diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/ExperimentalSettings.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/ExperimentalSettings.kt
index 070aae6d..d01b40b1 100644
--- a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/ExperimentalSettings.kt
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/ExperimentalSettings.kt
@@ -22,12 +22,26 @@ class ExperimentalSettings : PersistentStateComponent<ExperimentalSettings.State
     /**
      * Use the new list-of-successes grammar engine (Combinator.parse / validate) for value
      * validation instead of the original SyntacticMatch/SemanticMatch path.
+     *
+     * Release 1 of the rollout (GitHub #467) flips this default to `true` so every project that has
+     * not explicitly opted out lands on the new engine. The old engine remains reachable as an escape
+     * hatch: the settings checkbox, or the "Switch back to legacy validation" action on
+     * [NewGrammarEngineEditorNotificationProvider]. Once the bake period is over the old engine and
+     * this flag are removed and the new engine becomes unconditional.
      */
-    var useGrammarParseEngine: Boolean = false
+    var useGrammarParseEngine: Boolean = true
+
+    /**
+     * Set once the user has dismissed (or acted on) the "you are now on the new grammar engine"
+     * banner shown by [NewGrammarEngineEditorNotificationProvider], so it does not reappear on every
+     * unit file they open. Independent of [useGrammarParseEngine]: a user can dismiss the banner while
+     * staying on the new engine.
+     */
+    var newEngineBannerDismissed: Boolean = false
 
     /**
      * Underline the KEY of every option whose value is backed by a grammar validator
-     * ([GrammarOptionValue]), a debug aid for seeing which keys the new engine covers. Independent of
+     * ([net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.optionvalues.grammar.Combinator]), a debug aid for seeing which keys the new engine covers. Independent of
      * [useGrammarParseEngine]: the grammar validators exist in the registry regardless of which
      * validation path is active, so this can be toggled on its own.
      */
diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/NewGrammarEngineEditorNotificationProvider.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/NewGrammarEngineEditorNotificationProvider.kt
new file mode 100644
index 00000000..4d77b672
--- /dev/null
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/settings/NewGrammarEngineEditorNotificationProvider.kt
@@ -0,0 +1,67 @@
+package net.sjrx.intellij.plugins.systemdunitfiles.settings
+
+import com.intellij.codeInsight.daemon.DaemonCodeAnalyzer
+import com.intellij.openapi.fileEditor.FileEditor
+import com.intellij.openapi.project.Project
+import com.intellij.openapi.vfs.VirtualFile
+import com.intellij.psi.PsiManager
+import com.intellij.ui.EditorNotificationPanel
+import com.intellij.ui.EditorNotificationProvider
+import com.intellij.ui.EditorNotifications
+import net.sjrx.intellij.plugins.systemdunitfiles.UnitFileLanguage
+import java.util.function.Function
+import javax.swing.JComponent
+
+/**
+ * Release 1 rollout banner (GitHub #467).
+ *
+ * Shown at the top of a systemd unit file once the new grammar engine has become the default, so
+ * active users know the validation engine changed and have a one-click way out if it misbehaves.
+ * Because [EditorNotificationProvider]s are evaluated lazily (only when a matching file is actually
+ * opened), users who never touch a unit file never see it.
+ *
+ * Template to copy from: [PodmanNetworkEditorNotificationProvider] — same shape (gate on settings,
+ * detect the file, return a panel with action labels).
+ */
+class NewGrammarEngineEditorNotificationProvider : EditorNotificationProvider {
+
+  override fun collectNotificationData(
+    project: Project,
+    file: VirtualFile
+  ): Function<in FileEditor, out JComponent?>? {
+
+    val psiFile = PsiManager.getInstance(project).findFile(file) ?: return null
+
+    if (psiFile.language != UnitFileLanguage.INSTANCE) {
+      return null
+    }
+
+    val state = ExperimentalSettings.getInstance(project).state
+    if (!state.useGrammarParseEngine) {
+      return null
+    }
+
+    if (state.newEngineBannerDismissed) {
+      return null
+    }
+
+    return Function { fileEditor ->
+      val panel = EditorNotificationPanel(fileEditor, EditorNotificationPanel.Status.Info)
+
+      panel.text = "Unit File Support (systemd) use a new experimental engine for validating systemd values. Having problems, switch back and report it."
+      panel.createActionLabel("Dismiss") {
+        state.newEngineBannerDismissed = true
+        EditorNotifications.getInstance(project).updateAllNotifications()
+        DaemonCodeAnalyzer.getInstance(project).restart()
+      }
+
+      panel.createActionLabel("Switch back") {
+        state.useGrammarParseEngine = false
+        EditorNotifications.getInstance(project).updateAllNotifications()
+        DaemonCodeAnalyzer.getInstance(project).restart()
+      }
+      panel
+
+    }
+  }
+}
diff --git a/src/main/resources/META-INF/plugin.xml b/src/main/resources/META-INF/plugin.xml
index 72b2ed2a..d4902b93 100644
--- a/src/main/resources/META-INF/plugin.xml
+++ b/src/main/resources/META-INF/plugin.xml
@@ -102,6 +102,7 @@
                          displayName="systemd Unit Files"
                          parentId="tools"/>
     <editorNotificationProvider implementation="net.sjrx.intellij.plugins.systemdunitfiles.settings.PodmanNetworkEditorNotificationProvider"/>
+    <editorNotificationProvider implementation="net.sjrx.intellij.plugins.systemdunitfiles.settings.NewGrammarEngineEditorNotificationProvider"/>
   </extensions>
 
   <actions>
diff --git a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/AbstractUnitFileTest.kt b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/AbstractUnitFileTest.kt
index ad4ad0b9..571ee203 100644
--- a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/AbstractUnitFileTest.kt
+++ b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/AbstractUnitFileTest.kt
@@ -2,22 +2,34 @@ package net.sjrx.intellij.plugins.systemdunitfiles
 
 import com.intellij.codeInsight.completion.CompletionType
 import com.intellij.codeInsight.daemon.impl.HighlightInfo
-import com.intellij.codeInsight.lookup.LookupElement
 import com.intellij.codeInspection.LocalInspectionTool
 import com.intellij.psi.PsiElement
 import com.intellij.psi.PsiFile
 import com.intellij.psi.tree.IElementType
 import com.intellij.psi.util.PsiTreeUtil
 import com.intellij.testFramework.fixtures.BasePlatformTestCase
-import com.intellij.testFramework.utils.module.assertContains
-import junit.framework.TestCase
 import net.sjrx.intellij.plugins.systemdunitfiles.generated.UnitFileElementTypeHolder
+import net.sjrx.intellij.plugins.systemdunitfiles.settings.ExperimentalSettings
 import org.hamcrest.CoreMatchers.hasItem
 import org.hamcrest.MatcherAssert.assertThat
-import java.util.*
 import java.util.stream.Collectors
 
 abstract class AbstractUnitFileTest : BasePlatformTestCase() {
+
+  override fun setUp() {
+    super.setUp()
+    // Pin the grammar-engine flag to false (its historical default) so the shipped production default
+    // — flipped to true in #467 — does not leak into tests. This keeps the CI matrix meaningful:
+    //   * -Dsystemd.unit.grammarParseEngine=false  -> flag false + FORCE_PARSE_ENGINE false: the OLD
+    //     validation engine is exercised (still reachable in production via "Switch back"), so it stays
+    //     covered even though it is no longer the default.
+    //   * -Dsystemd.unit.grammarParseEngine=true   -> FORCE_PARSE_ENGINE forces the NEW validation
+    //     engine regardless of the flag.
+    // Tests that need the new engine's flag-gated features (grammar coloring, completion, IPv6
+    // inspection) set useGrammarParseEngine = true explicitly, overriding this.
+    ExperimentalSettings.getInstance(project).state.useGrammarParseEngine = false
+  }
+
   protected fun enableInspection(cls: Class<out LocalInspectionTool?>?) {
     myFixture.enableInspections(cls)
   }
@@ -63,15 +75,12 @@ abstract class AbstractUnitFileTest : BasePlatformTestCase() {
 
     @JvmStatic
     protected fun assertStringContains(subject: String, value: String) {
-      TestCase.assertTrue("Expected that $value contains $subject", value.contains(subject))
+      assertTrue("Expected that $value contains $subject", value.contains(subject))
     }
 
     @JvmStatic
     protected fun assertContainsQuickfix(info: HighlightInfo, quickfixName: String) {
-
-      var found = false
       val quickFixes = info.quickFixActionRanges.map {
-        it ->
         it.first.action.text
       }