diff --git a/router/src/main.rs b/router/src/main.rs
index 546019488..baabb5c49 100644
--- a/router/src/main.rs
+++ b/router/src/main.rs
@@ -87,7 +87,7 @@ struct Args {
     /// will refuse to start with an error instead of silently truncating sequences.
     ///
     /// Unused for gRPC servers
-    #[clap(long, env, default_value = "true")]
+    #[clap(long, env, default_value = "true", num_args = 0..=1, default_missing_value = "true")]
     auto_truncate: bool,
 
     /// The name of the prompt that should be used by default for encoding. If not set, no prompt