diff --git a/frontend/editor/vite.config.ts b/frontend/editor/vite.config.ts
index 3f51f7ba3..b6594c680 100644
--- a/frontend/editor/vite.config.ts
+++ b/frontend/editor/vite.config.ts
@@ -13,6 +13,34 @@ const gzipPromise = promisify(gzip);
 const brotliPromise = promisify(brotliCompress);
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
+// TEMP DEBUG — writes a build-info.txt into dist/ on every build so we can
+// curl https://<host>/<base>/build-info.txt to confirm a fresh deploy landed
+// and which env vars the build host actually received. Values are masked to
+// (set)/(unset) so nothing real is exposed via the publicly-served file; the
+// build log (vite-config-debug below) shows actual values for the project
+// owner. Timestamp guarantees the file differs every build, forcing upload.
+function buildInfoPlugin(): PluginOption {
+  const mask = (v: string | undefined) => (v ? "(set)" : "(unset)");
+  return {
+    name: "debug-build-info",
+    apply: "build" as const,
+    async closeBundle() {
+      const distDir = path.resolve(__dirname, "dist");
+      const contents = [
+        `BuildTime=${new Date().toISOString()}`,
+        `VITE_API_BASE_URL=${mask(process.env.VITE_API_BASE_URL)}`,
+        `RUN_SUBPATH=${mask(process.env.RUN_SUBPATH)}`,
+        `VITE_SUPABASE_URL=${mask(process.env.VITE_SUPABASE_URL)}`,
+        `VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY=${mask(process.env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY)}`,
+        `STIRLING_FLAVOR=${mask(process.env.STIRLING_FLAVOR)}`,
+        "",
+      ].join("\n");
+      await fs.mkdir(distDir, { recursive: true });
+      await fs.writeFile(path.join(distDir, "build-info.txt"), contents);
+    },
+  };
+}
+
 function compressStaticCopyPlugin(): PluginOption {
   return {
     name: "compress-static-copy",
@@ -97,6 +125,28 @@ export default defineConfig(async ({ mode }) => {
   // frontend/editor/ the cwd-based lookup would miss editor/.env*.
   const env = loadEnv(mode, import.meta.dirname, "");
 
+  // TEMP DEBUG — surface build-time env values so we can confirm what the
+  // build host (Cloudflare / CI) is actually passing through. Safe to remove
+  // once we've stopped chasing env-var routing bugs.
+  // eslint-disable-next-line no-console
+  console.log(
+    "[vite-config-debug]",
+    JSON.stringify(
+      {
+        mode,
+        VITE_API_BASE_URL_processEnv: process.env.VITE_API_BASE_URL ?? "(unset)",
+        VITE_API_BASE_URL_loadEnv: env.VITE_API_BASE_URL ?? "(unset)",
+        RUN_SUBPATH: process.env.RUN_SUBPATH ?? env.RUN_SUBPATH ?? "(unset)",
+        VITE_SUPABASE_URL: env.VITE_SUPABASE_URL ? "(set)" : "(unset)",
+        VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY: env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY
+          ? "(set)"
+          : "(unset)",
+      },
+      null,
+      2,
+    ),
+  );
+
   // Effective mode: --mode > STIRLING_FLAVOR > ENABLE_SAAS > DISABLE_ADDITIONAL_FEATURES > proprietary.
   const explicitMode = (VALID_MODES as readonly string[]).includes(mode)
     ? (mode as BuildMode)
@@ -202,6 +252,7 @@ export default defineConfig(async ({ mode }) => {
         ],
       }),
       compressStaticCopyPlugin(),
+      buildInfoPlugin(),
     ],
     server: {
       host: true,