diff --git a/runtime/lua/nvim/spellfile.lua b/runtime/lua/nvim/spellfile.lua
index 51c7a90f84..604ca1b0b6 100644
--- a/runtime/lua/nvim/spellfile.lua
+++ b/runtime/lua/nvim/spellfile.lua
@@ -1,22 +1,23 @@
 local M = {}
 
---- @class SpellfileConfig
+--- @class vim.spellfile.Config
 --- @field url string
 --- @field timeout_ms integer
 
----@class SpellInfo
+---@class vim.spellfile.Info
 ---@field files string[]
 ---@field key string
 ---@field lang string
 ---@field encoding string
 ---@field dir string
 
----@type SpellfileConfig
+---@type vim.spellfile.Config
 M.config = {
   url = 'https://ftp.nluug.nl/pub/vim/runtime/spell',
   timeout_ms = 15000,
 }
 
+--- TODO(justinmk): add on_done/on_err callbacks to download(), instead of exposing this?
 ---@type table<string, boolean>
 M._done = {}
 
@@ -25,15 +26,6 @@ local function rtp_list()
   return vim.opt.rtp:get()
 end
 
-function M.isDone(key)
-  return M._done[key]
-end
-
-function M.setup(opts)
-  M._done = {}
-  M.config = vim.tbl_extend('force', M.config, opts or {})
-end
-
 local function notify(msg, level)
   vim.notify(msg, level or vim.log.levels.INFO)
 end
@@ -45,37 +37,43 @@ local function normalize_lang(lang)
   return (l:match('^[^,%s]+') or l)
 end
 
+local function file_ok(path)
+  local s = vim.uv.fs_stat(path)
+  return s and s.type == 'file' and (s.size or 0) > 0
+end
+
+local function can_use_dir(dir)
+  return not not (vim.fn.isdirectory(dir) == 1 and vim.uv.fs_access(dir, 'W'))
+end
+
 local function writable_spell_dirs_from_rtp()
   local dirs = {}
   for _, dir in ipairs(rtp_list()) do
-    local spell = vim.fs.joinpath(vim.fn.fnamemodify(dir, ':p'), 'spell')
-    if vim.fn.isdirectory(spell) == 1 and vim.uv.fs_access(spell, 'W') then
+    local spell = vim.fs.joinpath(vim.fs.abspath(dir), 'spell')
+    if can_use_dir(spell) then
       table.insert(dirs, spell)
     end
   end
   return dirs
 end
 
-local function default_spell_dir()
-  return vim.fs.joinpath(vim.fn.stdpath('data'), 'site', 'spell')
-end
-
 local function ensure_target_dir()
+  local dir = vim.fs.abspath(vim.fs.joinpath(vim.fn.stdpath('data'), 'site/spell'))
+  if vim.fn.isdirectory(dir) == 0 and pcall(vim.fn.mkdir, dir, 'p') then
+    notify('Created ' .. dir)
+  end
+  if can_use_dir(dir) then
+    return dir
+  end
+
+  -- Else, look for a spell/ dir in 'runtimepath'.
   local dirs = writable_spell_dirs_from_rtp()
   if #dirs > 0 then
     return dirs[1]
   end
-  local target = default_spell_dir()
-  if vim.fn.isdirectory(target) ~= 1 then
-    vim.fn.mkdir(target, 'p')
-    notify('Created ' .. target)
-  end
-  return target
-end
 
-local function file_ok(path)
-  local s = vim.uv.fs_stat(path)
-  return s and s.type == 'file' and (s.size or 0) > 0
+  dir = vim.fn.fnamemodify(dir, ':~')
+  error(('cannot find a writable spell/ dir in runtimepath, and %s is not usable'):format(dir))
 end
 
 local function reload_spell_silent()
@@ -86,9 +84,12 @@ local function reload_spell_silent()
   vim.cmd('echo ""')
 end
 
---- Blocking GET to file with timeout; treats status==0 as success if file exists.
+--- Fetch file via blocking HTTP GET and write to `outpath`.
+---
+--- Treats status==0 as success if file exists.
+---
 --- @return boolean ok, integer|nil status, string|nil err
-local function http_get_to_file_sync(url, outpath, timeout_ms)
+local function fetch_file_sync(url, outpath, timeout_ms)
   local done, err, res = false, nil, nil
   vim.net.request(url, { outpath = outpath }, function(e, r)
     err, res, done = e, r, true
@@ -99,43 +100,10 @@ local function http_get_to_file_sync(url, outpath, timeout_ms)
 
   local status = res and res.status or 0
   local ok = (not err) and ((status >= 200 and status < 300) or (status == 0 and file_ok(outpath)))
-  return ok, (status ~= 0 and status or nil), err
+  return not not ok, (status ~= 0 and status or nil), err
 end
 
----@return string[]
-function M.directory_choices()
-  local opts = {}
-  for _, dir in ipairs(rtp_list()) do
-    local spelldir = vim.fs.joinpath(vim.fn.fnamemodify(dir, ':p'), 'spell')
-    if vim.fn.isdirectory(spelldir) == 1 then
-      table.insert(opts, spelldir)
-    end
-  end
-  return opts
-end
-
-function M.choose_directory()
-  local dirs = writable_spell_dirs_from_rtp()
-  if #dirs == 0 then
-    return ensure_target_dir()
-  elseif #dirs == 1 then
-    return dirs[1]
-  end
-  local prompt ---@type string[]
-  prompt = {}
-  for i, d in
-    ipairs(dirs --[[@as string[] ]])
-  do
-    prompt[i] = string.format('%d: %s', i, d)
-  end
-  local choice = vim.fn.inputlist(prompt)
-  if choice < 1 or choice > #dirs then
-    return dirs[1]
-  end
-  return dirs[choice]
-end
-
-function M.parse(lang)
+local function parse(lang)
   local code = normalize_lang(lang)
   local enc = 'utf-8'
   local dir = ensure_target_dir()
@@ -160,8 +128,8 @@ function M.parse(lang)
   }
 end
 
----@param info SpellInfo
-function M.download(info)
+---@param info vim.spellfile.Info
+local function download(info)
   local dir = info.dir or ensure_target_dir()
   if not dir then
     notify('No (writable) spell directory found and could not create one.', vim.log.levels.ERROR)
@@ -178,7 +146,7 @@ function M.download(info)
   local url_utf8 = M.config.url .. '/' .. spl_utf8
   local out_utf8 = vim.fs.joinpath(dir, spl_utf8)
   notify('Downloading ' .. spl_utf8 .. ' …')
-  local ok, st, err = http_get_to_file_sync(url_utf8, out_utf8, M.config.timeout_ms)
+  local ok, st, err = fetch_file_sync(url_utf8, out_utf8, M.config.timeout_ms)
   if not ok then
     notify(
       ('Could not get %s (status %s): trying %s …'):format(
@@ -189,7 +157,7 @@ function M.download(info)
     )
     local url_ascii = M.config.url .. '/' .. spl_ascii
     local out_ascii = vim.fs.joinpath(dir, spl_ascii)
-    local ok2, st2, err2 = http_get_to_file_sync(url_ascii, out_ascii, M.config.timeout_ms)
+    local ok2, st2, err2 = fetch_file_sync(url_ascii, out_ascii, M.config.timeout_ms)
     if not ok2 then
       notify(
         ('No spell file available for %s (utf8:%s ascii:%s) — %s'):format(
@@ -217,7 +185,7 @@ function M.download(info)
     local url_sug = M.config.url .. '/' .. sug_name
     local out_sug = vim.fs.joinpath(dir, sug_name)
     notify('Downloading ' .. sug_name .. ' …')
-    local ok3, st3, err3 = http_get_to_file_sync(url_sug, out_sug, M.config.timeout_ms)
+    local ok3, st3, err3 = fetch_file_sync(url_sug, out_sug, M.config.timeout_ms)
     if ok3 then
       notify('Saved ' .. sug_name .. ' to ' .. out_sug)
     else
@@ -244,7 +212,7 @@ function M.download(info)
 end
 
 function M.load_file(lang)
-  local info = M.parse(lang)
+  local info = parse(lang)
   if #info.files == 0 then
     return
   end
@@ -260,19 +228,9 @@ function M.load_file(lang)
     return
   end
 
-  M.download(info)
-end
+  download(info)
 
-function M.exists(filename)
-  local stat = (vim.uv or vim.loop).fs_stat
-  for _, dir in ipairs(M.directory_choices()) do
-    local p = vim.fs.joinpath(dir, filename)
-    local s = stat(p)
-    if s and s.type == 'file' then
-      return true
-    end
-  end
-  return false
+  return info
 end
 
 return M
diff --git a/runtime/plugin/nvim/spellfile.lua b/runtime/plugin/nvim/spellfile.lua
index cafa914163..9d8bc44511 100644
--- a/runtime/plugin/nvim/spellfile.lua
+++ b/runtime/plugin/nvim/spellfile.lua
@@ -1,6 +1,7 @@
 vim.g.loaded_spellfile_plugin = true
 
---- Callback for SpellFileMissing: download missing .spl
+--- Downloads missing .spl file.
+---
 --- @param args { bufnr: integer, match: string }
 local function on_spellfile_missing(args)
   local spellfile = require('nvim.spellfile')
diff --git a/test/functional/lua/spellfile_spec.lua b/test/functional/lua/spellfile_spec.lua
deleted file mode 100644
index 5aadd87945..0000000000
--- a/test/functional/lua/spellfile_spec.lua
+++ /dev/null
@@ -1,160 +0,0 @@
-local n = require('test.functional.testnvim')()
-local t = require('test.testutil')
-
-local exec = n.exec
-local exec_lua = n.exec_lua
-local mkdir_p = n.mkdir_p
-local write_file = t.write_file
-local eq = t.eq
-
-describe('nvim.spellfile', function()
-  before_each(function()
-    n.clear()
-  end)
-
-  it('no-op when .spl and .sug already exist on rtp', function()
-    mkdir_p('Xplug/spell')
-    write_file('Xplug/spell/en_gb.utf-8.spl', 'dummy')
-    write_file('Xplug/spell/en_gb.utf-8.sug', 'dummy')
-    exec('set rtp+=' .. 'Xplug')
-
-    local out = exec_lua([[
-    local s = require('nvim.spellfile')
-
-    local my_spell = vim.fs.joinpath(vim.fn.fnamemodify('Xplug', ':p'), 'spell')
-    local old_access = vim.uv.fs_access
-    vim.uv.fs_access = function(p, mode)
-      return p == my_spell
-    end
-
-    local prompted = false
-    vim.fn.input = function() prompted = true; return 'n' end
-
-    local requests = 0
-    local orig_req = vim.net.request
-    vim.net.request = function(...) requests = requests + 1 end
-
-    s.load_file('en_gb')
-
-    vim.uv.fs_access = old_access
-    vim.net.request = orig_req
-
-    return { prompted = prompted, requests = requests }
-    ]])
-
-    eq(false, out.prompted)
-    eq(0, out.requests)
-  end)
-
-  it(
-    'downloads UTF-8 .spl to stdpath(data)/site/spell when no rtp spelldir; .sug 404 is non-fatal; reloads',
-    function()
-      mkdir_p('Xempty')
-      exec('set rtp+=' .. 'Xempty')
-
-      local out = exec_lua([[
-      local s = require('nvim.spellfile')
-
-      local data_root = 'Xdata'
-      vim.fn.stdpath = function(k)
-        assert(k == 'data')
-        return data_root
-      end
-
-      local old_access = vim.uv.fs_access
-      vim.uv.fs_access = function(_, _) return false end
-
-      vim.fn.input = function() return 'y' end
-
-      local reloaded = false
-      local orig_cmd = vim.cmd
-      vim.cmd = function(c)
-        if c:match('setlocal%s+spell!') then reloaded = true end
-        return orig_cmd(c)
-      end
-
-      local orig_req = vim.net.request
-      vim.net.request = function(url, opts, cb)
-        local name = url:match('/([^/]+)$')
-        if name and name:find('%.spl$') then
-          vim.fn.mkdir(vim.fs.dirname(opts.outpath), 'p')
-          vim.fn.writefile({'ok'}, opts.outpath)
-          cb(nil, { status = 200 })
-        else
-          cb(nil, { status = 404 })
-        end
-      end
-
-      s.load_file('en_gb')
-
-      local spl = vim.fs.joinpath(data_root, 'site', 'spell', 'en_gb.utf-8.spl')
-      local sug = vim.fs.joinpath(data_root, 'site', 'spell', 'en_gb.utf-8.sug')
-      local has_spl = vim.uv.fs_stat(spl) ~= nil
-      local has_sug = vim.uv.fs_stat(sug) ~= nil
-
-      vim.net.request = orig_req
-      vim.cmd = orig_cmd
-      vim.uv.fs_access = old_access
-
-      return { spl = has_spl, sug = has_sug, reloaded = reloaded }
-      ]])
-
-      eq(true, out.spl)
-      eq(false, out.sug)
-      eq(true, out.reloaded)
-    end
-  )
-
-  it('dual-fail: UTF-8 and ASCII 404 -> warn once, mark done, no reload', function()
-    mkdir_p('Xempty2')
-    exec('set rtp+=' .. 'Xempty2')
-
-    local out = exec_lua([[
-      local s = require('nvim.spellfile')
-
-      local data_root = 'Xdata2'
-      vim.fn.stdpath = function(k)
-        assert(k == 'data')
-        return data_root
-      end
-
-      local old_access = vim.uv.fs_access
-      vim.uv.fs_access = function(_, _) return false end
-      local old_stat = vim.uv.fs_stat
-      vim.uv.fs_stat = function(p) return old_stat and old_stat(p) or nil end
-
-      vim.fn.input = function() return 'y' end
-
-      local warns = 0
-      local orig_notify = vim.notify
-      vim.notify = function(_, lvl)
-        if lvl and lvl >= vim.log.levels.WARN then warns = warns + 1 end
-      end
-
-      local reloaded = false
-      local orig_cmd = vim.cmd
-      vim.cmd = function(c)
-        if c:match('setlocal%s+spell!') then reloaded = true end
-        return orig_cmd(c)
-      end
-
-      local orig_req = vim.net.request
-      vim.net.request = function(_, _, cb) cb(nil, { status = 404 }) end
-
-      local key = s.parse('zz').key
-      s.load_file('zz')
-      local done = (s.isDone(key)) == true
-
-      vim.net.request = orig_req
-      vim.notify = orig_notify
-      vim.cmd = orig_cmd
-      vim.uv.fs_access = old_access
-
-      return { warns = warns, done = done, reloaded = reloaded }
-    ]])
-
-    eq(1, out.warns)
-    eq(true, out.done)
-    eq(false, out.reloaded)
-  end)
-end)
diff --git a/test/functional/plugin/spellfile_spec.lua b/test/functional/plugin/spellfile_spec.lua
new file mode 100644
index 0000000000..c891028deb
--- /dev/null
+++ b/test/functional/plugin/spellfile_spec.lua
@@ -0,0 +1,151 @@
+local n = require('test.functional.testnvim')()
+local t = require('test.testutil')
+
+local eq = t.eq
+local exec_lua = n.exec_lua
+
+describe('nvim.spellfile', function()
+  local data_root = 'Xtest_data'
+  local rtp_dir = 'Xtest_rtp'
+
+  before_each(function()
+    n.clear()
+    n.exec('set runtimepath+=' .. rtp_dir)
+  end)
+  after_each(function()
+    n.rmdir(data_root)
+    n.rmdir(rtp_dir)
+  end)
+
+  it('no-op when .spl and .sug already exist on runtimepath', function()
+    local my_spell = vim.fs.joinpath(vim.fs.abspath(rtp_dir), 'spell')
+    n.mkdir_p(my_spell)
+    t.retry(nil, nil, function()
+      assert(vim.uv.fs_stat(my_spell))
+    end)
+    t.write_file(my_spell .. '/en_gb.utf-8.spl', 'dummy')
+    t.write_file(my_spell .. '/en_gb.utf-8.sug', 'dummy')
+
+    local out = exec_lua(
+      [[
+      local rtp_dir = ...
+      local s = require('nvim.spellfile')
+      local my_spell = vim.fs.joinpath(vim.fs.abspath(rtp_dir), 'spell')
+
+      vim.uv.fs_access = function(p, mode)
+        return p == my_spell
+      end
+
+      local prompted = false
+      vim.fn.input = function() prompted = true; return 'n' end
+
+      local requests = 0
+      vim.net.request = function(...) requests = requests + 1 end
+
+      s.load_file('en_gb')
+
+      return { prompted = prompted, requests = requests }
+    ]],
+      rtp_dir
+    )
+
+    eq(false, out.prompted)
+    eq(0, out.requests)
+  end)
+
+  it('downloads .spl to stdpath(data)/site/spell, .sug 404 is non-fatal, reloads', function()
+    n.mkdir_p(rtp_dir)
+
+    local out = exec_lua(
+      [[
+        local data_root = ...
+        local s = require('nvim.spellfile')
+
+        vim.fn.stdpath = function(k)
+          assert(k == 'data')
+          return data_root
+        end
+
+        vim.fn.input = function() return 'y' end
+
+        local did_reload = false
+        local orig_cmd = vim.cmd
+        vim.cmd = function(cmd)
+          if cmd:match('setlocal%s+spell!') then
+            did_reload = true
+          end
+          return orig_cmd(cmd)
+        end
+
+        vim.net.request = function(url, opts, cb)
+          local name = url:match('/([^/]+)$')
+          if name and name:find('%.spl$') then
+            vim.fn.mkdir(vim.fs.dirname(opts.outpath), 'p')
+            vim.fn.writefile({'ok'}, opts.outpath)
+            cb(nil, { status = 200 })
+          else
+            cb(nil, { status = 404 })
+          end
+        end
+
+        s.load_file('en_gb')
+
+        local spl = vim.fs.joinpath(data_root, 'site/spell/en_gb.utf-8.spl')
+        local sug = vim.fs.joinpath(data_root, 'site/spell/en_gb.utf-8.sug')
+
+        return {
+          has_spl = vim.uv.fs_stat(spl) ~= nil,
+          has_sug = vim.uv.fs_stat(sug) ~= nil,
+          did_reload = did_reload,
+        }
+      ]],
+      data_root
+    )
+
+    eq(true, out.has_spl)
+    eq(false, out.has_sug)
+    eq(true, out.did_reload)
+  end)
+
+  it('failure mode: 404 for all files => warn once, mark done, no reload', function()
+    local out = exec_lua(
+      [[
+      local data_root = ...
+      local s = require('nvim.spellfile')
+
+      vim.fn.stdpath = function(k)
+        assert(k == 'data')
+        return data_root
+      end
+
+      vim.fn.input = function() return 'y' end
+
+      local warns = 0
+      vim.notify = function(_, lvl)
+        if lvl and lvl >= vim.log.levels.WARN then warns = warns + 1 end
+      end
+
+      local did_reload = false
+      local orig_cmd = vim.cmd
+      vim.cmd = function(c)
+        if c:match('setlocal%s+spell!') then
+          did_reload = true
+        end
+        return orig_cmd(c)
+      end
+
+      vim.net.request = function(_, _, cb) cb(nil, { status = 404 }) end
+
+      local info = s.load_file('zz')
+      local done = s._done[info.key] == true
+
+      return { warns = warns, done = done, did_reload = did_reload }
+    ]],
+      data_root
+    )
+
+    eq(1, out.warns)
+    eq(true, out.done)
+    eq(false, out.did_reload)
+  end)
+end)
diff --git a/test/functional/testnvim.lua b/test/functional/testnvim.lua
index 0449d3d5dc..abb2ffd86c 100644
--- a/test/functional/testnvim.lua
+++ b/test/functional/testnvim.lua
@@ -1017,7 +1017,9 @@ end
 --- @param path string
 --- @return boolean?
 function M.mkdir_p(path)
-  return os.execute((is_os('win') and 'mkdir ' .. path or 'mkdir -p ' .. path))
+  return os.execute(
+    (is_os('win') and 'mkdir ' .. string.gsub(path, '/', '\\') or 'mkdir -p ' .. path)
+  )
 end
 
 local testid = (function()