diff --git a/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ApiException.cs b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ApiException.cs
new file mode 100644
index 0000000..7e88849
--- /dev/null
+++ b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ApiException.cs
@@ -0,0 +1,32 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents an API-facing exception that carries HTTP and domain meta information.
+/// </summary>
+public class ApiException : Exception
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ApiException"/> class.
+    /// </summary>
+    /// <param name="statusCode">HTTP status code to return.</param>
+    /// <param name="metaCode">Domain meta code to return.</param>
+    /// <param name="message">Response message.</param>
+    public ApiException(int statusCode, MetaCode metaCode, string message)
+        : base(message)
+    {
+        StatusCode = statusCode;
+        MetaCode = metaCode;
+    }
+
+    /// <summary>
+    /// Gets the HTTP status code.
+    /// </summary>
+    public int StatusCode { get; }
+
+    /// <summary>
+    /// Gets the domain meta code.
+    /// </summary>
+    public MetaCode MetaCode { get; }
+}
diff --git a/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ConflictApiException.cs b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ConflictApiException.cs
new file mode 100644
index 0000000..937e56d
--- /dev/null
+++ b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ConflictApiException.cs
@@ -0,0 +1,19 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents a 409 API exception.
+/// </summary>
+public sealed class ConflictApiException : ApiException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ConflictApiException"/> class.
+    /// </summary>
+    /// <param name="metaCode">Domain meta code to return.</param>
+    /// <param name="message">Response message.</param>
+    public ConflictApiException(MetaCode metaCode, string message)
+        : base(409, metaCode, message)
+    {
+    }
+}
diff --git a/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/NotFoundApiException.cs b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/NotFoundApiException.cs
new file mode 100644
index 0000000..bbc75c7
--- /dev/null
+++ b/src/core/DigitalData.TaskFlow.Application/Common/Exceptions/NotFoundApiException.cs
@@ -0,0 +1,18 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents a 404 API exception.
+/// </summary>
+public sealed class NotFoundApiException : ApiException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="NotFoundApiException"/> class.
+    /// </summary>
+    /// <param name="message">Response message.</param>
+    public NotFoundApiException(string message)
+        : base(404, MetaCode.NotFound, message)
+    {
+    }
+}
diff --git a/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs b/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs
index c54e6f9..206b481 100644
--- a/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs
+++ b/src/core/DigitalData.TaskFlow.Application/Features/Documents/Queries/GetDocument/GetDocumentQuery.cs
@@ -2,6 +2,7 @@ using DigitalData.TaskFlow.Application.Common.Factories;
 using DigitalData.TaskFlow.Application.Common.Interfaces;
 using DigitalData.TaskFlow.Domain.Common;
 using DigitalData.TaskFlow.Domain.Documents;
+using DigitalData.TaskFlow.Domain.Enums;
 using FluentValidation;
 using MediatR;
 
diff --git a/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs b/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs
index 932bd5e..332c77b 100644
--- a/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs
+++ b/src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflow/CloseWorkflowCommand.cs
@@ -1,8 +1,10 @@
+using DigitalData.TaskFlow.Application.Common.Exceptions;
 using DigitalData.TaskFlow.Application.Common.Interfaces;
 using DigitalData.TaskFlow.Application.Common.Factories;
 using AutoMapper;
 using DigitalData.TaskFlow.Domain.Closing;
 using DigitalData.TaskFlow.Domain.Common;
+using DigitalData.TaskFlow.Domain.Enums;
 using FluentValidation;
 using MediatR;
 
@@ -77,6 +79,16 @@ public sealed class CloseWorkflowCommandHandler(IWorkflowRepository workflowRepo
             decision,
             cancellationToken);
 
+        if (result.ProcessId <= 0)
+        {
+            throw new NotFoundApiException("Vorgang nicht gefunden");
+        }
+
+        if (!result.IsClosed)
+        {
+            throw new ConflictApiException(MetaCode.AlreadyClosed, "Vorgang bereits abgeschlossen");
+        }
+
         return ApiEnvelopeFactory.Create(result);
     }
 }