feat: discard prompt starting with a slash (#5048)

This is does not consider lines starting with a space or containing
multiple `/` as commands
<img width="550" height="362" alt="Screenshot 2025-10-13 at 10 00 08"
src="https://github.com/user-attachments/assets/17f7347f-db24-47cb-9845-b0eb6fb139cb"
/>

codex-rs/tui/src/bottom_pane/chat_composer.rs

@@ -962,6 +962,7 @@ impl ChatComposer {
                 }
                 let mut text = self.textarea.text().to_string();
                 let original_input = text.clone();
+                let input_starts_with_space = original_input.starts_with(' ');
                 self.textarea.set_text("");
 
                 // Replace all pending pastes in the text
@@ -975,6 +976,35 @@ impl ChatComposer {
                 // If there is neither text nor attachments, suppress submission entirely.
                 let has_attachments = !self.attached_images.is_empty();
                 text = text.trim().to_string();
+                if let Some((name, _rest)) = parse_slash_name(&text) {
+                    let treat_as_plain_text = input_starts_with_space || name.contains('/');
+                    if !treat_as_plain_text {
+                        let is_builtin = built_in_slash_commands()
+                            .into_iter()
+                            .any(|(command_name, _)| command_name == name);
+                        let prompt_prefix = format!("{PROMPTS_CMD_PREFIX}:");
+                        let is_known_prompt = name
+                            .strip_prefix(&prompt_prefix)
+                            .map(|prompt_name| {
+                                self.custom_prompts
+                                    .iter()
+                                    .any(|prompt| prompt.name == prompt_name)
+                            })
+                            .unwrap_or(false);
+                        if !is_builtin && !is_known_prompt {
+                            let message = format!(
+                                r#"Unrecognized command '/{name}'. Type "/" for a list of supported commands."#
+                            );
+                            self.app_event_tx.send(AppEvent::InsertHistoryCell(Box::new(
+                                history_cell::new_info_event(message, None),
+                            )));
+                            self.textarea.set_text(&original_input);
+                            self.textarea.set_cursor(original_input.len());
+                            return (InputResult::None, true);
+                        }
+                    }
+                }
+
                 let expanded_prompt = match expand_custom_prompt(&text, &self.custom_prompts) {
                     Ok(expanded) => expanded,
                     Err(err) => {
@@ -2877,6 +2907,76 @@ mod tests {
         assert!(composer.textarea.is_empty());
     }
 
+    #[test]
+    fn slash_path_input_submits_without_command_error() {
+        use crossterm::event::KeyCode;
+        use crossterm::event::KeyEvent;
+        use crossterm::event::KeyModifiers;
+
+        let (tx, mut rx) = unbounded_channel::<AppEvent>();
+        let sender = AppEventSender::new(tx);
+        let mut composer = ChatComposer::new(
+            true,
+            sender,
+            false,
+            "Ask Codex to do anything".to_string(),
+            false,
+        );
+
+        composer
+            .textarea
+            .set_text("/Users/example/project/src/main.rs");
+
+        let (result, _needs_redraw) =
+            composer.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+        if let InputResult::Submitted(text) = result {
+            assert_eq!(text, "/Users/example/project/src/main.rs");
+        } else {
+            panic!("expected Submitted");
+        }
+        assert!(composer.textarea.is_empty());
+        match rx.try_recv() {
+            Ok(event) => panic!("unexpected event: {event:?}"),
+            Err(tokio::sync::mpsc::error::TryRecvError::Empty) => {}
+            Err(err) => panic!("unexpected channel state: {err:?}"),
+        }
+    }
+
+    #[test]
+    fn slash_with_leading_space_submits_as_text() {
+        use crossterm::event::KeyCode;
+        use crossterm::event::KeyEvent;
+        use crossterm::event::KeyModifiers;
+
+        let (tx, mut rx) = unbounded_channel::<AppEvent>();
+        let sender = AppEventSender::new(tx);
+        let mut composer = ChatComposer::new(
+            true,
+            sender,
+            false,
+            "Ask Codex to do anything".to_string(),
+            false,
+        );
+
+        composer.textarea.set_text(" /this-looks-like-a-command");
+
+        let (result, _needs_redraw) =
+            composer.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+        if let InputResult::Submitted(text) = result {
+            assert_eq!(text, "/this-looks-like-a-command");
+        } else {
+            panic!("expected Submitted");
+        }
+        assert!(composer.textarea.is_empty());
+        match rx.try_recv() {
+            Ok(event) => panic!("unexpected event: {event:?}"),
+            Err(tokio::sync::mpsc::error::TryRecvError::Empty) => {}
+            Err(err) => panic!("unexpected channel state: {err:?}"),
+        }
+    }
+
     #[test]
     fn custom_prompt_invalid_args_reports_error() {
         let (tx, mut rx) = unbounded_channel::<AppEvent>();