diff --git a/src/app.d.ts b/src/app.d.ts
index be7d5b8..53a4264 100644
--- a/src/app.d.ts
+++ b/src/app.d.ts
@@ -9,6 +9,9 @@ import type {
 import type { ThreadDetail } from "$lib/forum/thread";
 
 declare global {
+  // Injected by vite.config.ts from PUBLIC_SSR
+  const __SQUALK_SSR__: boolean;
+
   namespace App {
     // interface Error {}
     // interface Locals {}
diff --git a/src/lib/config.ts b/src/lib/config.ts
index 95f70f6..e736634 100644
--- a/src/lib/config.ts
+++ b/src/lib/config.ts
@@ -18,9 +18,9 @@ export const GROUP_ID = env.PUBLIC_GROUP_ID ?? "";
 if (MODE === "simple" && !GROUP_ID) {
   throw new Error("PUBLIC_GROUP_ID is required in simple mode");
 }
-// Server-side rendering: opt-in, and only meaningful when the build targets the
-// Node adapter (svelte.config.js reads the same variable).
-export const SSR_ENABLED = env.PUBLIC_SSR === "yes";
+// Server-side rendering: opt-in, decided at build time (vite.config.ts bakes
+// PUBLIC_SSR in; svelte.config.js picks the matching adapter).
+export const SSR_ENABLED = __SQUALK_SSR__;
 // Server-rendered pages and snapshots are anonymous, so a shared cache may
 // hold them: fresh for 5 minutes, served stale for an hour while revalidating.
 // Browsers always revalidate (max-age=0) so a login shows its content at once.
diff --git a/svelte.config.js b/svelte.config.js
index f3b9c64..75459b0 100644
--- a/svelte.config.js
+++ b/svelte.config.js
@@ -3,8 +3,8 @@ import adapterNode from "@sveltejs/adapter-node";
 import adapterStatic from "@sveltejs/adapter-static";
 
 // PUBLIC_SSR picks the deployment target: a Node server that renders pages
-// (yes) or a static single-page bundle (no, the default). The same flag turns
-// SSR on in +layout.ts.
+// (yes) or a static single-page bundle (no, the default). vite.config.ts bakes
+// the same flag into the app as SSR_ENABLED.
 const env = loadEnv(
   process.env.NODE_ENV ?? "production",
   process.cwd(),
diff --git a/vite.config.ts b/vite.config.ts
index 23797da..a02ee62 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -1,5 +1,15 @@
 import tailwindcss from "@tailwindcss/vite";
 import { sveltekit } from "@sveltejs/kit/vite";
-import { defineConfig } from "vite";
+import { defineConfig, loadEnv } from "vite";
 
-export default defineConfig({ plugins: [tailwindcss(), sveltekit()] });
+// PUBLIC_SSR is a build-time decision (it also picks the adapter in
+// svelte.config.js), so it is compiled in as a constant rather than read from
+// the environment when the server starts.
+export default defineConfig(({ mode }) => {
+  const env = loadEnv(mode, process.cwd(), "PUBLIC_");
+  const ssr = (process.env.PUBLIC_SSR ?? env.PUBLIC_SSR) === "yes";
+  return {
+    plugins: [tailwindcss(), sveltekit()],
+    define: { __SQUALK_SSR__: JSON.stringify(ssr) },
+  };
+});