diff --git a/DocumentOperator.Tests/Unit/Client/MockHttpMessageHandler.cs b/DocumentOperator.Tests/Unit/Client/MockHttpMessageHandler.cs
new file mode 100644
index 0000000..230beb7
--- /dev/null
+++ b/DocumentOperator.Tests/Unit/Client/MockHttpMessageHandler.cs
@@ -0,0 +1,56 @@
+using System.Net;
+using System.Net.Http;
+using System.Text;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Reusable fake <see cref="HttpMessageHandler"/> for unit-testing HTTP clients.
+/// Captures the outgoing request and returns the configured response.
+/// </summary>
+internal sealed class MockHttpMessageHandler : HttpMessageHandler
+{
+    private readonly HttpResponseMessage _response;
+
+    /// <summary>The last request that was sent through this handler.</summary>
+    public HttpRequestMessage? LastRequest { get; private set; }
+
+    public MockHttpMessageHandler(HttpResponseMessage response)
+    {
+        _response = response;
+    }
+
+    // ?? convenience factories ????????????????????????????????????????????????
+
+    /// <summary>Creates a handler that returns 200 OK with a JSON-serialised body.</summary>
+    public static MockHttpMessageHandler ReturningJson<T>(T body, HttpStatusCode status = HttpStatusCode.OK)
+    {
+        var json = JsonSerializer.Serialize(body, new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase });
+        var response = new HttpResponseMessage(status)
+        {
+            Content = new StringContent(json, Encoding.UTF8, "application/json")
+        };
+        return new MockHttpMessageHandler(response);
+    }
+
+    /// <summary>Creates a handler that returns 200 OK with raw bytes as the body.</summary>
+    public static MockHttpMessageHandler ReturningBytes(byte[] bytes, string mediaType = "application/octet-stream", HttpStatusCode status = HttpStatusCode.OK)
+    {
+        var response = new HttpResponseMessage(status)
+        {
+            Content = new ByteArrayContent(bytes) { Headers = { ContentType = new(mediaType) } }
+        };
+        return new MockHttpMessageHandler(response);
+    }
+
+    /// <summary>Creates a handler that returns the given status code with no body.</summary>
+    public static MockHttpMessageHandler ReturningStatus(HttpStatusCode status)
+        => new(new HttpResponseMessage(status));
+
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        LastRequest = request;
+        return Task.FromResult(_response);
+    }
+}
diff --git a/DocumentOperator.Tests/Unit/Client/PdfAttachmentClientTests.cs b/DocumentOperator.Tests/Unit/Client/PdfAttachmentClientTests.cs
new file mode 100644
index 0000000..81c9d65
--- /dev/null
+++ b/DocumentOperator.Tests/Unit/Client/PdfAttachmentClientTests.cs
@@ -0,0 +1,221 @@
+using DocumentService.Client.Clients;
+using DocumentService.Client.Interfaces;
+using DocumentService.Client.Models.Requests;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using System.IO.Compression;
+using System.Net;
+using System.Net.Http;
+using System.Text;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Unit tests for <see cref="PdfAttachmentClient"/>.
+/// All tests use a fake <see cref="MockHttpMessageHandler"/> — no real HTTP calls are made.
+/// </summary>
+public class PdfAttachmentClientTests
+{
+    // ?? helpers ?????????????????????????????????????????????????????????????
+
+    private static (PdfAttachmentClient client, MockHttpMessageHandler handler) BuildJson<T>(T body)
+    {
+        var handler = MockHttpMessageHandler.ReturningJson(body);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfAttachmentClient(httpClient, NullLogger<PdfAttachmentClient>.Instance);
+        return (client, handler);
+    }
+
+    private static (PdfAttachmentClient client, MockHttpMessageHandler handler) BuildBytes(byte[] bytes, string mediaType = "application/zip")
+    {
+        var handler = MockHttpMessageHandler.ReturningBytes(bytes, mediaType);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfAttachmentClient(httpClient, NullLogger<PdfAttachmentClient>.Instance);
+        return (client, handler);
+    }
+
+    private static byte[] FakePdfBytes() => "fake-pdf-content"u8.ToArray();
+
+    /// <summary>Builds a minimal valid ZIP containing the given entries.</summary>
+    private static byte[] BuildZip(Dictionary<string, string> entries)
+    {
+        using var ms = new MemoryStream();
+        using (var archive = new ZipArchive(ms, ZipArchiveMode.Create, leaveOpen: true))
+        {
+            foreach (var (name, content) in entries)
+            {
+                var entry = archive.CreateEntry(name);
+                using var writer = new StreamWriter(entry.Open());
+                writer.Write(content);
+            }
+        }
+        return ms.ToArray();
+    }
+
+    // ?? CheckAttachmentsAsync (Stream) ???????????????????????????????????????
+
+    [Fact]
+    public async Task CheckAttachmentsAsync_Stream_SendsMultipartPost()
+    {
+        // Arrange
+        var expected = new AttachmentCheckResult
+        {
+            HasAttachments = true,
+            AttachmentCount = 1,
+            Attachments = new List<AttachmentMetadata>
+            {
+                new() { FileName = "factur-x.xml", MimeType = "application/xml", Size = 512 }
+            }
+        };
+        var (client, handler) = BuildJson(expected);
+
+        // Act
+        var result = await client.CheckAttachmentsAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.HasAttachments.Should().BeTrue();
+        result.AttachmentCount.Should().Be(1);
+        result.Attachments[0].FileName.Should().Be("factur-x.xml");
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/attachments/check");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    [Fact]
+    public async Task CheckAttachmentsAsync_Stream_PdfWithNoAttachments_ReturnsFalse()
+    {
+        // Arrange
+        var expected = new AttachmentCheckResult { HasAttachments = false, AttachmentCount = 0 };
+        var (client, _) = BuildJson(expected);
+
+        // Act
+        var result = await client.CheckAttachmentsAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.HasAttachments.Should().BeFalse();
+        result.AttachmentCount.Should().Be(0);
+        result.Attachments.Should().BeEmpty();
+    }
+
+    // ?? CheckAttachmentsAsync (byte[]) ???????????????????????????????????????
+
+    [Fact]
+    public async Task CheckAttachmentsAsync_Bytes_SendsJsonWithBase64()
+    {
+        // Arrange
+        var expected = new AttachmentCheckResult { HasAttachments = false };
+        var (client, handler) = BuildJson(expected);
+
+        // Act
+        await client.CheckAttachmentsAsync(FakePdfBytes());
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdf").GetString().Should().NotBeNullOrEmpty();
+    }
+
+    // ?? ExtractAttachmentsAsync (Stream) — ZIP unzip ?????????????????????????
+
+    [Fact]
+    public async Task ExtractAttachmentsAsync_Stream_UnzipsAndReturnsDictionary()
+    {
+        // Arrange
+        var zipBytes = BuildZip(new Dictionary<string, string>
+        {
+            ["factur-x.xml"] = "<invoice>test</invoice>",
+            ["readme.txt"]   = "Hello World"
+        });
+        var (client, handler) = BuildBytes(zipBytes);
+
+        // Act
+        var result = await client.ExtractAttachmentsAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.Should().HaveCount(2);
+        result.Should().ContainKey("factur-x.xml");
+        result.Should().ContainKey("readme.txt");
+
+        using var xmlStream = result["factur-x.xml"];
+        var xmlContent = await new StreamReader(xmlStream).ReadToEndAsync();
+        xmlContent.Should().Be("<invoice>test</invoice>");
+
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/attachments/extract");
+
+        // Cleanup
+        foreach (var s in result.Values) s.Dispose();
+    }
+
+    [Fact]
+    public async Task ExtractAttachmentsAsync_Stream_WithSingleEntry_ReturnsOneItem()
+    {
+        // Arrange
+        var zipBytes = BuildZip(new Dictionary<string, string>
+        {
+            ["data.xml"] = "<root/>"
+        });
+        var (client, _) = BuildBytes(zipBytes);
+
+        // Act
+        var result = await client.ExtractAttachmentsAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.Should().HaveCount(1);
+        result.Should().ContainKey("data.xml");
+
+        foreach (var s in result.Values) s.Dispose();
+    }
+
+    // ?? ExtractAttachmentsAsync (byte[]) — ZIP unzip ?????????????????????????
+
+    [Fact]
+    public async Task ExtractAttachmentsAsync_Bytes_SendsJsonAndUnzips()
+    {
+        // Arrange
+        var zipBytes = BuildZip(new Dictionary<string, string>
+        {
+            ["invoice.xml"] = "<invoice/>"
+        });
+        var (client, handler) = BuildBytes(zipBytes);
+
+        // Act
+        var result = await client.ExtractAttachmentsAsync(FakePdfBytes());
+
+        // Assert
+        result.Should().ContainKey("invoice.xml");
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+
+        foreach (var s in result.Values) s.Dispose();
+    }
+
+    // ?? HTTP error propagation ???????????????????????????????????????????????
+
+    [Fact]
+    public async Task CheckAttachmentsAsync_WhenApiReturns404_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.NotFound);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfAttachmentClient(httpClient, NullLogger<PdfAttachmentClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.CheckAttachmentsAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+
+    [Fact]
+    public async Task ExtractAttachmentsAsync_WhenApiReturns500_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.InternalServerError);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfAttachmentClient(httpClient, NullLogger<PdfAttachmentClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.ExtractAttachmentsAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+}
diff --git a/DocumentOperator.Tests/Unit/Client/PdfOperationsClientTests.cs b/DocumentOperator.Tests/Unit/Client/PdfOperationsClientTests.cs
new file mode 100644
index 0000000..9288c15
--- /dev/null
+++ b/DocumentOperator.Tests/Unit/Client/PdfOperationsClientTests.cs
@@ -0,0 +1,236 @@
+using DocumentService.Client.Clients;
+using DocumentService.Client.Interfaces;
+using DocumentService.Client.Models.Requests;
+using DocumentService.Client.Models.ValueObjects;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using System.Net;
+using System.Net.Http;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Unit tests for <see cref="PdfOperationsClient"/>.
+/// All tests use a fake <see cref="MockHttpMessageHandler"/> — no real HTTP calls are made.
+/// </summary>
+public class PdfOperationsClientTests
+{
+    // ?? helpers ?????????????????????????????????????????????????????????????
+
+    private static (PdfOperationsClient client, MockHttpMessageHandler handler) BuildBytes(byte[] bytes = null!)
+    {
+        var handler = MockHttpMessageHandler.ReturningBytes(bytes ?? "merged-pdf"u8.ToArray(), "application/pdf");
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfOperationsClient(httpClient, NullLogger<PdfOperationsClient>.Instance);
+        return (client, handler);
+    }
+
+    private static byte[] FakePdfBytes() => "fake-pdf-content"u8.ToArray();
+
+    private static AddAnnotationBase64Request FakeAnnotationRequest() => new()
+    {
+        Base64Pdf    = string.Empty,
+        AnnotationType = AnnotationType.TextMarkup,
+        PageNumber   = 1,
+        X1 = 10, Y1 = 20, Width = 100, Height = 30,
+        Color        = "FFFF00",
+        TextMarkupStyle = TextMarkupStyle.Highlight,
+        Origin       = AnnotationOrigin.TopLeft
+    };
+
+    private static AddStampBase64Request FakeStampRequest() => new()
+    {
+        Base64Pdf  = string.Empty,
+        StampType  = StampType.Text,
+        X = 100, Y = 50,
+        Text       = "CONFIDENTIAL",
+        FontSize   = 24,
+        Color      = "FF0000",
+        Opacity    = 0.5,
+        Placement  = StampPlacement.Foreground,
+        Origin     = AnnotationOrigin.BottomLeft
+    };
+
+    // ?? MergeAsync (Streams) ?????????????????????????????????????????????????
+
+    [Fact]
+    public async Task MergeAsync_Streams_SendsMultipartWithAllFiles()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+        var streams = new List<Stream>
+        {
+            new MemoryStream(FakePdfBytes()),
+            new MemoryStream(FakePdfBytes())
+        };
+
+        // Act
+        using var result = await client.MergeAsync(streams);
+
+        // Assert
+        result.Should().NotBeNull();
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/operations/merge");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+
+        foreach (var s in streams) s.Dispose();
+    }
+
+    [Fact]
+    public async Task MergeAsync_Streams_ReturnsMergedPdfStream()
+    {
+        // Arrange
+        var expectedBytes = "merged-content"u8.ToArray();
+        var (client, _) = BuildBytes(expectedBytes);
+
+        // Act
+        using var result = await client.MergeAsync(new[] { new MemoryStream(FakePdfBytes()), new MemoryStream(FakePdfBytes()) });
+        var actualBytes = await result.ReadAllBytesAsync();
+
+        // Assert
+        actualBytes.Should().BeEquivalentTo(expectedBytes);
+    }
+
+    // ?? MergeAsync (byte[][]) ????????????????????????????????????????????????
+
+    [Fact]
+    public async Task MergeAsync_ByteArrays_SendsJsonWithBase64List()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+
+        // Act
+        using var result = await client.MergeAsync(new[] { FakePdfBytes(), FakePdfBytes() });
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdfs").GetArrayLength().Should().Be(2);
+    }
+
+    [Fact]
+    public async Task MergeAsync_ByteArrays_WithPageRanges_IncludesPageRangesInJson()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+        var pageRanges = new List<string?> { "1-2", null };
+
+        // Act
+        await client.MergeAsync(new[] { FakePdfBytes(), FakePdfBytes() }, pageRanges);
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        var body = await handler.LastRequest!.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("pageRanges").GetArrayLength().Should().Be(2);
+    }
+
+    // ?? AnnotateAsync (Stream) ????????????????????????????????????????????????
+
+    [Fact]
+    public async Task AnnotateAsync_Stream_SendsMultipartWithAnnotationFields()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+        var request = FakeAnnotationRequest();
+
+        // Act
+        using var result = await client.AnnotateAsync(new MemoryStream(FakePdfBytes()), request);
+
+        // Assert
+        result.Should().NotBeNull();
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/operations/annotate");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    // ?? AnnotateAsync (byte[]) ????????????????????????????????????????????????
+
+    [Fact]
+    public async Task AnnotateAsync_Bytes_SendsJsonWithBase64Pdf()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+        var request = FakeAnnotationRequest();
+
+        // Act
+        await client.AnnotateAsync(FakePdfBytes(), request);
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdf").GetString().Should().NotBeNullOrEmpty();
+        // annotationType serializes as integer by default (TextMarkup = 0)
+        doc.RootElement.GetProperty("annotationType").GetInt32().Should().Be((int)AnnotationType.TextMarkup);
+    }
+
+    // ?? StampAsync (Stream) ???????????????????????????????????????????????????
+
+    [Fact]
+    public async Task StampAsync_Stream_SendsMultipartWithStampFields()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+        var request = FakeStampRequest();
+
+        // Act
+        using var result = await client.StampAsync(new MemoryStream(FakePdfBytes()), request);
+
+        // Assert
+        result.Should().NotBeNull();
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/operations/stamp");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    // ?? StampAsync (byte[]) ???????????????????????????????????????????????????
+
+    [Fact]
+    public async Task StampAsync_Bytes_SendsJsonWithBase64Pdf()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes();
+        var request = FakeStampRequest();
+
+        // Act
+        await client.StampAsync(FakePdfBytes(), request);
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdf").GetString().Should().NotBeNullOrEmpty();
+        // stampType serializes as integer by default (Text = 0)
+        doc.RootElement.GetProperty("stampType").GetInt32().Should().Be((int)StampType.Text);
+    }
+
+    // ?? HTTP error propagation ???????????????????????????????????????????????
+
+    [Fact]
+    public async Task MergeAsync_WhenApiReturns400_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.BadRequest);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfOperationsClient(httpClient, NullLogger<PdfOperationsClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.MergeAsync(new[] { new MemoryStream(FakePdfBytes()), new MemoryStream(FakePdfBytes()) }))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+}
+
+// ?? local helper extension ???????????????????????????????????????????????????
+
+file static class StreamHelper
+{
+    public static async Task<byte[]> ReadAllBytesAsync(this Stream stream)
+    {
+        using var ms = new MemoryStream();
+        await stream.CopyToAsync(ms);
+        return ms.ToArray();
+    }
+}
diff --git a/DocumentOperator.Tests/Unit/Client/PdfValidationClientTests.cs b/DocumentOperator.Tests/Unit/Client/PdfValidationClientTests.cs
new file mode 100644
index 0000000..dedca82
--- /dev/null
+++ b/DocumentOperator.Tests/Unit/Client/PdfValidationClientTests.cs
@@ -0,0 +1,168 @@
+using DocumentService.Client.Clients;
+using DocumentService.Client.Interfaces;
+using DocumentService.Client.Models.Requests;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using System.Net;
+using System.Net.Http;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Unit tests for <see cref="PdfValidationClient"/>.
+/// All tests use a fake <see cref="MockHttpMessageHandler"/> — no real HTTP calls are made.
+/// </summary>
+public class PdfValidationClientTests
+{
+    // ?? helpers ?????????????????????????????????????????????????????????????
+
+    private static (PdfValidationClient client, MockHttpMessageHandler handler) Build<T>(T responseBody)
+    {
+        var handler = MockHttpMessageHandler.ReturningJson(responseBody);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfValidationClient(httpClient, NullLogger<PdfValidationClient>.Instance);
+        return (client, handler);
+    }
+
+    private static byte[] FakePdfBytes() => "fake-pdf-content"u8.ToArray();
+
+    // ?? ValidatePdfAsync (Stream) ????????????????????????????????????????????
+
+    [Fact]
+    public async Task ValidatePdfAsync_Stream_SendsMultipartPost()
+    {
+        // Arrange
+        var expected = new PdfValidationResult { PageCount = 3, PdfVersion = "1.7", FileSizeBytes = 2048 };
+        var (client, handler) = Build(expected);
+
+        // Act
+        var result = await client.ValidatePdfAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.PageCount.Should().Be(3);
+        result.PdfVersion.Should().Be("1.7");
+        handler.LastRequest!.Method.Should().Be(HttpMethod.Post);
+        handler.LastRequest.RequestUri!.PathAndQuery.Should().Be("/api/pdf/validation/validate");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    [Fact]
+    public async Task ValidatePdfAsync_Stream_ThrowsWhenApiReturnsNull()
+    {
+        // Arrange — API returns JSON null
+        var handler = MockHttpMessageHandler.ReturningJson<PdfValidationResult?>(null);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfValidationClient(httpClient, NullLogger<PdfValidationClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.ValidatePdfAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<InvalidOperationException>();
+    }
+
+    // ?? ValidatePdfAsync (byte[]) ????????????????????????????????????????????
+
+    [Fact]
+    public async Task ValidatePdfAsync_Bytes_SendsJsonWithBase64()
+    {
+        // Arrange
+        var expected = new PdfValidationResult { PageCount = 1, IsEncrypted = false };
+        var (client, handler) = Build(expected);
+
+        // Act
+        var result = await client.ValidatePdfAsync(FakePdfBytes());
+
+        // Assert
+        result.PageCount.Should().Be(1);
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdf").GetString().Should().NotBeNullOrEmpty();
+    }
+
+    // ?? ValidatePdfAAsync (Stream) ???????????????????????????????????????????
+
+    [Fact]
+    public async Task ValidatePdfAAsync_Stream_SendsMultipartPost()
+    {
+        // Arrange
+        var expected = new PdfAValidationResult { IsValid = true, PdfAVersion = "PDF/A-3b", PageCount = 2 };
+        var (client, handler) = Build(expected);
+
+        // Act
+        var result = await client.ValidatePdfAAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.IsValid.Should().BeTrue();
+        result.PdfAVersion.Should().Be("PDF/A-3b");
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/validation/validate-pdfa");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    [Fact]
+    public async Task ValidatePdfAAsync_Stream_WithErrors_ReturnsErrors()
+    {
+        // Arrange
+        var expected = new PdfAValidationResult
+        {
+            IsValid = false,
+            Errors = new List<string> { "Missing embedded font", "Encryption not allowed" }
+        };
+        var (client, _) = Build(expected);
+
+        // Act
+        var result = await client.ValidatePdfAAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.IsValid.Should().BeFalse();
+        result.Errors.Should().HaveCount(2).And.Contain("Missing embedded font");
+    }
+
+    // ?? ValidatePdfAAsync (byte[]) ???????????????????????????????????????????
+
+    [Fact]
+    public async Task ValidatePdfAAsync_Bytes_SendsJson()
+    {
+        // Arrange
+        var expected = new PdfAValidationResult { IsValid = true };
+        var (client, handler) = Build(expected);
+
+        // Act
+        await client.ValidatePdfAAsync(FakePdfBytes());
+
+        // Assert
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/validation/validate-pdfa");
+        handler.LastRequest.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+    }
+
+    // ?? HTTP error propagation ???????????????????????????????????????????????
+
+    [Fact]
+    public async Task ValidatePdfAsync_WhenApiReturns400_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.BadRequest);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfValidationClient(httpClient, NullLogger<PdfValidationClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.ValidatePdfAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+
+    [Fact]
+    public async Task ValidatePdfAsync_WhenApiReturns500_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.InternalServerError);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new PdfValidationClient(httpClient, NullLogger<PdfValidationClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.ValidatePdfAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+}
diff --git a/DocumentOperator.Tests/Unit/Client/SwissQrCodeClientTests.cs b/DocumentOperator.Tests/Unit/Client/SwissQrCodeClientTests.cs
new file mode 100644
index 0000000..d252a29
--- /dev/null
+++ b/DocumentOperator.Tests/Unit/Client/SwissQrCodeClientTests.cs
@@ -0,0 +1,123 @@
+using DocumentService.Client.Clients;
+using DocumentService.Client.Interfaces;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using System.Net;
+using System.Net.Http;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Unit tests for <see cref="SwissQrCodeClient"/>.
+/// All tests use a fake <see cref="MockHttpMessageHandler"/> — no real HTTP calls are made.
+/// </summary>
+public class SwissQrCodeClientTests
+{
+    // ?? helpers ?????????????????????????????????????????????????????????????
+
+    private static (SwissQrCodeClient client, MockHttpMessageHandler handler) Build<T>(T body)
+    {
+        var handler = MockHttpMessageHandler.ReturningJson(body);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new SwissQrCodeClient(httpClient, NullLogger<SwissQrCodeClient>.Instance);
+        return (client, handler);
+    }
+
+    private static byte[] FakePdfBytes() => "fake-pdf-content"u8.ToArray();
+
+    // ?? ExtractSwissQrCodeAsync (Stream) — parsed Bill ???????????????????????
+
+    [Fact]
+    public async Task ExtractSwissQrCodeAsync_Stream_ParsedMode_SendsMultipartToCorrectEndpoint()
+    {
+        // Arrange
+        var expected = new SwissQrCodeExtractionResult { Bill = new { Iban = "CH93-0076-2011-6238-5295-7" } };
+        var (client, handler) = Build(expected);
+
+        // Act
+        var result = await client.ExtractSwissQrCodeAsync(new MemoryStream(FakePdfBytes()), raw: false);
+
+        // Assert
+        result.Should().NotBeNull();
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/qr-code/extract-swiss?raw=False");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    [Fact]
+    public async Task ExtractSwissQrCodeAsync_Stream_RawMode_SendsRawFlagInUrl()
+    {
+        // Arrange
+        var expected = new SwissQrCodeExtractionResult { RawLines = new List<string> { "SPC", "0200", "1" } };
+        var (client, handler) = Build(expected);
+
+        // Act
+        var result = await client.ExtractSwissQrCodeAsync(new MemoryStream(FakePdfBytes()), raw: true);
+
+        // Assert
+        result.RawLines.Should().HaveCount(3).And.StartWith("SPC");
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/qr-code/extract-swiss?raw=True");
+    }
+
+    // ?? ExtractSwissQrCodeAsync (byte[]) ?????????????????????????????????????
+
+    [Fact]
+    public async Task ExtractSwissQrCodeAsync_Bytes_SendsJsonWithBase64()
+    {
+        // Arrange
+        var expected = new SwissQrCodeExtractionResult();
+        var (client, handler) = Build(expected);
+
+        // Act
+        await client.ExtractSwissQrCodeAsync(FakePdfBytes(), raw: false);
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdf").GetString().Should().NotBeNullOrEmpty();
+    }
+
+    [Fact]
+    public async Task ExtractSwissQrCodeAsync_Bytes_RawMode_IncludesRawFlagInUrl()
+    {
+        // Arrange
+        var expected = new SwissQrCodeExtractionResult { RawLines = new List<string> { "SPC" } };
+        var (client, handler) = Build(expected);
+
+        // Act
+        await client.ExtractSwissQrCodeAsync(FakePdfBytes(), raw: true);
+
+        // Assert
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/qr-code/extract-swiss?raw=True");
+    }
+
+    // ?? HTTP error propagation ???????????????????????????????????????????????
+
+    [Fact]
+    public async Task ExtractSwissQrCodeAsync_WhenApiReturns404_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.NotFound);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new SwissQrCodeClient(httpClient, NullLogger<SwissQrCodeClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.ExtractSwissQrCodeAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+
+    [Fact]
+    public async Task ExtractSwissQrCodeAsync_WhenApiReturns500_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.InternalServerError);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new SwissQrCodeClient(httpClient, NullLogger<SwissQrCodeClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.ExtractSwissQrCodeAsync(FakePdfBytes()))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+}
diff --git a/DocumentOperator.Tests/Unit/Client/ZugferdClientTests.cs b/DocumentOperator.Tests/Unit/Client/ZugferdClientTests.cs
new file mode 100644
index 0000000..cc852c0
--- /dev/null
+++ b/DocumentOperator.Tests/Unit/Client/ZugferdClientTests.cs
@@ -0,0 +1,199 @@
+using DocumentService.Client.Clients;
+using DocumentService.Client.Interfaces;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using System.Net;
+using System.Net.Http;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Unit tests for <see cref="ZugferdClient"/>.
+/// All tests use a fake <see cref="MockHttpMessageHandler"/> — no real HTTP calls are made.
+/// </summary>
+public class ZugferdClientTests
+{
+    // ?? helpers ?????????????????????????????????????????????????????????????
+
+    private static (ZugferdClient client, MockHttpMessageHandler handler) BuildJson<T>(T body)
+    {
+        var handler = MockHttpMessageHandler.ReturningJson(body);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new ZugferdClient(httpClient, NullLogger<ZugferdClient>.Instance);
+        return (client, handler);
+    }
+
+    private static (ZugferdClient client, MockHttpMessageHandler handler) BuildBytes(byte[] bytes, string mediaType = "application/xml")
+    {
+        var handler = MockHttpMessageHandler.ReturningBytes(bytes, mediaType);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new ZugferdClient(httpClient, NullLogger<ZugferdClient>.Instance);
+        return (client, handler);
+    }
+
+    private static byte[] FakePdfBytes() => "fake-pdf-content"u8.ToArray();
+    private static byte[] FakeXmlBytes() => "<invoice>test</invoice>"u8.ToArray();
+
+    // ?? HasZugferdAsync (Stream) ?????????????????????????????????????????????
+
+    [Fact]
+    public async Task HasZugferdAsync_Stream_WhenZugferdPresent_ReturnsTrue()
+    {
+        // Arrange
+        var expected = new ZugferdCheckResult { HasZugferd = true, Version = "2.1", Profile = "EN 16931" };
+        var (client, handler) = BuildJson(expected);
+
+        // Act
+        var result = await client.HasZugferdAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.HasZugferd.Should().BeTrue();
+        result.Version.Should().Be("2.1");
+        result.Profile.Should().Be("EN 16931");
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/zugferd/has-zugferd");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    [Fact]
+    public async Task HasZugferdAsync_Stream_WhenNoZugferd_ReturnsFalse()
+    {
+        // Arrange
+        var expected = new ZugferdCheckResult { HasZugferd = false };
+        var (client, _) = BuildJson(expected);
+
+        // Act
+        var result = await client.HasZugferdAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.HasZugferd.Should().BeFalse();
+        result.Version.Should().BeNull();
+    }
+
+    // ?? HasZugferdAsync (byte[]) ?????????????????????????????????????????????
+
+    [Fact]
+    public async Task HasZugferdAsync_Bytes_SendsJsonWithBase64()
+    {
+        // Arrange
+        var expected = new ZugferdCheckResult { HasZugferd = true };
+        var (client, handler) = BuildJson(expected);
+
+        // Act
+        await client.HasZugferdAsync(FakePdfBytes());
+
+        // Assert
+        handler.LastRequest!.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+        var body = await handler.LastRequest.Content!.ReadAsStringAsync();
+        var doc = JsonDocument.Parse(body);
+        doc.RootElement.GetProperty("base64Pdf").GetString().Should().NotBeNullOrEmpty();
+    }
+
+    // ?? ExtractZugferdAsync (Stream) — raw XML stream ?????????????????????????
+
+    [Fact]
+    public async Task ExtractZugferdAsync_Stream_SendsMultipartWithAsFileTrue()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes(FakeXmlBytes());
+
+        // Act
+        using var result = await client.ExtractZugferdAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.Should().NotBeNull();
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/zugferd/extract?asFile=true");
+        handler.LastRequest.Content.Should().BeOfType<MultipartFormDataContent>();
+    }
+
+    [Fact]
+    public async Task ExtractZugferdAsync_Stream_ReturnsXmlContent()
+    {
+        // Arrange
+        var xmlBytes = "<root><invoice/></root>"u8.ToArray();
+        var (client, _) = BuildBytes(xmlBytes);
+
+        // Act
+        using var result = await client.ExtractZugferdAsync(new MemoryStream(FakePdfBytes()));
+        var content = await new StreamReader(result).ReadToEndAsync();
+
+        // Assert
+        content.Should().Be("<root><invoice/></root>");
+    }
+
+    // ?? ExtractZugferdAsync (byte[]) ?????????????????????????????????????????
+
+    [Fact]
+    public async Task ExtractZugferdAsync_Bytes_SendsJsonWithFormatFile()
+    {
+        // Arrange
+        var (client, handler) = BuildBytes(FakeXmlBytes());
+
+        // Act
+        using var result = await client.ExtractZugferdAsync(FakePdfBytes());
+
+        // Assert
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/zugferd/extract?format=file");
+        handler.LastRequest.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+    }
+
+    // ?? ExtractZugferdAsResultAsync (Stream) ?????????????????????????????????
+
+    [Fact]
+    public async Task ExtractZugferdAsResultAsync_Stream_ReturnsStructuredResult()
+    {
+        // Arrange
+        var expected = new ZugferdExtractionResult
+        {
+            FileName   = "factur-x.xml",
+            XmlContent = "<invoice/>",
+            Version    = "2.1",
+            Profile    = "EN 16931"
+        };
+        var (client, handler) = BuildJson(expected);
+
+        // Act
+        var result = await client.ExtractZugferdAsResultAsync(new MemoryStream(FakePdfBytes()));
+
+        // Assert
+        result.FileName.Should().Be("factur-x.xml");
+        result.XmlContent.Should().Be("<invoice/>");
+        result.Version.Should().Be("2.1");
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/zugferd/extract?asFile=false");
+    }
+
+    // ?? ExtractZugferdAsResultAsync (byte[]) ?????????????????????????????????
+
+    [Fact]
+    public async Task ExtractZugferdAsResultAsync_Bytes_SendsJsonWithFormatJson()
+    {
+        // Arrange
+        var expected = new ZugferdExtractionResult { FileName = "zugferd.xml", XmlContent = "<x/>" };
+        var (client, handler) = BuildJson(expected);
+
+        // Act
+        await client.ExtractZugferdAsResultAsync(FakePdfBytes());
+
+        // Assert
+        handler.LastRequest!.RequestUri!.PathAndQuery.Should().Be("/api/pdf/zugferd/extract?format=json");
+        handler.LastRequest.Content.Should().NotBeNull();
+        handler.LastRequest.Content!.Headers.ContentType!.MediaType.Should().Be("application/json");
+    }
+
+    // ?? HTTP error propagation ???????????????????????????????????????????????
+
+    [Fact]
+    public async Task HasZugferdAsync_WhenApiReturns400_ThrowsHttpRequestException()
+    {
+        // Arrange
+        var handler = MockHttpMessageHandler.ReturningStatus(HttpStatusCode.BadRequest);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("http://localhost") };
+        var client = new ZugferdClient(httpClient, NullLogger<ZugferdClient>.Instance);
+
+        // Act & Assert
+        await client.Invoking(c => c.HasZugferdAsync(new MemoryStream(FakePdfBytes())))
+            .Should().ThrowAsync<HttpRequestException>();
+    }
+}