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tmux Setup

Atomic works inside tmux, but tmux strips modifier information from certain keys by default. Without configuration, SHIFT+Enter and CTRL+Enter are usually indistinguishable from plain Enter. Add to ~/.tmux.conf: 
set -g extended-keys on
set -g extended-keys-format csi-u
Then restart tmux fully: 
tmux kill-server
tmux
Atomic requests extended key reporting automatically when Kitty keyboard protocol is not available. With extended-keys-format csi-u, tmux forwards modified keys in CSI-u format, which is the most reliable configuration. With only: 
set -g extended-keys on
tmux defaults to extended-keys-format xterm. When an application requests extended key reporting, modified keys are forwarded in xterm modifyOtherKeys format such as:
  • CTRL+C → \x1b[27;5;99~
  • CTRL+D → \x1b[27;5;100~
  • CTRL+Enter → \x1b[27;5;13~
With extended-keys-format csi-u, the same keys are forwarded as:
  • CTRL+C → \x1b[99;5u
  • CTRL+D → \x1b[100;5u
  • CTRL+Enter → \x1b[13;5u
Atomic supports both formats, but csi-u is the recommended tmux setup.

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What This Fixes

Without tmux extended keys, modified Enter keys collapse to legacy sequences:
KeyWithout extkeysWith csi-u
Enter\r\r
SHIFT+Enter\r\x1b[13;2u
CTRL+Enter\r\x1b[13;5u
Alt/Option+Enter\x1b\r\x1b[13;3u
This affects the default keybindings (Enter to submit, SHIFT+Enter for newline) and any custom keybindings using modified Enter.

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Requirements

  • tmux 3.2 or later (run tmux -V to check)
  • A terminal emulator that supports extended keys (Ghostty, Kitty, iTerm2, WezTerm, Windows Terminal)