How to Enable Undercurl in Neovim: Terminal and Tmux Setup Guide

If you're using Neovim as your default code editor or IDE, you've likely encountered the concept of undercurl. But what exactly is undercurl? Let's dive in!

---

What is Undercurl?

Think of the red squiggly lines you see in VS Code to highlight code errors or typos:

Undercurl brings that same functionality to your terminal-based Neovim setup. It's a visual cue for spelling or code errors—right inside your terminal.

---

Setting Up Undercurl in Neovim (Terminal)

Step 1: Use a True Color Terminal

To enable undercurl, you'll need a True Color Terminal, such as:

• iTerm2
• Alacritty
• Ghostty
• Windows Terminal

To check if your terminal supports undercurl, run the following command:

echo -e "\e[4:3mThis text has an undercurl\e[0m"

If your terminal supports undercurl, you'll see the text undercurled.

---

Step 2: Enable Undercurl in Neovim

Add the following lines to your Neovim configuration (options.lua or init.vim):

-- Undercurl
vim.cmd([[let &t_Cs = "\e[4:3m"]])
vim.cmd([[let &t_Ce = "\e[4:0m"]])

-- Enable spell check
vim.opt.spell = true
vim.opt.spelllang = { "en_us" }

This setup enables undercurl for spelling errors.

Don't forget to reload neovim after config changes above.

You can see below the word undercurl is undercurled.

---

Step 3: Configure Terminal for Undercurl

If undercurl still doesn’t work, follow these steps:

• Identify Your Terminal

Run echo $TERM to find your terminal type (e.g., xterm-256color or xterm-ghostty depending on your terminal).

• Check for Smulx Support

Run:

infocmp -l -x | grep Smulx

If it produces no output, proceed.

• Generate a Terminfo File

infocmp > /tmp/${TERM}.ti

Replace ${TERM} with your terminal type (e.g., xterm-256color).
Here in this case

infocmp > /tmp/xterm-256color.ti

• Edit the Terminfo File

Open /tmp/${TERM}.ti in your editor:

nvim /tmp/xterm-256color.ti

Add the following line after smul=\E[4m,:

Smulx=\E[4:%p1%dm,

• Reload the Terminfo

tic -x /tmp/${TERM}.ti

Replace ${TERM} with your terminal type which is the output from echo $TERM. Here in this case

tic -x /tmp/xterm-256color.ti

• Verify Smulx Support Run:

infocmp -l -x | grep Smulx

You should now see output confirming Smulx.

Open Neovim and enjoy undercurl support for spelling and code errors.

---

BTW, if you love my neovim and tmux setup above, do check my dotfiles on GitHub.

---

Enabling Undercurl in Neovim with Tmux

Undercurl may not work out of the box in Tmux. Here’s how to fix that:

Step 1: Update Your tmux.conf

Add the following lines to your ~/.tmux.conf or ~/.config/tmux/tmux.conf:

set -g default-terminal "screen-256color"

# Undercurl support
set -as terminal-overrides ',*:Smulx=\E[4::%p1%dm'
set -as terminal-overrides ',*:Setulc=\E[58::2::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m'

Check if you are able to see undercurl in Neovim inside tmux session. If not follow the below steps.

Step 2: Configure the Terminfo

• Run echo $TERM (should return screen-256color).
• Follow the terminal configuration steps. Remember your $TERM is now screen-256color so replace ${TERM} with screen-256color from step 3 onwards.

Step 3: Verify Smulx in Tmux

Run:

infocmp -l -x | grep Smulx

If successful, you’ll see the correct Smulx configuration.

Step 4: Test Undercurl in Tmux

Open Neovim inside a Tmux session, and voilà! Undercurl now works in Neovim + Tmux.

---

With these steps, you've unlocked the power of undercurl in Neovim, both in standalone terminals and Tmux sessions. Happy coding!

---

Questions or Feedback?

Feel free to leave a comment or ask any questions if you have doubts! I’d love to hear your thoughts or help you out. 😊