diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Transport/IStaleInstanceProbe.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Transport/IStaleInstanceProbe.cs
new file mode 100644
index 00000000..d7e8903b
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Transport/IStaleInstanceProbe.cs
@@ -0,0 +1,32 @@
+namespace ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
+
+/// <summary>
+/// Computes the CURRENT flattened-config revision hash of a deployed instance —
+/// the same value the deployment pipeline produces and stores in
+/// <c>DeployedConfigSnapshot.RevisionHash</c>. Transport's bundle importer uses
+/// this to enumerate stale instances after a template overwrite (#16): an
+/// instance is stale when its freshly-computed hash no longer matches the hash
+/// captured at deploy time.
+/// <para>
+/// The seam lives in Commons so the Transport component can depend on it without
+/// referencing the DeploymentManager project (where the flattening pipeline that
+/// implements it lives). The implementation re-flattens the instance via the
+/// existing flattening pipeline over the SAME scoped <c>DbContext</c>, so it sees
+/// any template rows the in-flight import has staged on the change tracker but
+/// not yet committed.
+/// </para>
+/// </summary>
+public interface IStaleInstanceProbe
+{
+    /// <summary>
+    /// Computes the current flattened-config revision hash for the given instance,
+    /// or <c>null</c> when the hash cannot be computed (e.g. the instance or its
+    /// template chain cannot be resolved). Callers treat a <c>null</c> result as
+    /// "cannot determine staleness" and skip the instance — staleness is
+    /// informational, never a reason to abort an import.
+    /// </summary>
+    /// <param name="instanceId">The target instance's surrogate id.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The current revision hash (e.g. <c>sha256:...</c>), or <c>null</c>.</returns>
+    Task<string?> GetCurrentRevisionHashAsync(int instanceId, CancellationToken cancellationToken = default);
+}
diff --git a/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/ServiceCollectionExtensions.cs b/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/ServiceCollectionExtensions.cs
index 916af136..5b8d5195 100644
--- a/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/ServiceCollectionExtensions.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/ServiceCollectionExtensions.cs
@@ -1,4 +1,5 @@
 using Microsoft.Extensions.DependencyInjection;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
 
 namespace ZB.MOM.WW.ScadaBridge.DeploymentManager;
 
@@ -40,6 +41,13 @@ public static class ServiceCollectionExtensions
         services.AddScoped<IFlatteningPipeline, FlatteningPipeline>();
         services.AddScoped<DeploymentService>();
         services.AddScoped<ArtifactDeploymentService>();
+
+        // #16 (M8 D2): expose the flattening pipeline's revision-hash computation
+        // to the Transport bundle importer through the Commons IStaleInstanceProbe
+        // seam, so a template overwrite can enumerate the deployed instances whose
+        // flattened config has drifted from their deployed snapshot. Scoped so it
+        // shares the per-request DbContext with the in-flight import.
+        services.AddScoped<IStaleInstanceProbe, StaleInstanceProbe>();
         return services;
     }
 
diff --git a/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/StaleInstanceProbe.cs b/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/StaleInstanceProbe.cs
new file mode 100644
index 00000000..87d501ee
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.DeploymentManager/StaleInstanceProbe.cs
@@ -0,0 +1,38 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
+
+namespace ZB.MOM.WW.ScadaBridge.DeploymentManager;
+
+/// <summary>
+/// <see cref="IStaleInstanceProbe"/> backed by the deployment flattening pipeline.
+/// Re-flattens an instance through <see cref="IFlatteningPipeline"/> and returns
+/// the computed revision hash — the exact same value
+/// <c>DeploymentService.GetDeploymentComparisonAsync</c> compares against the
+/// deployed snapshot to decide staleness. Hosted in DeploymentManager so the
+/// Transport component (which references only Commons + TemplateEngine) can probe
+/// staleness without taking a DeploymentManager project reference.
+/// </summary>
+public sealed class StaleInstanceProbe : IStaleInstanceProbe
+{
+    private readonly IFlatteningPipeline _flatteningPipeline;
+
+    /// <summary>Initializes a new <see cref="StaleInstanceProbe"/>.</summary>
+    /// <param name="flatteningPipeline">The deployment flattening pipeline used to recompute the current revision hash.</param>
+    public StaleInstanceProbe(IFlatteningPipeline flatteningPipeline)
+    {
+        _flatteningPipeline = flatteningPipeline ?? throw new ArgumentNullException(nameof(flatteningPipeline));
+    }
+
+    /// <inheritdoc />
+    public async Task<string?> GetCurrentRevisionHashAsync(int instanceId, CancellationToken cancellationToken = default)
+    {
+        // The pipeline returns a Result; a flatten failure (e.g. unresolvable
+        // template chain mid-import) yields null so the caller treats the
+        // instance as "staleness indeterminate" and skips it. Flattening reuses
+        // the scoped ITemplateEngineRepository, so it observes template rows the
+        // in-flight import has staged on the shared change tracker.
+        var result = await _flatteningPipeline
+            .FlattenAndValidateAsync(instanceId, cancellationToken)
+            .ConfigureAwait(false);
+        return result.IsSuccess ? result.Value.RevisionHash : null;
+    }
+}
diff --git a/src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleImporter.cs b/src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleImporter.cs
index 634ace03..c91d64e5 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleImporter.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleImporter.cs
@@ -1,6 +1,7 @@
 using System.IO.Compression;
 using System.Security.Cryptography;
 using System.Text.Json;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.ExternalSystems;
@@ -73,6 +74,11 @@ public sealed class BundleImporter : IBundleImporter
     private readonly IOptions<TransportOptions> _options;
     private readonly TimeProvider _timeProvider;
     private readonly SemanticValidator _semanticValidator;
+    // #16 (M8 D2): optional. Implemented in DeploymentManager (where the flattening
+    // pipeline lives) and surfaced via the Commons IStaleInstanceProbe seam. Null
+    // in a host that registers Transport without DeploymentManager — staleness is
+    // then best-effort empty (informational only, never gates the import).
+    private readonly IStaleInstanceProbe? _staleInstanceProbe;
     private readonly ILogger<BundleImporter>? _logger;
 
     /// <summary>
@@ -95,6 +101,8 @@ public sealed class BundleImporter : IBundleImporter
     /// <param name="correlationContext">Correlation context that carries the active BundleImportId.</param>
     /// <param name="dbContext">EF Core context used to commit the import transaction.</param>
     /// <param name="semanticValidator">Validates template references before applying.</param>
+    /// <param name="staleInstanceProbe">Optional (#16): recomputes a deployed instance's current revision hash so the importer can enumerate instances stale-ed by a template overwrite. Null when no flattening pipeline is registered (Transport-without-DeploymentManager hosts) — staleness is then skipped.</param>
+    /// <param name="logger">Optional logger.</param>
     public BundleImporter(
         BundleSerializer bundleSerializer,
         ManifestValidator manifestValidator,
@@ -113,6 +121,7 @@ public sealed class BundleImporter : IBundleImporter
         IAuditCorrelationContext correlationContext,
         ScadaBridgeDbContext dbContext,
         SemanticValidator semanticValidator,
+        IStaleInstanceProbe? staleInstanceProbe = null,
         ILogger<BundleImporter>? logger = null)
     {
         _bundleSerializer = bundleSerializer ?? throw new ArgumentNullException(nameof(bundleSerializer));
@@ -132,6 +141,7 @@ public sealed class BundleImporter : IBundleImporter
         _correlationContext = correlationContext ?? throw new ArgumentNullException(nameof(correlationContext));
         _dbContext = dbContext ?? throw new ArgumentNullException(nameof(dbContext));
         _semanticValidator = semanticValidator ?? throw new ArgumentNullException(nameof(semanticValidator));
+        _staleInstanceProbe = staleInstanceProbe;
         _logger = logger;
     }
 
@@ -1047,6 +1057,8 @@ public sealed class BundleImporter : IBundleImporter
             // Array.Empty<int>() placeholder). The single deferred SaveChangesAsync
             // is the next statement, so a read against the change tracker here sees
             // the full post-apply graph before commit.
+            var staleInstanceIds = await ComputeStaleInstanceIdsAsync(
+                content, resolutionMap, ct).ConfigureAwait(false);
 
             await _auditService.LogAsync(
                 user: user,
@@ -1089,7 +1101,7 @@ public sealed class BundleImporter : IBundleImporter
                 Overwritten: summary.Overwritten,
                 Skipped: summary.Skipped,
                 Renamed: summary.Renamed,
-                StaleInstanceIds: Array.Empty<int>(),
+                StaleInstanceIds: staleInstanceIds,
                 AuditEventCorrelation: bundleImportId.ToString(),
                 ApiKeysIgnored: apiKeysIgnored);
         }
@@ -1197,6 +1209,113 @@ public sealed class BundleImporter : IBundleImporter
         }
     }
 
+    /// <summary>
+    /// #16 (M8 D2): enumerates the deployed instances stale-ed by this import.
+    /// Overwriting a template changes the flattened-config hash of every instance
+    /// of that template, so a deployed instance's freshly-computed revision hash
+    /// will no longer match the hash captured in its <c>DeployedConfigSnapshot</c>
+    /// at deploy time — it surfaces as "stale" on the Deployments page. This walks
+    /// the templates THIS import OVERWROTE, finds each one's deployed instances
+    /// (State <see cref="InstanceState.Enabled"/>/<see cref="InstanceState.Disabled"/>
+    /// with a snapshot), recomputes their current hash through
+    /// <see cref="IStaleInstanceProbe"/> (which reads the just-staged template
+    /// rows on the shared change tracker), and collects the ids whose hash drifted.
+    /// <para>
+    /// Freshly-IMPORTED instances are excluded structurally: they enter the target
+    /// <see cref="InstanceState.NotDeployed"/> (no snapshot), so they're "new", not
+    /// "stale". Best-effort: a probe that returns null or throws for one instance
+    /// is skipped (logged), never aborting the import — staleness is informational
+    /// and the import's own success path is authoritative. Returns an empty list
+    /// when no probe is wired (Transport-without-DeploymentManager hosts).
+    /// </para>
+    /// </summary>
+    private async Task<IReadOnlyList<int>> ComputeStaleInstanceIdsAsync(
+        BundleContentDto content,
+        Dictionary<(string EntityType, string Name), ImportResolution> resolutionMap,
+        CancellationToken ct)
+    {
+        // No flattening pipeline registered → can't recompute hashes. Staleness is
+        // informational, so degrade gracefully to "none".
+        if (_staleInstanceProbe is null || content.Templates.Count == 0)
+        {
+            return Array.Empty<int>();
+        }
+
+        // Which templates did THIS import overwrite? A bundle template is
+        // overwritten when its resolution Action == Overwrite AND a target template
+        // of that name already exists (otherwise the Overwrite branch falls through
+        // to Add — see ApplyTemplatesAsync). Map those names to their target ids.
+        var targetTemplateIdByName = (await _templateRepo.GetAllTemplatesAsync(ct).ConfigureAwait(false))
+            .GroupBy(t => t.Name, StringComparer.Ordinal)
+            .ToDictionary(g => g.Key, g => g.First().Id, StringComparer.Ordinal);
+
+        var overwrittenTemplateIds = new HashSet<int>();
+        foreach (var dto in content.Templates)
+        {
+            var resolution = ResolveOrDefault(resolutionMap, "Template", dto.Name);
+            if (resolution.Action != ResolutionAction.Overwrite) continue;
+            if (targetTemplateIdByName.TryGetValue(dto.Name, out var templateId))
+            {
+                overwrittenTemplateIds.Add(templateId);
+            }
+        }
+
+        if (overwrittenTemplateIds.Count == 0)
+        {
+            return Array.Empty<int>();
+        }
+
+        var staleIds = new List<int>();
+        foreach (var templateId in overwrittenTemplateIds)
+        {
+            var instances = await _templateRepo
+                .GetInstancesByTemplateIdAsync(templateId, ct).ConfigureAwait(false);
+            foreach (var instance in instances)
+            {
+                // NotDeployed instances (including everything this import just
+                // created/overwrote) are "new", not "stale" — skip them.
+                if (instance.State is not (InstanceState.Enabled or InstanceState.Disabled))
+                {
+                    continue;
+                }
+
+                try
+                {
+                    // A deployed instance has a snapshot; its absence means the
+                    // instance is not actually deployed (defensive — State already
+                    // gated this) so there's no baseline to compare against.
+                    var snapshot = await _dbContext.DeployedConfigSnapshots
+                        .AsNoTracking()
+                        .Where(s => s.InstanceId == instance.Id)
+                        .OrderByDescending(s => s.DeployedAt)
+                        .FirstOrDefaultAsync(ct)
+                        .ConfigureAwait(false);
+                    if (snapshot is null) continue;
+
+                    var currentHash = await _staleInstanceProbe
+                        .GetCurrentRevisionHashAsync(instance.Id, ct).ConfigureAwait(false);
+                    // null => hash indeterminate; don't claim staleness either way.
+                    if (currentHash is null) continue;
+
+                    if (!string.Equals(currentHash, snapshot.RevisionHash, StringComparison.Ordinal))
+                    {
+                        staleIds.Add(instance.Id);
+                    }
+                }
+                catch (Exception ex)
+                {
+                    // Best-effort: a single instance's hash failure must not abort
+                    // the (already-successful) import. Skip it and move on.
+                    _logger?.LogWarning(ex,
+                        "Stale-instance hash computation failed for instance {InstanceId} (template {TemplateId}); skipping.",
+                        instance.Id, templateId);
+                }
+            }
+        }
+
+        return staleIds;
+    }
+
     /// <summary>Mutable per-apply counter struct, accumulated through every helper.</summary>
     private sealed class ImportSummary
     {
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/BundleImporterApplyTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/BundleImporterApplyTests.cs
index 023511b6..2c8c0db2 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/BundleImporterApplyTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/BundleImporterApplyTests.cs
@@ -2,7 +2,9 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Deployment;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.ExternalSystems;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Instances;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Scripts;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Templates;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
@@ -13,6 +15,8 @@ using ZB.MOM.WW.ScadaBridge.Commons.Types.Transport;
 using ZB.MOM.WW.ScadaBridge.ConfigurationDatabase;
 using ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Repositories;
 using ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Services;
+using ZB.MOM.WW.ScadaBridge.DeploymentManager;
+using ZB.MOM.WW.ScadaBridge.TemplateEngine;
 using ZB.MOM.WW.ScadaBridge.Transport;
 using ZB.MOM.WW.ScadaBridge.Transport.Import;
 using ZB.MOM.WW.ScadaBridge.Transport.Serialization;
@@ -64,9 +68,16 @@ public sealed class BundleImporterApplyTests : IDisposable
         // M8: DependencyResolver now injects ISiteRepository to walk the
         // site/data-connection/instance closure; register it or activation fails.
         services.AddScoped<ISiteRepository, SiteRepository>();
+        services.AddScoped<IDeploymentManagerRepository, DeploymentManagerRepository>();
         services.AddScoped<IAuditCorrelationContext, AuditCorrelationContext>();
         services.AddScoped<IAuditService, AuditService>();
         services.AddTransport();
+        // #16 (M8 D2): the stale-instance probe is implemented in DeploymentManager
+        // and the flatten/hash primitives in TemplateEngine — register both so
+        // BundleImporter resolves a real IStaleInstanceProbe and ApplyAsync can
+        // compute StaleInstanceIds against genuine revision hashes.
+        services.AddTemplateEngine();
+        services.AddDeploymentManager();
 
         _provider = services.BuildServiceProvider();
     }
@@ -926,4 +937,238 @@ public sealed class BundleImporterApplyTests : IDisposable
         Assert.Contains(preview.Items, item =>
             item.EntityType == "ApiMethod" && item.Name == "GetStatus");
     }
+
+    // ─────────────────────────────────────────────────────────────────────
+    // #16 (M8 D2): StaleInstanceIds enumeration. Overwriting a template changes
+    // its flattened-config hash, so deployed instances of that template drift
+    // from their DeployedConfigSnapshot.RevisionHash and must surface as stale
+    // in the import result. NotDeployed (incl. freshly-imported) instances are
+    // never stale.
+    // ─────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Seeds an Enabled instance of <paramref name="templateName"/> and a
+    /// <see cref="DeployedConfigSnapshot"/> whose RevisionHash is the GENUINE hash
+    /// the deployment flattening pipeline computes for the instance's CURRENT
+    /// (pre-import) config. Returns the instance id. Mirrors how the deploy path
+    /// captures a snapshot so the staleness comparison is faithful.
+    /// </summary>
+    private async Task<int> SeedDeployedInstanceWithRealSnapshotAsync(
+        string templateName, string instanceName)
+    {
+        int instanceId;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var template = await ctx.Templates.SingleAsync(t => t.Name == templateName);
+            var instance = new Instance(instanceName)
+            {
+                TemplateId = template.Id,
+                SiteId = (await ctx.Sites.FirstAsync()).Id,
+                State = InstanceState.Enabled,
+            };
+            ctx.Instances.Add(instance);
+            await ctx.SaveChangesAsync();
+            instanceId = instance.Id;
+
+            // Compute the REAL current revision hash via the same pipeline the
+            // deploy path uses, then persist it as the deployed snapshot so the
+            // import's drift comparison runs against a genuine baseline.
+            var pipeline = scope.ServiceProvider.GetRequiredService<IFlatteningPipeline>();
+            var flattened = await pipeline.FlattenAndValidateAsync(instanceId);
+            Assert.True(flattened.IsSuccess,
+                $"Seed flatten failed: {(flattened.IsFailure ? flattened.Error : "(success)")}");
+            ctx.DeployedConfigSnapshots.Add(new DeployedConfigSnapshot(
+                deploymentId: Guid.NewGuid().ToString(),
+                revisionHash: flattened.Value.RevisionHash,
+                configurationJson: System.Text.Json.JsonSerializer.Serialize(flattened.Value.Configuration))
+            {
+                InstanceId = instanceId,
+                DeployedAt = DateTimeOffset.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+        return instanceId;
+    }
+
+    private async Task<int> SeedSiteAsync()
+    {
+        await using var scope = _provider.CreateAsyncScope();
+        var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+        var site = new ZB.MOM.WW.ScadaBridge.Commons.Entities.Sites.Site("Plant 1", "plant-1")
+        {
+            NodeAAddress = "akka://site@10.0.0.1:2552",
+            NodeBAddress = "akka://site@10.0.0.2:2552",
+        };
+        ctx.Sites.Add(site);
+        await ctx.SaveChangesAsync();
+        return site.Id;
+    }
+
+    [Fact]
+    public async Task ApplyAsync_overwriting_template_marks_deployed_instance_stale()
+    {
+        // Arrange — seed Pump with the BUNDLE's (new) attribute shape, export it,
+        // then mutate the target template + reseed the deployed snapshot to the
+        // OLD shape so an Overwrite restores the new shape and drifts the deployed
+        // instance off its captured hash.
+        await SeedSiteAsync();
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var t = new Template("Pump") { Description = "v2" };
+            t.Attributes.Add(new TemplateAttribute("Flow") { DataType = DataType.Float, Value = "1.0" });
+            t.Attributes.Add(new TemplateAttribute("Pressure") { DataType = DataType.Float, Value = "2.0" }); // extra attr in bundle
+            ctx.Templates.Add(t);
+            await ctx.SaveChangesAsync();
+        }
+        var sessionId = await ExportAndLoadAsync();
+
+        // Mutate the target template to the OLD shape (drop Pressure, change Flow),
+        // then deploy an instance capturing the OLD config's hash.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var t = await ctx.Templates.Include(x => x.Attributes).SingleAsync(x => x.Name == "Pump");
+            t.Description = "v1";
+            var pressure = t.Attributes.Single(a => a.Name == "Pressure");
+            t.Attributes.Remove(pressure);
+            ctx.TemplateAttributes.Remove(pressure);
+            t.Attributes.Single(a => a.Name == "Flow").Value = "0.0";
+            await ctx.SaveChangesAsync();
+        }
+        var deployedInstanceId = await SeedDeployedInstanceWithRealSnapshotAsync("Pump", "Pump-Deployed");
+
+        // Act — Overwrite Pump back to the bundle's v2 shape.
+        ImportResult result;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var importer = scope.ServiceProvider.GetRequiredService<IBundleImporter>();
+            result = await importer.ApplyAsync(sessionId,
+                new List<ImportResolution> { new("Template", "Pump", ResolutionAction.Overwrite, null) },
+                user: "bob");
+        }
+
+        // Assert — the deployed instance drifted off its snapshot hash and is stale.
+        Assert.Equal(1, result.Overwritten);
+        Assert.Contains(deployedInstanceId, result.StaleInstanceIds);
+    }
+
+    [Fact]
+    public async Task ApplyAsync_overwriting_template_does_not_mark_unchanged_template_instance_stale()
+    {
+        // Two templates: Pump (overwritten with a CHANGED shape) and Valve
+        // (overwritten with an IDENTICAL shape — its instance must NOT be stale).
+        // The export carries both at their seeded shape; we mutate only Pump in
+        // the target so its overwrite changes the hash, while Valve's overwrite
+        // is a no-op hash-wise.
+        await SeedSiteAsync();
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var pump = new Template("Pump") { Description = "v2" };
+            pump.Attributes.Add(new TemplateAttribute("Flow") { DataType = DataType.Float, Value = "1.0" });
+            pump.Attributes.Add(new TemplateAttribute("Pressure") { DataType = DataType.Float, Value = "2.0" });
+            ctx.Templates.Add(pump);
+            var valve = new Template("Valve") { Description = "stable" };
+            valve.Attributes.Add(new TemplateAttribute("Position") { DataType = DataType.Float, Value = "0.5" });
+            ctx.Templates.Add(valve);
+            await ctx.SaveChangesAsync();
+        }
+        var sessionId = await ExportAndLoadAsync();
+
+        // Mutate ONLY Pump in the target so its overwrite changes the hash; leave
+        // Valve identical to the bundle so its overwrite is hash-neutral.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var pump = await ctx.Templates.Include(x => x.Attributes).SingleAsync(x => x.Name == "Pump");
+            var pressure = pump.Attributes.Single(a => a.Name == "Pressure");
+            pump.Attributes.Remove(pressure);
+            ctx.TemplateAttributes.Remove(pressure);
+            await ctx.SaveChangesAsync();
+        }
+        var pumpInstanceId = await SeedDeployedInstanceWithRealSnapshotAsync("Pump", "Pump-Deployed");
+        var valveInstanceId = await SeedDeployedInstanceWithRealSnapshotAsync("Valve", "Valve-Deployed");
+
+        // Act — Overwrite BOTH templates.
+        ImportResult result;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var importer = scope.ServiceProvider.GetRequiredService<IBundleImporter>();
+            result = await importer.ApplyAsync(sessionId,
+                new List<ImportResolution>
+                {
+                    new("Template", "Pump", ResolutionAction.Overwrite, null),
+                    new("Template", "Valve", ResolutionAction.Overwrite, null),
+                },
+                user: "bob");
+        }
+
+        // Assert — only Pump's deployed instance is stale; Valve's is not.
+        Assert.Contains(pumpInstanceId, result.StaleInstanceIds);
+        Assert.DoesNotContain(valveInstanceId, result.StaleInstanceIds);
+    }
+
+    [Fact]
+    public async Task ApplyAsync_freshly_imported_NotDeployed_instance_is_never_stale()
+    {
+        // A bundle that carries a template AND an instance of it: the imported
+        // instance enters the target NotDeployed (no snapshot), so even though the
+        // template is "new" to the target it must NOT appear in StaleInstanceIds.
+        // We also overwrite an EXISTING deployed instance's template to prove the
+        // stale list contains the deployed one but never the fresh one.
+        await SeedSiteAsync();
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var t = new Template("Pump") { Description = "v2" };
+            t.Attributes.Add(new TemplateAttribute("Flow") { DataType = DataType.Float, Value = "1.0" });
+            ctx.Templates.Add(t);
+            await ctx.SaveChangesAsync();
+        }
+        var sessionId = await ExportAndLoadAsync();
+
+        // Mutate the target template, then deploy an instance capturing the OLD hash.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var t = await ctx.Templates.Include(x => x.Attributes).SingleAsync(x => x.Name == "Pump");
+            t.Attributes.Single(a => a.Name == "Flow").Value = "0.0";
+            await ctx.SaveChangesAsync();
+        }
+        var deployedInstanceId = await SeedDeployedInstanceWithRealSnapshotAsync("Pump", "Pump-Deployed");
+
+        // Also seed a NotDeployed instance of the same template (simulates a
+        // never-deployed / freshly-imported instance) — it has no snapshot.
+        int notDeployedInstanceId;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            var t = await ctx.Templates.SingleAsync(x => x.Name == "Pump");
+            var fresh = new Instance("Pump-Fresh")
+            {
+                TemplateId = t.Id,
+                SiteId = (await ctx.Sites.FirstAsync()).Id,
+                State = InstanceState.NotDeployed,
+            };
+            ctx.Instances.Add(fresh);
+            await ctx.SaveChangesAsync();
+            notDeployedInstanceId = fresh.Id;
+        }
+
+        // Act — Overwrite Pump.
+        ImportResult result;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var importer = scope.ServiceProvider.GetRequiredService<IBundleImporter>();
+            result = await importer.ApplyAsync(sessionId,
+                new List<ImportResolution> { new("Template", "Pump", ResolutionAction.Overwrite, null) },
+                user: "bob");
+        }
+
+        // Assert — deployed instance is stale; the NotDeployed one is excluded.
+        Assert.Contains(deployedInstanceId, result.StaleInstanceIds);
+        Assert.DoesNotContain(notDeployedInstanceId, result.StaleInstanceIds);
+    }
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/SiteInstanceImportTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/SiteInstanceImportTests.cs
index 2604d5a4..aedbf179 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/SiteInstanceImportTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.Transport.IntegrationTests/Import/SiteInstanceImportTests.cs
@@ -323,7 +323,8 @@ public sealed class SiteInstanceImportTests : IDisposable
         // Counts: site + connection + instance added (template was Skipped).
         Assert.Equal(3, result.Added);
         Assert.Equal(1, result.Skipped);
-        // D2 has not run yet — StaleInstanceIds stays empty.
+        // No template was overwritten and the imported instance is NotDeployed, so
+        // the D2 (#16) stale-instance enumeration finds nothing.
         Assert.Empty(result.StaleInstanceIds);
     }
 
@@ -866,6 +867,131 @@ public sealed class SiteInstanceImportTests : IDisposable
         }
     }
 
+    // ──────────────────────────────────────────────────────────────────────
+    // M-4 — native-alarm-source ConnectionNameOverride RENAME-REDIRECT
+    // ──────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ApplyAsync_native_alarm_override_connection_name_rewritten_to_redirected_target_name()
+    {
+        // M-4 regression. The existing tests only cover the same-name identity case
+        // (source connection name == target connection name), so a bug that fails to
+        // rewrite ConnectionNameOverride on a RENAME-REDIRECT would go unnoticed.
+        // Here the operator maps the source connection "SourceConn" to a
+        // DIFFERENTLY-named existing target connection "TargetConn" via an explicit
+        // MapToExisting. After import, the imported instance's native-alarm-source
+        // override (and its connection binding) must carry the TARGET name, not the
+        // source's.
+        int targetConnId;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+            ctx.Templates.Add(new Template("Pump") { Description = "pump tpl" });
+            var site = new Site("Plant 1", "plant-1");
+            ctx.Sites.Add(site);
+            await ctx.SaveChangesAsync();
+            // The target connection is named DIFFERENTLY from the source's.
+            var conn = new DataConnection("TargetConn", "OpcUa", site.Id)
+            {
+                PrimaryConfiguration = "{\"endpoint\":\"opc.tcp://target\"}",
+            };
+            ctx.DataConnections.Add(conn);
+            await ctx.SaveChangesAsync();
+            targetConnId = conn.Id;
+        }
+
+        // Hand-pack a bundle whose instance references the SOURCE connection name
+        // "SourceConn" (both in its binding and its native-alarm override). The
+        // bundle does NOT carry the connection — the redirect resolves it against
+        // the existing target "TargetConn".
+        var content = new BundleContentDto(
+            TemplateFolders: Array.Empty<TemplateFolderDto>(),
+            Templates: Array.Empty<TemplateDto>(),
+            SharedScripts: Array.Empty<SharedScriptDto>(),
+            ExternalSystems: Array.Empty<ExternalSystemDto>(),
+            DatabaseConnections: Array.Empty<DatabaseConnectionDto>(),
+            NotificationLists: Array.Empty<NotificationListDto>(),
+            SmtpConfigs: Array.Empty<SmtpConfigDto>(),
+            ApiMethods: Array.Empty<ApiMethodDto>())
+        {
+            Sites = new[]
+            {
+                new SiteDto("plant-1", "Plant 1", null, null, null, null, null),
+            },
+            DataConnections = Array.Empty<DataConnectionDto>(),
+            Instances = new[]
+            {
+                new InstanceDto(
+                    UniqueName: "Pump-01",
+                    TemplateName: "Pump",
+                    SiteIdentifier: "plant-1",
+                    AreaName: null,
+                    State: InstanceState.Enabled,
+                    AttributeOverrides: Array.Empty<InstanceAttributeOverrideDto>(),
+                    AlarmOverrides: Array.Empty<InstanceAlarmOverrideDto>(),
+                    NativeAlarmSourceOverrides: new[]
+                    {
+                        new InstanceNativeAlarmSourceOverrideDto(
+                            SourceCanonicalName: "NativeSrc",
+                            ConnectionNameOverride: "SourceConn",
+                            SourceReferenceOverride: "ns=3;s=Pump.Alarm",
+                            ConditionFilterOverride: null),
+                    },
+                    ConnectionBindings: new[]
+                    {
+                        new InstanceConnectionBindingDto(
+                            AttributeName: "Flow",
+                            ConnectionName: "SourceConn",
+                            DataSourceReferenceOverride: "ns=3;s=Pump.Flow"),
+                    }),
+            },
+        };
+        var sessionId = await PackAndLoadAsync(content);
+
+        // Explicit redirect: source "SourceConn" → target "TargetConn" (DIFFERENT name).
+        var nameMap = new BundleNameMap(
+            Sites: new[] { new SiteMapping("plant-1", MappingAction.MapToExisting, "plant-1") },
+            Connections: new[]
+            {
+                new ConnectionMapping("plant-1", "SourceConn", MappingAction.MapToExisting, "TargetConn"),
+            });
+
+        var result = await ApplyAsync(
+            sessionId,
+            new List<ImportResolution>
+            {
+                new("Site", "plant-1", ResolutionAction.Skip, null),
+                new("Instance", "Pump-01", ResolutionAction.Add, null),
+            },
+            nameMap);
+
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaBridgeDbContext>();
+
+            // No new connection created — the redirect resolved the existing target.
+            var conn = Assert.Single(await ctx.DataConnections.ToListAsync());
+            Assert.Equal("TargetConn", conn.Name);
+            Assert.Equal(targetConnId, conn.Id);
+
+            var inst = await ctx.Instances
+                .Include(i => i.ConnectionBindings)
+                .Include(i => i.NativeAlarmSourceOverrides)
+                .SingleAsync(i => i.UniqueName == "Pump-01");
+
+            // THE FIX: the native-alarm-source override carries the TARGET name,
+            // not the source's "SourceConn".
+            var native = Assert.Single(inst.NativeAlarmSourceOverrides);
+            Assert.Equal("TargetConn", native.ConnectionNameOverride);
+
+            // The connection binding FK rewires to the redirected target connection.
+            var binding = Assert.Single(inst.ConnectionBindings);
+            Assert.Equal(targetConnId, binding.DataConnectionId);
+        }
+
+        Assert.Equal(1, result.Added);
+    }
+
     // ──────────────────────────────────────────────────────────────────────
     // C2 — two sites with same-named connections resolve independently
     // ──────────────────────────────────────────────────────────────────────