diff --git a/.github/workflows/pr_check.yml b/.github/workflows/pr_check.yml
index 8048e58..7687edc 100644
--- a/.github/workflows/pr_check.yml
+++ b/.github/workflows/pr_check.yml
@@ -39,5 +39,23 @@ jobs:
 
       - name: test
         run: |
-          luacheck lua
-
+          luacheck lua specs
+
+  tests:
+    strategy:
+      fail-fast: false
+      matrix:
+        version:
+          - stable
+          - nightly
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Neovim
+        uses: rhysd/action-setup-vim@v1
+        id: neovim
+        with:
+          neovim: true
+          version: ${{ matrix.version }}
+      - name: Run tests
+        run: make specs
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..22d0d82
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+vendor
diff --git a/.luacheckrc b/.luacheckrc
index 35b522b..7d82b11 100644
--- a/.luacheckrc
+++ b/.luacheckrc
@@ -1,2 +1,2 @@
-globals = { "vim", "_" }
+globals = { "vim", "_", "assert", "describe", "before_each", "after_each", "it" }
 max_line_length = false
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..f34e254
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,20 @@
+ifndef VERBOSE
+.SILENT:
+endif
+
+specs: dependencies
+	@echo "Running lsp-format specs..."
+	timeout 300 nvim -e \
+		--headless \
+		-u specs/minimal_init.vim \
+		-c "PlenaryBustedDirectory specs/features {minimal_init = 'specs/minimal_init.vim'}"
+
+dependencies:
+	if [ ! -d vendor ]; then \
+		git clone --depth 1 \
+			https://github.com/nvim-lua/plenary.nvim \
+			vendor/pack/vendor/start/plenary.nvim; \
+		git clone --depth 1 \
+			https://github.com/neovim/nvim-lspconfig \
+			vendor/pack/vendor/start/nvim-lspconfig; \
+	fi
diff --git a/README.md b/README.md
index 0ea58d7..5b4cea7 100644
--- a/README.md
+++ b/README.md
@@ -89,6 +89,10 @@ formatting.
 
 `sync` turns on synchronous formatting. The editor will block until the formatting is done.
 
+#### `force` format option
+
+`force` will write the format result to the buffer, even if the buffer changed after the format request started.
+
 ## Notes
 
 #### Make sure you remove any old format on save code
diff --git a/doc/format.txt b/doc/format.txt
index 4e72aae..e83f9a7 100644
--- a/doc/format.txt
+++ b/doc/format.txt
@@ -2,7 +2,7 @@
 
 
 Author: Lukas Reineke <lukas@reineke.jp>
-Version: 2.3.3
+Version: 2.4.0
 
 ==============================================================================
 CONTENTS                                                          *lsp-format*
@@ -87,6 +87,20 @@ sync                                                         *lsp-format-sync*
           }
       }
 
+------------------------------------------------------------------------------
+force                                                       *lsp-format-force*
+
+  `force` is a boolean flag. When on the format result will always be written
+  to the buffer, even if the buffer changed.
+
+  Example: >
+
+      require "lsp-format".setup {
+          go = {
+              force = true
+          }
+      }
+
 ==============================================================================
  4. COMMANDS                                             *lsp-format-commands*
 
@@ -121,6 +135,9 @@ sync                                                         *lsp-format-sync*
 ==============================================================================
  5. CHANGELOG                                           *lsp-format-changelog*
 
+2.4.0
+   Add `force` option
+
 2.3.3
    Use "vim.lsp.log" for logging
    Don't attach servers that don't support formatting
diff --git a/lua/lsp-format/init.lua b/lua/lsp-format/init.lua
index 8bcddf2..6449b83 100644
--- a/lua/lsp-format/init.lua
+++ b/lua/lsp-format/init.lua
@@ -29,6 +29,27 @@ M.setup = function(format_options)
     )
 end
 
+M._parse_value = function(key, value)
+    if not value then
+        return true
+    end
+    if key == "order" or key == "exclude" then
+        return vim.split(value, ",")
+    end
+    local int_value = tonumber(value)
+    if int_value then
+        return int_value
+    end
+    if value == "false" then
+        return false
+    end
+    if value == "true" then
+        return true
+    end
+
+    return value
+end
+
 M.format = function(options)
     if vim.b.format_saving or M.disabled or M.disabled_filetypes[vim.bo.filetype] then
         return
@@ -38,10 +59,7 @@ M.format = function(options)
     local format_options = vim.deepcopy(M.format_options[vim.bo.filetype] or {})
     for _, option in ipairs(options.fargs or {}) do
         local key, value = unpack(vim.split(option, "="))
-        if key == "order" or key == "exclude" then
-            value = vim.split(value, ",")
-        end
-        format_options[key] = value or true
+        format_options[key] = M._parse_value(key, value)
     end
 
     local clients = vim.tbl_values(vim.lsp.buf_get_clients())
@@ -144,10 +162,14 @@ M._handler = function(err, result, ctx)
     if not vim.api.nvim_buf_is_loaded(ctx.bufnr) then
         vim.fn.bufload(ctx.bufnr)
         vim.api.nvim_buf_set_var(ctx.bufnr, "format_changedtick", vim.api.nvim_buf_get_var(ctx.bufnr, "changedtick"))
-    elseif
-        vim.api.nvim_buf_get_var(ctx.bufnr, "format_changedtick")
-            ~= vim.api.nvim_buf_get_var(ctx.bufnr, "changedtick")
-        or vim.startswith(vim.api.nvim_get_mode().mode, "i")
+    end
+    if
+        not ctx.params.options.force
+        and (
+            vim.api.nvim_buf_get_var(ctx.bufnr, "format_changedtick")
+                ~= vim.api.nvim_buf_get_var(ctx.bufnr, "changedtick")
+            or vim.startswith(vim.api.nvim_get_mode().mode, "i")
+        )
     then
         M._next()
         return
diff --git a/specs/features/main_spec.lua b/specs/features/main_spec.lua
new file mode 100644
index 0000000..57494ce
--- /dev/null
+++ b/specs/features/main_spec.lua
@@ -0,0 +1,198 @@
+local mock = require "luassert.mock"
+local match = require "luassert.match"
+local spy = require "luassert.spy"
+local f = require "lsp-format"
+
+local mock_client = {
+    id = 1,
+    name = "lsp-client-test",
+    request = function(_, _, _, _) end,
+    request_sync = function(_, _, _, _) end,
+    supports_method = function(_) end,
+    setup = function() end,
+}
+
+vim.lsp.buf_get_clients = function()
+    local clients = {}
+    clients[mock_client.name] = mock_client
+    return clients
+end
+
+describe("lsp-format", function()
+    local c
+    local api
+
+    before_each(function()
+        c = mock(mock_client, true)
+        api = mock(vim.api)
+        c.supports_method = function(_)
+            return true
+        end
+        f.setup {}
+        f.on_attach(c)
+    end)
+
+    after_each(function()
+        mock.revert(c)
+        mock.revert(api)
+    end)
+
+    it("sends a valid format request", function()
+        f.format {}
+        assert.stub(c.request).was_called(1)
+        assert.stub(c.request).was_called_with("textDocument/formatting", {
+            options = {
+                insertSpaces = false,
+                tabSize = 8,
+            },
+            textDocument = {
+                uri = "file://",
+            },
+        }, match.is_ref(f._handler), 1)
+    end)
+
+    it("FormatToggle prevent/allow formatting", function()
+        f.toggle { args = "" }
+        f.format {}
+        assert.stub(c.request).was_called(0)
+
+        f.toggle { args = "" }
+        f.format {}
+        assert.stub(c.request).was_called(1)
+    end)
+
+    it("FormatDisable/Enable prevent/allow formatting", function()
+        f.disable { args = "" }
+        f.format {}
+        assert.stub(c.request).was_called(0)
+
+        f.enable { args = "" }
+        f.format {}
+        assert.stub(c.request).was_called(1)
+    end)
+
+    it("sends default format options", function()
+        f.setup {
+            lua = {
+                bool_test = true,
+                int_test = 1,
+                string_test = "string",
+            },
+        }
+        vim.bo.filetype = "lua"
+        f.format {}
+        assert.stub(c.request).was_called(1)
+        assert.stub(c.request).was_called_with("textDocument/formatting", {
+            options = {
+                insertSpaces = false,
+                tabSize = 8,
+                bool_test = true,
+                int_test = 1,
+                string_test = "string",
+            },
+            textDocument = {
+                uri = "file://",
+            },
+        }, match.is_ref(f._handler), 1)
+    end)
+
+    it("sends format options", function()
+        f.format {
+            fargs = { "bool_test", "int_test=1", "string_test=string" },
+        }
+        assert.stub(c.request).was_called(1)
+        assert.stub(c.request).was_called_with("textDocument/formatting", {
+            options = {
+                insertSpaces = false,
+                tabSize = 8,
+                bool_test = true,
+                int_test = 1,
+                string_test = "string",
+            },
+            textDocument = {
+                uri = "file://",
+            },
+        }, match.is_ref(f._handler), 1)
+    end)
+
+    it("overwrites default format options", function()
+        f.setup {
+            lua = {
+                bool_test = true,
+                int_test = 1,
+                string_test = "string",
+            },
+        }
+        vim.bo.filetype = "lua"
+        f.format {
+            fargs = { "bool_test=false", "int_test=2", "string_test=another_string" },
+        }
+        assert.stub(c.request).was_called(1)
+        assert.stub(c.request).was_called_with("textDocument/formatting", {
+            options = {
+                insertSpaces = false,
+                tabSize = 8,
+                bool_test = false,
+                int_test = 2,
+                string_test = "another_string",
+            },
+            textDocument = {
+                uri = "file://",
+            },
+        }, match.is_ref(f._handler), 1)
+    end)
+
+    it("does not overwrite changes", function()
+        local apply_text_edits = spy.on(vim.lsp.util, "apply_text_edits")
+        c.request = function(_, params, handler, bufnr)
+            api.nvim_buf_get_var = function(_, var)
+                if var == "format_changedtick" then
+                    return 9999
+                end
+                return 1
+            end
+            handler(nil, {}, { bufnr = bufnr, params = params })
+        end
+        f.format {}
+        assert.spy(apply_text_edits).was.called(0)
+    end)
+
+    it("does overwrite changes with force", function()
+        local apply_text_edits = spy.on(vim.lsp.util, "apply_text_edits")
+        c.request = function(_, params, handler, bufnr)
+            api.nvim_buf_get_var = function(_, var)
+                if var == "format_changedtick" then
+                    return 9999
+                end
+                return 1
+            end
+            handler(nil, {}, { bufnr = bufnr, params = params })
+        end
+        f.format { fargs = { "force=true" } }
+        assert.spy(apply_text_edits).was.called(1)
+    end)
+
+    it("does not overwrite when in insert mode", function()
+        local apply_text_edits = spy.on(vim.lsp.util, "apply_text_edits")
+        c.request = function(_, params, handler, bufnr)
+            api.nvim_get_mode = function()
+                return "insert"
+            end
+            handler(nil, {}, { bufnr = bufnr, params = params })
+        end
+        f.format {}
+        assert.spy(apply_text_edits).was.called(0)
+    end)
+
+    it("does overwrite when in insert mode with force", function()
+        local apply_text_edits = spy.on(vim.lsp.util, "apply_text_edits")
+        c.request = function(_, params, handler, bufnr)
+            api.nvim_get_mode = function()
+                return "insert"
+            end
+            handler(nil, {}, { bufnr = bufnr, params = params })
+        end
+        f.format { fargs = { "force=true" } }
+        assert.spy(apply_text_edits).was.called(1)
+    end)
+end)
diff --git a/specs/minimal_init.vim b/specs/minimal_init.vim
new file mode 100644
index 0000000..3507561
--- /dev/null
+++ b/specs/minimal_init.vim
@@ -0,0 +1,13 @@
+set rtp-=~/.config/nvim
+set rtp-=~/.local/share/nvim/site
+set rtp+=.
+set noswapfile
+
+let $lsp_format = getcwd()
+let $specs = getcwd() .. "/specs"
+let $vendor = getcwd() .. "/vendor"
+
+set rtp+=$lsp_format,$specs
+set packpath=$vendor
+
+packloadall