[BUG] Fullscreen rendering corrupts display / mouse tracking under tmux inside AWS SSM Session Manager

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?

/tui fullscreen (fullscreen rendering) corrupts the display and its mouse tracking emits garbage escape sequences — but only when tmux runs inside an AWS SSM Session Manager session.

I isolated it by varying one layer at a time:

• SSM → claude directly (no tmux): fullscreen works fine
• tmux locally (no SSM): fullscreen works fine
• SSM → tmux → claude: fullscreen breaks — the only failing combination

Control test (key evidence): in the identical SSM → tmux session, htop — which also uses alt-screen and mouse tracking — works correctly. This shows the transport plus tmux can carry alt-screen and mouse escape sequences properly, so the fault is specific to fullscreen rendering's mouse handling over this transport, not a terminal/terminfo limitation.

Hypothesis (not bisected — leaving the finer diagnosis to maintainers): fullscreen rendering appears to enable more aggressive mouse tracking than conventional alt-screen apps like htop (e.g. hover / any-motion, mode 1003), whose higher-frequency event stream may not survive the buffered SSM channel intact.

see the image:
<img width="1728" height="1084" alt="Image" src="https://github.com/user-attachments/assets/a1aa4317-a7cf-4b2d-aff2-4fa0cb994d78" />

What Should Happen?

Fullscreen rendering should render and handle mouse input correctly when running under tmux inside an AWS SSM Session Manager session — the same way it does over other transports, and the same way conventional alt-screen + mouse apps (e.g. htop) already work in that exact environment.

Error Messages/Logs

Steps to Reproduce

1. aws ssm start-session to an Amazon Linux 2023 host
2. Launch tmux (3.6a)
3. Run claude, then /tui fullscreen
4. Interact with the mouse (move / click / scroll)
5. Display corruption and garbage escape sequences appear

Isolation performed:

• Not reproducible with SSM directly (no tmux)
• Not reproducible with tmux locally (no SSM)
• Reproducible only with tmux inside SSM Session Manager

Control: htop (alt-screen + mouse tracking) works correctly in the identical SSM → tmux session.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.201 (Claude Code)

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

iTerm2

Additional Information

Transport: AWS SSM Session Manager (session-manager-plugin) — not SSH.
Multiplexer: tmux 3.6a
OS: Amazon Linux 2023 (2023.12.20260629), al2023
Kernel: 6.18.35-68.129.amzn2023.aarch64
Local terminal emulator (SSM client host): iTerm2 3.6.11 on macOS 26.5.2

$TERM without tmux: xterm-256color
$TERM with tmux: tmux-256color
tmux default-terminal: tmux-256color
tmux terminal-overrides: linux*:AX@
tmux terminal-features: xterm:clipboard:ccolour:cstyle:focus:title / screen:title / rxvt*:ignorefkeys

Config is not the usual culprit: default-terminal is the modern tmux-256color (not legacy screen-256color), tmux is current (3.6a), and the htop control confirms the transport is capable — pointing at fullscreen rendering rather than environment misconfiguration.

Workaround: /tui default (classic renderer) works correctly in the same environment.

Related: #27995 (mouse/scroll capture over SSH+tmux) — this report is a different transport (SSM). This was also submitted via the in-app /feedback command (Anthropic internal); filing here for public tracking — same report, not a separate bug.