diff --git a/tests/ScadaLink.Transport.IntegrationTests/ConflictResolutionTests.cs b/tests/ScadaLink.Transport.IntegrationTests/ConflictResolutionTests.cs
new file mode 100644
index 0000000..616f195
--- /dev/null
+++ b/tests/ScadaLink.Transport.IntegrationTests/ConflictResolutionTests.cs
@@ -0,0 +1,231 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Diagnostics;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using ScadaLink.Commons.Entities.Templates;
+using ScadaLink.Commons.Interfaces.Repositories;
+using ScadaLink.Commons.Interfaces.Services;
+using ScadaLink.Commons.Interfaces.Transport;
+using ScadaLink.Commons.Types.Transport;
+using ScadaLink.ConfigurationDatabase;
+using ScadaLink.ConfigurationDatabase.Repositories;
+using ScadaLink.ConfigurationDatabase.Services;
+using ScadaLink.Transport;
+
+namespace ScadaLink.Transport.IntegrationTests;
+
+/// <summary>
+/// T26 — integration conflict-resolution tests. The unit-level Apply paths
+/// in <c>BundleImporterApplyTests</c> exercise hand-built sessions; this
+/// suite drives the full export→load→apply pipeline so the wire-level
+/// session round-trip is part of the assertion.
+/// </summary>
+public sealed class ConflictResolutionTests : IDisposable
+{
+    private readonly ServiceProvider _provider;
+
+    public ConflictResolutionTests()
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<IConfiguration>(
+            new ConfigurationBuilder().AddInMemoryCollection().Build());
+
+        var dbName = $"ConflictResolutionTests_{Guid.NewGuid()}";
+        services.AddDbContext<ScadaLinkDbContext>(opts => opts
+            .UseInMemoryDatabase(dbName)
+            .ConfigureWarnings(w => w.Ignore(InMemoryEventId.TransactionIgnoredWarning)));
+
+        services.AddScoped<ITemplateEngineRepository, TemplateEngineRepository>();
+        services.AddScoped<IExternalSystemRepository, ExternalSystemRepository>();
+        services.AddScoped<INotificationRepository, NotificationRepository>();
+        services.AddScoped<IInboundApiRepository, InboundApiRepository>();
+        services.AddScoped<IAuditCorrelationContext, AuditCorrelationContext>();
+        services.AddScoped<IAuditService, AuditService>();
+        services.AddTransport();
+
+        _provider = services.BuildServiceProvider();
+    }
+
+    public void Dispose() => _provider.Dispose();
+
+    /// <summary>
+    /// Exports the current set of templates+shared scripts to a freshly built
+    /// bundle and immediately loads it into a session. Returns the
+    /// <see cref="BundleSession.SessionId"/> the caller can hand to ApplyAsync.
+    /// </summary>
+    private async Task<Guid> ExportAndLoadAsync()
+    {
+        byte[] bundleBytes;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var exporter = scope.ServiceProvider.GetRequiredService<IBundleExporter>();
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var templateIds = await ctx.Templates.Select(t => t.Id).ToListAsync();
+            var sharedScriptIds = await ctx.SharedScripts.Select(s => s.Id).ToListAsync();
+
+            var selection = new ExportSelection(
+                TemplateIds: templateIds,
+                SharedScriptIds: sharedScriptIds,
+                ExternalSystemIds: Array.Empty<int>(),
+                DatabaseConnectionIds: Array.Empty<int>(),
+                NotificationListIds: Array.Empty<int>(),
+                SmtpConfigurationIds: Array.Empty<int>(),
+                ApiKeyIds: Array.Empty<int>(),
+                ApiMethodIds: Array.Empty<int>(),
+                IncludeDependencies: false);
+
+            var stream = await exporter.ExportAsync(selection,
+                user: "alice", sourceEnvironment: "dev",
+                passphrase: null, cancellationToken: CancellationToken.None);
+
+            using var ms = new MemoryStream();
+            await stream.CopyToAsync(ms);
+            bundleBytes = ms.ToArray();
+        }
+
+        await using var loadScope = _provider.CreateAsyncScope();
+        var importer = loadScope.ServiceProvider.GetRequiredService<IBundleImporter>();
+        using var input = new MemoryStream(bundleBytes, writable: false);
+        var session = await importer.LoadAsync(input, passphrase: null);
+        return session.SessionId;
+    }
+
+    [Fact]
+    public async Task Overwrite_replaces_existing_template_description()
+    {
+        // Arrange: seed Pump with Description="new", export it (the bundle
+        // therefore carries "new"), then mutate the target's Pump to "old".
+        // Apply with Overwrite must restore "new".
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            ctx.Templates.Add(new Template("Pump") { Description = "new" });
+            await ctx.SaveChangesAsync();
+        }
+
+        var sessionId = await ExportAndLoadAsync();
+
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var existing = await ctx.Templates.SingleAsync(t => t.Name == "Pump");
+            existing.Description = "old";
+            await ctx.SaveChangesAsync();
+        }
+
+        // Act
+        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
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var pump = await ctx.Templates.SingleAsync(t => t.Name == "Pump");
+            Assert.Equal("new", pump.Description);
+        }
+        Assert.Equal(1, result.Overwritten);
+    }
+
+    [Fact]
+    public async Task Skip_leaves_existing_template_unchanged()
+    {
+        // Arrange: seed Pump with Description="keep", export, then mutate to
+        // "replace" so the bundle's body diverges. Skip must NOT touch the
+        // target row, and the summary must report Skipped=1.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            ctx.Templates.Add(new Template("Pump") { Description = "replace" });
+            await ctx.SaveChangesAsync();
+        }
+        var sessionId = await ExportAndLoadAsync();
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var existing = await ctx.Templates.SingleAsync(t => t.Name == "Pump");
+            existing.Description = "keep";
+            await ctx.SaveChangesAsync();
+        }
+
+        // Act
+        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.Skip, null),
+                },
+                user: "bob");
+        }
+
+        // Assert
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var pump = await ctx.Templates.SingleAsync(t => t.Name == "Pump");
+            Assert.Equal("keep", pump.Description);
+        }
+        Assert.Equal(1, result.Skipped);
+        Assert.Equal(0, result.Added);
+        Assert.Equal(0, result.Overwritten);
+    }
+
+    [Fact]
+    public async Task Rename_creates_new_template_alongside_existing()
+    {
+        // Arrange: seed Pump, export, mutate description so the rename target
+        // is obviously the bundle's version. The original Pump must survive
+        // untouched and a second Pump.Imported template must materialise.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            ctx.Templates.Add(new Template("Pump") { Description = "from-bundle" });
+            await ctx.SaveChangesAsync();
+        }
+        var sessionId = await ExportAndLoadAsync();
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var existing = await ctx.Templates.SingleAsync(t => t.Name == "Pump");
+            existing.Description = "kept-original";
+            await ctx.SaveChangesAsync();
+        }
+
+        // Act
+        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.Rename, "Pump.Imported"),
+                },
+                user: "bob");
+        }
+
+        // Assert
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var original = await ctx.Templates.SingleAsync(t => t.Name == "Pump");
+            Assert.Equal("kept-original", original.Description);
+
+            var renamed = await ctx.Templates.SingleAsync(t => t.Name == "Pump.Imported");
+            Assert.Equal("from-bundle", renamed.Description);
+        }
+        Assert.Equal(1, result.Renamed);
+    }
+}
diff --git a/tests/ScadaLink.Transport.IntegrationTests/ValidationFailureTests.cs b/tests/ScadaLink.Transport.IntegrationTests/ValidationFailureTests.cs
new file mode 100644
index 0000000..bfacca5
--- /dev/null
+++ b/tests/ScadaLink.Transport.IntegrationTests/ValidationFailureTests.cs
@@ -0,0 +1,174 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Diagnostics;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using ScadaLink.Commons.Entities.Scripts;
+using ScadaLink.Commons.Entities.Templates;
+using ScadaLink.Commons.Interfaces.Repositories;
+using ScadaLink.Commons.Interfaces.Services;
+using ScadaLink.Commons.Interfaces.Transport;
+using ScadaLink.Commons.Types.Transport;
+using ScadaLink.ConfigurationDatabase;
+using ScadaLink.ConfigurationDatabase.Repositories;
+using ScadaLink.ConfigurationDatabase.Services;
+using ScadaLink.Transport;
+using ScadaLink.Transport.Import;
+
+namespace ScadaLink.Transport.IntegrationTests;
+
+/// <summary>
+/// T26 — integration validation-failure rollback test. Complements the unit
+/// rollback covered by <c>BundleImporterApplyTests</c> by driving the full
+/// export → load → apply path, so the failing validation runs against a
+/// real export-side serialised bundle.
+/// </summary>
+public sealed class ValidationFailureTests : IDisposable
+{
+    private readonly ServiceProvider _provider;
+
+    public ValidationFailureTests()
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<IConfiguration>(
+            new ConfigurationBuilder().AddInMemoryCollection().Build());
+
+        var dbName = $"ValidationFailureTests_{Guid.NewGuid()}";
+        services.AddDbContext<ScadaLinkDbContext>(opts => opts
+            .UseInMemoryDatabase(dbName)
+            .ConfigureWarnings(w => w.Ignore(InMemoryEventId.TransactionIgnoredWarning)));
+
+        services.AddScoped<ITemplateEngineRepository, TemplateEngineRepository>();
+        services.AddScoped<IExternalSystemRepository, ExternalSystemRepository>();
+        services.AddScoped<INotificationRepository, NotificationRepository>();
+        services.AddScoped<IInboundApiRepository, InboundApiRepository>();
+        services.AddScoped<IAuditCorrelationContext, AuditCorrelationContext>();
+        services.AddScoped<IAuditService, AuditService>();
+        services.AddTransport();
+
+        _provider = services.BuildServiceProvider();
+    }
+
+    public void Dispose() => _provider.Dispose();
+
+    [Fact]
+    public async Task Semantic_validation_failure_rolls_back_all_writes()
+    {
+        // Arrange:
+        //   - Seed a template whose script body references MissingHelper(),
+        //     a SharedScript that is NOT in the bundle (we don't seed one) and
+        //     also NOT in the target after the wipe.
+        //   - Export the template only (no helper alongside).
+        //   - Wipe the target so the apply runs through Add-paths.
+        //   - Snapshot row counts so the rollback assertion is unambiguous.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var t = new Template("BrokenPump") { Description = "calls a missing helper" };
+            t.Scripts.Add(new TemplateScript("init", "var x = MissingHelper();"));
+            ctx.Templates.Add(t);
+            await ctx.SaveChangesAsync();
+        }
+
+        // Export only the broken template. IncludeDependencies=false guarantees
+        // MissingHelper is NOT pulled in even if one happened to exist.
+        byte[] bundleBytes;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var exporter = scope.ServiceProvider.GetRequiredService<IBundleExporter>();
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var ids = await ctx.Templates.Select(t => t.Id).ToListAsync();
+            var selection = new ExportSelection(
+                TemplateIds: ids,
+                SharedScriptIds: Array.Empty<int>(),
+                ExternalSystemIds: Array.Empty<int>(),
+                DatabaseConnectionIds: Array.Empty<int>(),
+                NotificationListIds: Array.Empty<int>(),
+                SmtpConfigurationIds: Array.Empty<int>(),
+                ApiKeyIds: Array.Empty<int>(),
+                ApiMethodIds: Array.Empty<int>(),
+                IncludeDependencies: false);
+
+            var stream = await exporter.ExportAsync(selection,
+                user: "alice", sourceEnvironment: "dev",
+                passphrase: null, cancellationToken: CancellationToken.None);
+            using var ms = new MemoryStream();
+            await stream.CopyToAsync(ms);
+            bundleBytes = ms.ToArray();
+        }
+
+        // Wipe target so the apply attempts an Add (the validation failure must
+        // not let it land).
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            ctx.TemplateScripts.RemoveRange(ctx.TemplateScripts);
+            ctx.Templates.RemoveRange(ctx.Templates);
+            ctx.SharedScripts.RemoveRange(ctx.SharedScripts);
+            await ctx.SaveChangesAsync();
+        }
+
+        int templatesBefore;
+        int sharedScriptsBefore;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            templatesBefore = await ctx.Templates.CountAsync();
+            sharedScriptsBefore = await ctx.SharedScripts.CountAsync();
+        }
+        Assert.Equal(0, templatesBefore);
+        Assert.Equal(0, sharedScriptsBefore);
+
+        // Load the bundle into a session.
+        Guid sessionId;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var importer = scope.ServiceProvider.GetRequiredService<IBundleImporter>();
+            using var input = new MemoryStream(bundleBytes, writable: false);
+            var session = await importer.LoadAsync(input, passphrase: null);
+            sessionId = session.SessionId;
+        }
+
+        // Act — apply with Add. Validation MUST throw.
+        SemanticValidationException ex = default!;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var importer = scope.ServiceProvider.GetRequiredService<IBundleImporter>();
+            ex = await Assert.ThrowsAsync<SemanticValidationException>(() =>
+                importer.ApplyAsync(sessionId,
+                    new List<ImportResolution>
+                    {
+                        new("Template", "BrokenPump", ResolutionAction.Add, null),
+                    },
+                    user: "bob"));
+        }
+
+        // Assert
+        // 1. Errors list calls out the missing dependency by name.
+        Assert.NotEmpty(ex.Errors);
+        Assert.Contains(ex.Errors,
+            err => err.Contains("MissingHelper", StringComparison.Ordinal));
+
+        // 2. No new Template or SharedScript row was committed.
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            Assert.Equal(templatesBefore, await ctx.Templates.CountAsync());
+            Assert.Equal(sharedScriptsBefore, await ctx.SharedScripts.CountAsync());
+
+            // 3. A BundleImportFailed audit row exists. The BundleImporter
+            //    contract (T17) is: failure row's BundleImportId is NULL (the
+            //    rolled-back id is intentionally disowned) and its
+            //    AfterStateJson.Reason calls out the validation failure.
+            var failure = await ctx.AuditLogEntries
+                .SingleAsync(a => a.Action == "BundleImportFailed");
+            Assert.Equal("Bundle", failure.EntityType);
+            Assert.Null(failure.BundleImportId);
+            Assert.NotNull(failure.AfterStateJson);
+            // The exception message lands in the Reason field of the payload
+            // (BundleImporter.ApplyAsync wires it through). Spot-check for
+            // "validation" so the row's correlation to the failure is visible.
+            Assert.Contains("validation", failure.AfterStateJson!,
+                StringComparison.OrdinalIgnoreCase);
+        }
+    }
+}