Unrecognized slash command prevents message from being sent

Bug Report

When a user types a message starting with / followed by an unrecognized command name (e.g., /xxx), the message cannot be sent. Instead, the composer shows an error message and restores the input text, leaving the user stuck with no way to submit that text as a regular message.

Steps to Reproduce

1. Open Codex CLI
2. Type a message starting with an unrecognized slash command, e.g., /xxx hello world
3. Press Enter to submit

Actual Behavior

The message is blocked with an error:

Unrecognized command '/xxx'. Type "/" for a list of supported commands.

The input text is restored to the composer, and the user cannot send it as a regular message.

Expected Behavior

When a slash command is not recognized, the text should be treated as a regular user message and sent to the model, rather than being blocked entirely. This is especially important because:

• Users may want to discuss slash commands in natural language (e.g., "how do I use /diff?")
• Users may accidentally type a leading slash before a message
• The current behavior creates a dead end where the user must manually delete the leading / to send their message

Relevant Code

The issue is in codex-rs/tui/src/bottom_pane/chat_composer.rs in the prepare_submission_text_with_options method (around line 2721-2751):

if slash_validation == SlashValidation::Immediate
    && self.slash_commands_enabled()
    && let Some((name, _rest, _rest_offset)) = parse_slash_name(&text)
{
    let treat_as_plain_text = input_starts_with_space || name.contains('/');
    if !treat_as_plain_text {
        let is_known = find_slash_command(
            name,
            self.builtin_command_flags(),
            &self.service_tier_commands,
        )
        .is_some();
        if !is_known {
            // This blocks the message entirely
            let message = format!(
                r#"Unrecognized command '/{name}'. Type "/" for a list of supported commands."#
            );
            // ... restores input and returns None
        }
    }
}

Similarly, try_dispatch_bare_slash_command (line 2937) and try_dispatch_slash_command_with_args (line 2973) also block unknown commands without falling back to regular message submission.

Suggested Fix

When a slash command is not recognized, treat the input as a regular user message instead of blocking it. This could be done by:

1. Removing the early return that blocks unknown slash commands in prepare_submission_text_with_options
2. Allowing the text to proceed through normal submission as a regular message
3. Optionally showing a brief hint that the command was not recognized, but still sending the message

Environment

• Codex CLI version: latest main
• Platform: all platforms affected

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This is a UX issue that significantly impacts the interactive experience when users accidentally type unknown slash commands or want to discuss slash command syntax with the model.