diff --git a/src/AnthropicClient/Models/CharacterLocationCitation.cs b/src/AnthropicClient/Models/CharacterLocationCitation.cs
index 0b461cb..678326e 100644
--- a/src/AnthropicClient/Models/CharacterLocationCitation.cs
+++ b/src/AnthropicClient/Models/CharacterLocationCitation.cs
@@ -18,4 +18,12 @@ public class CharacterLocationCitation : Citation
   /// </summary>
   [JsonPropertyName("end_char_index")]
   public int EndCharIndex { get; init; }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="CharacterLocationCitation"/> class.
+  /// </summary>
+  /// <returns>A new instance of <see cref="CharacterLocationCitation"/>.</returns>
+  public CharacterLocationCitation() : base(CitationType.CharacterLocation)
+  {
+  }
 }
diff --git a/src/AnthropicClient/Models/Citation.cs b/src/AnthropicClient/Models/Citation.cs
index ec7ae73..485f917 100644
--- a/src/AnthropicClient/Models/Citation.cs
+++ b/src/AnthropicClient/Models/Citation.cs
@@ -29,4 +29,19 @@ public abstract class Citation
   /// </summary>
   [JsonPropertyName("document_title")]
   public string DocumentTitle { get; init; } = string.Empty;
+
+  [JsonConstructor]
+  internal Citation()
+  {
+  }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="Citation"/> class with a specified type.
+  /// </summary>
+  /// <param name="type">The type of the citation.</param>
+  /// <returns>A new instance of <see cref="Citation"/>.</returns>
+  protected Citation(string type)
+  {
+    Type = type;
+  }
 }
diff --git a/src/AnthropicClient/Models/CitationDelta.cs b/src/AnthropicClient/Models/CitationDelta.cs
index 1d27bd0..e299d97 100644
--- a/src/AnthropicClient/Models/CitationDelta.cs
+++ b/src/AnthropicClient/Models/CitationDelta.cs
@@ -22,6 +22,9 @@ public class CitationDelta : ContentDelta
   /// <summary>
   /// Initializes a new instance of the <see cref="CitationDelta"/> class.
   /// </summary>
+  /// <param name="citation">The citation to associate with this delta.</param>
+  /// <exception cref="ArgumentNullException">Thrown when <paramref name="citation"/> is null.</exception>
+  /// <returns>A new instance of <see cref="CitationDelta"/>.</returns>
   public CitationDelta(Citation citation) : base(ContentDeltaType.CitationDelta)
   {
     ArgumentValidator.ThrowIfNull(citation, nameof(citation));
diff --git a/src/AnthropicClient/Models/ContentBlockLocationCitation.cs b/src/AnthropicClient/Models/ContentBlockLocationCitation.cs
index 0cbb73a..f0383bb 100644
--- a/src/AnthropicClient/Models/ContentBlockLocationCitation.cs
+++ b/src/AnthropicClient/Models/ContentBlockLocationCitation.cs
@@ -18,4 +18,12 @@ public class ContentBlockLocationCitation : Citation
   /// </summary>
   [JsonPropertyName("end_block_index")]
   public int EndBlockIndex { get; init; }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="ContentBlockLocationCitation"/> class.
+  /// </summary>
+  /// <returns>A new instance of <see cref="ContentBlockLocationCitation"/>.</returns>
+  public ContentBlockLocationCitation() : base(CitationType.ContentBlockLocation)
+  {
+  }
 }
\ No newline at end of file
diff --git a/src/AnthropicClient/Models/PageLocationCitation.cs b/src/AnthropicClient/Models/PageLocationCitation.cs
index 040c262..29a9ce1 100644
--- a/src/AnthropicClient/Models/PageLocationCitation.cs
+++ b/src/AnthropicClient/Models/PageLocationCitation.cs
@@ -18,4 +18,12 @@ public class PageLocationCitation : Citation
   /// </summary>
   [JsonPropertyName("end_page_number")]
   public int EndPageNumber { get; init; }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="PageLocationCitation"/> class.
+  /// </summary>
+  /// <returns>A new instance of <see cref="PageLocationCitation"/>.</returns>
+  public PageLocationCitation() : base(CitationType.PageLocation)
+  {
+  }
 }
diff --git a/tests/AnthropicClient.Tests/Unit/Models/Base64SourceTests.cs b/tests/AnthropicClient.Tests/Unit/Models/Base64SourceTests.cs
new file mode 100644
index 0000000..1a6972b
--- /dev/null
+++ b/tests/AnthropicClient.Tests/Unit/Models/Base64SourceTests.cs
@@ -0,0 +1,39 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class Base64SourceTests
+{
+  [Fact]
+  public void Constructor_WhenCalledWithValidArguments_ItShouldSetProperties()
+  {
+    var mediaType = "application/pdf";
+    var data = "base64data";
+
+    var source = new Base64Source(mediaType, data);
+
+    source.MediaType.Should().Be(mediaType);
+    source.Data.Should().Be(data);
+    source.Type.Should().Be("base64");
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledWithNullMediaType_ItShouldThrowArgumentNullException()
+  {
+    string? mediaType = null;
+    var data = "base64data";
+
+    var action = () => new Base64Source(mediaType!, data);
+
+    action.Should().Throw<ArgumentNullException>();
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledWithNullData_ItShouldThrowArgumentNullException()
+  {
+    var mediaType = "application/pdf";
+    string? data = null;
+
+    var action = () => new Base64Source(mediaType, data!);
+
+    action.Should().Throw<ArgumentNullException>();
+  }
+}
\ No newline at end of file
diff --git a/tests/AnthropicClient.Tests/Unit/Models/CharacterLocationCitationTests.cs b/tests/AnthropicClient.Tests/Unit/Models/CharacterLocationCitationTests.cs
new file mode 100644
index 0000000..0043ea4
--- /dev/null
+++ b/tests/AnthropicClient.Tests/Unit/Models/CharacterLocationCitationTests.cs
@@ -0,0 +1,17 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class CharacterLocationCitationTests
+{
+  [Fact]
+  public void Constructor_WhenCalled_ItShouldSetProperties()
+  {
+    var result = new CharacterLocationCitation();
+
+    result.Type.Should().Be("char_location");
+    result.CitedText.Should().BeEmpty();
+    result.DocumentIndex.Should().Be(0);
+    result.DocumentTitle.Should().BeEmpty();
+    result.StartCharIndex.Should().Be(0);
+    result.EndCharIndex.Should().Be(0);
+  }
+}
\ No newline at end of file
diff --git a/tests/AnthropicClient.Tests/Unit/Models/CitationDeltaTests.cs b/tests/AnthropicClient.Tests/Unit/Models/CitationDeltaTests.cs
new file mode 100644
index 0000000..8352ce8
--- /dev/null
+++ b/tests/AnthropicClient.Tests/Unit/Models/CitationDeltaTests.cs
@@ -0,0 +1,31 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class CitationDeltaTests
+{
+  [Fact]
+  public void Constructor_WhenCalled_ItShouldSetProperties()
+  {
+    var result = new CitationDelta();
+
+    result.Type.Should().Be("citations_delta");
+    result.Citation.Should().BeEquivalentTo(new CharacterLocationCitation());
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledWithCitation_ItShouldSetProperties()
+  {
+    var citation = new ContentBlockLocationCitation();
+    var result = new CitationDelta(citation);
+
+    result.Type.Should().Be("citations_delta");
+    result.Citation.Should().BeSameAs(citation);
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledWithNullCitation_ItShouldThrowException()
+  {
+    var act = () => new CitationDelta(null!);
+
+    act.Should().Throw<ArgumentNullException>();
+  }
+}
\ No newline at end of file
diff --git a/tests/AnthropicClient.Tests/Unit/Models/CitationOptionTests.cs b/tests/AnthropicClient.Tests/Unit/Models/CitationOptionTests.cs
new file mode 100644
index 0000000..36ff5d8
--- /dev/null
+++ b/tests/AnthropicClient.Tests/Unit/Models/CitationOptionTests.cs
@@ -0,0 +1,12 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class CitationOptionTests
+{
+  [Fact]
+  public void Constructor_WhenCalled_ItShouldSetProperties()
+  {
+    var result = new CitationOption();
+
+    result.Enabled.Should().BeFalse();
+  }
+}
\ No newline at end of file
diff --git a/tests/AnthropicClient.Tests/Unit/Models/CitationTypeTests.cs b/tests/AnthropicClient.Tests/Unit/Models/CitationTypeTests.cs
new file mode 100644
index 0000000..c0fb416
--- /dev/null
+++ b/tests/AnthropicClient.Tests/Unit/Models/CitationTypeTests.cs
@@ -0,0 +1,22 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class CitationTypeTests
+{
+  [Fact]
+  public void ContentBlockLocation_WhenCalled_ItShouldReturnCorrectValue()
+  {
+    CitationType.ContentBlockLocation.Should().Be("content_block_location");
+  }
+
+  [Fact]
+  public void CharacterLocation_WhenCalled_ItShouldReturnCorrectValue()
+  {
+    CitationType.CharacterLocation.Should().Be("char_location");
+  }
+
+  [Fact]
+  public void PageLocation_WhenCalled_ItShouldReturnCorrectValue()
+  {
+    CitationType.PageLocation.Should().Be("page_location");
+  }
+}
\ No newline at end of file
diff --git a/tests/AnthropicClient.Tests/Unit/Models/ContentBlockLocationCitationTests.cs b/tests/AnthropicClient.Tests/Unit/Models/ContentBlockLocationCitationTests.cs
new file mode 100644
index 0000000..45fc169
--- /dev/null
+++ b/tests/AnthropicClient.Tests/Unit/Models/ContentBlockLocationCitationTests.cs
@@ -0,0 +1,17 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class ContentBlockLocationCitationTests
+{
+  [Fact]
+  public void Constructor_WhenCalled_ItShouldSetProperties()
+  {
+    var result = new ContentBlockLocationCitation();
+
+    result.Type.Should().Be("content_block_location");
+    result.CitedText.Should().BeEmpty();
+    result.DocumentIndex.Should().Be(0);
+    result.DocumentTitle.Should().BeEmpty();
+    result.StartBlockIndex.Should().Be(0);
+    result.EndBlockIndex.Should().Be(0);
+  }
+}
\ No newline at end of file
diff --git a/tests/AnthropicClient.Tests/Unit/Models/ContentDeltaTypeTests.cs b/tests/AnthropicClient.Tests/Unit/Models/ContentDeltaTypeTests.cs
index 32a0023..bb46b42 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/ContentDeltaTypeTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/ContentDeltaTypeTests.cs
@@ -21,4 +21,14 @@ public class ContentDeltaTypeTests
 
     actual.Should().Be(expected);
   }
+
+  [Fact]
+  public void CitationDelta_WhenCalled_ItShouldReturnExpectedValue()
+  {
+    var expected = "citations_delta";
+
+    var actual = ContentDeltaType.CitationDelta;
+
+    actual.Should().Be(expected);
+  }
 }
\ No newline at end of file