[BUG] TUI rendering corruption (dropped/overlapping characters) inside tmux, only with full animated UI — --ax-screen-reader works fine

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?

Description

Claude Code's interactive TUI renders correctly in a plain terminal, but shows character corruption (dropped characters, overlapping/garbled text, e.g. "Claude Code will be able to" rendering as "Claude Code'll be able to") when run inside tmux. The corruption is present on initial render and worsens after resizing the pane while Claude Code is running.

Environment

• Claude Code version: 2.1.204 (confirmed up to date via claude update)
• OS: macOS (Darwin 25.5.0)
• Terminal: iTerm2 Build 3.6.11
• Multiplexer: tmux 3.7a
• TERM: xterm-256color (confirmed identical both inside and outside tmux)
• Locale: en_US.UTF-8 (confirmed correct via locale)

Steps to reproduce

1. Open iTerm2, start a plain (non-tmux) shell, run claude — renders correctly.
2. Open iTerm2, start a tmux session (single full-size pane, no splits), run claude — rendering is corrupted (dropped/overlapping characters) from the initial welcome screen onward.
3. Resizing the tmux pane while Claude Code is running makes the corruption worse.

Isolation performed (ruling out local environment causes)

• Disabling iTerm2 GPU (Metal) rendering — no change
• Disabling tmux mouse mode (mouse off) — no change
• Verified locale is UTF-8 — not the cause
• Verified TERM is identical (xterm-256color) in both working and broken cases — rules out terminfo/capability mismatch
• Reproduces in a single full-size tmux pane (not specific to split-pane layouts/narrow widths)
• Other full-screen TUI apps in the same tmux session (vim, htop) render perfectly — rules out a general tmux/terminal misconfiguration
• Running with claude --ax-screen-reader (flat text, no decorative borders/animations) renders correctly with no corruption — strongly suggests the issue is specific to the incremental/animated redraw path (spinners, borders, cursor-relative partial redraws) conflicting with tmux's own screen-diffing model, rather than a general output/encoding problem

Expected behavior
Full animated TUI renders correctly inside tmux, matching behavior outside tmux.

Actual behavior
Characters are dropped/overwritten during redraws inside tmux; only the flat --ax-screen-reader mode avoids it.

Workaround
Running with claude --ax-screen-reader avoids the corruption but loses decorative borders/animations.

What Should Happen?

Expected behavior
Full animated TUI renders correctly inside tmux, matching behavior outside tmux.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce

1. Open iTerm2, start a plain (non-tmux) shell, run claude — renders correctly.
2. Open iTerm2, start a tmux session (single full-size pane, no splits), run claude — rendering is corrupted (dropped/overlapping characters) from the initial welcome screen onward.
3. Resizing the tmux pane while Claude Code is running makes the corruption worse.

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.204

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_