diff --git a/anthropic-java-vertex/src/main/kotlin/com/anthropic/vertex/backends/VertexBackend.kt b/anthropic-java-vertex/src/main/kotlin/com/anthropic/vertex/backends/VertexBackend.kt
index 6967781da..bb6ab0614 100644
--- a/anthropic-java-vertex/src/main/kotlin/com/anthropic/vertex/backends/VertexBackend.kt
+++ b/anthropic-java-vertex/src/main/kotlin/com/anthropic/vertex/backends/VertexBackend.kt
@@ -29,6 +29,7 @@ private constructor(
     @get:JvmName("googleCredentials") val googleCredentials: GoogleCredentials,
     @get:JvmName("region") val region: String,
     @get:JvmName("project") val project: String,
+    private val baseUrlOverride: String?,
 ) : Backend {
 
     private val jsonMapper = jsonMapper()
@@ -45,12 +46,13 @@ private constructor(
     }
 
     override fun baseUrl(): String =
-        when (region) {
-            "global" -> "https://aiplatform.googleapis.com"
-            "us" -> "https://aiplatform.us.rep.googleapis.com"
-            "eu" -> "https://aiplatform.eu.rep.googleapis.com"
-            else -> "https://$region-aiplatform.googleapis.com"
-        }
+        baseUrlOverride
+            ?: when (region) {
+                "global" -> "https://aiplatform.googleapis.com"
+                "us" -> "https://aiplatform.us.rep.googleapis.com"
+                "eu" -> "https://aiplatform.eu.rep.googleapis.com"
+                else -> "https://$region-aiplatform.googleapis.com"
+            }
 
     override fun prepareRequest(request: HttpRequest): HttpRequest {
         val pathSegments = request.pathSegments
@@ -150,8 +152,8 @@ private constructor(
      *
      * The credentials can be extracted from the environment and set on the builder by calling
      * [fromEnv] before calling [build] to create the [VertexBackend]. Alternatively, set the Google
-     * credentials, region and project explicitly via [googleCredentials], [region] and [project]
-     * before calling [build].
+     * credentials, region and project explicitly via [googleCredentials], [region] and [project].
+     * If requests should use a non-standard endpoint, set [baseUrl] before calling [build].
      */
     class Builder internal constructor() {
         private var googleCredentials: GoogleCredentials? = null
@@ -160,6 +162,8 @@ private constructor(
 
         private var project: String? = null
 
+        private var baseUrlOverride: String? = null
+
         fun fromEnv() = apply {
             try {
                 googleCredentials = GoogleCredentials.getApplicationDefault()
@@ -188,11 +192,15 @@ private constructor(
 
         fun project(project: String) = apply { this.project = project }
 
+        /** Overrides the base URL for requests. */
+        fun baseUrl(baseUrl: String) = apply { this.baseUrlOverride = baseUrl }
+
         fun build(): VertexBackend =
             VertexBackend(
                 checkRequired("googleCredentials", googleCredentials),
                 checkRequired("region", region),
                 checkRequired("project", project),
+                baseUrlOverride,
             )
     }
 }
diff --git a/anthropic-java-vertex/src/test/kotlin/com/anthropic/vertex/backends/VertexBackendTest.kt b/anthropic-java-vertex/src/test/kotlin/com/anthropic/vertex/backends/VertexBackendTest.kt
index 9eebe63a0..e1b3ba1f0 100644
--- a/anthropic-java-vertex/src/test/kotlin/com/anthropic/vertex/backends/VertexBackendTest.kt
+++ b/anthropic-java-vertex/src/test/kotlin/com/anthropic/vertex/backends/VertexBackendTest.kt
@@ -24,6 +24,7 @@ internal class VertexBackendTest {
         private const val GC_PROJECT = "vertex-project-12345-b6"
         private const val MODEL_ID = "claude-3-sonnet"
         private const val GC_REGION = "us-central1"
+        private const val CUSTOM_BASE_URL = "https://vertex.example.com"
         private const val ANTHROPIC_VERSION = "vertex-2023-10-16"
     }
 
@@ -106,6 +107,19 @@ internal class VertexBackendTest {
         assertThat(backend2.baseUrl()).isEqualTo("https://$otherRegion-aiplatform.googleapis.com")
     }
 
+    @Test
+    fun baseUrlOverride() {
+        val backend =
+            VertexBackend.builder()
+                .googleCredentials(createCredentials())
+                .region(GC_REGION)
+                .project(GC_PROJECT)
+                .baseUrl(CUSTOM_BASE_URL)
+                .build()
+
+        assertThat(backend.baseUrl()).isEqualTo(CUSTOM_BASE_URL)
+    }
+
     @Test
     fun prepareRequestMissingJSON() {
         val backend = createBackend()