diff --git a/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/annotators/GrammarEngineKeyAnnotator.kt b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/annotators/GrammarEngineKeyAnnotator.kt
new file mode 100644
index 00000000..4942a841
--- /dev/null
+++ b/src/main/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/annotators/GrammarEngineKeyAnnotator.kt
@@ -0,0 +1,49 @@
+package net.sjrx.intellij.plugins.systemdunitfiles.annotators
+
+import com.intellij.lang.annotation.AnnotationHolder
+import com.intellij.lang.annotation.Annotator
+import com.intellij.lang.annotation.HighlightSeverity
+import com.intellij.openapi.editor.DefaultLanguageHighlighterColors
+import com.intellij.openapi.editor.colors.TextAttributesKey
+import com.intellij.psi.PsiElement
+import com.intellij.psi.util.PsiTreeUtil
+import net.sjrx.intellij.plugins.systemdunitfiles.psi.UnitFileProperty
+import net.sjrx.intellij.plugins.systemdunitfiles.psi.UnitFileSectionType
+import net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.SemanticDataRepository
+import net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.fileClass
+import net.sjrx.intellij.plugins.systemdunitfiles.semanticdata.optionvalues.grammar.GrammarOptionValue
+import net.sjrx.intellij.plugins.systemdunitfiles.settings.ExperimentalSettings
+
+/**
+ * Debug aid (#467): while the experimental grammar engine is enabled, mark the KEY of every option
+ * whose value is validated by that engine (a [GrammarOptionValue]), so it is obvious at a glance
+ * which keys exercise the new parse() path versus the original SyntacticMatch/SemanticMatch one.
+ *
+ * Does nothing when the flag is off, so it has no effect for normal users.
+ */
+class GrammarEngineKeyAnnotator : Annotator {
+
+  override fun annotate(element: PsiElement, holder: AnnotationHolder) {
+    if (element !is UnitFileProperty) return
+    if (!ExperimentalSettings.getInstance(element.project).state.useGrammarParseEngine) return
+
+    val section = PsiTreeUtil.getParentOfType(element, UnitFileSectionType::class.java) ?: return
+    val fileClass = element.containingFile.fileClass()
+    val validator = SemanticDataRepository.instance.getOptionValidator(fileClass, section.sectionName, element.key)
+
+    if (validator is GrammarOptionValue) {
+      holder.newSilentAnnotation(HighlightSeverity.INFORMATION)
+        .range(element.keyNode.psi)
+        .textAttributes(NEW_ENGINE_KEY)
+        .create()
+    }
+  }
+
+  companion object {
+    // Layered on top of the normal key color; METADATA gives a distinct, theme-aware tint.
+    val NEW_ENGINE_KEY: TextAttributesKey = TextAttributesKey.createTextAttributesKey(
+      "SYSTEMD_UNIT_FILE_NEW_GRAMMAR_ENGINE_KEY",
+      DefaultLanguageHighlighterColors.METADATA,
+    )
+  }
+}
diff --git a/src/main/resources/META-INF/plugin.xml b/src/main/resources/META-INF/plugin.xml
index 94771c62..2fdc75e5 100644
--- a/src/main/resources/META-INF/plugin.xml
+++ b/src/main/resources/META-INF/plugin.xml
@@ -60,6 +60,7 @@
     <annotator language="Unit File (systemd)" implementationClass="net.sjrx.intellij.plugins.systemdunitfiles.annotators.LineContinuationCharacterFollowedByWhitespaceAnnotator"/>
     <annotator language="Unit File (systemd)" implementationClass="net.sjrx.intellij.plugins.systemdunitfiles.annotators.PropertyIsNotInSectionAnnotator"/>
     <annotator language="Unit File (systemd)" implementationClass="net.sjrx.intellij.plugins.systemdunitfiles.annotators.PidFileOptionWarning"/>
+    <annotator language="Unit File (systemd)" implementationClass="net.sjrx.intellij.plugins.systemdunitfiles.annotators.GrammarEngineKeyAnnotator"/>
     <localInspection implementationClass="net.sjrx.intellij.plugins.systemdunitfiles.inspections.UnknownKeyInSectionInspection"
                      groupPath="Unit files (systemd)" language="Unit File (systemd)"
                      shortName="UnknownKeyInSection"     displayName="Unknown option in section"
diff --git a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/annotators/GrammarEngineKeyAnnotatorTest.kt b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/annotators/GrammarEngineKeyAnnotatorTest.kt
new file mode 100644
index 00000000..cd9c769b
--- /dev/null
+++ b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/annotators/GrammarEngineKeyAnnotatorTest.kt
@@ -0,0 +1,39 @@
+package net.sjrx.intellij.plugins.systemdunitfiles.annotators
+
+import com.intellij.lang.annotation.HighlightSeverity
+import net.sjrx.intellij.plugins.systemdunitfiles.AbstractUnitFileTest
+import net.sjrx.intellij.plugins.systemdunitfiles.settings.ExperimentalSettings
+import org.junit.Test
+
+class GrammarEngineKeyAnnotatorTest : AbstractUnitFileTest() {
+
+  // The light-test project is shared across classes, so don't leak the opt-in.
+  override fun tearDown() {
+    try {
+      ExperimentalSettings.getInstance(project).state.useGrammarParseEngine = false
+    } finally {
+      super.tearDown()
+    }
+  }
+
+  private fun markedKey(highlights: List<com.intellij.codeInsight.daemon.impl.HighlightInfo>, key: String) =
+    highlights.any { it.severity == HighlightSeverity.INFORMATION && it.text == key }
+
+  @Test
+  fun testGrammarBackedKeyIsMarkedWhenFlagOn() {
+    ExperimentalSettings.getInstance(project).state.useGrammarParseEngine = true
+    // RestrictAddressFamilies is validated by a GrammarOptionValue; PrivateTmp's value is not.
+    setupFileInEditor("file.service", "[Service]\nRestrictAddressFamilies=AF_INET\n")
+
+    val highlights = myFixture.doHighlighting()
+    assertTrue(markedKey(highlights, "RestrictAddressFamilies"))
+  }
+
+  @Test
+  fun testNotMarkedWhenFlagOff() {
+    setupFileInEditor("file.service", "[Service]\nRestrictAddressFamilies=AF_INET\n")
+
+    val highlights = myFixture.doHighlighting()
+    assertFalse(markedKey(highlights, "RestrictAddressFamilies"))
+  }
+}
diff --git a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/inspections/GrammarParseEngineInspectionTest.kt b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/inspections/GrammarParseEngineInspectionTest.kt
index 1a3831c7..7d822977 100644
--- a/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/inspections/GrammarParseEngineInspectionTest.kt
+++ b/src/test/kotlin/net/sjrx/intellij/plugins/systemdunitfiles/inspections/GrammarParseEngineInspectionTest.kt
@@ -1,5 +1,6 @@
 package net.sjrx.intellij.plugins.systemdunitfiles.inspections
 
+import com.intellij.lang.annotation.HighlightSeverity
 import net.sjrx.intellij.plugins.systemdunitfiles.AbstractUnitFileTest
 import net.sjrx.intellij.plugins.systemdunitfiles.settings.ExperimentalSettings
 import org.junit.Test
@@ -42,7 +43,7 @@ class GrammarParseEngineInspectionTest : AbstractUnitFileTest() {
     setupFileInEditor("file.service", file)
     enableInspection(InvalidValueInspection::class.java)
 
-    assertSize(0, myFixture.doHighlighting())
+    assertSize(0, myFixture.doHighlighting().filter { it.severity != HighlightSeverity.INFORMATION })
   }
 
   @Test
@@ -61,7 +62,7 @@ class GrammarParseEngineInspectionTest : AbstractUnitFileTest() {
     setupFileInEditor("file.service", file)
     enableInspection(InvalidValueInspection::class.java)
 
-    assertSize(3, myFixture.doHighlighting())
+    assertSize(3, myFixture.doHighlighting().filter { it.severity != HighlightSeverity.INFORMATION })
   }
 
   @Test
@@ -76,6 +77,6 @@ class GrammarParseEngineInspectionTest : AbstractUnitFileTest() {
     setupFileInEditor("file.service", file)
     enableInspection(InvalidValueInspection::class.java)
 
-    assertSize(0, myFixture.doHighlighting())
+    assertSize(0, myFixture.doHighlighting().filter { it.severity != HighlightSeverity.INFORMATION })
   }
 }