diff --git a/packages/codemagic_app_preview/lib/src/commands/post_command.dart b/packages/codemagic_app_preview/lib/src/commands/post_command.dart
index e0aafaa..a194d16 100644
--- a/packages/codemagic_app_preview/lib/src/commands/post_command.dart
+++ b/packages/codemagic_app_preview/lib/src/commands/post_command.dart
@@ -70,7 +70,7 @@ class PostCommand extends Command {
   Future<void> run({DateTime? now}) async {
     if (!_isPullRequest()) {
       stderr.writeln(
-          '"CM_PULL_REQUEST" is not set to "true". Seems like the current build is not building a pull request. Aborting.');
+          '"CM_PULL_REQUEST_NUMBER" is not set. Seems like the current build is not building a pull request. Aborting.');
       exitCode = 1;
       return;
     }
@@ -133,12 +133,12 @@ class PostCommand extends Command {
   /// Returns `true` if the current build is building a pull request, `false`
   /// otherwise.
   bool _isPullRequest() {
-    // Set to "true" if the current build is building a pull request, "false"
-    // otherwise.
-    //
-    // https://docs.codemagic.io/flutter-configuration/built-in-variables/
-    final isPullRequest = environmentVariableAccessor.get('CM_PULL_REQUEST');
-    return isPullRequest == 'true';
+    // We don't use CM_PULL_REQUEST here because this would require to set
+    // an additional environment variable in the workflow to trigger the
+    // build when using a label.
+    final pullRequestId =
+        environmentVariableAccessor.get('CM_PULL_REQUEST_NUMBER') as String?;
+    return pullRequestId != null && pullRequestId.isNotEmpty;
   }
 
   Future<GitHostRepository?> _getGitHostRepository(GitRepo gitRepo) async {
diff --git a/packages/codemagic_app_preview/test/post_command_test.dart b/packages/codemagic_app_preview/test/post_command_test.dart
index c6eaac2..6256064 100644
--- a/packages/codemagic_app_preview/test/post_command_test.dart
+++ b/packages/codemagic_app_preview/test/post_command_test.dart
@@ -46,8 +46,6 @@ void main() {
 
     test('sets exit code to 1 when not setting the token', () async {
       const pullRequestId = '24';
-      environmentVariableAccessor.environmentVariables['CM_PULL_REQUEST'] =
-          'true';
       environmentVariableAccessor
           .environmentVariables['CM_PULL_REQUEST_NUMBER'] = pullRequestId;
       when(() => gitRepo.getHost()).thenAnswer((_) async => GitHost.github);
@@ -71,10 +69,6 @@ void main() {
           '62877273178d247b70405cb0';
       environmentVariableAccessor.environmentVariables['FCI_COMMIT'] =
           '50b04d910c6b73472f7dfc1fee38a67e7132bf32';
-      environmentVariableAccessor.environmentVariables['CM_PULL_REQUEST'] =
-          'true';
-      environmentVariableAccessor
-          .environmentVariables['CM_PULL_REQUEST_NUMBER'] = pullRequestId;
       environmentVariableAccessor.environmentVariables['CM_ARTIFACT_LINKS'] =
           '[]'; // no artifacts
 
@@ -94,9 +88,6 @@ void main() {
 
     test('sets exit code to 1 when build is not executed in pull request',
         () async {
-      const pullRequestId = '24';
-      environmentVariableAccessor
-          .environmentVariables['CM_PULL_REQUEST_NUMBER'] = pullRequestId;
       when(() => gitRepo.getHost()).thenAnswer((_) async => GitHost.github);
       environmentVariableAccessor.environmentVariables['FCI_PROJECT_ID'] =
           '6274fcfc87c748ce531c7376';
@@ -104,10 +95,8 @@ void main() {
           '62877273178d247b70405cb0';
       environmentVariableAccessor.environmentVariables['FCI_COMMIT'] =
           '50b04d910c6b73472f7dfc1fee38a67e7132bf32';
-      environmentVariableAccessor.environmentVariables['CM_PULL_REQUEST'] =
-          null;
       environmentVariableAccessor
-          .environmentVariables['CM_PULL_REQUEST_NUMBER'] = pullRequestId;
+          .environmentVariables['CM_PULL_REQUEST_NUMBER'] = null;
       environmentVariableAccessor.environmentVariables['CM_ARTIFACT_LINKS'] =
           '[]'; // no artifacts