Summary

Add a narrow undo last paste feature to the TUI composer.

This is not a full editor undo stack. It only reverts the most recent successful paste transaction, including paste side effects beyond raw text.

Problem

The TUI already treats paste as a first-class composer operation:

• normal pasted text
• oversized paste placeholders backed by pending_pastes
• pasted image paths that become local image attachments
• shortcut image paste via Ctrl+V / Alt+V

Today, if a paste is accidental or wrong, the recovery options are inconsistent:

• plain text may be manually deleted
• oversized paste placeholders are not a normal text edit
• pasted image attachments are not just text
• there is no single standard shortcut to “undo the paste I just did”

The existing Ctrl+U / Ctrl+K / Ctrl+W / Ctrl+Y behavior is helpful, but it is kill/yank behavior, not paste undo.

Proposal

Add a composer-level undo last paste shortcut:

• primary binding: Ctrl+_
• accept terminal-friendly aliases: Ctrl+/ and raw ^_

Behavior:

• only undoes the most recent successful paste transaction
• restores the entire pre-paste draft state
• includes:
  • text
  • text elements
  • local image attachments
  • remote image URLs
  • mention bindings
  • pending_pastes
  • cursor position
• cursor-only movement after paste should not invalidate undo
• any content mutation after paste should invalidate it
• single-level only
• no redo
• no general-purpose edit undo stack

Why composer-level, not textarea-level

Paste in the TUI is not only text insertion.

A general textarea undo would not naturally cover:

• large-paste placeholder bookkeeping
• local image attach/remove
• shortcut image paste
• attachment relabeling and related draft state

So the smallest coherent feature is “undo last paste transaction” at the composer layer.

Suggested implementation shape

• capture a full draft snapshot before a successful paste
• capture a post-paste fingerprint after it
• on Ctrl+_ / alias:
  • if the current draft still matches the post-paste fingerprint, restore the pre-paste snapshot
  • otherwise treat the undo-paste state as stale and do nothing

This keeps the feature predictable and avoids introducing a broad edit-history system.

Test cases

• undo after plain text paste
• undo after oversized paste with placeholder + pending_pastes
• undo after image-path paste that attached a local image
• undo after shortcut image paste
• cursor movement does not block undo
• content edits after paste invalidate undo
• one-shot behavior
• Ctrl+Z remains suspend, not undo

Rationale / prior art

This key choice follows established terminal/editor conventions:

• Emacs documents undo on C-/, with C-_ as the terminal-friendly equivalent on systems where Ctrl+/ is delivered that way.
• The current Codex TUI already uses readline/emacs-style editing keys (Ctrl+U, Ctrl+K, Ctrl+W, Ctrl+Y), so a narrow paste undo on the same family of bindings is a reasonable fit.
• Ctrl+Z should not be repurposed because it is already used for terminal suspend in the TUI.

Local status

I also have a local implementation of this behavior in a fork, verified with:

• just fmt
• cargo clippy -p codex-tui --tests
• cargo test -p codex-tui

Reference implementation:

8d0fdd5f

If this direction matches the intended UX, I can turn it into a PR if invited.