codex-fill.nvim

Small Neovim plugin that uses the codex CLI to fill in a selected code range.

Select a stub, empty function, or partial implementation, run Codex Fill, and the plugin replaces only the selected lines with Codex's final answer.

Why doing a _99 clone?

ThePrimeagen's _99 has been haunting me for half a year now. I knew that this concept was really good, but I did not want to use his version mainly because of skill issues (I have a disgust for lua unfortunately, I don't like it and I don't want to learn it). By coincidence, I have a quite big codex subscription (AFAIK his project does not support the codex yet) and as a result of this, the project has been shamelessly vibecoded.

Requirements

• Neovim 0.10+ with Lua support
• codex available on your PATH
• Codex CLI already logged in and working from a terminal

Check Codex:

codex --version
codex exec "Say OK"

Install

lazy.nvim

For local development, point lazy.nvim at this directory:

{
  dir = "/home/domanskiba/codex-fill",
  name = "codex-fill.nvim",
  config = function()
    require("codex_fill").setup()
  end,
}

If you use Kickstart.nvim, put that spec in ~/.config/nvim/lua/custom/plugins/codex-fill.lua:

return {
  {
    dir = "/home/domanskiba/codex-fill",
    name = "codex-fill.nvim",
    config = function()
      require("codex_fill").setup {
        keymap = "<leader>cf",
        model = nil,
      }
    end,
  },
}

Make sure Kickstart imports custom plugins in init.lua:

{ import = "custom.plugins" },

Manual runtimepath

You can also load it without a plugin manager:

vim.opt.runtimepath:prepend("/home/domanskiba/codex-fill")
require("codex_fill").setup()

Usage

1. Open a code file.
2. Select the code to replace with visual mode.
3. Press <leader>cf.
4. Enter an instruction, or press Enter with an empty prompt for the default fill behavior.

Example:

fn main() {
    say_hello_world();
}

fn say_hello_world() {

}

Select the whole say_hello_world function, press <leader>cf, and leave the instruction empty. Codex Fill asks Codex to infer the missing body from the function name and surrounding buffer.

You can also provide a visual range command:

:'<,'>CodexFill print hello world

Configuration

Default setup:

require("codex_fill").setup {
  codex_cmd = "codex",
  keymap = "<leader>cf",
  model = nil,
  notify = true,
}

Options:

• codex_cmd: executable name or path for the Codex CLI.
• keymap: visual mode mapping. Set to false or "" to skip keymap creation.
• model: optional Codex model. When nil, Codex uses your normal CLI config.
• notify: show Neovim notifications for start/success/failure.

Behavior

• Runs codex exec asynchronously.
• Uses the nearest git root as Codex working directory.
• Runs Codex with --sandbox read-only.
• Sends Codex the selected range, current file path, filetype, and full current buffer.
• Applies only Codex's final response to the selected line range.
• Strips a single surrounding markdown code fence when Codex returns one.
• Leaves the buffer unchanged if Codex fails, returns empty output, returns identical text, or if the buffer changed while Codex was running.

Current limits

• True rectangular Visual Block selections are rejected for now. Use visual line or visual character selection instead.
• The plugin replaces whole touched lines, not arbitrary character spans.
• It does not autoformat after replacement.
• It does not preview diffs before applying.

Test

Run the headless smoke test:

nvim --headless -u NONE -S test/headless.lua

Load through your Neovim config:

nvim --headless -u ~/.config/nvim/init.lua '+lua require("codex_fill")' +qa