diff --git a/src/compiler/io/compilerMessage.ml b/src/compiler/io/compilerMessage.ml
index b5f13e8529f..259c7b1decd 100644
--- a/src/compiler/io/compilerMessage.ml
+++ b/src/compiler/io/compilerMessage.ml
@@ -85,11 +85,7 @@ let replay_message com cm =
 	| MKWarning(w, options) ->
 		com.warning ~depth:cm.cm_depth w options cm.cm_message cm.cm_pos
 	| _ ->
-		let is_diagnostics_only = match cm.cm_diagnostics_kind with
-			| MessageKind.DKMissingFields | MessageKind.DKUnresolvedIdentifier -> true
-			| _ -> false
-		in
-		if not is_diagnostics_only || is_diagnostics com then
+		if not (Message.cm_is_diagnostics_only cm) || is_diagnostics com then
 			com.part_scope.messages <- cm :: com.part_scope.messages
 
 (* Default handlers *)
diff --git a/src/compiler/server/serverCache.ml b/src/compiler/server/serverCache.ml
index 116ea9a55b4..4d63075f283 100644
--- a/src/compiler/server/serverCache.ml
+++ b/src/compiler/server/serverCache.ml
@@ -230,8 +230,21 @@ let check_module sctx com m_path m_extra p =
 				| Some reason -> raise (Dirty (DependencyDirty(mpath,reason)))
 			) m_extra.m_deps
 		in
+		(* Errors like missing fields are downgraded to diagnostics-only messages when typing in
+		   diagnostics mode. Such a module cannot be reused by an actual compilation, which has to
+		   type it again in order to produce the real error (#12995). *)
+		let check_diagnostics_only_errors () =
+			if not (is_diagnostics com) && DynArray.fold_left (fun acc cbo -> acc || match cbo with
+				| Message cm ->
+					Message.cm_is_diagnostics_only cm
+				| _ ->
+					false
+			) false m_extra.m_cache_bound_objects then
+				raise (Dirty (Tainted DiagnosticsOnlyErrors))
+		in
 		let check () =
 			try
+				check_diagnostics_only_errors();
 				check_module_path();
 				if not (has_policy NoFileSystemCheck) || Path.file_extension (Path.UniqueKey.lazy_path m_extra.m_file) <> "hx" then check_file();
 				if (get_typing_mode com m_extra) = FullTyping then check_dependencies();
diff --git a/src/context/common.ml b/src/context/common.ml
index c34adbfc85e..05acc818566 100644
--- a/src/context/common.ml
+++ b/src/context/common.ml
@@ -831,9 +831,7 @@ let is_compilation com = com.display.dms_kind = DMNone && not (is_diagnostics co
     not prevent context caching. *)
 let has_error_to_report com =
 	let has_reportable_message = List.exists (fun cm ->
-		match cm.cm_diagnostics_kind with
-		| MessageKind.DKMissingFields | MessageKind.DKUnresolvedIdentifier -> false
-		| _ -> true
+		not (Message.cm_is_diagnostics_only cm)
 	) com.part_scope.messages in
 	com.part_scope.has_error && (is_compilation com || has_reportable_message)
 
diff --git a/src/core/message.ml b/src/core/message.ml
index 381bab21d83..9815da46eec 100644
--- a/src/core/message.ml
+++ b/src/core/message.ml
@@ -67,6 +67,12 @@ type t = {
 
 let cm_severity cm = message_kind_severity cm.cm_message_kind
 
+(** Messages which only exist in diagnostics mode, where the corresponding error is downgraded to
+    diagnostics data instead of being reported. *)
+let cm_is_diagnostics_only cm = match cm.cm_diagnostics_kind with
+	| MessageKind.DKMissingFields | MessageKind.DKUnresolvedIdentifier -> true
+	| _ -> false
+
 let cm_code cm = match cm.cm_message_kind with
 	| MKWarning(w,_) ->
 		let wobj = WarningList.warning_obj w in
diff --git a/src/core/tPrinting.ml b/src/core/tPrinting.ml
index b91205d1d8f..2ac242e2f8e 100644
--- a/src/core/tPrinting.ml
+++ b/src/core/tPrinting.ml
@@ -651,6 +651,7 @@ module Printer = struct
 		| ServerInvalidate -> "server/invalidate"
 		| ServerInvalidateFiles -> "server_invalidate_files"
 		| ServerInvalidateModule -> "server_invalidate_module"
+		| DiagnosticsOnlyErrors -> "diagnostics_only_errors"
 
 	let s_module_skip_reason reason =
 		let rec loop stack = function
diff --git a/src/core/tType.ml b/src/core/tType.ml
index e7efeec516e..e52db71e6dd 100644
--- a/src/core/tType.ml
+++ b/src/core/tType.ml
@@ -38,6 +38,7 @@ type module_tainting_reason =
 	| ServerInvalidate
 	| ServerInvalidateFiles
 	| ServerInvalidateModule
+	| DiagnosticsOnlyErrors
 
 type module_skip_reason =
 	| DependencyDirty of path * module_skip_reason
diff --git a/tests/server/src/cases/issues/Issue12995.hx b/tests/server/src/cases/issues/Issue12995.hx
new file mode 100644
index 00000000000..8591cd97296
--- /dev/null
+++ b/tests/server/src/cases/issues/Issue12995.hx
@@ -0,0 +1,17 @@
+package cases.issues;
+
+class Issue12995 extends TestCase {
+	function test(_) {
+		vfs.putContent("Main.hx", getTemplate("issues/Issue12995/Main.hx"));
+		var args = ["-main", "Main"];
+
+		runHaxe(args);
+		assertErrorMessage("Class<Foo> has no field bar");
+
+		runHaxeJson(args, DisplayMethods.Diagnostics, {file: new FsPath("Main.hx")});
+
+		// The diagnostics request must not cache the erroring module (#12995)
+		runHaxe(args);
+		assertErrorMessage("Class<Foo> has no field bar");
+	}
+}
diff --git a/tests/server/test/templates/issues/Issue12995/Main.hx b/tests/server/test/templates/issues/Issue12995/Main.hx
new file mode 100644
index 00000000000..0dda6517bb7
--- /dev/null
+++ b/tests/server/test/templates/issues/Issue12995/Main.hx
@@ -0,0 +1,7 @@
+class Foo {}
+
+class Main {
+	static function main() {
+		Foo.bar();
+	}
+}