[BUG] CSI u (Fixterms) shifted sequences not recognised

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When using Claude Code inside tmux with extended-keys on, alt+shift+<letter> keybindings (e.g. alt+shift+g) are treated as the unshifted binding (e.g. alt+g).

This is true even when tmux is configured to output CSI u via extended-keys-format csi-u
This is because the tmux's CSI u uses:

• the Fixterms encoding (Alt+Shift+g = ^[[71;3u meaning codepoint 'G', modifier Alt)

but Claude code only seem to understand:

• the Kitty Keyboard Protocol Flag 1 encoding (Alt+Shift+g = ^[[103;4u meaning codepoint 'g', modifier Alt + Shift)
• the Kitty Keyboard Protocol Flag 5 encoding (Alt+Shift+g = ^[[103:71;4u meaning the same but with "alternate keys" enabled)
• the legacy, non CSI u encoding (Alt+Shift+g = ^[G)
• This means that tmux with extended-keys off will work, but then we miss out on the whole range of extended key bindings that Claude Code supports

TL;DR Claude Code doesn't seem to support the original CSI u specification (FixTerms) that tmux uses. Causing it to interpret ^[[71;3u as a plain Alt + g (doesn't disambiguate from ^[[103;3u), even though shift is pressed.

What Should Happen?

If Tmux is to be supported by Claude Code, then Alt+Shift+<letter> bindings should work inside Tmux even when extended-keys is set to on.

Tmux by default outputs extended keys in the way that xterm does, but with extended-keys-format csi-u we can have it output in CSI u FixTerms format, both of which currently are not recognized by Claude Code when using Tmux.

Suggested fix: when a CSI u key event arrives without alternate-key data and its codepoint is a shifted character (e.g. 'G' rather than 'g'), treat it as unshifted character (base key) with the Shift bit set.
This makes ESC[71;3u equivalent to ESC[103;4u.

Error Messages/Logs

Steps to Reproduce

Any terminal should work for this example since ^[g and ^[G are distinguishable even in the legacy encoding, it is Tmux that converts the various input encodings into the Fixterms CSI u encoding.

1. Claude Code configured to use alt+shift+g and alt+g keybinding (actions arbitrarily chosen):

{
  "$schema": "https://www.schemastore.org/claude-code-keybindings.json",
  "$docs": "https://code.claude.com/docs/en/keybindings",
  "bindings": [
    {
      "context": "Global",
      "bindings": {
        "alt+g": "chat:cycleMode",
        "alt+shift+g": "chat:modelPicker"
      }
    }
  ]
}

1. Tmux should have the following configuration to enable extended keys to be passed into running programs:

set -as terminal-features ',*xterm*:extkeys'
set -s extended-keys on
set -s extended-keys-format csi-u   # Optional, both the default `xterm` and `csi-u` fail to work on Claude Code

1. Open Claude with claude
2. Notice that the alt+shift+g binding performs chat:cycleMode instead of chat:modelPicker

Bonus peculiar behavior: Spamming Alt+Shift+G immediately after running claude while Claude Code is running will cause the correct keybind to trigger once.

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.295 (Claude Code)

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

Other

Additional Information

_No response_