app: fix f-string.

Author: xavierog

CHANGELOG.md

@@ -6,6 +6,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html) for its CLI (Command-Line Interface), i.e. for the `moulti` command.
 Although Moulti's Python packages, modules and functions are obviously available, they do not constitute a public API yet.
 
+## Unreleased
+
+### Fixed
+
+- Fix rare "unknown action" error message.
+
 ## [1.34.0] - 2025-03-16
 
 ### Changed

src/moulti/app.py

@@ -699,7 +699,7 @@ def handle_message(self, connection: Socket, raddr: str, message: Message, file_
 						method_name = f'cli_action_{action.replace("-", "_")}'
 						method = getattr(step, method_name, False)
 						if not callable(method):
-							raise MoultiMessageException('unknown action {action}')
+							raise MoultiMessageException(f'unknown action {action}')
 						# Provide the method with helper functions that abstract away app/network-specific stuff:
 						def reply(**kwargs: Any) -> None:
 							self.reply(connection, raddr, message, **kwargs)