeedPattern =
+        new(@"^akka(?:\.[a-z0-9]+)?://[^@/]+@(?<host>[^:/]+):(?<port>\d+)", RegexOptions.IgnoreCase | RegexOptions.Compiled);
+
+    /// <summary>The analyzed bootstrap role of this node within its seed list.</summary>
+    /// <param name="Applies">True only when the guard engages: exactly two seeds, one of them this node.</param>
+    /// <param name="IsFounder">True when this node is the preferred founder (lower address) — form immediately, no probe.</param>
+    /// <param name="PartnerHost">The partner's advertised host (to probe when this node is the higher one); null when not applicable.</param>
+    /// <param name="PartnerPort">The partner's Akka port.</param>
+    /// <param name="SelfFirstOrder"><c>[self, partner]</c> — self-first; forms a new cluster when no peer answers.</param>
+    /// <param name="PeerFirstOrder"><c>[partner, self]</c> — peer-first; joins the partner's cluster, never self-forms while it is up.</param>
+    public sealed record BootstrapRole(
+        bool Applies,
+        bool IsFounder,
+        string? PartnerHost,
+        int PartnerPort,
+        string[] SelfFirstOrder,
+        string[] PeerFirstOrder);
+
+    /// <summary>
+    /// Analyzes the configured seed list to decide this node's bootstrap role. The guard engages only
+    /// for the Phase-6 pair shape (exactly two seeds, one of them this node); any other shape
+    /// (single-seed legacy site node, three+ seeds, self absent) returns <see cref="BootstrapRole.Applies"/>
+    /// = false and the caller joins the configured seeds unchanged.
+    /// </summary>
+    /// <param name="seeds">The configured seed URIs (self-first by convention, but order is not relied on here).</param>
+    /// <param name="selfHost">This node's advertised host (<see cref="AkkaClusterOptions.PublicHostname"/>).</param>
+    /// <param name="selfPort">This node's Akka port (<see cref="AkkaClusterOptions.Port"/>).</param>
+    /// <returns>The analyzed role; never null.</returns>
+    public static BootstrapRole Analyze(IReadOnlyList<string> seeds, string selfHost, int selfPort)
+    {
+        ArgumentNullException.ThrowIfNull(seeds);
+        ArgumentException.ThrowIfNullOrWhiteSpace(selfHost);
+
+        var na = new BootstrapRole(false, false, null, 0, Array.Empty<string>(), Array.Empty<string>());
+        if (seeds.Count != 2) return na;
+
+        var selfKey = Key(selfHost, selfPort);
+        string? self = null, partner = null;
+        string? partnerHost = null;
+        var partnerPort = 0;
+
+        foreach (var seed in seeds)
+        {
+            if (!TryParse(seed, out var host, out var port)) return na;
+            if (string.Equals(Key(host, port), selfKey, StringComparison.OrdinalIgnoreCase))
+                self = seed;
+            else
+            {
+                partner = seed;
+                partnerHost = host;
+                partnerPort = port;
+            }
+        }
+
+        // Self must be exactly one of the two, and the other must be a distinct partner.
+        if (self is null || partner is null || partnerHost is null) return na;
+
+        var selfFirst = new[] { self, partner };
+        var peerFirst = new[] { partner, self };
+
+        // Deterministic tie-break: the lower canonical address is the preferred founder. Both nodes
+        // compute the same comparison (same two seeds), so exactly one is the founder. Case-INSENSITIVE
+        // to match how self/partner are classified above (and AkkaClusterOptionsValidator.IsSelf):
+        // container/DNS hostnames are conventionally case-insensitive, and a casing difference between
+        // the two sides' seed config must NOT make both think they are the founder (which would reopen
+        // the very split this guard closes).
+        var isFounder = string.Compare(
+            Key(selfHost, selfPort),
+            Key(partnerHost, partnerPort),
+            StringComparison.OrdinalIgnoreCase) < 0;
+
+        return new BootstrapRole(true, isFounder, partnerHost, partnerPort, selfFirst, peerFirst);
+    }
+
+    /// <summary>
+    /// The order the higher (non-founder) node uses once it has learned whether its partner is
+    /// reachable: peer-first when the founder is up (join it), self-first when the founder is absent
+    /// (form alone — cold-start-alone preserved).
+    /// </summary>
+    /// <param name="role">The analyzed role (must be applicable and NOT the founder).</param>
+    /// <param name="partnerReachable">Whether the partner's Akka endpoint became reachable within the probe window.</param>
+    /// <returns>The seed order to pass to <c>Cluster.JoinSeedNodes</c>.</returns>
+    public static string[] HigherNodeOrder(BootstrapRole role, bool partnerReachable)
+    {
+        ArgumentNullException.ThrowIfNull(role);
+        return partnerReachable ? role.PeerFirstOrder : role.SelfFirstOrder;
+    }
+
+    /// <summary>Parses <c>akka.tcp://system@host:port[/...]</c> into host + port.</summary>
+    /// <param name="seed">The seed URI.</param>
+    /// <param name="host">The parsed advertised host.</param>
+    /// <param name="port">The parsed Akka port.</param>
+    /// <returns>True when the seed matched the expected shape.</returns>
+    public static bool TryParse(string? seed, out string host, out int port)
+    {
+        host = string.Empty;
+        port = 0;
+        if (string.IsNullOrWhiteSpace(seed)) return false;
+
+        var m = SeedPattern.Match(seed.Trim());
+        if (!m.Success) return false;
+        if (!int.TryParse(m.Groups["port"].Value, out port)) return false;
+        host = m.Groups["host"].Value;
+        return true;
+    }
+
+    private static string Key(string host, int port) => $"{host}:{port}";
+}

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/ServiceCollectionExtensions.cs

@@ -241,7 +241,13 @@ public static class ServiceCollectionExtensions
 
         return new ClusterOptions
         {
-            SeedNodes = options.SeedNodes,
+            // When the bootstrap guard is on, DO NOT hand Akka the seed list: Akka.Cluster.Hosting
+            // auto-joins from ClusterOptions.SeedNodes the instant the ActorSystem starts, which is
+            // exactly the self-first race the guard exists to arbitrate. Left empty, the node starts
+            // unjoined and ClusterBootstrapCoordinator picks the order (founder self-first / joiner
+            // peer-first, after a reachability probe) and issues the single Cluster.JoinSeedNodes.
+            // The config seed list is still read — by the coordinator, off AkkaClusterOptions.SeedNodes.
+            SeedNodes = options.BootstrapGuard.Enabled ? Array.Empty<string>() : options.SeedNodes,
             Roles = options.Roles,
             SplitBrainResolver = IsKeepOldest(options) ? new KeepOldestOption { DownIfAlone = true } : null,
         };

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -390,6 +390,16 @@ builder.Services.AddAkka("otopcua", (ab, sp) =>
     }
 });
 
+// Simultaneous-cold-start split-brain guard (dark switch Cluster:BootstrapGuard:Enabled, default off).
+// Registered AFTER AddAkka so its StartAsync runs once Akka's hosted service has built (and, when the
+// guard is off, already auto-joined) the ActorSystem. When on, the node started with no config seeds
+// (BuildClusterOptions) and this coordinator issues the single reachability-gated JoinSeedNodes. It
+// no-ops when the guard is off, so registering it unconditionally is safe.
+builder.Services.AddHostedService(sp => new ClusterBootstrapCoordinator(
+    () => sp.GetRequiredService<ActorSystem>(),
+    sp.GetRequiredService<IOptions<AkkaClusterOptions>>(),
+    sp.GetRequiredService<ILogger<ClusterBootstrapCoordinator>>()));
+
 // Down-if-alone recovery watchdog (#459). Registered AFTER AddAkka so it starts after Akka's own
 // hosted service has built the ActorSystem; it resolves the system lazily (never at construction) so
 // it can't race startup. On an unexpected SBR self-down it stops the host so the service supervisor

tests/Core/ZB.MOM.WW.OtOpcUa.Cluster.Tests/AkkaClusterOptionsValidatorTests.cs

@@ -174,4 +174,44 @@ public sealed class AkkaClusterOptionsValidatorTests
 
         result.Failed.ShouldBeFalse(result.FailureMessage);
     }
+
+    /// <summary>The bootstrap-guard timing knobs default to positive values and pass when enabled.</summary>
+    [Fact]
+    public void Bootstrap_guard_defaults_pass()
+    {
+        var options = CentralNode("site-a-1", Seed("site-a-1"), Seed("site-a-2"));
+        options.BootstrapGuard = new ClusterBootstrapGuardOptions { Enabled = true };
+
+        var result = new AkkaClusterOptionsValidator().Validate(null, options);
+
+        result.Failed.ShouldBeFalse(result.FailureMessage);
+    }
+
+    /// <summary>
+    /// A zero <c>PartnerProbeSeconds</c> silently degrades the guard to "never wait, form alone
+    /// immediately" — re-opening the split. It must fail the host at boot, not run degraded.
+    /// </summary>
+    [Fact]
+    public void Bootstrap_guard_with_non_positive_probe_seconds_fails()
+    {
+        var options = CentralNode("site-a-1", Seed("site-a-1"), Seed("site-a-2"));
+        options.BootstrapGuard = new ClusterBootstrapGuardOptions { Enabled = true, PartnerProbeSeconds = 0 };
+
+        var result = new AkkaClusterOptionsValidator().Validate(null, options);
+
+        result.Failed.ShouldBeTrue();
+        result.FailureMessage.ShouldContain("PartnerProbeSeconds");
+    }
+
+    /// <summary>The timing knobs are NOT validated when the guard is off — a disabled guard is inert.</summary>
+    [Fact]
+    public void Bootstrap_guard_disabled_does_not_validate_timings()
+    {
+        var options = CentralNode("site-a-1", Seed("site-a-1"), Seed("site-a-2"));
+        options.BootstrapGuard = new ClusterBootstrapGuardOptions { Enabled = false, PartnerProbeSeconds = 0 };
+
+        var result = new AkkaClusterOptionsValidator().Validate(null, options);
+
+        result.Failed.ShouldBeFalse(result.FailureMessage);
+    }
 }

tests/Core/ZB.MOM.WW.OtOpcUa.Cluster.Tests/ClusterBootstrapCoordinatorTests.cs

@@ -0,0 +1,204 @@
+using System.Net;
+using System.Net.Sockets;
+using Akka.Actor;
+using Akka.Cluster;
+using Akka.Hosting;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Cluster.Tests;
+
+/// <summary>
+/// Real-ActorSystem tests for the simultaneous-cold-start split-brain guard
+/// (<see cref="ClusterBootstrapCoordinator"/> + <see cref="ClusterBootstrapGuard"/>), started through
+/// the SAME wiring as production (<see cref="ServiceCollectionExtensions.WithOtOpcUaClusterBootstrap"/>
+/// with <c>Cluster:BootstrapGuard:Enabled</c> + the coordinator hosted service). This subsystem has a
+/// history of bugs invisible to pure unit tests, so the load-bearing correctness properties are
+/// asserted against running nodes, mirroring <see cref="SelfFirstSeedBootstrapTests"/>.
+/// </summary>
+/// <remarks>
+/// Ephemeral loopback ports are 5-digit, so numeric order equals the guard's ordinal "host:port"
+/// string order — <c>Min(port)</c> is the founder, <c>Max(port)</c> is the higher (probing) node.
+/// </remarks>
+public sealed class ClusterBootstrapCoordinatorTests
+{
+    private static int FreePort()
+    {
+        var listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+        listener.Stop();
+        return port;
+    }
+
+    /// <summary>
+    /// Starts a node with the bootstrap guard ENABLED, wired exactly as Program.cs does: empty Akka
+    /// seed list (via <c>BuildClusterOptions</c>) plus the coordinator hosted service that issues the
+    /// reachability-gated <c>JoinSeedNodes</c>. Seeds are listed self-first (as every shipped config
+    /// is); the guard, not the order, decides founder vs. joiner.
+    /// </summary>
+    private static async Task<IHost> StartGuardedNodeAsync(
+        string systemName, int selfPort, int peerPort, int probeSeconds = 4)
+    {
+        var self = $"akka.tcp://{systemName}@127.0.0.1:{selfPort}";
+        var peer = $"akka.tcp://{systemName}@127.0.0.1:{peerPort}";
+        var options = new AkkaClusterOptions
+        {
+            SystemName = systemName,
+            Hostname = "127.0.0.1",
+            PublicHostname = "127.0.0.1",
+            Port = selfPort,
+            Roles = new[] { "driver" },
+            SeedNodes = new[] { self, peer },
+            BootstrapGuard = new ClusterBootstrapGuardOptions
+            {
+                Enabled = true,
+                PartnerProbeSeconds = probeSeconds,
+                PartnerProbeIntervalMs = 250,
+                ProbeConnectTimeoutMs = 500,
+            },
+        };
+
+        var builder = Host.CreateDefaultBuilder();
+        builder.ConfigureServices(services =>
+        {
+            services.AddSingleton<IOptions<AkkaClusterOptions>>(Options.Create(options));
+            services.AddAkka(systemName, (ab, sp) => ab.WithOtOpcUaClusterBootstrap(sp));
+            // Mirror Program.cs — the coordinator is what drives the guarded join.
+            services.AddHostedService(sp => new ClusterBootstrapCoordinator(
+                () => sp.GetRequiredService<ActorSystem>(),
+                sp.GetRequiredService<IOptions<AkkaClusterOptions>>(),
+                sp.GetRequiredService<ILogger<ClusterBootstrapCoordinator>>()));
+        });
+
+        var host = builder.Build();
+        await host.StartAsync();
+        return host;
+    }
+
+    private static async Task<bool> WaitForUpMembersAsync(IHost host, int expected, TimeSpan timeout)
+    {
+        var cluster = Akka.Cluster.Cluster.Get(host.Services.GetRequiredService<ActorSystem>());
+        var deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            if (cluster.State.Members.Count(m => m.Status == MemberStatus.Up) >= expected) return true;
+            await Task.Delay(200);
+        }
+        return cluster.State.Members.Count(m => m.Status == MemberStatus.Up) >= expected;
+    }
+
+    private static async Task StopAsync(IHost host)
+    {
+        try { await host.StopAsync(); }
+        catch (Exception) { /* teardown only */ }
+        host.Dispose();
+    }
+
+    /// <summary>
+    /// The FOUNDER (lower address) with the guard on forms a cluster alone when its partner is dead —
+    /// exactly as un-guarded self-first does. The guard must not regress the preferred founder.
+    /// </summary>
+    [Fact]
+    public async Task Founder_forms_alone_when_partner_is_dead()
+    {
+        var low = FreePort();
+        var high = FreePort();
+        var founderPort = Math.Min(low, high);
+        var deadPeerPort = Math.Max(low, high); // nothing listening
+
+        var host = await StartGuardedNodeAsync("otopcua-guard-1", founderPort, deadPeerPort);
+        try
+        {
+            (await WaitForUpMembersAsync(host, 1, TimeSpan.FromSeconds(30)))
+                .ShouldBeTrue("the founder (lower address) must self-form immediately when its partner is down");
+        }
+        finally { await StopAsync(host); }
+    }
+
+    /// <summary>
+    /// THE LOAD-BEARING CASE. The HIGHER node with the guard on must still cold-start ALONE when its
+    /// partner is genuinely dead: it probes the (dead) founder, times out, and falls back to self-first.
+    /// If the guard's peer-first logic were wrong, this node would hang in JoinSeedNodeProcess forever —
+    /// the very regression a naive "always let the lower node found" rule would introduce.
+    /// </summary>
+    [Fact]
+    public async Task Higher_node_forms_alone_when_partner_is_dead_after_probing()
+    {
+        var low = FreePort();
+        var high = FreePort();
+        var higherPort = Math.Max(low, high);
+        var deadFounderPort = Math.Min(low, high); // the founder is dead
+
+        var host = await StartGuardedNodeAsync("otopcua-guard-2", higherPort, deadFounderPort, probeSeconds: 3);
+        try
+        {
+            // Must come Up AFTER the probe window (~3 s) expires and it falls back to self-first.
+            (await WaitForUpMembersAsync(host, 1, TimeSpan.FromSeconds(30)))
+                .ShouldBeTrue("the higher node must fall back to self-first and form alone once the dead partner never answers the probe");
+        }
+        finally { await StopAsync(host); }
+    }
+
+    /// <summary>
+    /// FALSIFIABILITY CONTROL for the test above: the higher node does NOT form alone during the probe
+    /// window — it is genuinely waiting/probing, not self-forming immediately (which would mean the
+    /// guard is inert). If this ever starts seeing a member before the window, the probe is not gating.
+    /// </summary>
+    [Fact]
+    public async Task Higher_node_does_not_form_before_the_probe_window_expires()
+    {
+        var low = FreePort();
+        var high = FreePort();
+        var higherPort = Math.Max(low, high);
+        var deadFounderPort = Math.Min(low, high);
+
+        var host = await StartGuardedNodeAsync("otopcua-guard-3", higherPort, deadFounderPort, probeSeconds: 10);
+        try
+        {
+            // Well within the 10 s probe window: still no cluster, because it is probing the dead founder.
+            (await WaitForUpMembersAsync(host, 1, TimeSpan.FromSeconds(3)))
+                .ShouldBeFalse("the higher node must probe for the founder, not self-form immediately");
+        }
+        finally { await StopAsync(host); }
+    }
+
+    /// <summary>
+    /// The higher node JOINS a live founder rather than forming a second cluster: start the founder,
+    /// then the higher node — its probe finds the founder reachable, it commits peer-first, and the
+    /// pair converges to ONE cluster of two.
+    /// </summary>
+    [Fact]
+    public async Task Higher_node_joins_a_live_founder()
+    {
+        const string systemName = "otopcua-guard-4";
+        var low = FreePort();
+        var high = FreePort();
+        var founderPort = Math.Min(low, high);
+        var higherPort = Math.Max(low, high);
+
+        var founder = await StartGuardedNodeAsync(systemName, founderPort, higherPort);
+        IHost? higher = null;
+        try
+        {
+            (await WaitForUpMembersAsync(founder, 1, TimeSpan.FromSeconds(30)))
+                .ShouldBeTrue("the founder must be Up before the higher node probes it");
+
+            higher = await StartGuardedNodeAsync(systemName, higherPort, founderPort);
+
+            (await WaitForUpMembersAsync(higher, 2, TimeSpan.FromSeconds(60)))
+                .ShouldBeTrue("the higher node must join the founder, forming one cluster of two");
+            (await WaitForUpMembersAsync(founder, 2, TimeSpan.FromSeconds(60)))
+                .ShouldBeTrue("the founder must see the higher node as a member of ITS cluster");
+        }
+        finally
+        {
+            if (higher is not null) await StopAsync(higher);
+            await StopAsync(founder);
+        }
+    }
+}

tests/Core/ZB.MOM.WW.OtOpcUa.Cluster.Tests/ClusterBootstrapGuardTests.cs

@@ -0,0 +1,137 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Cluster;
+
+namespace ZB.MOM.WW.OtOpcUa.Cluster.Tests;
+
+/// <summary>
+/// Unit tests for <see cref="ClusterBootstrapGuard"/> — the pure decision core of the
+/// simultaneous-cold-start split-brain guard. The runtime probe + JoinSeedNodes wiring lives in
+/// <c>ClusterBootstrapCoordinator</c> and is covered by the live gate.
+/// </summary>
+public sealed class ClusterBootstrapGuardTests
+{
+    private const string A = "akka.tcp://otopcua@site-a-1:4053";
+    private const string B = "akka.tcp://otopcua@site-a-2:4053";
+
+    [Fact]
+    public void Lower_address_node_is_the_founder_and_needs_no_probe()
+    {
+        // site-a-1 < site-a-2, so on site-a-1 the guard makes it the founder.
+        var role = ClusterBootstrapGuard.Analyze(new[] { A, B }, "site-a-1", 4053);
+
+        role.Applies.ShouldBeTrue();
+        role.IsFounder.ShouldBeTrue();
+        role.SelfFirstOrder.ShouldBe(new[] { A, B });
+    }
+
+    [Fact]
+    public void Higher_address_node_is_not_the_founder_and_targets_the_partner_for_probing()
+    {
+        // On site-a-2, the partner is site-a-1 (the lower/founder).
+        var role = ClusterBootstrapGuard.Analyze(new[] { B, A }, "site-a-2", 4053);
+
+        role.Applies.ShouldBeTrue();
+        role.IsFounder.ShouldBeFalse();
+        role.PartnerHost.ShouldBe("site-a-1");
+        role.PartnerPort.ShouldBe(4053);
+    }
+
+    [Fact]
+    public void Tie_break_is_symmetric_regardless_of_seed_order()
+    {
+        // Both nodes must agree on who founds no matter how each lists its seeds.
+        var onLower = ClusterBootstrapGuard.Analyze(new[] { A, B }, "site-a-1", 4053);
+        var onHigher = ClusterBootstrapGuard.Analyze(new[] { B, A }, "site-a-2", 4053);
+
+        onLower.IsFounder.ShouldBeTrue();
+        onHigher.IsFounder.ShouldBeFalse();
+        // Exactly one founder.
+        (onLower.IsFounder ^ onHigher.IsFounder).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Higher_node_joins_the_founder_when_partner_is_reachable()
+    {
+        var role = ClusterBootstrapGuard.Analyze(new[] { B, A }, "site-a-2", 4053);
+
+        // Reachable ⇒ peer-first ⇒ JoinSeedNodeProcess ⇒ joins the founder, never self-forms.
+        ClusterBootstrapGuard.HigherNodeOrder(role, partnerReachable: true)
+            .ShouldBe(new[] { A, B }); // partner (site-a-1) first
+    }
+
+    [Fact]
+    public void Higher_node_forms_alone_when_partner_is_unreachable()
+    {
+        var role = ClusterBootstrapGuard.Analyze(new[] { B, A }, "site-a-2", 4053);
+
+        // Unreachable after the window ⇒ partner is dead ⇒ self-first ⇒ cold-start-alone preserved.
+        ClusterBootstrapGuard.HigherNodeOrder(role, partnerReachable: false)
+            .ShouldBe(new[] { B, A }); // self (site-a-2) first
+    }
+
+    [Fact]
+    public void Guard_does_not_apply_to_a_single_seed_node()
+    {
+        // A driver-only site node seeded solely by central-1 is not a pair seed — guard is inert.
+        var role = ClusterBootstrapGuard.Analyze(new[] { "akka.tcp://otopcua@central-1:4053" }, "site-x", 4053);
+
+        role.Applies.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Guard_does_not_apply_when_self_is_absent_from_the_two_seeds()
+    {
+        var role = ClusterBootstrapGuard.Analyze(new[] { A, B }, "central-1", 4053);
+
+        role.Applies.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Guard_does_not_apply_to_three_or_more_seeds()
+    {
+        var role = ClusterBootstrapGuard.Analyze(
+            new[] { A, B, "akka.tcp://otopcua@site-a-3:4053" }, "site-a-1", 4053);
+
+        role.Applies.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Guard_does_not_apply_when_a_seed_is_malformed()
+    {
+        var role = ClusterBootstrapGuard.Analyze(new[] { A, "not-a-seed" }, "site-a-1", 4053);
+
+        role.Applies.ShouldBeFalse();
+    }
+
+    [Theory]
+    [InlineData("akka.tcp://otopcua@site-a-1:4053", "site-a-1", 4053)]
+    [InlineData("akka.tcp://otopcua@10.0.0.5:4053/", "10.0.0.5", 4053)]
+    [InlineData("  akka.tcp://otopcua@host.lan:1234  ", "host.lan", 1234)]
+    public void TryParse_extracts_host_and_port(string seed, string expectedHost, int expectedPort)
+    {
+        ClusterBootstrapGuard.TryParse(seed, out var host, out var port).ShouldBeTrue();
+        host.ShouldBe(expectedHost);
+        port.ShouldBe(expectedPort);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("garbage")]
+    [InlineData("akka.tcp://otopcua@host")] // no port
+    public void TryParse_rejects_malformed_seeds(string seed)
+    {
+        ClusterBootstrapGuard.TryParse(seed, out _, out _).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Port_participates_in_the_tie_break_when_hosts_are_equal()
+    {
+        // Shared-host loopback pair (test rig): same host, different ports — port breaks the tie.
+        const string low = "akka.tcp://otopcua@127.0.0.1:4053";
+        const string high = "akka.tcp://otopcua@127.0.0.1:4054";
+
+        ClusterBootstrapGuard.Analyze(new[] { low, high }, "127.0.0.1", 4053).IsFounder.ShouldBeTrue();
+        ClusterBootstrapGuard.Analyze(new[] { high, low }, "127.0.0.1", 4054).IsFounder.ShouldBeFalse();
+    }
+}