diff --git a/packages/dev-server/src/index.ts b/packages/dev-server/src/index.ts
index 2113e7fc..5a98f0fd 100644
--- a/packages/dev-server/src/index.ts
+++ b/packages/dev-server/src/index.ts
@@ -272,7 +272,11 @@ export async function startServer(opts: ServeOptions): Promise<RunningServer> {
 
   const pluginContributions = await pluginRunner.contributions();
   const virtualModules = new Map<string, string>();
-  const virtualDir = join(appRoot, ".wrnexus", "virtual");
+  // A live dev server and a test process can share an app root. Keeping the
+  // generated artifacts in a process-specific directory prevents a test boot
+  // from deleting the running server's client modules.
+  const cacheDir = join(appRoot, `.wrnexus-${process.pid}`);
+  const virtualDir = join(cacheDir, "virtual");
   mkdirSync(virtualDir, { recursive: true });
   for (const [index, module] of pluginContributions.virtualModules.entries()) {
     const output = join(virtualDir, `plugin-${index}.ts`);
@@ -327,12 +331,13 @@ export async function startServer(opts: ServeOptions): Promise<RunningServer> {
 
   resetDevCache({
     rootDir: appRoot,
+    cacheDir,
     enabled: process.env.WRNEXUS_PRESERVE_CACHE !== "1",
   });
 
   // Compile every `.wrn` into ONE cache dir at the project root, instead of a
   // `.wrnexus/` next to each source file (and inside node_modules UI dirs).
-  setCompileCacheDir(join(appRoot, ".wrnexus"));
+  setCompileCacheDir(cacheDir);
   const theme = resolveThemeConfig(opts.theme);
   const uiStylesPath = uiCssPath();