diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 7b4364e..b6d6755 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -184,7 +184,7 @@ jobs:
       - uses: ./.github/actions/test-setup
       - uses: cachix/install-nix-action@v27
       - name: Install tools
-        run: nix profile install nixpkgs#nixfmt-rfc-style
+        run: nix profile install nixpkgs#nixfmt
       - name: Run tests
         run: |
           export LUA_PATH="lua/?.lua;lua/?/init.lua;$LUA_PATH"
diff --git a/README.md b/README.md
index a2734e4..d7780da 100644
--- a/README.md
+++ b/README.md
@@ -36,7 +36,7 @@
 - [latexindent](https://github.com/cmhughes/latexindent.pl)
 - [mdformat](https://github.com/executablebooks/mdformat)
 - [mixformat](https://github.com/elixir-lang/elixir/)
-- [nixfmt](https://github.com/serokell/nixfmt)
+- [nixfmt](https://github.com/NixOS/nixfmt)
 - [npm_groovy_lint](https://github.com/nvuillam/npm-groovy-lint) as `npm-groovy-lint --format`
 - [npm_groovy_lint_fix](https://github.com/nvuillam/npm-groovy-lint) as `npm-groovy-lint --fix`
 - [ormolu](https://hackage.haskell.org/package/ormolu)
diff --git a/lua/guard-collection/formatter.lua b/lua/guard-collection/formatter.lua
index ff61fe8..a597d4e 100644
--- a/lua/guard-collection/formatter.lua
+++ b/lua/guard-collection/formatter.lua
@@ -186,29 +186,9 @@ M.prettier = {
 }
 
 M.prettierd = {
-  fn = function(buf, range)
-    if vim.fn.has('nvim-0.10') ~= 1 then
-      vim.notify('[guard-collection]: prettierd uses vim.system introduced in nvim 0.10', 4)
-      return
-    end
-    local srow = range and range['start'][1] or 0
-    local erow = range and range['end'][1] or -1
-    local handle = vim.system(
-      { 'prettierd', vim.api.nvim_buf_get_name(buf) },
-      {
-        stdin = true,
-      },
-      vim.schedule_wrap(function(result)
-        if result.code ~= 0 then
-          return
-        end
-        vim.api.nvim_buf_set_lines(buf, srow, erow, false, vim.split(result.stdout, '\r?\n'))
-        vim.cmd('silent! noautocmd write!')
-      end)
-    )
-    handle:write(table.concat(vim.api.nvim_buf_get_lines(buf, srow, erow, false), '\n'))
-    handle:write(nil)
-  end,
+  cmd = 'prettierd',
+  stdin = true,
+  fname = true,
 }
 
 M.rubocop = {
diff --git a/lua/guard-collection/linter/checkmake.lua b/lua/guard-collection/linter/checkmake.lua
index af62824..565bc01 100644
--- a/lua/guard-collection/linter/checkmake.lua
+++ b/lua/guard-collection/linter/checkmake.lua
@@ -1,17 +1,9 @@
 local lint = require('guard.lint')
 
 return {
-  fn = function(_, fname)
-    local co = assert(coroutine.running())
-    vim.system({
-      'checkmake',
-      '--format={{.FileName}}:{{.LineNumber}}: [{{.Rule}}] {{.Violation}}\n',
-      fname,
-    }, {}, function(result)
-      coroutine.resume(co, result.stdout or '')
-    end)
-    return coroutine.yield()
-  end,
+  cmd = 'checkmake',
+  args = { '--format={{.FileName}}:{{.LineNumber}}: [{{.Rule}}] {{.Violation}}\n' },
+  fname = true,
   parse = function(result, bufnr)
     local diags = {}
     for line in result:gmatch('[^\n]+') do
diff --git a/test/binary/checkmake_spec.lua b/test/binary/checkmake_spec.lua
index adda48a..12b5166 100644
--- a/test/binary/checkmake_spec.lua
+++ b/test/binary/checkmake_spec.lua
@@ -1,25 +1,13 @@
 describe('checkmake', function()
   it('can lint', function()
-    local linter = require('test.helper').get_linter('checkmake')
-    local tmpfile = '/tmp/guard-test.make'
-    local input = {
+    local helper = require('test.helper')
+    local buf, diagnostics = helper.run_lint('checkmake', 'make', {
       [[all:]],
       [[\techo hello]],
-    }
-    vim.fn.writefile(input, tmpfile)
-    local bufnr = vim.api.nvim_create_buf(false, true)
-    local result = vim
-      .system({
-        'checkmake',
-        '--format={{.FileName}}:{{.LineNumber}}: [{{.Rule}}] {{.Violation}}\n',
-        tmpfile,
-      }, {})
-      :wait()
-    local output = result.stdout or ''
-    local diagnostics = linter.parse(output, bufnr)
+    })
     assert.is_true(#diagnostics > 0)
     for _, d in ipairs(diagnostics) do
-      assert.equal(bufnr, d.bufnr)
+      assert.equal(buf, d.bufnr)
       assert.equal('checkmake', d.source)
       assert.is_number(d.lnum)
       assert.is_string(d.message)
diff --git a/test/binary/zsh_spec.lua b/test/binary/zsh_spec.lua
index 78332d2..5e40c7c 100644
--- a/test/binary/zsh_spec.lua
+++ b/test/binary/zsh_spec.lua
@@ -1,16 +1,10 @@
 describe('zsh', function()
   it('can lint', function()
-    local linter = require('test.helper').get_linter('zsh')
-    local tmpfile = '/tmp/guard-test.zsh'
-    vim.fn.writefile({ 'if true; then' }, tmpfile)
-    local bufnr = vim.api.nvim_create_buf(false, true)
-    local result = vim.system({ 'zsh', '-n', tmpfile }, {}):wait()
-    local output = result.stderr or ''
-    local diagnostics = linter.parse(output, bufnr)
+    local helper = require('test.helper')
+    local bufnr, diagnostics = helper.run_lint_fn('zsh', 'zsh', { 'if true; then' })
     assert.is_true(#diagnostics > 0)
     for _, d in ipairs(diagnostics) do
-      assert.equal(bufnr, d.bufnr)
-      assert.equal('zsh', d.source)
+      helper.assert_diag(d, { bufnr = bufnr, source = 'zsh' })
       assert.is_number(d.lnum)
       assert.is_string(d.message)
     end
diff --git a/test/helper.lua b/test/helper.lua
index d57204a..1b9b55b 100644
--- a/test/helper.lua
+++ b/test/helper.lua
@@ -55,6 +55,28 @@ function M.run_lint(name, ft, input, opts)
   return bufnr, diags
 end
 
+function M.run_lint_fn(name, ft, input, opts)
+  opts = opts or {}
+  local linter = require('guard-collection.linter')[name]
+  assert(linter, 'unknown linter: ' .. name)
+  assert(linter.fn, name .. ' does not use custom fn')
+  local dir = opts.tmpdir or '/tmp'
+  local tmpfile = dir .. '/guard-test.' .. ft
+  vim.fn.writefile(input, tmpfile)
+  local bufnr = api.nvim_create_buf(false, true)
+  local output
+  local co = coroutine.create(function()
+    output = linter.fn(nil, tmpfile)
+  end)
+  coroutine.resume(co)
+  vim.wait(5000, function()
+    return output ~= nil
+  end)
+  assert(output ~= nil, name .. ' fn timed out')
+  local diags = linter.parse(output, bufnr)
+  return bufnr, diags
+end
+
 function M.assert_diag(d, expect)
   local a = require('luassert')
   if expect.bufnr then
diff --git a/test/npm/prettierd_spec.lua b/test/npm/prettierd_spec.lua
index 61cecd6..a13b997 100644
--- a/test/npm/prettierd_spec.lua
+++ b/test/npm/prettierd_spec.lua
@@ -1,20 +1,12 @@
 describe('prettierd', function()
   it('can format', function()
-    local tmpfile = '/tmp/guard-test.js'
-    local input = {
+    local formatted = require('test.helper').run_fmt('prettierd', 'js', {
       'const x={a:1,b:2,c:3}',
       'const y = [1,2,3,4,5]',
-    }
-    vim.fn.writefile(input, tmpfile)
-    local result = vim
-      .system({ 'prettierd', tmpfile }, { stdin = table.concat(input, '\n') })
-      :wait()
-    assert.equal(0, result.code)
-    local expected = table.concat({
+    })
+    assert.are.same({
       'const x = { a: 1, b: 2, c: 3 };',
       'const y = [1, 2, 3, 4, 5];',
-      '',
-    }, '\n')
-    assert.equal(expected, result.stdout)
+    }, formatted)
   end)
 end)
diff --git a/test/pip/cpplint_spec.lua b/test/pip/cpplint_spec.lua
index cb0095a..452f3ad 100644
--- a/test/pip/cpplint_spec.lua
+++ b/test/pip/cpplint_spec.lua
@@ -1,17 +1,10 @@
 describe('cpplint', function()
   it('can lint', function()
-    local linter = require('test.helper').get_linter('cpplint')
-    local tmpfile = '/tmp/guard-test.cpp'
-    local input = { [[int main(){int x=1;}]] }
-    vim.fn.writefile(input, tmpfile)
-    local bufnr = vim.api.nvim_create_buf(false, true)
-    local result = vim.system({ 'cpplint', '--filter=-legal/copyright', tmpfile }, {}):wait()
-    local output = result.stderr or ''
-    local diagnostics = linter.parse(output, bufnr)
+    local helper = require('test.helper')
+    local bufnr, diagnostics = helper.run_lint_fn('cpplint', 'cpp', { [[int main(){int x=1;}]] })
     assert.is_true(#diagnostics > 0)
     for _, d in ipairs(diagnostics) do
-      assert.equal(bufnr, d.bufnr)
-      assert.equal('cpplint', d.source)
+      helper.assert_diag(d, { bufnr = bufnr, source = 'cpplint' })
       assert.is_number(d.lnum)
       assert.is_string(d.message)
     end