diff --git a/lib/cmd/vite/generate-bootstrap.js b/lib/cmd/vite/generate-bootstrap.js
index 1496fec..22a0385 100644
--- a/lib/cmd/vite/generate-bootstrap.js
+++ b/lib/cmd/vite/generate-bootstrap.js
@@ -4,6 +4,7 @@ const fs = require('fs-extra');
 const path = require('path');
 const { globSync } = require('glob');
 const { getConfigValue } = require('../../config-file-helpers');
+const { generateViteEnvInit } = require('./generate-env-init');
 
 const CWD = process.cwd();
 const CLAY_DIR = path.join(CWD, '.clay');
@@ -106,6 +107,8 @@ mountComponentModules().catch(console.error);
  * @returns {Promise<string>} absolute path to the written bootstrap file
  */
 async function generateViteBootstrap() {
+  await generateViteEnvInit();
+
   const clientFiles = [
     ...globSync(path.join(CWD, 'components', '**', 'client.js')),
     ...globSync(path.join(CWD, 'layouts', '**', 'client.js')),
@@ -152,6 +155,8 @@ ${stickyListeners}
     ? "import './_globals-init.js';\n"
     : '// no global/js — skipping _globals-init\n';
 
+  const envInitImport = "import './_env-init.js';\n";
+
   // Clay-kiln's preloader does `Object.keys(window.modules)` to find
   // component model.js / kiln.js registrations that Browserify's megabundler
   // assigns there at page load. Under Vite there is no Browserify runtime,
@@ -174,6 +179,7 @@ ${stickyListeners}
     '// present on the current page — keeping initial parse cost low.',
     '',
     kilnCompatStub,
+    envInitImport,
     globalsImport,
     stickyShimBlock,
     'const _clayClientModules = {',
diff --git a/lib/cmd/vite/generate-env-init.js b/lib/cmd/vite/generate-env-init.js
new file mode 100644
index 0000000..61f9021
--- /dev/null
+++ b/lib/cmd/vite/generate-env-init.js
@@ -0,0 +1,56 @@
+'use strict';
+
+const fs = require('fs-extra');
+const path = require('path');
+
+const CWD = process.cwd();
+const CLAY_DIR = path.join(CWD, '.clay');
+const ENV_INIT_FILE = path.join(CLAY_DIR, '_env-init.js');
+
+/**
+ * Generate .clay/_env-init.js — runtime hydration for window.process.env.
+ *
+ * Why:
+ * - Universal modules can be evaluated before late env injection in some
+ *   browser paths under ESM (especially edit-mode/kiln startup), and any
+ *   top-level `const X = process.env.X` captures undefined permanently.
+ * - Hydrating window.process.env in a dedicated side-effect module imported
+ *   first by Vite entries ensures env is present before those modules run.
+ *
+ * Sources (in priority order):
+ * 1) window.kiln.preloadData._envVars (authoritative edit-mode payload from amphora-html)
+ * 2) existing window.process.env (if already injected by any earlier script)
+ *
+ * @returns {Promise<string>} absolute path to the generated file
+ */
+async function generateViteEnvInit() {
+  const content = [
+    '// AUTO-GENERATED — clay vite env init (do not edit)',
+    `// ${new Date().toISOString()}`,
+    ';(function clayViteEnvInit() {',
+    '  if (typeof window === "undefined") return;',
+    '',
+    '  var fromKiln = window.kiln && window.kiln.preloadData && window.kiln.preloadData._envVars;',
+    '  var existing = window.process && window.process.env;',
+    '',
+    '  var source = (fromKiln && typeof fromKiln === "object")',
+    '    ? fromKiln',
+    '    : ((existing && typeof existing === "object") ? existing : {});',
+    '',
+    '  window.process = window.process || {};',
+    '  window.process.env = Object.assign({}, source, window.process.env || {});',
+    '}());',
+    '',
+  ].join('\n');
+
+  await fs.ensureDir(CLAY_DIR);
+  await fs.writeFile(ENV_INIT_FILE, content, 'utf8');
+
+  return ENV_INIT_FILE;
+}
+
+module.exports = {
+  generateViteEnvInit,
+  ENV_INIT_FILE,
+};
+
diff --git a/lib/cmd/vite/generate-kiln-edit.js b/lib/cmd/vite/generate-kiln-edit.js
index 5bb16fc..7b2f65f 100644
--- a/lib/cmd/vite/generate-kiln-edit.js
+++ b/lib/cmd/vite/generate-kiln-edit.js
@@ -3,6 +3,7 @@
 const fs = require('fs-extra');
 const path = require('path');
 const { globSync } = require('glob');
+const { generateViteEnvInit } = require('./generate-env-init');
 
 const CWD = process.cwd();
 const CLAY_DIR = path.join(CWD, '.clay');
@@ -27,6 +28,8 @@ const KILN_EDIT_ENTRY_KEY = '.clay/vite-kiln-edit-init';
  * @returns {Promise<string>} absolute path to the generated file
  */
 async function generateViteKilnEditEntry() {
+  await generateViteEnvInit();
+
   const modelFiles = [
     ...globSync(path.join(CWD, 'components', '**', 'model.js')),
     ...globSync(path.join(CWD, 'layouts', '**', 'model.js')),
@@ -54,6 +57,7 @@ async function generateViteKilnEditEntry() {
   // Use namespace imports (import * as) so both CJS modules (which @rollup/plugin-commonjs
   // gives a namespace object with a .default property) and ESM modules work uniformly.
   // Then resolve the actual export via _resolveDefault() at runtime.
+  lines.push('import \'./_env-init.js\';');
   modelFiles.forEach((f, i) => lines.push(`import * as _m${i} from ${JSON.stringify(toRel(f))};`));
   kilnjsFiles.forEach((f, i) => lines.push(`import * as _k${i} from ${JSON.stringify(toRel(f))};`));
   if (hasKilnPlugin) {