diff --git a/examples/rails/vite.config.ts b/examples/rails/vite.config.ts
index b6c707e7..2c1d831a 100644
--- a/examples/rails/vite.config.ts
+++ b/examples/rails/vite.config.ts
@@ -4,6 +4,8 @@ import rails from 'vite-plugin-rails'
 import WindiCSS from 'vite-plugin-windicss'
 import BugsnagPlugins from './plugins/bugsnag'
 
+const administratorAssetsPath = process.env.ADMINISTRATOR_ASSETS_PATH
+
 export default defineConfig({
   plugins: [
     BugsnagPlugins,
@@ -26,13 +28,13 @@ export default defineConfig({
   ],
   // Example: Importing assets from arbitrary paths.
   resolve: {
-    alias: {
-      '@administrator/': `${process.env.ADMINISTRATOR_ASSETS_PATH}/`,
-    },
+    alias: administratorAssetsPath
+      ? { '@administrator/': `${administratorAssetsPath}/` }
+      : {},
   },
   server: {
     fs: {
-      allow: [process.env.ADMINISTRATOR_ASSETS_PATH!],
+      allow: administratorAssetsPath ? [administratorAssetsPath] : [],
     },
   },
 })
diff --git a/vite-plugin-rails/tests/build.spec.ts b/vite-plugin-rails/tests/build.spec.ts
index 4373364d..bd59b2ed 100644
--- a/vite-plugin-rails/tests/build.spec.ts
+++ b/vite-plugin-rails/tests/build.spec.ts
@@ -22,7 +22,6 @@ describe('config', () => {
       'assets/external-BwssHmjP.js',
       'assets/external-BwssHmjP.js.br',
       'assets/external-BwssHmjP.js.gz',
-      'assets/external-BwssHmjP.js.map',
       'assets/index-qTzjl5TV.js',
       'assets/index-qTzjl5TV.js.br',
       'assets/index-qTzjl5TV.js.gz',
@@ -34,7 +33,6 @@ describe('config', () => {
       'assets/main-Ddvw3iap.js',
       'assets/main-Ddvw3iap.js.br',
       'assets/main-Ddvw3iap.js.gz',
-      'assets/main-Ddvw3iap.js.map',
       'assets/sassy-D5kz_As0.css',
       'assets/sassy-D5kz_As0.css.br',
       'assets/sassy-D5kz_As0.css.gz',
@@ -47,7 +45,6 @@ describe('config', () => {
       'assets/vue-CoJ_KGkH.js',
       'assets/vue-CoJ_KGkH.js.br',
       'assets/vue-CoJ_KGkH.js.gz',
-      'assets/vue-CoJ_KGkH.js.map',
       'index.html',
       'index.html.br',
       'index.html.gz',
diff --git a/vite-plugin-ruby/src/index.ts b/vite-plugin-ruby/src/index.ts
index 4a370d2f..fb2f372e 100644
--- a/vite-plugin-ruby/src/index.ts
+++ b/vite-plugin-ruby/src/index.ts
@@ -60,7 +60,6 @@ function config (userConfig: UserConfig, env: ConfigEnv): UserConfig {
 
   const build = {
     emptyOutDir: userConfig.build?.emptyOutDir ?? (ssrBuild || isLocal),
-    sourcemap: !isLocal,
     ...userConfig.build,
     assetsDir,
     manifest: !ssrBuild,
@@ -108,6 +107,7 @@ function configureServer (server: ViteDevServer) {
       res.statusCode = 404
       const file = readFileSync(resolve(__dirname, 'dev-server-index.html'), 'utf-8')
       res.end(file)
+      return
     }
 
     next()
diff --git a/vite-plugin-ruby/tests/index.spec.ts b/vite-plugin-ruby/tests/index.spec.ts
index 10f3c572..22597c67 100644
--- a/vite-plugin-ruby/tests/index.spec.ts
+++ b/vite-plugin-ruby/tests/index.spec.ts
@@ -8,21 +8,23 @@ describe('config', () => {
     const pluginConfig = plugin[0].config
     defaultConfig.configPath = './default.vite.json'
 
+    // Sourcemap is no longer set by the plugin (Vite's safe default of `false`
+    // applies in all modes). Users opt in via `build.sourcemap` in their vite.config.
     const productionConfig = pluginConfig(defaultConfig, { mode: 'production' })
     expect(productionConfig.build.emptyOutDir).toBe(false)
-    expect(productionConfig.build.sourcemap).toBe(true)
+    expect(productionConfig.build.sourcemap).toBeUndefined()
 
     const stagingConfig = pluginConfig(defaultConfig, { mode: 'staging' })
     expect(stagingConfig.build.emptyOutDir).toBe(false)
-    expect(stagingConfig.build.sourcemap).toBe(true)
+    expect(stagingConfig.build.sourcemap).toBeUndefined()
 
     const developmentConfig = pluginConfig(defaultConfig, { mode: 'development' })
     expect(developmentConfig.build.emptyOutDir).toBe(true)
-    expect(developmentConfig.build.sourcemap).toBe(false)
+    expect(developmentConfig.build.sourcemap).toBeUndefined()
 
     const testConfig = pluginConfig(defaultConfig, { mode: 'test' })
     expect(testConfig.build.emptyOutDir).toBe(true)
-    expect(testConfig.build.sourcemap).toBe(false)
+    expect(testConfig.build.sourcemap).toBeUndefined()
 
     expect(() => {
       pluginConfig({ ...defaultConfig, build: { ssr: true } }, { mode: 'production' })
diff --git a/vite_ruby/lib/vite_ruby.rb b/vite_ruby/lib/vite_ruby.rb
index 9c16e02a..daf6631f 100644
--- a/vite_ruby/lib/vite_ruby.rb
+++ b/vite_ruby/lib/vite_ruby.rb
@@ -69,6 +69,7 @@ def framework_libraries
 
   def initialize(**config_options)
     @config_options = config_options
+    @running_mutex = Mutex.new
   end
 
   def logger
@@ -85,15 +86,18 @@ def digest
   # NOTE: Checks only once every second since every lookup calls this method.
   def dev_server_running?
     return false unless run_proxy?
-    return @running if defined?(@running) && Time.now - @running_checked_at < 1
-
-    begin
-      Socket.tcp(config.host, config.port, connect_timeout: config.dev_server_connect_timeout).close
-      @running = true
-    rescue
-      @running = false
-    ensure
-      @running_checked_at = Time.now
+
+    @running_mutex.synchronize do
+      return @running if defined?(@running) && Time.now - @running_checked_at < 1
+
+      begin
+        Socket.tcp(config.host, config.port, connect_timeout: config.dev_server_connect_timeout).close
+        @running = true
+      rescue
+        @running = false
+      ensure
+        @running_checked_at = Time.now
+      end
     end
   end