diff --git a/.luarc.json b/.luarc.json
index ece73b80..4149a4f2 100644
--- a/.luarc.json
+++ b/.luarc.json
@@ -8,6 +8,7 @@
       "lua",
       "$VIMRUNTIME",
       "${3rd}/busted/library",
+      "${3rd}/luassert/library",
       "${3rd}/luv/library"
     ],
     "checkThirdParty": false
diff --git a/test/client/session.lua b/test/client/session.lua
index 75179ad8..4b43637e 100644
--- a/test/client/session.lua
+++ b/test/client/session.lua
@@ -1,11 +1,11 @@
-local uv = require('luv')
+local uv = vim.loop
 local MsgpackRpcStream = require('test.client.msgpack_rpc_stream')
 
---- @class Session
+--- @class NvimSession
 --- @field private _msgpack_rpc_stream MsgpackRpcStream
 --- @field private _pending_messages string[]
---- @field private _prepare uv_prepare_t
---- @field private _timer uv_timer_t
+--- @field private _prepare uv.uv_prepare_t
+--- @field private _timer uv.uv_timer_t
 --- @field private _is_running boolean
 local Session = {}
 Session.__index = Session
@@ -57,7 +57,17 @@ function Session.new(stream)
   }, Session)
 end
 
+--- @param timeout integer
+--- @return string
 function Session:next_message(timeout)
+  if self._is_running then
+    error('Event loop already running')
+  end
+
+  if #self._pending_messages > 0 then
+    return table.remove(self._pending_messages, 1)
+  end
+
   local function on_request(method, args, response)
     table.insert(self._pending_messages, {'request', method, args, response})
     uv.stop()
@@ -68,14 +78,6 @@ function Session:next_message(timeout)
     uv.stop()
   end
 
-  if self._is_running then
-    error('Event loop already running')
-  end
-
-  if #self._pending_messages > 0 then
-    return table.remove(self._pending_messages, 1)
-  end
-
   self:_run(on_request, on_notification, timeout)
   return table.remove(self._pending_messages, 1)
 end
@@ -100,6 +102,10 @@ function Session:request(method, ...)
   return true, result
 end
 
+---@param request_cb fun()?
+---@param notification_cb fun()?
+---@param setup_cb fun()?
+---@param timeout integer?
 function Session:run(request_cb, notification_cb, setup_cb, timeout)
   local function on_request(method, args, response)
     coroutine_exec(request_cb, method, args, function(status, result, flag)
diff --git a/test/gitsigns_spec.lua b/test/gitsigns_spec.lua
index eddce370..21755240 100644
--- a/test/gitsigns_spec.lua
+++ b/test/gitsigns_spec.lua
@@ -35,8 +35,8 @@ local eq              = helpers.eq
 helpers.env()
 
 describe('gitsigns', function()
-  local screen
-  local config
+  local screen --- @type NvimScreen
+  local config --- @type table
 
   before_each(function()
     clear()
diff --git a/test/helpers.lua b/test/helpers.lua
index d35855df..8e7d048c 100644
--- a/test/helpers.lua
+++ b/test/helpers.lua
@@ -1,6 +1,5 @@
 local assert = require('luassert')
-local busted = require('busted')
-local luv = require('luv')
+local luv = vim.loop
 local Session = require('test.client.session')
 local uv_stream = require('test.client.uv_stream')
 local ChildProcessStream = uv_stream.ChildProcessStream
@@ -14,36 +13,16 @@ function M.sleep(ms)
   luv.sleep(ms)
 end
 
--- Calls fn() until it succeeds, up to `max` times or until `max_ms`
--- milliseconds have passed.
-function M.retry(max, max_ms, fn)
-  assert(max == nil or max > 0)
-  assert(max_ms == nil or max_ms > 0)
-  local tries = 1
-  local timeout = (max_ms and max_ms or 10000)
-  local start_time = luv.now()
-  while true do
-    local status, result = pcall(fn)
-    if status then
-      return result
-    end
-    luv.update_time()  -- Update cached value of luv.now() (libuv: uv_now()).
-    if (max and tries >= max) or (luv.now() - start_time > timeout) then
-      busted.fail(string.format("retry() attempts: %d\n%s", tries, tostring(result)), 2)
-    end
-    tries = tries + 1
-    luv.sleep(20)  -- Avoid hot loop...
-  end
-end
-
 M.eq = assert.are.same
 M.neq = assert.are_not.same
 
 local function epicfail(state, arguments, _)
+  --- @diagnostic disable-next-line
   state.failure_message = arguments[1]
   return false
 end
 
+--- @diagnostic disable-next-line:missing-parameter
 assert:register("assertion", "epicfail", epicfail)
 
 function M.matches(pat, actual)
@@ -53,17 +32,21 @@ function M.matches(pat, actual)
   error(string.format('Pattern does not match.\nPattern:\n%s\nActual:\n%s', pat, actual))
 end
 
--- Reads text lines from `filename` into a table.
---
--- filename: path to file
--- start: start line (1-indexed), negative means "lines before end" (tail)
+--- Reads text lines from `filename` into a table.
+---
+--- filename: path to file
+--- start: start line (1-indexed), negative means "lines before end" (tail)
+--- @param filename string
+--- @param start? integer
+--- @return string[]?
 local function read_file_list(filename, start)
-  local lnum = (start ~= nil and type(start) == 'number') and start or 1
-  local tail = (lnum < 0)
+  local lnum = start or 1
+  local tail = lnum < 0
   local maxlines = tail and math.abs(lnum) or nil
   local file = io.open(filename, 'r')
+
   if not file then
-    return nil
+    return
   end
 
   -- There is no need to read more than the last 2MB of the log file, so seek
@@ -75,10 +58,10 @@ local function read_file_list(filename, start)
   end
   file:seek("set", offset)
 
-  local lines = {}
+  local lines = {} --- @type string[]
   local i = 1
   local line = file:read("*l")
-  while line ~= nil do
+  while line do
     if i >= start then
       table.insert(lines, line)
       if #lines > maxlines then
@@ -92,6 +75,11 @@ local function read_file_list(filename, start)
   return lines
 end
 
+--- @generic R
+--- @param fn fun(...): R
+--- @param ... any arguments
+--- @return boolean
+--- @return R|string
 function M.pcall(fn, ...)
   assert(type(fn) == 'function')
   local status, rv = pcall(fn, ...)
@@ -119,19 +107,23 @@ function M.pcall(fn, ...)
   return status, errmsg
 end
 
--- Invokes `fn` and returns the error string (with truncated paths), or raises
--- an error if `fn` succeeds.
---
--- Replaces line/column numbers with zero:
---     shared.lua:0: in function 'gsplit'
---     shared.lua:0: in function <shared.lua:0>'
---
--- Usage:
---    -- Match exact string.
---    eq('e', pcall_err(function(a, b) error('e') end, 'arg1', 'arg2'))
---    -- Match Lua pattern.
---    matches('e[or]+$', pcall_err(function(a, b) error('some error') end, 'arg1', 'arg2'))
---
+--- Invokes `fn` and returns the error string (with truncated paths), or raises
+--- an error if `fn` succeeds.
+---
+--- Replaces line/column numbers with zero:
+---     shared.lua:0: in function 'gsplit'
+---     shared.lua:0: in function <shared.lua:0>'
+---
+--- Usage:
+---    -- Match exact string.
+---    eq('e', pcall_err(function(a, b) error('e') end, 'arg1', 'arg2'))
+---    -- Match Lua pattern.
+---    matches('e[or]+$', pcall_err(function(a, b) error('some error') end, 'arg1', 'arg2'))
+---
+--- @generic R
+--- @param fn fun(...): R
+--- @param ... any arguments
+--- @return R
 local function pcall_err_withfile(fn, ...)
   assert(type(fn) == 'function')
   local status, rv = M.pcall(fn, ...)
@@ -171,9 +163,12 @@ function M.concat_tables(...)
   return ret
 end
 
+--- @param str string
+--- @param leave_indent? integer
+--- @return string
 function M.dedent(str, leave_indent)
   -- find minimum common indent across lines
-  local indent = nil
+  local indent = nil --- @type string?
   for line in str:gmatch('[^\n]+') do
     local line_indent = line:match('^%s+') or ''
     if indent == nil or #line_indent < #indent then
@@ -240,31 +235,23 @@ local nvim_argv = {
   '--cmd', nvim_set,
   '--cmd', 'mapclear',
   '--cmd', 'mapclear!',
-  '--embed'
+  '--embed',
+  '--headless'
 }
 
-local prepend_argv
-
-if prepend_argv then
-  local new_nvim_argv = {}
-  local len = #prepend_argv
-  for i = 1, len do
-    new_nvim_argv[i] = prepend_argv[i]
-  end
-  for i = 1, #nvim_argv do
-    new_nvim_argv[i + len] = nvim_argv[i]
-  end
-  nvim_argv = new_nvim_argv
-  M.prepend_argv = prepend_argv
-end
-
-local session, loop_running, last_error
+local session --- @type NvimSession?
+local loop_running = false
+local last_error --- @type string?
 
 function M.get_session()
   return session
 end
 
+--- @param method string
+--- @param ... any
+--- @return any[]
 local function request(method, ...)
+  assert(session)
   local status, rv = session:request(method, ...)
   if not status then
     if loop_running then
@@ -287,8 +274,14 @@ local function call_and_stop_on_error(lsession, ...)
   return result
 end
 
+--- @param lsession NvimSession
+--- @param request_cb fun()?
+--- @param notification_cb fun()?
+--- @param timeout integer
+--- @return unknown
 function M.run_session(lsession, request_cb, notification_cb, timeout)
-  local on_request, on_notification
+  local on_request --- @type fun()
+  local on_notification --- @type fun()
 
   if request_cb then
     function on_request(method, args)
@@ -314,20 +307,26 @@ function M.run_session(lsession, request_cb, notification_cb, timeout)
   return lsession.eof_err
 end
 
----- Executes an ex-command. VimL errors manifest as client (lua) errors, but
----- v:errmsg will not be updated.
+--- Executes an ex-command. VimL errors manifest as client (lua) errors, but
+--- v:errmsg will not be updated.
+--- @param cmd string
 function M.command(cmd)
   request('nvim_command', cmd)
 end
 
----- Evaluates a VimL expression.
----- Fails on VimL error, but does not update v:errmsg.
+--- Evaluates a VimL expression.
+--- Fails on VimL error, but does not update v:errmsg.
+--- @param expr string
+--- @return any[]
 function M.eval(expr)
   return request('nvim_eval', expr)
 end
 
----- Executes a VimL function via RPC.
----- Fails on VimL error, but does not update v:errmsg.
+--- Executes a VimL function via RPC.
+--- Fails on VimL error, but does not update v:errmsg.
+--- @param name string
+--- @param ... any
+--- @return any[]
 function M.call(name, ...)
   return request('nvim_call_function', name, {...})
 end
@@ -337,8 +336,9 @@ local function assert_alive()
   assert(2 == M.eval('1+1'), 'crash? request failed')
 end
 
----- Sends user input to Nvim.
----- Does not fail on VimL error, but v:errmsg will be updated.
+--- Sends user input to Nvim.
+--- Does not fail on VimL error, but v:errmsg will be updated.
+--- @param input string
 local function nvim_feed(input)
   while #input > 0 do
     local written = request('nvim_input', input)
@@ -350,77 +350,21 @@ local function nvim_feed(input)
   end
 end
 
+--- @param ... string
 function M.feed(...)
   for _, v in ipairs({...}) do
     nvim_feed(M.dedent(v))
   end
 end
 
+--- @param ... string
 local function rawfeed(...)
   for _, v in ipairs({...}) do
     nvim_feed(M.dedent(v))
   end
 end
 
-local function merge_args(...)
-  local i = 1
-  local argv = {}
-  for anum = 1,select('#', ...) do
-    local args = select(anum, ...)
-    if args then
-      for _, arg in ipairs(args) do
-        argv[i] = arg
-        i = i + 1
-      end
-    end
-  end
-  return argv
-end
-
---  Removes Nvim startup args from `args` matching items in `args_rm`.
---
---  - Special case: "-u", "-i", "--cmd" are treated specially: their "values" are also removed.
---  - Special case: "runtimepath" will remove only { '--cmd', 'set runtimepath^=…', }
---
---  Example:
---      args={'--headless', '-u', 'NONE'}
---      args_rm={'--cmd', '-u'}
---  Result:
---      {'--headless'}
---
---  All matching cases are removed.
---
---  Example:
---      args={'--cmd', 'foo', '-N', '--cmd', 'bar'}
---      args_rm={'--cmd', '-u'}
---  Result:
---      {'-N'}
-local function remove_args(args, args_rm)
-  local new_args = {}
-  local skip_following = {'-u', '-i', '-c', '--cmd', '-s', '--listen'}
-  if not args_rm or #args_rm == 0 then
-    return {unpack(args)}
-  end
-  for _, v in ipairs(args_rm) do
-    assert(type(v) == 'string')
-  end
-  local last = ''
-  for _, arg in ipairs(args) do
-    if vim.tbl_contains(skip_following, last) then
-      last = ''
-    elseif vim.tbl_contains(args_rm, arg) then
-      last = arg
-    elseif arg == runtime_set and vim.tbl_contains(args_rm, 'runtimepath') then
-      table.remove(new_args)  -- Remove the preceding "--cmd".
-      last = ''
-    else
-      table.insert(new_args, arg)
-    end
-  end
-  return new_args
-end
-
-function M.check_close()
+local function check_close()
   if not session then
     return
   end
@@ -437,100 +381,14 @@ function M.check_close()
   session = nil
 end
 
---- @param io_extra used for stdin_fd, see :help ui-option
-function M.spawn(argv, merge, env, keep, io_extra)
-  if not keep then
-    M.check_close()
-  end
-
-  local child_stream = ChildProcessStream.spawn(
-      merge and merge_args(prepend_argv, argv) or argv,
-      env, io_extra)
-  return Session.new(child_stream)
-end
-
--- Builds an argument list for use in clear().
---
----@see clear() for parameters.
-local function new_argv(...)
-  local args = {unpack(nvim_argv)}
-  table.insert(args, '--headless')
-  if _G._nvim_test_id then
-    -- Set the server name to the test-id for logging. #8519
-    table.insert(args, '--listen')
-    table.insert(args, _G._nvim_test_id)
-  end
-  local new_args
-  local io_extra
-  local env = nil
-  local opts = select(1, ...)
-  if type(opts) ~= 'table' then
-    new_args = {...}
-  else
-    args = remove_args(args, opts.args_rm)
-    if opts.env then
-      local env_opt = {}
-      for k, v in pairs(opts.env) do
-        assert(type(k) == 'string')
-        assert(type(v) == 'string')
-        env_opt[k] = v
-      end
-      for _, k in ipairs({
-        'HOME',
-        'ASAN_OPTIONS',
-        'TSAN_OPTIONS',
-        'MSAN_OPTIONS',
-        'LD_LIBRARY_PATH',
-        'PATH',
-        'NVIM_LOG_FILE',
-        'NVIM_RPLUGIN_MANIFEST',
-        'GCOV_ERROR_FILE',
-        'XDG_DATA_DIRS',
-        'TMPDIR',
-        'VIMRUNTIME',
-      }) do
-        -- Set these from the environment unless the caller defined them.
-        if not env_opt[k] then
-          env_opt[k] = os.getenv(k)
-        end
-      end
-      env = {}
-      for k, v in pairs(env_opt) do
-        env[#env + 1] = k .. '=' .. v
-      end
-    end
-    new_args = opts.args or {}
-    io_extra = opts.io_extra
-  end
-  for _, arg in ipairs(new_args) do
-    table.insert(args, arg)
-  end
-  return args, env, io_extra
-end
-
--- same params as clear, but does returns the session instead
--- of replacing the default session
-local function spawn_argv(keep, ...)
-  local argv, env, io_extra = new_argv(...)
-  return M.spawn(argv, nil, env, keep, io_extra)
-end
-
--- Starts a new global Nvim session.
---
--- Parameters are interpreted as startup args, OR a map with these keys:
---    args:       List: Args appended to the default `nvim_argv` set.
---    args_rm:    List: Args removed from the default set. All cases are
---                removed, e.g. args_rm={'--cmd'} removes all cases of "--cmd"
---                (and its value) from the default set.
---    env:        Map: Defines the environment of the new session.
---
--- Example:
---    clear('-e')
---    clear{args={'-e'}, args_rm={'-i'}, env={TERM=term}}
-function M.clear(...)
-  session = spawn_argv(false, ...)
+--- Starts a new global Nvim session.
+function M.clear()
+  check_close()
+  local child_stream = ChildProcessStream.spawn(nvim_argv)
+  session = Session.new(child_stream)
 end
 
+---@param ... string
 function M.insert(...)
   nvim_feed('i')
   for _, v in ipairs({...}) do
@@ -541,8 +399,7 @@ function M.insert(...)
 end
 
 function M.create_callindex(func)
-  local table = {}
-  setmetatable(table, {
+  return setmetatable({}, {
     __index = function(tbl, arg1)
       local ret = function(...)
         return func(arg1, ...)
@@ -551,7 +408,6 @@ function M.create_callindex(func)
       return ret
     end,
   })
-  return table
 end
 
 function M.nvim(method, ...)
@@ -609,7 +465,7 @@ function M.exec_lua(code, ...)
   return M.meths.exec_lua(code, {...})
 end
 
---- @param after_each fun(name:string,block:fun())
+--- @param after_each fun(block:fun())
 function M.after_each(after_each)
   after_each(function()
     if not session then
diff --git a/test/screen.lua b/test/screen.lua
index 3c173e9f..5be916c8 100644
--- a/test/screen.lua
+++ b/test/screen.lua
@@ -88,6 +88,7 @@ local function isempty(v)
   return type(v) == 'table' and next(v) == nil
 end
 
+--- @class NvimScreen
 local Screen = {}
 Screen.__index = Screen