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

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

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

Failed Attempts

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

Verification (Critical!)

Never trust return codes. Always verify visually:

# 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:

1. The prompt text in output
2. Processing indicator (e.g., "Thinking...")
3. Actual response content
4. 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