diff --git a/src/Elastic.Markdown/Myst/CodeBlocks/EnhancedCodeBlockParser.cs b/src/Elastic.Markdown/Myst/CodeBlocks/EnhancedCodeBlockParser.cs
index 03bafceea..7b6fe182a 100644
--- a/src/Elastic.Markdown/Myst/CodeBlocks/EnhancedCodeBlockParser.cs
+++ b/src/Elastic.Markdown/Myst/CodeBlocks/EnhancedCodeBlockParser.cs
@@ -116,6 +116,11 @@ private static void ProcessAppliesToDirective(AppliesToDirective appliesToDirect
 		{
 			var applicableTo = YamlSerialization.Deserialize<ApplicableTo>(yaml);
 			appliesToDirective.AppliesTo = applicableTo;
+			if (appliesToDirective.AppliesTo.Warnings is null)
+				return;
+			foreach (var warning in appliesToDirective.AppliesTo.Warnings)
+				appliesToDirective.EmitWarning(warning);
+			applicableTo.Warnings = null;
 		}
 		catch (Exception e)
 		{
diff --git a/src/Elastic.Markdown/Myst/FrontMatter/ApplicableTo.cs b/src/Elastic.Markdown/Myst/FrontMatter/ApplicableTo.cs
index f0f241cee..ef618ccad 100644
--- a/src/Elastic.Markdown/Myst/FrontMatter/ApplicableTo.cs
+++ b/src/Elastic.Markdown/Myst/FrontMatter/ApplicableTo.cs
@@ -24,6 +24,8 @@ public record ApplicableTo
 	[YamlMember(Alias = "product")]
 	public ApplicabilityOverTime? Product { get; set; }
 
+	public string[]? Warnings { get; set; }
+
 	public static ApplicableTo All { get; } = new()
 	{
 		Stack = ApplicabilityOverTime.GenerallyAvailable,
@@ -87,6 +89,11 @@ public record ServerlessProjectApplicability
 
 public class ApplicableToConverter : IYamlTypeConverter
 {
+	private static readonly string[] KnownKeys =
+		["stack", "deployment", "serverless", "product", "ece",
+			"eck", "ess", "self", "elasticsearch", "observability","security"
+		];
+
 	public bool Accepts(Type type) => type == typeof(ApplicableTo);
 
 	public object? ReadYaml(IParser parser, Type type, ObjectDeserializer rootDeserializer)
@@ -103,7 +110,18 @@ public class ApplicableToConverter : IYamlTypeConverter
 		if (deserialized is not Dictionary<object, object?> { Count: > 0 } dictionary)
 			return null;
 
+
 		var applicableTo = new ApplicableTo();
+		var warnings = new List<string>();
+
+		var keys = dictionary.Keys.OfType<string>().ToArray();
+		var oldStyleKeys = keys.Where(k => k.StartsWith(":")).ToList();
+		if (oldStyleKeys.Count > 0)
+			warnings.Add($"Applies block does not use valid yaml keys: {string.Join(", ", oldStyleKeys)}");
+		var unknownKeys = keys.Except(KnownKeys).Except(oldStyleKeys).ToList();
+		if (unknownKeys.Count > 0)
+			warnings.Add($"Applies block does not support the following keys: {string.Join(", ", unknownKeys)}");
+
 		if (TryGetApplicabilityOverTime(dictionary, "stack", out var stackAvailability))
 			applicableTo.Stack = stackAvailability;
 
@@ -119,6 +137,8 @@ public class ApplicableToConverter : IYamlTypeConverter
 		if (TryGetProjectApplicability(dictionary, out var serverless))
 			applicableTo.Serverless = serverless;
 
+		if (warnings.Count > 0)
+			applicableTo.Warnings = warnings.ToArray();
 		return applicableTo;
 	}
 
diff --git a/tests/authoring/Applicability/AppliesToDirective.fs b/tests/authoring/Applicability/AppliesToDirective.fs
index fb3148618..006c1c70c 100644
--- a/tests/authoring/Applicability/AppliesToDirective.fs
+++ b/tests/authoring/Applicability/AppliesToDirective.fs
@@ -55,4 +55,30 @@ serverless:
                 Elasticsearch=ApplicabilityOverTime.op_Explicit "beta 9.1.0",
                 Observability=ApplicabilityOverTime.op_Explicit "discontinued 9.2.0"
             )
-        ))
\ No newline at end of file
+        ))
+
+type ``warns on old syntax`` () =
+    static let markdown = Setup.Markdown """
+```{applies_to}
+:hosted: all
+```
+"""
+    [<Fact>]
+    let ``has no errors`` () = markdown |> hasNoErrors
+
+    [<Fact>]
+    let ``warns on bad syntax`` () =
+        markdown |> hasWarning "Applies block does not use valid yaml keys: :hosted"
+
+type ``warns on invalid keys`` () =
+    static let markdown = Setup.Markdown """
+```{applies_to}
+hosted: all
+```
+"""
+    [<Fact>]
+    let ``has no errors`` () = markdown |> hasNoErrors
+
+    [<Fact>]
+    let ``warns on bad syntax`` () =
+        markdown |> hasWarning "Applies block does not support the following keys: hosted"