diff --git a/Makefile b/Makefile
index c9092f9..50f2d2c 100644
--- a/Makefile
+++ b/Makefile
@@ -1,12 +1,12 @@
 LUAROCKS_PATH_CMD = luarocks path --no-bin --lua-version 5.1
-BUSTED = eval $$(luarocks path --no-bin --lua-version 5.1) && busted --lua nlua
+TEST = luarocks test --local --lua-version=5.1
 TEST_DIR = spec
 
 .PHONY: test
 test:
 	@echo "Running tests..."
 	@if [ -n "$(file)" ]; then \
-		$(BUSTED) $(file); \
+		$(TEST) $(file); \
 	else \
-		$(BUSTED) $(TEST_DIR); \
+		$(TEST); \
 	fi
diff --git a/doc/guard.nvim.txt b/doc/guard.nvim.txt
index de36435..b4a0035 100644
--- a/doc/guard.nvim.txt
+++ b/doc/guard.nvim.txt
@@ -1,4 +1,4 @@
-*guard.nvim.txt*         For NVIM v0.8.0         Last change: 2025 November 13
+*guard.nvim.txt*         For NVIM v0.8.0         Last change: 2025 December 15
 
 ==============================================================================
 Table of Contents                               *guard.nvim-table-of-contents*
diff --git a/lua/guard/_async.lua b/lua/guard/_async.lua
index 47f37b6..7a643fc 100644
--- a/lua/guard/_async.lua
+++ b/lua/guard/_async.lua
@@ -46,7 +46,7 @@ function M.run(func, on_finish)
   return {
     --- @param timeout? integer
     --- @return any ... return values of `func`
-    wait = function(_self, timeout)
+    wait = function(_, timeout)
       vim.wait(timeout or max_timeout, function()
         return res ~= nil
       end)
diff --git a/lua/guard/format.lua b/lua/guard/format.lua
index ae2b7ed..897f512 100644
--- a/lua/guard/format.lua
+++ b/lua/guard/format.lua
@@ -201,7 +201,7 @@ local function do_fmt(buf)
         fail(err)
         return
       end
-      new_lines = output
+      new_lines = assert(output)
     end
 
     async.await(1, function(callback)
diff --git a/lua/guard/lsp.lua b/lua/guard/lsp.lua
index 1c0a112..8575cfc 100644
--- a/lua/guard/lsp.lua
+++ b/lua/guard/lsp.lua
@@ -1,13 +1,13 @@
+local async = require('guard._async')
 local M = {}
 local api = vim.api
 local apply = vim.lsp.util.apply_text_edits
 
 ---@param buf number
----@param range table
+---@param range table?
 ---@param acc string
 ---@return string
 function M.format(buf, range, acc)
-  local co = assert(coroutine.running())
   local clients = vim.lsp.get_clients({ bufnr = buf, method = 'textDocument/formatting' })
   if #clients == 0 then
     return acc
@@ -19,30 +19,30 @@ function M.format(buf, range, acc)
   api.nvim_buf_set_lines(scratch, 0, -1, false, vim.split(acc, '\r?\n'))
   local line_offset = range and range.start[1] - 1 or 0
 
-  ---@diagnostic disable-next-line: duplicate-set-field
-  vim.lsp.util.apply_text_edits = function(text_edits, _, offset_encoding)
-    -- the target buffer must be buf, we apply it to our scratch buffer
-    n_edits = n_edits - 1
-    vim.tbl_map(function(edit)
-      edit.range.start.line = edit.range.start.line - line_offset
-      edit.range['end'].line = edit.range['end'].line - line_offset
-    end, text_edits)
-    apply(text_edits, scratch, offset_encoding)
-    if n_edits == 0 then
-      vim.lsp.util.apply_text_edits = apply
-      local lines = api.nvim_buf_get_lines(scratch, 0, -1, false)
-      api.nvim_command('silent! bwipe! ' .. scratch)
-      coroutine.resume(co, table.concat(lines, '\n'))
+  return async.await(1, function(callback)
+    ---@diagnostic disable-next-line: duplicate-set-field
+    vim.lsp.util.apply_text_edits = function(text_edits, _, offset_encoding)
+      -- the target buffer must be buf, we apply it to our scratch buffer
+      n_edits = n_edits - 1
+      vim.tbl_map(function(edit)
+        edit.range.start.line = edit.range.start.line - line_offset
+        edit.range['end'].line = edit.range['end'].line - line_offset
+      end, text_edits)
+      apply(text_edits, scratch, offset_encoding)
+      if n_edits == 0 then
+        vim.lsp.util.apply_text_edits = apply
+        local lines = api.nvim_buf_get_lines(scratch, 0, -1, false)
+        api.nvim_command('silent! bwipe! ' .. scratch)
+        callback(table.concat(lines, '\n'))
+      end
     end
-  end
-
-  vim.lsp.buf.format({
-    bufnr = buf,
-    range = range,
-    async = true,
-  })
 
-  return (coroutine.yield())
+    vim.lsp.buf.format({
+      bufnr = buf,
+      range = range,
+      async = true,
+    })
+  end)
 end
 
 return M
diff --git a/spec/lsp_spec.lua b/spec/lsp_spec.lua
new file mode 100644
index 0000000..ba213d6
--- /dev/null
+++ b/spec/lsp_spec.lua
@@ -0,0 +1,61 @@
+-- ---@diagnostic disable: undefined-field, undefined-global
+-- require('plugin.guard')
+-- local api = vim.api
+-- local same = assert.are.same
+-- local ft = require('guard.filetype')
+-- local gapi = require('guard.api')
+--
+-- describe('format module', function()
+--   local bufnr
+--   local ill_c = {
+--     'int main(     void ) {return 0;}',
+--   }
+--
+--   before_each(function()
+--     for k, _ in pairs(ft) do
+--       ft[k] = nil
+--     end
+--     vim.g.guard_config = {
+--       lsp_as_default_formatter = true,
+--     }
+--
+--     vim.lsp.config('clangd', {
+--       cmd = { 'clangd' },
+--       filetypes = { 'c' },
+--       capabilities = {
+--         textDocument = {
+--           formatting = {
+--             dynamicRegistration = true,
+--           },
+--         },
+--       },
+--     })
+--     vim.lsp.enable('clangd')
+--
+--     bufnr = api.nvim_create_buf(true, false)
+--
+--     vim.bo[bufnr].filetype = 'c'
+--     api.nvim_set_current_buf(bufnr)
+--     vim.cmd('silent! write! /tmp/lsp_spec_test.c')
+--   end)
+--
+--   after_each(function()
+--     vim.lsp.enable('clangd', false)
+--   end)
+--
+--   local function getlines()
+--     return api.nvim_buf_get_lines(bufnr, 0, -1, false)
+--   end
+--
+--   local function setlines(lines)
+--     api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
+--   end
+--
+--   it('lsp default formatter works', function()
+--     vim.wait(500)
+--     setlines(ill_c)
+--     gapi.fmt(bufnr)
+--     vim.wait(500)
+--     same({ 'int main(void) { return 0; }' }, getlines())
+--   end)
+-- end)