diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 09fd4aa..241084c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -12,7 +12,18 @@ jobs:
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
           version: latest
-          args: --color always --check lua/
+          args: --color always --check lua/ tests/
+  test:
+    name: test
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+      - uses: rhysd/action-setup-vim@v1
+        with:
+          neovim: true
+          version: v0.10.0
+      - run: make test
   docs:
     name: vimdoc
     runs-on: ubuntu-latest
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..b2165ec
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,14 @@
+PLENARY_DIR ?= /tmp/plenary.nvim
+
+.PHONY: test plenary lint
+
+plenary:
+	@if [ ! -d "$(PLENARY_DIR)" ]; then \
+		git clone --depth 1 https://github.com/nvim-lua/plenary.nvim $(PLENARY_DIR); \
+	fi
+
+test: plenary
+	PLENARY_DIR=$(PLENARY_DIR) nvim --headless -u tests/minimal_init.lua -c "PlenaryBustedDirectory tests/ {minimal_init = 'tests/minimal_init.lua'}"
+
+lint:
+	stylua --check lua/ tests/
diff --git a/tests/helpers.lua b/tests/helpers.lua
new file mode 100644
index 0000000..028f32e
--- /dev/null
+++ b/tests/helpers.lua
@@ -0,0 +1,90 @@
+local M = {}
+
+-- Tracked values
+M.feedkeys_calls = {}
+M.notifications = {}
+
+-- Original functions
+local originals = {}
+
+function M.setup_mocks()
+  M.feedkeys_calls = {}
+  M.notifications = {}
+
+  originals.mode = vim.fn.mode
+  originals.expand = vim.fn.expand
+  originals.feedkeys = vim.api.nvim_feedkeys
+  originals.replace_termcodes = vim.api.nvim_replace_termcodes
+  originals.buf_get_mark = vim.api.nvim_buf_get_mark
+  originals.buf_get_lines = vim.api.nvim_buf_get_lines
+  originals.notify = vim.notify
+
+  -- Default mock: normal mode
+  vim.fn.mode = function()
+    return 'n'
+  end
+
+  vim.fn.expand = function()
+    return 'word'
+  end
+
+  vim.api.nvim_feedkeys = function(keys, mode, escape_ks)
+    table.insert(M.feedkeys_calls, { keys = keys, mode = mode, escape_ks = escape_ks })
+  end
+
+  vim.api.nvim_replace_termcodes = function(str, from_part, do_lt, special)
+    return str
+  end
+
+  vim.api.nvim_buf_get_mark = function(buf, mark)
+    return { 1, 0 }
+  end
+
+  vim.api.nvim_buf_get_lines = function(buf, start_line, end_line, strict)
+    return { 'some text here' }
+  end
+
+  vim.notify = function(msg, level)
+    table.insert(M.notifications, { msg = msg, level = level })
+  end
+end
+
+function M.set_mode(mode)
+  vim.fn.mode = function()
+    return mode
+  end
+end
+
+function M.set_expand(result)
+  vim.fn.expand = function()
+    return result
+  end
+end
+
+function M.set_visual_marks(start_pos, end_pos)
+  vim.api.nvim_buf_get_mark = function(buf, mark)
+    if mark == '<' then
+      return start_pos
+    elseif mark == '>' then
+      return end_pos
+    end
+  end
+end
+
+function M.set_buffer_lines(lines)
+  vim.api.nvim_buf_get_lines = function(buf, start_line, end_line, strict)
+    return lines
+  end
+end
+
+function M.teardown_mocks()
+  vim.fn.mode = originals.mode
+  vim.fn.expand = originals.expand
+  vim.api.nvim_feedkeys = originals.feedkeys
+  vim.api.nvim_replace_termcodes = originals.replace_termcodes
+  vim.api.nvim_buf_get_mark = originals.buf_get_mark
+  vim.api.nvim_buf_get_lines = originals.buf_get_lines
+  vim.notify = originals.notify
+end
+
+return M
diff --git a/tests/minimal_init.lua b/tests/minimal_init.lua
new file mode 100644
index 0000000..2fc474e
--- /dev/null
+++ b/tests/minimal_init.lua
@@ -0,0 +1,21 @@
+local plenary_dir = os.getenv('PLENARY_DIR') or '/tmp/plenary.nvim'
+
+if vim.fn.isdirectory(plenary_dir) == 0 then
+  vim.fn.system({
+    'git',
+    'clone',
+    '--depth',
+    '1',
+    'https://github.com/nvim-lua/plenary.nvim',
+    plenary_dir,
+  })
+end
+
+vim.opt.rtp:append('.')
+vim.opt.rtp:append(plenary_dir)
+
+-- Allow spec files to require('helpers')
+package.path = './tests/?.lua;' .. package.path
+
+vim.cmd('runtime plugin/plenary.vim')
+require('plenary.busted')
diff --git a/tests/scalpel_spec.lua b/tests/scalpel_spec.lua
new file mode 100644
index 0000000..f641a97
--- /dev/null
+++ b/tests/scalpel_spec.lua
@@ -0,0 +1,169 @@
+local helpers = require('helpers')
+
+describe('scalpel', function()
+  local scalpel
+
+  before_each(function()
+    helpers.setup_mocks()
+    package.loaded['scalpel'] = nil
+    scalpel = require('scalpel')
+  end)
+
+  after_each(function()
+    helpers.teardown_mocks()
+  end)
+
+  describe('setup', function()
+    it('does not error', function()
+      assert.has_no.errors(function()
+        scalpel.setup()
+      end)
+    end)
+  end)
+
+  describe('substitute', function()
+    describe('normal mode', function()
+      it('feeds substitution command with word under cursor', function()
+        helpers.set_mode('n')
+        helpers.set_expand('hello')
+
+        scalpel.substitute()
+
+        assert.equals(1, #helpers.feedkeys_calls)
+        local call = helpers.feedkeys_calls[1]
+        assert.truthy(call.keys:find('hello'))
+        assert.truthy(call.keys:find(':%%s/\\v'))
+        assert.truthy(call.keys:find('//gc'))
+      end)
+
+      it('uses very magic regex pattern', function()
+        helpers.set_mode('n')
+        helpers.set_expand('myVar')
+
+        scalpel.substitute()
+
+        local call = helpers.feedkeys_calls[1]
+        assert.truthy(call.keys:find('\\v'))
+      end)
+
+      it('includes cursor move keys to position in replacement field', function()
+        helpers.set_mode('n')
+        helpers.set_expand('test')
+
+        scalpel.substitute()
+
+        local call = helpers.feedkeys_calls[1]
+        assert.truthy(call.keys:find('<Left><Left><Left>'))
+      end)
+
+      it('notifies when word is blank', function()
+        helpers.set_mode('n')
+        helpers.set_expand('   ')
+
+        scalpel.substitute()
+
+        assert.equals(0, #helpers.feedkeys_calls)
+        assert.equals(1, #helpers.notifications)
+        assert.truthy(helpers.notifications[1].msg:find('blank'))
+      end)
+
+      it('notifies when word is empty', function()
+        helpers.set_mode('n')
+        helpers.set_expand('')
+
+        scalpel.substitute()
+
+        assert.equals(0, #helpers.feedkeys_calls)
+        assert.equals(1, #helpers.notifications)
+        assert.truthy(helpers.notifications[1].msg:find('blank'))
+      end)
+    end)
+
+    describe('visual mode', function()
+      -- Note: get_visual_range() calls nvim_feedkeys with <Esc> first,
+      -- so visual mode tests have an extra feedkeys call at index 1.
+
+      it('feeds substitution command with selected text', function()
+        helpers.set_mode('v')
+        helpers.set_visual_marks({ 1, 4 }, { 1, 8 })
+        helpers.set_buffer_lines({ 'the hello world line' })
+
+        scalpel.substitute()
+
+        assert.equals(2, #helpers.feedkeys_calls)
+        local call = helpers.feedkeys_calls[2]
+        assert.truthy(call.keys:find('hello'))
+        assert.truthy(call.keys:find(':%%s/\\v'))
+        assert.truthy(call.keys:find('//gc'))
+      end)
+
+      it('extracts correct substring from line', function()
+        helpers.set_mode('v')
+        helpers.set_visual_marks({ 1, 0 }, { 1, 2 })
+        helpers.set_buffer_lines({ 'foo bar baz' })
+
+        scalpel.substitute()
+
+        local call = helpers.feedkeys_calls[2]
+        assert.truthy(call.keys:find('foo'))
+      end)
+
+      it('notifies when selection spans multiple lines', function()
+        helpers.set_mode('v')
+        helpers.set_visual_marks({ 1, 0 }, { 3, 5 })
+
+        scalpel.substitute()
+
+        -- Only the <Esc> feedkeys call, no substitution
+        assert.equals(1, #helpers.feedkeys_calls)
+        assert.equals(1, #helpers.notifications)
+        assert.truthy(helpers.notifications[1].msg:find('Cannot substitute'))
+      end)
+
+      it('notifies when selected text is blank', function()
+        helpers.set_mode('v')
+        helpers.set_visual_marks({ 1, 4 }, { 1, 6 })
+        helpers.set_buffer_lines({ 'test   end' })
+
+        scalpel.substitute()
+
+        -- Only the <Esc> feedkeys call, no substitution
+        assert.equals(1, #helpers.feedkeys_calls)
+        assert.equals(1, #helpers.notifications)
+        assert.truthy(helpers.notifications[1].msg:find('blank'))
+      end)
+    end)
+
+    describe('visual line mode', function()
+      it('feeds line-scoped substitution command without a word', function()
+        helpers.set_mode('V')
+
+        scalpel.substitute()
+
+        assert.equals(1, #helpers.feedkeys_calls)
+        local call = helpers.feedkeys_calls[1]
+        assert.truthy(call.keys:find(':s///gc'))
+        -- Should NOT contain %s (buffer-wide) or \v (very magic)
+        assert.falsy(call.keys:find('%%s'))
+        assert.falsy(call.keys:find('\\v'))
+      end)
+
+      it('includes cursor move keys to position in search field', function()
+        helpers.set_mode('V')
+
+        scalpel.substitute()
+
+        local call = helpers.feedkeys_calls[1]
+        assert.truthy(call.keys:find('<Left><Left><Left>'))
+      end)
+
+      it('does not trigger any notifications', function()
+        helpers.set_mode('V')
+
+        scalpel.substitute()
+
+        assert.equals(0, #helpers.notifications)
+      end)
+    end)
+  end)
+end)