[BUG] claude-code 2.1.126 doesn't send DEC 2026 messages on terminals that support it

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?

Claude Code has supported the DEC 2026 synchronized output terminal extension for some time, which can help to reduce screen tearing and other artifacts when Claude redraws the scrollback buffer.

Since Claude Code 2.1.110, I've noticed Claude is no longer sending DEC 2026 enable/disable commands on my terminal which supports it. My previous issue was closed with the issue reported fixed, but it was never actually fixed. I re-tested with the latest 2.1.126 and confirmed the issue is still present.I use a modified version of the Emacs eat terminal emulator, which uses a TERM value of eat-truecolor. Its terminfo advertises the Sync capability, and it responds to DECRQM queries to test for the availability of the feature. I also tested setting my TERM value to xterm-256color to match iTerm, and Claude also does not send the sync messages.

Meanwhile, if I set my TERM to xterm-ghostty or xterm-kitty, Claude continues to send DEC 2026 instructions.

What Should Happen?

I assume Claude Code is now relying on a hardcoded list of terminal which support this feature, but this penalizes us users who run more obscure terminal emulators. Claude should support checking for DEC 2026 support using the DECRQM instruction, or a hardcoded list plus looking for the Sync feature in the terminfo (ghostty includes this feature in its custom terminfo, but other emulators such as iTerm that rely on a standard terminfo such as xterm-256color do not).

Ideally, Anthropic should document the terminal standards/behaviors Claude Code supports so terminal emulator developers can produce compatible software.

Error Messages/Logs

Steps to Reproduce

Launch claude in an affected terminal. When Claude produces a large amount of output, screen tearing will occur.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.109

Claude Code Version

2.1.126

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_