optimize vitest for parallel execution in non-coverage mode

Author: jdalton

.config/vitest.config.mts

@@ -1,5 +1,11 @@
 import { defineConfig } from 'vitest/config'
 
+// Check if coverage is enabled via CLI flags or environment.
+const isCoverageEnabled =
+  process.env['COVERAGE'] === 'true' ||
+  process.env['npm_lifecycle_event']?.includes('coverage') ||
+  process.argv.some(arg => arg.includes('coverage'))
+
 export default defineConfig({
   resolve: {
     preserveSymlinks: false,
@@ -13,6 +19,22 @@ export default defineConfig({
       'src/**/*.test.{js,ts,mjs,cjs,mts}',
     ],
     reporters: ['default'],
+    // Improve memory usage by running tests sequentially in CI.
+    pool: 'forks',
+    poolOptions: {
+      forks: {
+        // Use single fork for coverage to reduce memory, parallel otherwise.
+        singleFork: isCoverageEnabled,
+        maxForks: isCoverageEnabled ? 1 : undefined,
+        // Isolate tests to prevent memory leaks between test files.
+        isolate: true,
+      },
+      threads: {
+        // Use single thread for coverage to reduce memory, parallel otherwise.
+        singleThread: isCoverageEnabled,
+        maxThreads: isCoverageEnabled ? 1 : undefined,
+      },
+    },
     testTimeout: 60_000,
     hookTimeout: 60_000,
     coverage: {