diff --git a/Google.GenAI.Tests/ClientTest.cs b/Google.GenAI.Tests/ClientTest.cs
index 0475351e..a4822ca2 100644
--- a/Google.GenAI.Tests/ClientTest.cs
+++ b/Google.GenAI.Tests/ClientTest.cs
@@ -15,6 +15,7 @@
  */
 
 using System;
+using System.Net.Http;
 
 using Google.Apis.Auth.OAuth2;
 using Google.GenAI;
@@ -666,6 +667,16 @@ public void Constructor_HttpOptionsProvided_Timeout() {
       Assert.AreEqual(1000, client._apiClient.HttpOptions.Timeout);
     }
 
+    [TestMethod]
+    public void Constructor_HttpOptionsProvided_HttpClientFactory() {
+      Func<HttpClient> factory = () => new HttpClient();
+      var options = new HttpOptions { HttpClientFactory = factory };
+
+      var client = new Client(vertexAI: false, apiKey: "key", httpOptions: options);
+
+      Assert.AreSame(factory, client._apiClient.HttpOptions.HttpClientFactory);
+    }
+
 #endregion
 
 #region Successful Instantiation(all modules)
diff --git a/Google.GenAI.Tests/HttpApiClientTest.cs b/Google.GenAI.Tests/HttpApiClientTest.cs
index a49dcda7..f50506cd 100644
--- a/Google.GenAI.Tests/HttpApiClientTest.cs
+++ b/Google.GenAI.Tests/HttpApiClientTest.cs
@@ -54,6 +54,7 @@ public void GeminiConstructor_WithApiKey_SetsPropertiesCorrectly() {
       Assert.AreEqual("https://generativelanguage.googleapis.com", client.HttpOptions.BaseUrl);
       Assert.AreEqual("v1beta", client.HttpOptions.ApiVersion);
       Assert.IsNull(client.HttpOptions.Timeout);
+      Assert.IsNull(client.HttpOptions.HttpClientFactory);
     }
 
     [TestMethod]
@@ -70,13 +71,16 @@ public void GeminiConstructor_WithApiKeyFromEnvironment_SetsPropertiesCorrectly(
       Assert.AreEqual("https://generativelanguage.googleapis.com", client.HttpOptions.BaseUrl);
       Assert.AreEqual("v1beta", client.HttpOptions.ApiVersion);
       Assert.IsNull(client.HttpOptions.Timeout);
+      Assert.IsNull(client.HttpOptions.HttpClientFactory);
     }
 
     [TestMethod]
     public void GeminiConstructor_WithHttpOptionsProvided_SetsPropertiesCorrectly() {
       System.Environment.SetEnvironmentVariable(EnvApiKeyName, TestApiKey);
+      var httpClientFactory = () => new HttpClient();
       var customHttpOptions = new Types.HttpOptions { BaseUrl = "https://custom-url.com",
-                                                      ApiVersion = "v2", Timeout = 6000 };
+                                                      ApiVersion = "v2", Timeout = 6000,
+                                                      HttpClientFactory = httpClientFactory };
 
       var client = new HttpApiClient(vertexAI: false, httpOptions: customHttpOptions);
 
@@ -88,6 +92,7 @@ public void GeminiConstructor_WithHttpOptionsProvided_SetsPropertiesCorrectly()
       Assert.AreEqual("https://custom-url.com", client.HttpOptions.BaseUrl);
       Assert.AreEqual("v2", client.HttpOptions.ApiVersion);
       Assert.AreEqual(6000, client.HttpOptions.Timeout);
+      Assert.AreSame(httpClientFactory, client.HttpOptions.HttpClientFactory);
     }
 
     [TestMethod]
@@ -127,6 +132,7 @@ public void VertexConstructor_WithProjectLocationCredentials_SetsPropertiesCorre
                       client.HttpOptions.BaseUrl);
       Assert.AreEqual("v1beta1", client.HttpOptions.ApiVersion);
       Assert.IsNull(client.HttpOptions.Timeout);
+      Assert.IsNull(client.HttpOptions.HttpClientFactory);
     }
 
     [TestMethod]
@@ -150,6 +156,7 @@ public void VertexConstructor_WithProjectLocationFromEnvironment_SetsPropertiesC
                       client.HttpOptions.BaseUrl);
       Assert.AreEqual("v1beta1", client.HttpOptions.ApiVersion);
       Assert.IsNull(client.HttpOptions.Timeout);
+      Assert.IsNull(client.HttpOptions.HttpClientFactory);
     }
 
     [TestMethod]
@@ -213,8 +220,10 @@ public void VertexConstructor_WithCustomHttpOptions_OverridesDefaults() {
           .Setup(c => c.GetAccessTokenForRequestAsync(
                      It.IsAny<string?>(), It.IsAny<System.Threading.CancellationToken>()))
           .ReturnsAsync("mock-access-token");
+      var httpClientFactory = () => new HttpClient();
       var customOptions = new Types.HttpOptions { BaseUrl = "https://custom.vertex.ai",
-                                                  ApiVersion = "v2alpha", Timeout = 8000 };
+                                                  ApiVersion = "v2alpha", Timeout = 8000,
+                                                  HttpClientFactory = httpClientFactory };
 
       var client =
           new HttpApiClient(vertexAI: true, project: TestProject, location: TestLocation, credentials: mockCredential.Object, httpOptions: customOptions);
@@ -227,6 +236,7 @@ public void VertexConstructor_WithCustomHttpOptions_OverridesDefaults() {
       Assert.AreEqual("https://custom.vertex.ai", client.HttpOptions.BaseUrl);
       Assert.AreEqual("v2alpha", client.HttpOptions.ApiVersion);
       Assert.AreEqual(8000, client.HttpOptions.Timeout);
+      Assert.AreSame(httpClientFactory, client.HttpOptions.HttpClientFactory);
     }
 
     [TestMethod]
diff --git a/Google.GenAI/ApiClient.cs b/Google.GenAI/ApiClient.cs
index 8be3c6a0..0a6d0539 100644
--- a/Google.GenAI/ApiClient.cs
+++ b/Google.GenAI/ApiClient.cs
@@ -184,7 +184,7 @@ protected ApiClient(
 
     private static HttpClient CreateHttpClient(HttpOptions httpOptions)
     {
-      var client = new HttpClient();
+      var client = httpOptions.HttpClientFactory?.Invoke() ?? new HttpClient();
       if (httpOptions.Timeout != null)
       {
         client.Timeout = System.TimeSpan.FromMilliseconds(httpOptions.Timeout.Value);
@@ -263,6 +263,10 @@ protected HttpOptions MergeHttpOptions(HttpOptions? optionsToApply)
       {
         mergedOptions.BaseUrlResourceScope = optionsToApply?.BaseUrlResourceScope;
       }
+      if (optionsToApply?.HttpClientFactory != null)
+      {
+        mergedOptions.HttpClientFactory = optionsToApply?.HttpClientFactory;
+      }
 
       var currentHeaders = this.HttpOptions.Headers ?? new Dictionary<string, string>();
       var newHeaders = optionsToApply?.Headers ?? new Dictionary<string, string>();
diff --git a/Google.GenAI/UploadClient.cs b/Google.GenAI/UploadClient.cs
index 0909f37a..793351f1 100644
--- a/Google.GenAI/UploadClient.cs
+++ b/Google.GenAI/UploadClient.cs
@@ -86,7 +86,8 @@ public static HttpOptions BuildResumableUploadHttpOptions(
         ApiVersion = "",
         Headers = mergedHeaders,
         BaseUrl = userOptions?.BaseUrl,
-        Timeout = userOptions?.Timeout
+        Timeout = userOptions?.Timeout,
+        HttpClientFactory = userOptions?.HttpClientFactory
       };
     }
 
diff --git a/Google.GenAI/types/HttpOptions.cs b/Google.GenAI/types/HttpOptions.cs
index 5a8b3187..c721b167 100644
--- a/Google.GenAI/types/HttpOptions.cs
+++ b/Google.GenAI/types/HttpOptions.cs
@@ -74,6 +74,16 @@ public int
             get; set;
           }
 
+    /// <summary>
+    /// A factory function to create HttpClient instances.
+    /// This allows for custom configuration of HttpClient, such as setting default headers, timeouts, or using a custom message handler.
+    /// </summary>
+    [JsonIgnore]
+    public Func<HttpClient>
+      ? HttpClientFactory {
+          get; set;
+        }
+
     /// <summary>
     /// Deserializes a JSON string to a HttpOptions object.
     /// </summary>