[BUG] Shift+Enter inserts literal CSI u-protocol sequence instead of newline

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 pressing Shift+Enter in Claude Code while using WezTerm with enhanced keyboard protocols enabled (enable_kitty_keyboard and enable_csi_u_key_encoding), the literal character sequence [13;2u is inserted instead of a newline.

What Should Happen?

Shift+Enter should insert a newline character, allowing multi-line input in Claude Code.

Steps to Reproduce

1. Enable enhanced keyboard protocols in WezTerm:

``lua
config.enable_kitty_keyboard = true
config.enable_csi_u_key_encoding = true
``

1. Launch Claude Code in WezTerm
2. Press Shift+Enter in the input area
3. Observe that [13;2u is inserted instead of a newline

The sequence [13;2u is a CSI u-protocol code representing:

• 13 = Enter key (carriage return)
• 2 = Shift modifier
• u = CSI u-protocol terminator

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.35 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

The CSI u-protocol (also known as "fixterms" or "kitty keyboard protocol") is an enhanced keyboard encoding protocol that provides more detailed information about key events. The sequence ESC[13;2u should be decoded by the application as "Shift+Enter" and handled appropriately.

Workaround: Add a WezTerm key binding that sends a plain newline for Shift+Enter:

{ 'Enter', 'SHIFT', act.SendString('\n') }

However, this prevents users from benefiting from enhanced keyboard protocols in their shell/editor configurations.

Claude Code's input handler should properly decode CSI u-protocol sequences, particularly for common key combinations like Shift+Enter. This would ensure compatibility with modern terminal emulators that use enhanced keyboard protocols by default.

Additional details:

• WezTerm version: 20240203-110809-5046fc22
• macOS version: Darwin 25.0.0
• Regular Enter (without Shift) works correctly
• This issue likely affects other terminal emulators with enhanced keyboard protocol support (e.g., kitty, foot, ghostty)