diff --git a/runtime.md b/runtime.md
index c46c91f..b7141c5 100644
--- a/runtime.md
+++ b/runtime.md
@@ -9,8 +9,11 @@ $ funC [global-options] <COMMAND> [command-specific-options] <command-specific-a
 
 ## Global options
 
-None are required, but the runtime may support options that start with at least one hyphen.
-Global options may take positional arguments (e.g. `--log-level debug`), but the option parsing must be such that `funC <COMMAND>` is unambiguously an invocation of `<COMMAND>` for any `<COMMAND>` that does not start with a hyphen (including commands not specified in this document).
+None are required, but the runtime MAY support options that start with at least one hyphen.
+Global options MAY take positional arguments (e.g. `--log-level debug`).
+Command names MUST not start with hyphens.
+The option parsing MUST be such that `funC <COMMAND>` is unambiguously an invocation of `<COMMAND>` (even for commands not specified in this document).
+If the runtime is invoked with an unrecognized command, it MUST exit with a nonzero exit code and MAY log a warning to stderr.
 
 ## Character encodings