Shell mode should only be triggered when "!" is typed with the cursor at position 0

What version of Codex CLI is running?

v0.151.0

What subscription do you have?

Pro

Which model were you using?

gpt-5.6-sol high

What platform is your computer?

Linux 7.1.10-arch1-1 x86_64 unknown

What terminal emulator and version are you using (if applicable)?

Kitty

Codex doctor report

Not relevant.

What issue are you seeing?

Shell mode is triggered any time a "!" touches the left side of the command prompt, regardless of whether it was due to a user typing "!" at that position, or them deleting text in front of a "!".

Once shell mode is triggered, even via this unintentional route, it persists, even after the "!" is deleted, and requires stopping and hitting backspace at position 0 to return to normal input mode.

What steps can reproduce the bug?

Starting from an empty input:

1. Write "Yes! I agree."
2. Press <home> or <ctrl-a> to position the cursor at 0
3. Press <del><del><del><del>

At step 3, after deleting the first 3 characters, the "!" touches the left side and codex enters shell mode.
9/10 times, this is due to editing text with "!", not an attempt to enter shell mode.

If I then press <del> again, I would expect that shell mode would exit and codex would resume normal text editing, but it doesn't -- shell mode persists for the remainder of the input until I explicit press <backspace> from position 0.

The net effect:

• shell mode works great when I trigger it intentionally, by starting a prompt with "!"
• any time I'm editing text that contains a "!" and that "!" accidentally touches the left side of the input field, I end up in command mode and have to jump through some extra hoops before I can actually enter the edited prompt.

What is the expected behavior?

• "!" entered by a user at position 0 triggers shell mode
• deleting characters preceding a "!" either (a) does not trigger shell mode, or (b) triggers shell mode unless a user deletes another character, which is a good sign they were just editing text.

Additional information

No response

Reproduce the editing sequence in the Codex CLI: type “Yes! I agree.”, move to position 0, and delete characters until the exclamation mark reaches the prompt start. Trace the input-handling entry point that switches to shell mode, then verify that typing “!” at position 0 still works while deletion does not leave shell mode active after the exclamation mark is removed.