Tmux Enter Key Handling

What This Solves

When sending input to tmux panes running interactive applications (Claude Code, Codex, vim, etc.), the Enter keyword in tmux send-keys fails silently. Text appears but is never submitted.

Working Approach

Send Enter as hex 0x0d (carriage return) in a separate command:

Bash

bash

# ✅ CORRECT - send text, then Enter as hex
tmux send-keys -t "session-name" "Hello world"
tmux send-keys -t "session-name" -H 0d

Python

python

import subprocess

# Send text
subprocess.run(['tmux', 'send-keys', '-t', session, prompt])

# Send Enter as hex (REQUIRED for TUIs)
subprocess.run(['tmux', 'send-keys', '-t', session, '-H', '0d'])

Other Special Keys

Key	Hex Code	Usage
Enter	`0d`	`tmux send-keys -t sess -H 0d`
Escape	`1b`	`tmux send-keys -t sess -H 1b`
Tab	`09`	`tmux send-keys -t sess -H 09`

Failed Attempts

⚠️ Read this first - These approaches don't work:

Attempt	Why It Failed	Time Wasted
`tmux send-keys -t sess "text" Enter`	Enter keyword ignored by TUI apps	2+ hours debugging
`subprocess.run([..., prompt, 'Enter'])`	Same issue - Python passes it but TUI ignores	1 hour
Checking return code for success	returncode=0 means tmux ran, NOT that input was submitted	30 min false confidence

Verification (Critical!)

Never trust return codes. Always verify visually:

bash

# Send prompt
tmux send-keys -t sess "Write a haiku"
tmux send-keys -t sess -H 0d

# Wait for response
sleep 5

# VERIFY the response actually appeared
tmux capture-pane -t sess -p -S -50

Only report success when you SEE:

•The prompt text in output
•Processing indicator (e.g., "Thinking...")
•Actual response content
•New input prompt ready

Environment & Dependencies

•Applies to: Any tmux version
•Affected apps: Claude Code, Codex CLI, vim, emacs, any ncurses/TUI app
•Works fine with: Regular shell prompts (bash, zsh) - but hex is safer everywhere

Session Reference

•Date: 2024
•Source: tmux_sdk development, multiple debugging sessions
•Original issue: Commands appeared typed but never executed