diff --git a/mypy/plugin.py b/mypy/plugin.py
index 858795addb7f..a1af7fa76350 100644
--- a/mypy/plugin.py
+++ b/mypy/plugin.py
@@ -240,7 +240,7 @@ def type_context(self) -> list[Type | None]:
 
     @abstractmethod
     def fail(
-        self, msg: str | ErrorMessage, ctx: Context, *, code: ErrorCode | None = None
+        self, msg: str | ErrorMessage, ctx: Context, /, *, code: ErrorCode | None = None
     ) -> None:
         """Emit an error message at given location."""
         raise NotImplementedError