diff --git a/src/ScadaLink.Transport/Serialization/EntityDtos.cs b/src/ScadaLink.Transport/Serialization/EntityDtos.cs
new file mode 100644
index 0000000..864c1ee
--- /dev/null
+++ b/src/ScadaLink.Transport/Serialization/EntityDtos.cs
@@ -0,0 +1,155 @@
+using ScadaLink.Commons.Entities.ExternalSystems;
+using ScadaLink.Commons.Entities.InboundApi;
+using ScadaLink.Commons.Entities.Notifications;
+using ScadaLink.Commons.Entities.Scripts;
+using ScadaLink.Commons.Entities.Templates;
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Transport.Serialization;
+
+/// <summary>
+/// In-memory aggregate of all bundle-eligible Commons entities. Matches the
+/// shape of <see cref="BundleContentDto"/> but uses the real persistence-
+/// ignorant POCO types — what <see cref="EntitySerializer"/> consumes/produces
+/// on the application side of the bundle boundary.
+/// </summary>
+public sealed record EntityAggregate(
+    IReadOnlyList<TemplateFolder> TemplateFolders,
+    IReadOnlyList<Template> Templates,
+    IReadOnlyList<SharedScript> SharedScripts,
+    IReadOnlyList<ExternalSystemDefinition> ExternalSystems,
+    IReadOnlyList<ExternalSystemMethod> ExternalSystemMethods,
+    IReadOnlyList<DatabaseConnectionDefinition> DatabaseConnections,
+    IReadOnlyList<NotificationList> NotificationLists,
+    IReadOnlyList<SmtpConfiguration> SmtpConfigurations,
+    IReadOnlyList<ApiKey> ApiKeys,
+    IReadOnlyList<ApiMethod> ApiMethods);
+
+/// <summary>
+/// Top-level serializable bundle payload. Lists are sequenced in dependency
+/// order so importers can apply them inline. Lists are never null on the wire
+/// — empty arrays are preferred over nulls so JSON consumers can rely on each
+/// property being present.
+/// </summary>
+public sealed record BundleContentDto(
+    IReadOnlyList<TemplateFolderDto> TemplateFolders,
+    IReadOnlyList<TemplateDto> Templates,
+    IReadOnlyList<SharedScriptDto> SharedScripts,
+    IReadOnlyList<ExternalSystemDto> ExternalSystems,
+    IReadOnlyList<DatabaseConnectionDto> DatabaseConnections,
+    IReadOnlyList<NotificationListDto> NotificationLists,
+    IReadOnlyList<SmtpConfigDto> SmtpConfigs,
+    IReadOnlyList<ApiKeyDto> ApiKeys,
+    IReadOnlyList<ApiMethodDto> ApiMethods);
+
+/// <summary>
+/// Carved-off secret values for an entity. The outer DTO carries all non-
+/// sensitive fields; secrets land here so a future "share without secrets"
+/// export mode can drop this block without touching anything else.
+/// </summary>
+public sealed record SecretsBlock(IReadOnlyDictionary<string, string> Values);
+
+public sealed record TemplateFolderDto(
+    string Name,
+    string? ParentName,
+    int SortOrder);
+
+public sealed record TemplateDto(
+    string Name,
+    string? FolderName,
+    string? BaseTemplateName,
+    string? Description,
+    IReadOnlyList<TemplateAttributeDto> Attributes,
+    IReadOnlyList<TemplateAlarmDto> Alarms,
+    IReadOnlyList<TemplateScriptDto> Scripts,
+    IReadOnlyList<TemplateCompositionDto> Compositions);
+
+public sealed record TemplateAttributeDto(
+    string Name,
+    string? Value,
+    DataType DataType,
+    bool IsLocked,
+    string? Description,
+    string? DataSourceReference);
+
+public sealed record TemplateAlarmDto(
+    string Name,
+    string? Description,
+    int PriorityLevel,
+    AlarmTriggerType TriggerType,
+    string? TriggerConfiguration,
+    bool IsLocked);
+
+public sealed record TemplateScriptDto(
+    string Name,
+    string Code,
+    string? TriggerType,
+    string? TriggerConfiguration,
+    string? ParameterDefinitions,
+    string? ReturnDefinition,
+    bool IsLocked);
+
+public sealed record TemplateCompositionDto(
+    string InstanceName,
+    string ComposedTemplateName);
+
+public sealed record SharedScriptDto(
+    string Name,
+    string Code,
+    string? ParameterDefinitions,
+    string? ReturnDefinition);
+
+public sealed record ExternalSystemDto(
+    string Name,
+    string BaseUrl,
+    string AuthType,
+    IReadOnlyList<ExternalSystemMethodDto> Methods,
+    SecretsBlock? Secrets);
+
+public sealed record ExternalSystemMethodDto(
+    string Name,
+    string HttpMethod,
+    string Path,
+    string? ParameterDefinitions,
+    string? ReturnDefinition);
+
+public sealed record DatabaseConnectionDto(
+    string Name,
+    int MaxRetries,
+    TimeSpan RetryDelay,
+    SecretsBlock? Secrets);
+
+public sealed record NotificationListDto(
+    string Name,
+    NotificationType Type,
+    IReadOnlyList<NotificationRecipientDto> Recipients);
+
+public sealed record NotificationRecipientDto(
+    string Name,
+    string EmailAddress);
+
+public sealed record SmtpConfigDto(
+    string Host,
+    int Port,
+    string AuthType,
+    string FromAddress,
+    string? TlsMode,
+    int ConnectionTimeoutSeconds,
+    int MaxConcurrentConnections,
+    int MaxRetries,
+    TimeSpan RetryDelay,
+    SecretsBlock? Secrets);
+
+public sealed record ApiKeyDto(
+    string Name,
+    string KeyHash,
+    bool IsEnabled,
+    SecretsBlock? Secrets);
+
+public sealed record ApiMethodDto(
+    string Name,
+    string Script,
+    string? ApprovedApiKeyIds,
+    string? ParameterDefinitions,
+    string? ReturnDefinition,
+    int TimeoutSeconds);
diff --git a/src/ScadaLink.Transport/Serialization/EntitySerializer.cs b/src/ScadaLink.Transport/Serialization/EntitySerializer.cs
new file mode 100644
index 0000000..7e856de
--- /dev/null
+++ b/src/ScadaLink.Transport/Serialization/EntitySerializer.cs
@@ -0,0 +1,344 @@
+using ScadaLink.Commons.Entities.ExternalSystems;
+using ScadaLink.Commons.Entities.InboundApi;
+using ScadaLink.Commons.Entities.Notifications;
+using ScadaLink.Commons.Entities.Scripts;
+using ScadaLink.Commons.Entities.Templates;
+
+namespace ScadaLink.Transport.Serialization;
+
+/// <summary>
+/// Converts between the persistence-shaped <see cref="EntityAggregate"/> and
+/// the wire-shaped <see cref="BundleContentDto"/>. The conversion strips EF
+/// identity (FK ints) and references inter-entity links by name so bundles
+/// remain portable across environments. Secrets are carved into a per-entity
+/// <see cref="SecretsBlock"/> so the bundle's secret-free public surface can
+/// be inspected (or, in future, exported without secrets) without touching the
+/// rest of the payload.
+/// </summary>
+public sealed class EntitySerializer
+{
+    public BundleContentDto ToBundleContent(EntityAggregate aggregate)
+    {
+        ArgumentNullException.ThrowIfNull(aggregate);
+
+        var folderNameById = aggregate.TemplateFolders.ToDictionary(f => f.Id, f => f.Name);
+        var templateNameById = aggregate.Templates.ToDictionary(t => t.Id, t => t.Name);
+
+        return new BundleContentDto(
+            TemplateFolders: aggregate.TemplateFolders.Select(f => new TemplateFolderDto(
+                Name: f.Name,
+                ParentName: f.ParentFolderId is { } pid && folderNameById.TryGetValue(pid, out var pname) ? pname : null,
+                SortOrder: f.SortOrder)).ToList(),
+            Templates: aggregate.Templates.Select(t => new TemplateDto(
+                Name: t.Name,
+                FolderName: t.FolderId is { } fid && folderNameById.TryGetValue(fid, out var fname) ? fname : null,
+                BaseTemplateName: t.ParentTemplateId is { } btid && templateNameById.TryGetValue(btid, out var bname) ? bname : null,
+                Description: t.Description,
+                Attributes: t.Attributes.Select(a => new TemplateAttributeDto(
+                    Name: a.Name,
+                    Value: a.Value,
+                    DataType: a.DataType,
+                    IsLocked: a.IsLocked,
+                    Description: a.Description,
+                    DataSourceReference: a.DataSourceReference)).ToList(),
+                Alarms: t.Alarms.Select(a => new TemplateAlarmDto(
+                    Name: a.Name,
+                    Description: a.Description,
+                    PriorityLevel: a.PriorityLevel,
+                    TriggerType: a.TriggerType,
+                    TriggerConfiguration: a.TriggerConfiguration,
+                    IsLocked: a.IsLocked)).ToList(),
+                Scripts: t.Scripts.Select(s => new TemplateScriptDto(
+                    Name: s.Name,
+                    Code: s.Code,
+                    TriggerType: s.TriggerType,
+                    TriggerConfiguration: s.TriggerConfiguration,
+                    ParameterDefinitions: s.ParameterDefinitions,
+                    ReturnDefinition: s.ReturnDefinition,
+                    IsLocked: s.IsLocked)).ToList(),
+                Compositions: t.Compositions.Select(c => new TemplateCompositionDto(
+                    InstanceName: c.InstanceName,
+                    ComposedTemplateName: templateNameById.TryGetValue(c.ComposedTemplateId, out var cn) ? cn : string.Empty)).ToList())).ToList(),
+            SharedScripts: aggregate.SharedScripts.Select(s => new SharedScriptDto(
+                Name: s.Name,
+                Code: s.Code,
+                ParameterDefinitions: s.ParameterDefinitions,
+                ReturnDefinition: s.ReturnDefinition)).ToList(),
+            ExternalSystems: aggregate.ExternalSystems.Select(sys =>
+            {
+                var methods = aggregate.ExternalSystemMethods
+                    .Where(m => m.ExternalSystemDefinitionId == sys.Id)
+                    .Select(m => new ExternalSystemMethodDto(
+                        Name: m.Name,
+                        HttpMethod: m.HttpMethod,
+                        Path: m.Path,
+                        ParameterDefinitions: m.ParameterDefinitions,
+                        ReturnDefinition: m.ReturnDefinition))
+                    .ToList();
+
+                SecretsBlock? secrets = null;
+                if (!string.IsNullOrEmpty(sys.AuthConfiguration))
+                {
+                    secrets = new SecretsBlock(new Dictionary<string, string>(StringComparer.Ordinal)
+                    {
+                        ["AuthConfiguration"] = sys.AuthConfiguration,
+                    });
+                }
+
+                return new ExternalSystemDto(
+                    Name: sys.Name,
+                    BaseUrl: sys.EndpointUrl,
+                    AuthType: sys.AuthType,
+                    Methods: methods,
+                    Secrets: secrets);
+            }).ToList(),
+            DatabaseConnections: aggregate.DatabaseConnections.Select(dc => new DatabaseConnectionDto(
+                Name: dc.Name,
+                MaxRetries: dc.MaxRetries,
+                RetryDelay: dc.RetryDelay,
+                // Connection strings typically embed credentials inline; treat the entire
+                // string as secret rather than try to parse out the password fragment.
+                Secrets: new SecretsBlock(new Dictionary<string, string>(StringComparer.Ordinal)
+                {
+                    ["ConnectionString"] = dc.ConnectionString,
+                }))).ToList(),
+            NotificationLists: aggregate.NotificationLists.Select(nl => new NotificationListDto(
+                Name: nl.Name,
+                Type: nl.Type,
+                Recipients: nl.Recipients.Select(r => new NotificationRecipientDto(
+                    Name: r.Name,
+                    EmailAddress: r.EmailAddress)).ToList())).ToList(),
+            SmtpConfigs: aggregate.SmtpConfigurations.Select(smtp =>
+            {
+                SecretsBlock? secrets = null;
+                if (!string.IsNullOrEmpty(smtp.Credentials))
+                {
+                    secrets = new SecretsBlock(new Dictionary<string, string>(StringComparer.Ordinal)
+                    {
+                        ["Credentials"] = smtp.Credentials,
+                    });
+                }
+
+                return new SmtpConfigDto(
+                    Host: smtp.Host,
+                    Port: smtp.Port,
+                    AuthType: smtp.AuthType,
+                    FromAddress: smtp.FromAddress,
+                    TlsMode: smtp.TlsMode,
+                    ConnectionTimeoutSeconds: smtp.ConnectionTimeoutSeconds,
+                    MaxConcurrentConnections: smtp.MaxConcurrentConnections,
+                    MaxRetries: smtp.MaxRetries,
+                    RetryDelay: smtp.RetryDelay,
+                    Secrets: secrets);
+            }).ToList(),
+            // ApiKey stores only KeyHash already; no plaintext to carve. SecretsBlock
+            // stays null per design — KeyHash is on the public DTO.
+            ApiKeys: aggregate.ApiKeys.Select(k => new ApiKeyDto(
+                Name: k.Name,
+                KeyHash: k.KeyHash,
+                IsEnabled: k.IsEnabled,
+                Secrets: null)).ToList(),
+            ApiMethods: aggregate.ApiMethods.Select(m => new ApiMethodDto(
+                Name: m.Name,
+                Script: m.Script,
+                ApprovedApiKeyIds: m.ApprovedApiKeyIds,
+                ParameterDefinitions: m.ParameterDefinitions,
+                ReturnDefinition: m.ReturnDefinition,
+                TimeoutSeconds: m.TimeoutSeconds)).ToList());
+    }
+
+    public EntityAggregate FromBundleContent(BundleContentDto content)
+    {
+        ArgumentNullException.ThrowIfNull(content);
+
+        // Folders: assign synthetic ids by ordinal, resolve parent pointers by name.
+        var folders = content.TemplateFolders
+            .Select((dto, ix) => new TemplateFolder(dto.Name) { Id = ix + 1, SortOrder = dto.SortOrder })
+            .ToList();
+        var folderIdByName = folders.ToDictionary(f => f.Name, f => f.Id, StringComparer.Ordinal);
+        for (var i = 0; i < folders.Count; i++)
+        {
+            var parentName = content.TemplateFolders[i].ParentName;
+            if (parentName is not null && folderIdByName.TryGetValue(parentName, out var parentId))
+            {
+                folders[i].ParentFolderId = parentId;
+            }
+        }
+
+        // Templates: same pattern — assign ids, second pass resolves base + folder.
+        var templates = content.Templates
+            .Select((dto, ix) =>
+            {
+                var t = new Template(dto.Name) { Id = ix + 1, Description = dto.Description };
+                foreach (var a in dto.Attributes)
+                {
+                    t.Attributes.Add(new TemplateAttribute(a.Name)
+                    {
+                        TemplateId = t.Id,
+                        Value = a.Value,
+                        DataType = a.DataType,
+                        IsLocked = a.IsLocked,
+                        Description = a.Description,
+                        DataSourceReference = a.DataSourceReference,
+                    });
+                }
+                foreach (var al in dto.Alarms)
+                {
+                    t.Alarms.Add(new TemplateAlarm(al.Name)
+                    {
+                        TemplateId = t.Id,
+                        Description = al.Description,
+                        PriorityLevel = al.PriorityLevel,
+                        TriggerType = al.TriggerType,
+                        TriggerConfiguration = al.TriggerConfiguration,
+                        IsLocked = al.IsLocked,
+                    });
+                }
+                foreach (var s in dto.Scripts)
+                {
+                    t.Scripts.Add(new TemplateScript(s.Name, s.Code)
+                    {
+                        TemplateId = t.Id,
+                        TriggerType = s.TriggerType,
+                        TriggerConfiguration = s.TriggerConfiguration,
+                        ParameterDefinitions = s.ParameterDefinitions,
+                        ReturnDefinition = s.ReturnDefinition,
+                        IsLocked = s.IsLocked,
+                    });
+                }
+                return t;
+            })
+            .ToList();
+        var templateIdByName = templates.ToDictionary(t => t.Name, t => t.Id, StringComparer.Ordinal);
+        for (var i = 0; i < templates.Count; i++)
+        {
+            var dto = content.Templates[i];
+            if (dto.FolderName is not null && folderIdByName.TryGetValue(dto.FolderName, out var fid))
+            {
+                templates[i].FolderId = fid;
+            }
+            if (dto.BaseTemplateName is not null && templateIdByName.TryGetValue(dto.BaseTemplateName, out var btid))
+            {
+                templates[i].ParentTemplateId = btid;
+            }
+            foreach (var compDto in dto.Compositions)
+            {
+                var comp = new TemplateComposition(compDto.InstanceName)
+                {
+                    TemplateId = templates[i].Id,
+                };
+                if (templateIdByName.TryGetValue(compDto.ComposedTemplateName, out var ctid))
+                {
+                    comp.ComposedTemplateId = ctid;
+                }
+                templates[i].Compositions.Add(comp);
+            }
+        }
+
+        var sharedScripts = content.SharedScripts
+            .Select((dto, ix) => new SharedScript(dto.Name, dto.Code)
+            {
+                Id = ix + 1,
+                ParameterDefinitions = dto.ParameterDefinitions,
+                ReturnDefinition = dto.ReturnDefinition,
+            })
+            .ToList();
+
+        var externalSystems = new List<ExternalSystemDefinition>();
+        var externalSystemMethods = new List<ExternalSystemMethod>();
+        for (var ix = 0; ix < content.ExternalSystems.Count; ix++)
+        {
+            var dto = content.ExternalSystems[ix];
+            var sys = new ExternalSystemDefinition(dto.Name, dto.BaseUrl, dto.AuthType)
+            {
+                Id = ix + 1,
+                AuthConfiguration = dto.Secrets?.Values.TryGetValue("AuthConfiguration", out var auth) == true ? auth : null,
+            };
+            externalSystems.Add(sys);
+            foreach (var m in dto.Methods)
+            {
+                externalSystemMethods.Add(new ExternalSystemMethod(m.Name, m.HttpMethod, m.Path)
+                {
+                    ExternalSystemDefinitionId = sys.Id,
+                    ParameterDefinitions = m.ParameterDefinitions,
+                    ReturnDefinition = m.ReturnDefinition,
+                });
+            }
+        }
+
+        var databaseConnections = content.DatabaseConnections
+            .Select((dto, ix) =>
+            {
+                var connStr = dto.Secrets?.Values.TryGetValue("ConnectionString", out var cs) == true ? cs : string.Empty;
+                return new DatabaseConnectionDefinition(dto.Name, connStr)
+                {
+                    Id = ix + 1,
+                    MaxRetries = dto.MaxRetries,
+                    RetryDelay = dto.RetryDelay,
+                };
+            })
+            .ToList();
+
+        var notificationLists = content.NotificationLists
+            .Select((dto, ix) =>
+            {
+                var list = new NotificationList(dto.Name) { Id = ix + 1, Type = dto.Type };
+                foreach (var r in dto.Recipients)
+                {
+                    list.Recipients.Add(new NotificationRecipient(r.Name, r.EmailAddress)
+                    {
+                        NotificationListId = list.Id,
+                    });
+                }
+                return list;
+            })
+            .ToList();
+
+        var smtpConfigurations = content.SmtpConfigs
+            .Select((dto, ix) => new SmtpConfiguration(dto.Host, dto.AuthType, dto.FromAddress)
+            {
+                Id = ix + 1,
+                Port = dto.Port,
+                Credentials = dto.Secrets?.Values.TryGetValue("Credentials", out var cred) == true ? cred : null,
+                TlsMode = dto.TlsMode,
+                ConnectionTimeoutSeconds = dto.ConnectionTimeoutSeconds,
+                MaxConcurrentConnections = dto.MaxConcurrentConnections,
+                MaxRetries = dto.MaxRetries,
+                RetryDelay = dto.RetryDelay,
+            })
+            .ToList();
+
+        var apiKeys = content.ApiKeys
+            .Select((dto, ix) =>
+            {
+                var key = ApiKey.FromHash(dto.Name, dto.KeyHash);
+                key.Id = ix + 1;
+                key.IsEnabled = dto.IsEnabled;
+                return key;
+            })
+            .ToList();
+
+        var apiMethods = content.ApiMethods
+            .Select((dto, ix) => new ApiMethod(dto.Name, dto.Script)
+            {
+                Id = ix + 1,
+                ApprovedApiKeyIds = dto.ApprovedApiKeyIds,
+                ParameterDefinitions = dto.ParameterDefinitions,
+                ReturnDefinition = dto.ReturnDefinition,
+                TimeoutSeconds = dto.TimeoutSeconds,
+            })
+            .ToList();
+
+        return new EntityAggregate(
+            TemplateFolders: folders,
+            Templates: templates,
+            SharedScripts: sharedScripts,
+            ExternalSystems: externalSystems,
+            ExternalSystemMethods: externalSystemMethods,
+            DatabaseConnections: databaseConnections,
+            NotificationLists: notificationLists,
+            SmtpConfigurations: smtpConfigurations,
+            ApiKeys: apiKeys,
+            ApiMethods: apiMethods);
+    }
+}
diff --git a/tests/ScadaLink.Transport.Tests/Serialization/EntitySerializerTests.cs b/tests/ScadaLink.Transport.Tests/Serialization/EntitySerializerTests.cs
new file mode 100644
index 0000000..65ba701
--- /dev/null
+++ b/tests/ScadaLink.Transport.Tests/Serialization/EntitySerializerTests.cs
@@ -0,0 +1,189 @@
+using ScadaLink.Commons.Entities.ExternalSystems;
+using ScadaLink.Commons.Entities.InboundApi;
+using ScadaLink.Commons.Entities.Notifications;
+using ScadaLink.Commons.Entities.Scripts;
+using ScadaLink.Commons.Entities.Templates;
+using ScadaLink.Commons.Types.Enums;
+using ScadaLink.Transport.Serialization;
+
+namespace ScadaLink.Transport.Tests.Serialization;
+
+public sealed class EntitySerializerTests
+{
+    private static EntityAggregate MakeEmptyAggregate() => new(
+        TemplateFolders: Array.Empty<TemplateFolder>(),
+        Templates: Array.Empty<Template>(),
+        SharedScripts: Array.Empty<SharedScript>(),
+        ExternalSystems: Array.Empty<ExternalSystemDefinition>(),
+        ExternalSystemMethods: Array.Empty<ExternalSystemMethod>(),
+        DatabaseConnections: Array.Empty<DatabaseConnectionDefinition>(),
+        NotificationLists: Array.Empty<NotificationList>(),
+        SmtpConfigurations: Array.Empty<SmtpConfiguration>(),
+        ApiKeys: Array.Empty<ApiKey>(),
+        ApiMethods: Array.Empty<ApiMethod>());
+
+    [Fact]
+    public void ToDto_carves_external_system_credentials_into_secrets_block()
+    {
+        var sys = new ExternalSystemDefinition("erp", "https://erp/api", "ApiKey")
+        {
+            Id = 1,
+            AuthConfiguration = "{\"apiKey\":\"super-secret\"}",
+        };
+        var aggregate = MakeEmptyAggregate() with { ExternalSystems = new[] { sys } };
+
+        var sut = new EntitySerializer();
+        var dto = sut.ToBundleContent(aggregate);
+
+        var dtoSys = Assert.Single(dto.ExternalSystems);
+        Assert.NotNull(dtoSys.Secrets);
+        Assert.True(dtoSys.Secrets!.Values.ContainsKey("AuthConfiguration"));
+        Assert.Equal("{\"apiKey\":\"super-secret\"}", dtoSys.Secrets.Values["AuthConfiguration"]);
+        // Public part does not carry the secret.
+        Assert.Equal("erp", dtoSys.Name);
+        Assert.Equal("https://erp/api", dtoSys.BaseUrl);
+        Assert.Equal("ApiKey", dtoSys.AuthType);
+    }
+
+    [Fact]
+    public void ToDto_carves_smtp_password_into_secrets_block()
+    {
+        var smtp = new SmtpConfiguration("smtp.example.com", "Basic", "noreply@example.com")
+        {
+            Id = 1,
+            Port = 587,
+            Credentials = "user:p@ssw0rd",
+        };
+        var aggregate = MakeEmptyAggregate() with { SmtpConfigurations = new[] { smtp } };
+
+        var dto = new EntitySerializer().ToBundleContent(aggregate);
+
+        var dtoSmtp = Assert.Single(dto.SmtpConfigs);
+        Assert.NotNull(dtoSmtp.Secrets);
+        Assert.Equal("user:p@ssw0rd", dtoSmtp.Secrets!.Values["Credentials"]);
+        Assert.Equal("smtp.example.com", dtoSmtp.Host);
+        Assert.Equal(587, dtoSmtp.Port);
+    }
+
+    [Fact]
+    public void Roundtrip_template_preserves_attributes_alarms_scripts_composition()
+    {
+        var folder = new TemplateFolder("root") { Id = 1, SortOrder = 0 };
+        var basic = new Template("Basic") { Id = 1, FolderId = 1, Description = "base" };
+        basic.Attributes.Add(new TemplateAttribute("Pressure")
+        {
+            Id = 1,
+            TemplateId = 1,
+            DataType = DataType.Double,
+            Value = "0",
+            IsLocked = true,
+            Description = "PSI",
+        });
+        basic.Alarms.Add(new TemplateAlarm("High")
+        {
+            Id = 1,
+            TemplateId = 1,
+            PriorityLevel = 2,
+            TriggerType = AlarmTriggerType.RangeViolation,
+            TriggerConfiguration = "{\"threshold\":100}",
+            IsLocked = false,
+        });
+        basic.Scripts.Add(new TemplateScript("OnUpdate", "return 1;")
+        {
+            Id = 1,
+            TemplateId = 1,
+            TriggerType = "Periodic",
+            ParameterDefinitions = "[]",
+            ReturnDefinition = "void",
+            IsLocked = false,
+        });
+
+        var assembly = new Template("Assembly") { Id = 2, FolderId = 1 };
+        assembly.Compositions.Add(new TemplateComposition("MotorA")
+        {
+            Id = 1,
+            TemplateId = 2,
+            ComposedTemplateId = 1, // refers to "Basic".
+        });
+
+        var aggregate = MakeEmptyAggregate() with
+        {
+            TemplateFolders = new[] { folder },
+            Templates = new[] { basic, assembly },
+        };
+
+        var sut = new EntitySerializer();
+        var dto = sut.ToBundleContent(aggregate);
+        var roundTripped = sut.FromBundleContent(dto);
+
+        var rtBasic = Assert.Single(roundTripped.Templates, t => t.Name == "Basic");
+        var rtAttr = Assert.Single(rtBasic.Attributes);
+        Assert.Equal("Pressure", rtAttr.Name);
+        Assert.Equal(DataType.Double, rtAttr.DataType);
+        Assert.Equal("0", rtAttr.Value);
+        Assert.True(rtAttr.IsLocked);
+
+        var rtAlarm = Assert.Single(rtBasic.Alarms);
+        Assert.Equal("High", rtAlarm.Name);
+        Assert.Equal(AlarmTriggerType.RangeViolation, rtAlarm.TriggerType);
+        Assert.Equal("{\"threshold\":100}", rtAlarm.TriggerConfiguration);
+        Assert.Equal(2, rtAlarm.PriorityLevel);
+
+        var rtScript = Assert.Single(rtBasic.Scripts);
+        Assert.Equal("OnUpdate", rtScript.Name);
+        Assert.Equal("return 1;", rtScript.Code);
+        Assert.Equal("Periodic", rtScript.TriggerType);
+
+        var rtAssembly = Assert.Single(roundTripped.Templates, t => t.Name == "Assembly");
+        var rtComp = Assert.Single(rtAssembly.Compositions);
+        Assert.Equal("MotorA", rtComp.InstanceName);
+    }
+
+    [Fact]
+    public void Roundtrip_template_folder_preserves_hierarchy()
+    {
+        var root = new TemplateFolder("Root") { Id = 1, SortOrder = 0 };
+        var child = new TemplateFolder("Pumps") { Id = 2, ParentFolderId = 1, SortOrder = 1 };
+
+        var aggregate = MakeEmptyAggregate() with
+        {
+            TemplateFolders = new[] { root, child },
+        };
+
+        var sut = new EntitySerializer();
+        var dto = sut.ToBundleContent(aggregate);
+        var rt = sut.FromBundleContent(dto);
+
+        Assert.Equal(2, rt.TemplateFolders.Count);
+        var rtRoot = Assert.Single(rt.TemplateFolders, f => f.Name == "Root");
+        var rtChild = Assert.Single(rt.TemplateFolders, f => f.Name == "Pumps");
+        Assert.Null(rtRoot.ParentFolderId);
+        // Hierarchy is preserved by name reference; new local ids get assigned but
+        // the child's parent must still point at the row whose name is "Root".
+        Assert.NotNull(rtChild.ParentFolderId);
+        Assert.Equal(rtRoot.Id, rtChild.ParentFolderId);
+    }
+
+    [Fact]
+    public void FromDto_with_null_SecretsBlock_yields_entity_with_default_empty_secrets()
+    {
+        var dto = new BundleContentDto(
+            TemplateFolders: Array.Empty<TemplateFolderDto>(),
+            Templates: Array.Empty<TemplateDto>(),
+            SharedScripts: Array.Empty<SharedScriptDto>(),
+            ExternalSystems: new[]
+            {
+                new ExternalSystemDto("erp", "https://x", "None", Array.Empty<ExternalSystemMethodDto>(), Secrets: null),
+            },
+            DatabaseConnections: Array.Empty<DatabaseConnectionDto>(),
+            NotificationLists: Array.Empty<NotificationListDto>(),
+            SmtpConfigs: Array.Empty<SmtpConfigDto>(),
+            ApiKeys: Array.Empty<ApiKeyDto>(),
+            ApiMethods: Array.Empty<ApiMethodDto>());
+
+        var aggregate = new EntitySerializer().FromBundleContent(dto);
+
+        var sys = Assert.Single(aggregate.ExternalSystems);
+        Assert.Null(sys.AuthConfiguration);
+    }
+}