[Vertex AI] Use ephemeral URLSession config on iOS 18.4 sim

FirebaseVertexAI/CHANGELOG.md

@@ -6,6 +6,8 @@
   Note: This feature is in Public Preview and relies on experimental models,
   which means that it is not subject to any SLA or deprecation policy and could
   change in backwards-incompatible ways.
+- [fixed] Fixed an issue where network requests would fail in the iOS 18.4
+  simulator due to a `URLSession` bug introduced in Xcode 16.3. (#14677)
 
 # 11.11.0
 - [added] Emits a warning when attempting to use an incompatible model with

FirebaseVertexAI/Sources/GenAIURLSession.swift

@@ -0,0 +1,38 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+
+/// A namespace providing `URLSession` instances.
+enum GenAIURLSession {
+  /// The default `URLSession` instance for the SDK; returns `URLSession.shared` by default.
+  ///
+  /// - Important: On affected simulators (iOS 18.4+, visionOS 2.4+), this returns an ephemeral
+  ///   `URLSession` instance as a workaround for a known system bug.
+  static let `default` = {
+    #if targetEnvironment(simulator)
+      // The iOS 18.4 and visionOS 2.4 simulators (included in Xcode 16.3) contain a bug in
+      // `URLSession` causing requests to fail. The following workaround, using an ephemeral session
+      // resolves the issue. See https://developer.apple.com/forums/thread/777999 for more details.
+      //
+      // Note: This bug only impacts the simulator, not real devices, and does not impact watchOS
+      // or tvOS.
+      if #available(iOS 18.4, tvOS 100.0, watchOS 100.0, visionOS 2.4, *) {
+        return URLSession(configuration: URLSessionConfiguration.ephemeral)
+      }
+    #endif // targetEnvironment(simulator)
+
+    return URLSession.shared
+  }()
+}

FirebaseVertexAI/Sources/GenerativeModel.swift

@@ -83,7 +83,7 @@ public final class GenerativeModel: Sendable {
        toolConfig: ToolConfig? = nil,
        systemInstruction: ModelContent? = nil,
        requestOptions: RequestOptions,
-       urlSession: URLSession = .shared) {
+       urlSession: URLSession = GenAIURLSession.default) {
     self.modelName = modelName
     self.modelResourceName = modelResourceName
     self.apiConfig = apiConfig

FirebaseVertexAI/Sources/Types/Public/Imagen/ImagenModel.swift

@@ -53,7 +53,7 @@ public final class ImagenModel {
        generationConfig: ImagenGenerationConfig?,
        safetySettings: ImagenSafetySettings?,
        requestOptions: RequestOptions,
-       urlSession: URLSession = .shared) {
+       urlSession: URLSession = GenAIURLSession.default) {
     self.modelResourceName = modelResourceName
     self.apiConfig = apiConfig
     generativeAIService = GenerativeAIService(