diff --git a/src/AnthropicClient/Models/PagingRequest.cs b/src/AnthropicClient/Models/PagingRequest.cs
index f8b415a..52746df 100644
--- a/src/AnthropicClient/Models/PagingRequest.cs
+++ b/src/AnthropicClient/Models/PagingRequest.cs
@@ -34,6 +34,7 @@ public class PagingRequest
   /// <param name="beforeId">The ID of the item before which to start the page.</param>
   /// <param name="afterId">The ID of the item after which to start the page.</param>
   /// <param name="limit">The maximum number of items to return in the page.</param>
+  /// <exception cref="ArgumentException">Thrown when both <paramref name="beforeId"/> and <paramref name="afterId"/> are specified.</exception>
   /// <exception cref="ArgumentOutOfRangeException">Thrown when <paramref name="limit"/> is less than 1 or greater than 1000.</exception>
   /// <returns>A new instance of the <see cref="PagingRequest"/> class.</returns>
   public PagingRequest(
@@ -47,6 +48,11 @@ public class PagingRequest
       throw new ArgumentOutOfRangeException(nameof(limit), $"{nameof(limit)} must be between {LimitMinimum} and {LimitMaximum}.");
     }
 
+    if (string.IsNullOrEmpty(beforeId) is false && string.IsNullOrEmpty(afterId) is false)
+    {
+      throw new ArgumentException($"Only one of {nameof(beforeId)} or {nameof(afterId)} can be set.");
+    }
+
     BeforeId = beforeId;
     AfterId = afterId;
     Limit = limit;
diff --git a/tests/AnthropicClient.Tests/Integration/AnthropicApiClientTests.cs b/tests/AnthropicClient.Tests/Integration/AnthropicApiClientTests.cs
index 8c8f0b3..eaef353 100644
--- a/tests/AnthropicClient.Tests/Integration/AnthropicApiClientTests.cs
+++ b/tests/AnthropicClient.Tests/Integration/AnthropicApiClientTests.cs
@@ -549,13 +549,12 @@ public class AnthropicApiClientTests : IntegrationTest
   [Fact]
   public async Task ListModelsAsync_WhenCalledWithPagingRequestUsingCustomValues_ItShouldReturnListOfModels()
   {
-    var pagingRequest = new PagingRequest("prev_id", "next_id", 10);
+    var pagingRequest = new PagingRequest(afterId: "next_id", limit: 10);
 
     _mockHttpMessageHandler
       .WhenListModelsRequest()
       .WithQueryString(new Dictionary<string, string>
       {
-        { "before_id", pagingRequest.BeforeId },
         { "after_id", pagingRequest.AfterId },
         { "limit", pagingRequest.Limit.ToString() },
       })
diff --git a/tests/AnthropicClient.Tests/Unit/Models/PagingRequestTests.cs b/tests/AnthropicClient.Tests/Unit/Models/PagingRequestTests.cs
index 069120c..b63599b 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/PagingRequestTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/PagingRequestTests.cs
@@ -18,6 +18,14 @@ public class PagingRequestTests : SerializationTest
     act.Should().Throw<ArgumentOutOfRangeException>().WithMessage("limit must be between 1 and 1000. (Parameter 'limit')");
   }
 
+  [Fact]
+  public void Constructor_WhenBothBeforeIdAndAfterIdAreSet_ItShouldThrowArgumentException()
+  {
+    var act = () => new PagingRequest(beforeId: "before-id", afterId: "after-id");
+
+    act.Should().Throw<ArgumentException>().WithMessage("Only one of beforeId or afterId can be set.");
+  }
+
   [Fact]
   public void ToQueryParameters_WhenNoPropertiesSet_ItShouldReturnEmptyString()
   {
@@ -57,14 +65,4 @@ public class PagingRequestTests : SerializationTest
 
     result.Should().Be("limit=10");
   }
-
-  [Fact]
-  public void ToQueryParameters_WhenAllPropertiesAreSet_ItShouldReturnAllProperties()
-  {
-    var pagingRequest = new PagingRequest(beforeId: "before-id", afterId: "after-id", limit: 10);
-
-    var result = pagingRequest.ToQueryParameters();
-
-    result.Should().Be("before_id=before-id&after_id=after-id&limit=10");
-  }
 }
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