diff --git a/src/lib/furni/classname.ts b/src/lib/furni/classname.ts
index 08ec5d0b..fce009b6 100644
--- a/src/lib/furni/classname.ts
+++ b/src/lib/furni/classname.ts
@@ -28,6 +28,10 @@ export type FurnitureBundleExt = (typeof BUNDLE_EXT_CHOICES)[number];
  */
 function readBundleExtEnv(): string | undefined {
 	const name = "FURNI_BUNDLE_EXT";
+	// `classname.ts` is also imported by client components for its pure string
+	// helpers, where `process` does not exist at runtime. Reading env is a
+	// server concern; on the client fall back to the default extension.
+	if (typeof process === "undefined") return undefined;
 	return process.env[name]?.trim() || undefined;
 }