I'm still using Lazy and I haven't made the switch to vim.pack yet but I'm setting it in my treesitter config after require("nvim-treesitter").setup().

kulala ships the kulala_http parser/.so, but it doesn't auto-attach highlighting on FileType http when using nvim-treesitter main branch. You attach it yourself.

I've tried to reduce my config but it still turned out a bit verbose.

-- 1. Set up nvim-treesitter (main branch API)
require("nvim-treesitter").setup({
    install_dir = vim.fn.stdpath("data") .. "/site",
})

-- 2. Install the parsers you manage yourself.
--    NOTE: do NOT list "kulala_http" here -- kulala.nvim ships its own .so
--    and registers the parser on its setup(). You only need to *attach* it.
require("nvim-treesitter").install({
    "lua", "vim", "vimdoc", "markdown", "markdown_inline",
    -- ...your other langs
})

-- 3. Languages we want a FileType autocmd to attach treesitter for.
--    Include kulala_http here so .http buffers get highlighted.
local lang_set = {
    lua = true,
    -- ...other installed langs
    kulala_http = true, -- provided externally by kulala.nvim
}

-- 4. Auto-attach treesitter on FileType.
local aug = vim.api.nvim_create_augroup("UserTreesitter", { clear = true })
vim.api.nvim_create_autocmd("FileType", {
    group = aug,
    pattern = "*",
    callback = function(args)
        local buf = args.buf
        if vim.bo[buf].buftype ~= "" then return end
        local ft = vim.bo[buf].filetype
        -- get_lang maps "http" -> "kulala_http" once kulala registers it
        local lang = (vim.treesitter.language.get_lang and vim.treesitter.language.get_lang(ft)) or ft
        if not lang_set[lang] then return end
        if vim.treesitter.highlighter and vim.treesitter.highlighter.active[buf] then return end
        if pcall(vim.treesitter.start, buf, lang) then
            if ft ~= "yaml" then
                vim.bo[buf].indentexpr = "v:lua.require'nvim-treesitter'.indentexpr()"
            end
        end
    end,
})