diff --git a/GoogleSignIn/Sources/GIDApplication/API/GIDApplication.h b/GoogleSignIn/Sources/GIDApplication/API/GIDApplication.h
new file mode 100644
index 00000000..c9104803
--- /dev/null
+++ b/GoogleSignIn/Sources/GIDApplication/API/GIDApplication.h
@@ -0,0 +1,42 @@
+/*
+ * Copyright 2026 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 <TargetConditionals.h>
+
+#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
+
+#import <Foundation/Foundation.h>
+#import <UIKit/UIKit.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/// The subset of `UIApplication` that Google Sign-In's EMM error handling depends on.
+///
+/// Production code uses `GIDSharedApplication`, which forwards to
+/// `UIApplication.sharedApplication`; tests use `GIDFakeApplication`.
+@protocol GIDApplication <NSObject>
+
+/// The window that currently receives keyboard and other non-touch events, if any.
+- (nullable UIWindow *)keyWindow;
+
+/// Asks the system to open `url`.
+- (void)openURL:(NSURL *)url;
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST
diff --git a/GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.h b/GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.h
new file mode 100644
index 00000000..3fa45063
--- /dev/null
+++ b/GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.h
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2026 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 <TargetConditionals.h>
+
+#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
+
+#import <Foundation/Foundation.h>
+#import <UIKit/UIKit.h>
+
+#import "GoogleSignIn/Sources/GIDApplication/API/GIDApplication.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+/// An in-memory `GIDApplication` for tests.
+@interface GIDFakeApplication : NSObject <GIDApplication>
+
+/// The window returned by `-keyWindow`. Defaults to nil.
+@property(nonatomic, strong, nullable) UIWindow *keyWindow;
+
+/// Every URL passed to `-openURL:`, in call order.
+@property(nonatomic, readonly) NSArray<NSURL *> *openedURLs;
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST
diff --git a/GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.m b/GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.m
new file mode 100644
index 00000000..81649b4e
--- /dev/null
+++ b/GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.m
@@ -0,0 +1,50 @@
+/*
+ * Copyright 2026 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 <TargetConditionals.h>
+
+#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
+
+#import "GoogleSignIn/Sources/GIDApplication/Fake/GIDFakeApplication.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@implementation GIDFakeApplication {
+  // Every URL passed to `-openURL:`, in call order.
+  NSMutableArray<NSURL *> *_openedURLs;
+}
+
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _openedURLs = [NSMutableArray array];
+  }
+  return self;
+}
+
+- (NSArray<NSURL *> *)openedURLs {
+  return [_openedURLs copy];
+}
+
+- (void)openURL:(NSURL *)url {
+  [_openedURLs addObject:url];
+}
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST
diff --git a/GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.h b/GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.h
new file mode 100644
index 00000000..8242b4d9
--- /dev/null
+++ b/GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.h
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2026 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 <TargetConditionals.h>
+
+#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
+
+#import <Foundation/Foundation.h>
+
+#import "GoogleSignIn/Sources/GIDApplication/API/GIDApplication.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+/// A `GIDApplication` backed by `UIApplication.sharedApplication`.
+///
+/// `UIApplication.sharedApplication` is looked up on every call rather than captured at
+/// initialization, so instances can be created before the application object exists.
+@interface GIDSharedApplication : NSObject <GIDApplication>
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST
diff --git a/GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.m b/GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.m
new file mode 100644
index 00000000..cf1aa3a8
--- /dev/null
+++ b/GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.m
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2026 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 <TargetConditionals.h>
+
+#if TARGET_OS_IOS && !TARGET_OS_MACCATALYST
+
+#import "GoogleSignIn/Sources/GIDApplication/Implementation/GIDSharedApplication.h"
+
+#import <UIKit/UIKit.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@implementation GIDSharedApplication
+
+- (nullable UIWindow *)keyWindow {
+  for (UIScene *scene in UIApplication.sharedApplication.connectedScenes) {
+    if ([scene isKindOfClass:[UIWindowScene class]] &&
+        scene.activationState == UISceneActivationStateForegroundActive) {
+      return ((UIWindowScene *)scene).keyWindow;
+    }
+  }
+  return nil;
+}
+
+- (void)openURL:(NSURL *)url {
+  [UIApplication.sharedApplication openURL:url options:@{} completionHandler:nil];
+}
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif // TARGET_OS_IOS && !TARGET_OS_MACCATALYST