diff --git a/.changes/common/crowd-company-bead-authority.json b/.changes/common/crowd-company-bead-authority.json
new file mode 100644
index 00000000..8868b7e1
--- /dev/null
+++ b/.changes/common/crowd-company-bead-authority.json
@@ -0,0 +1 @@
+{"type":"MINOR","changes":["Add test-only APIController constructor for Vertex AI unit tests"]}
diff --git a/common/build.gradle.kts b/common/build.gradle.kts
index bd02f5fa..842e9301 100644
--- a/common/build.gradle.kts
+++ b/common/build.gradle.kts
@@ -90,6 +90,7 @@ dependencies {
     implementation("com.google.guava:listenablefuture:1.0")
     implementation("androidx.concurrent:concurrent-futures:1.2.0-alpha02")
     implementation("androidx.concurrent:concurrent-futures-ktx:1.2.0-alpha02")
+    compileOnly("io.ktor:ktor-client-mock:$ktorVersion")
     testImplementation("junit:junit:4.13.2")
     testImplementation("io.kotest:kotest-assertions-core:4.0.7")
     testImplementation("io.kotest:kotest-assertions-jvm:4.0.7")
diff --git a/common/src/main/kotlin/com/google/ai/client/generativeai/common/APIController.kt b/common/src/main/kotlin/com/google/ai/client/generativeai/common/APIController.kt
index d815f44a..aaf79634 100644
--- a/common/src/main/kotlin/com/google/ai/client/generativeai/common/APIController.kt
+++ b/common/src/main/kotlin/com/google/ai/client/generativeai/common/APIController.kt
@@ -17,12 +17,15 @@
 package com.google.ai.client.generativeai.common
 
 import android.util.Log
+import androidx.annotation.VisibleForTesting
 import com.google.ai.client.generativeai.common.server.FinishReason
 import com.google.ai.client.generativeai.common.util.decodeToFlow
 import com.google.ai.client.generativeai.common.util.fullModelName
 import io.ktor.client.HttpClient
 import io.ktor.client.call.body
 import io.ktor.client.engine.HttpClientEngine
+import io.ktor.client.engine.mock.MockEngine
+import io.ktor.client.engine.mock.respond
 import io.ktor.client.engine.okhttp.OkHttp
 import io.ktor.client.plugins.HttpTimeout
 import io.ktor.client.plugins.contentnegotiation.ContentNegotiation
@@ -35,9 +38,12 @@ import io.ktor.client.statement.HttpResponse
 import io.ktor.client.statement.bodyAsChannel
 import io.ktor.client.statement.bodyAsText
 import io.ktor.http.ContentType
+import io.ktor.http.HttpHeaders
 import io.ktor.http.HttpStatusCode
 import io.ktor.http.contentType
+import io.ktor.http.headersOf
 import io.ktor.serialization.kotlinx.json.json
+import io.ktor.utils.io.ByteChannel
 import kotlin.time.Duration
 import kotlinx.coroutines.CoroutineName
 import kotlinx.coroutines.TimeoutCancellationException
@@ -85,6 +91,24 @@ internal constructor(
     headerProvider: HeaderProvider? = null,
   ) : this(key, model, requestOptions, OkHttp.create(), apiClient, headerProvider)
 
+  @VisibleForTesting(otherwise = VisibleForTesting.NONE)
+  constructor(
+    key: String,
+    model: String,
+    requestOptions: RequestOptions,
+    apiClient: String,
+    headerProvider: HeaderProvider?,
+    channel: ByteChannel,
+    status: HttpStatusCode,
+  ) : this(
+    key,
+    model,
+    requestOptions,
+    MockEngine { respond(channel, status, headersOf(HttpHeaders.ContentType, "application/json")) },
+    apiClient,
+    headerProvider,
+  )
+
   private val model = fullModelName(model)
 
   private val client =
diff --git a/common/src/test/java/com/google/ai/client/generativeai/common/util/tests.kt b/common/src/test/java/com/google/ai/client/generativeai/common/util/tests.kt
index fa84d258..0726d959 100644
--- a/common/src/test/java/com/google/ai/client/generativeai/common/util/tests.kt
+++ b/common/src/test/java/com/google/ai/client/generativeai/common/util/tests.kt
@@ -28,11 +28,7 @@ import com.google.ai.client.generativeai.common.shared.Content
 import com.google.ai.client.generativeai.common.shared.TextPart
 import io.kotest.matchers.collections.shouldNotBeEmpty
 import io.kotest.matchers.nulls.shouldNotBeNull
-import io.ktor.client.engine.mock.MockEngine
-import io.ktor.client.engine.mock.respond
-import io.ktor.http.HttpHeaders
 import io.ktor.http.HttpStatusCode
-import io.ktor.http.headersOf
 import io.ktor.utils.io.ByteChannel
 import io.ktor.utils.io.close
 import io.ktor.utils.io.writeFully
@@ -105,17 +101,15 @@ internal fun commonTest(
   block: CommonTest,
 ) = doBlocking {
   val channel = ByteChannel(autoFlush = true)
-  val mockEngine = MockEngine {
-    respond(channel, status, headersOf(HttpHeaders.ContentType, "application/json"))
-  }
   val apiController =
     APIController(
       "super_cool_test_key",
       "gemini-pro",
       requestOptions,
-      mockEngine,
       TEST_CLIENT_ID,
       null,
+      channel,
+      status,
     )
   CommonTestScope(channel, apiController).block()
 }