fix(typecheck): log experimental feature warning once per run

packages/vitest/src/node/config/resolveConfig.ts

@@ -28,6 +28,8 @@ import { getWorkersCountByPercentage } from '../../utils/workers'
 import { BaseSequencer } from '../sequencers/BaseSequencer'
 import { RandomSequencer } from '../sequencers/RandomSequencer'
 
+const loggedExperimentalWarnings = new WeakSet<Logger>()
+
 function resolvePath(path: string, root: string) {
   // local-pkg (mlly)'s resolveModule("./file", { paths: ["/some/root"] }) tries
   // /some/file
@@ -796,7 +798,8 @@ export function resolveConfig(
   resolved.typecheck ??= {} as any
   resolved.typecheck.enabled ??= false
 
-  if (resolved.typecheck.enabled) {
+  if (resolved.typecheck.enabled && !loggedExperimentalWarnings.has(logger)) {
+    loggedExperimentalWarnings.add(logger)
     logger.console.warn(
       c.yellow(
         'Testing types with tsc and vue-tsc is an experimental feature.\nBreaking changes might not follow SemVer, please pin Vitest\'s version when using it.',

test/typescript/test/runner.test.ts

@@ -153,6 +153,24 @@ it('throws an error if typechecker process exists', async () => {
   }
 })
 
+it('logs experimental typecheck warning once for a run with multiple typecheck projects', async () => {
+  // Each project with typecheck.enabled triggers a separate resolveConfig call.
+  // Without dedup, PROJECT_COUNT warnings would be logged. Assert exactly one.
+  const PROJECT_COUNT = 5
+  const { stderr } = await runVitest({
+    root: resolve(import.meta.dirname, '..'),
+    projects: Array.from({ length: PROJECT_COUNT }, (_, i) => ({
+      test: {
+        name: `typecheck-project-${i + 1}`,
+        dir: resolve(import.meta.dirname, '../test-d'),
+        typecheck: { enabled: true },
+      },
+    })),
+  })
+  const warningCount = (stderr.match(/Testing types with tsc and vue-tsc is an experimental feature/g) ?? []).length
+  expect(warningCount).toBe(1)
+})
+
 function removeLines(log: string) {
   return log.replace(/⎯{2,}/g, '⎯⎯')
 }