`tty.WriteStream.getColorDepth()` returns 8 inside tmux despite COLORTERM=truecolor

[jacobcxdev]

Bug

In lib/internal/tty.js, getColorDepth() checks env.TMUX (line 174) before env.COLORTERM (line 215). When TMUX is set, it returns COLORS_256 (8) immediately, never reaching the COLORTERM === 'truecolor' check that would return COLORS_16m (24).

tmux has supported true color since v2.2 (2016). Modern tmux sets COLORTERM=truecolor when the outer terminal supports RGB via terminal-features or terminal-overrides. The current ordering ignores this.

Note: PR #30474 previously reordered COLORTERM before the TERM check but did not address the TMUX check ordering.

Reproduction

const tty = require('tty');
console.log('TMUX:', process.env.TMUX || '(unset)');
console.log('COLORTERM:', process.env.COLORTERM || '(unset)');
console.log('getColorDepth():', process.stdout.getColorDepth());

Inside tmux (with COLORTERM=truecolor):

TMUX: /private/tmp/tmux-501/default,9842,1
COLORTERM: truecolor
getColorDepth(): 8    # ← wrong, should be 24

With TMUX= unset:

TMUX: (unset)
COLORTERM: truecolor
getColorDepth(): 24   # ← correct

Suggested fix

Move the COLORTERM truecolor/24bit check (line 215) before the TMUX check (line 174):

// Check COLORTERM first — tmux sets this when RGB is supported
if (env.COLORTERM === 'truecolor' || env.COLORTERM === '24bit') {
  return COLORS_16m;
}

if (env.TMUX) {
  return COLORS_256;
}

This preserves the TMUX → 256 fallback for tmux sessions that don't have truecolor configured, while correctly returning 16m for those that do.

Related

• PR tty: truecolor check moved before 256 check #30474 — moved truecolor check before TERM 256 check (but not before TMUX)
• Bun has the same bug (filed at getColorDepth() returns 8 (256-color) inside tmux despite COLORTERM=truecolor oven-sh/bun#28463), inherited from this code

[semimikoh]

Thanks for the detailed bug report! I'd like to work on fixing this issue.

My plan:

1. Move the COLORTERM truecolor/24bit check before the TMUX check in lib/internal/tty.js
2. Add test cases for TMUX + COLORTERM=truecolor combination in test/pseudo-tty/test-tty-color-support.js

Is it okay if I submit a PR for this?

[jacobcxdev]

@semimikoh yeah that's fine by me—thanks for taking it on!