diff --git a/config_spec.lua b/config_spec.lua
index ae1c46a..f7b8eae 100644
--- a/config_spec.lua
+++ b/config_spec.lua
@@ -1,4 +1,3 @@
--- Unit tests for configuration management
 describe('cp.config', function()
   local config
 
@@ -7,40 +6,84 @@ describe('cp.config', function()
   end)
 
   describe('setup', function()
-    it('returns default config when no user config provided', function()
-      -- Test default configuration values
+    it('returns defaults with nil input', function()
+      local result = config.setup()
+
+      assert.equals('table', type(result.contests))
+      assert.equals('table', type(result.snippets))
+      assert.equals('table', type(result.hooks))
+      assert.equals('table', type(result.scrapers))
+      assert.is_false(result.debug)
+      assert.is_nil(result.filename)
     end)
 
     it('merges user config with defaults', function()
-      -- Test config merging behavior
+      local user_config = {
+        debug = true,
+        contests = { test_contest = { cpp = { extension = 'cpp' } } }
+      }
+
+      local result = config.setup(user_config)
+
+      assert.is_true(result.debug)
+      assert.equals('table', type(result.contests.test_contest))
+      assert.equals('table', type(result.scrapers))
     end)
 
-    it('validates contest configurations', function()
-      -- Test contest config validation
+    it('validates extension against supported filetypes', function()
+      local invalid_config = {
+        contests = {
+          test_contest = {
+            cpp = { extension = 'invalid' }
+          }
+        }
+      }
+
+      assert.has_error(function()
+        config.setup(invalid_config)
+      end)
     end)
 
-    it('handles invalid config gracefully', function()
-      -- Test error handling for bad configs
+    it('validates scraper platforms', function()
+      local invalid_config = {
+        scrapers = { invalid_platform = true }
+      }
+
+      assert.has_error(function()
+        config.setup(invalid_config)
+      end)
+    end)
+
+    it('validates scraper values are booleans', function()
+      local invalid_config = {
+        scrapers = { atcoder = 'not_boolean' }
+      }
+
+      assert.has_error(function()
+        config.setup(invalid_config)
+      end)
+    end)
+
+    it('validates hook functions', function()
+      local invalid_config = {
+        hooks = { before_run = 'not_a_function' }
+      }
+
+      assert.has_error(function()
+        config.setup(invalid_config)
+      end)
     end)
   end)
 
-  describe('platform validation', function()
-    it('accepts valid platforms', function()
-      -- Test platform validation
+  describe('default_filename', function()
+    it('generates lowercase contest filename', function()
+      local result = config.default_filename('ABC123')
+      assert.equals('abc123', result)
     end)
 
-    it('rejects invalid platforms', function()
-      -- Test platform rejection
-    end)
-  end)
-
-  describe('language configurations', function()
-    it('provides correct file extensions for languages', function()
-      -- Test language -> extension mappings
-    end)
-
-    it('provides correct compile commands', function()
-      -- Test compile command generation
+    it('combines contest and problem ids', function()
+      local result = config.default_filename('ABC123', 'A')
+      assert.equals('abc123a', result)
     end)
   end)
 end)