diff --git a/__tests__/setupQuietActionsCore.ts b/__tests__/setupQuietActionsCore.ts
new file mode 100644
index 00000000..53136ee7
--- /dev/null
+++ b/__tests__/setupQuietActionsCore.ts
@@ -0,0 +1,32 @@
+import * as core from '@actions/core'
+import { beforeEach, vi } from 'vitest'
+
+// setup.ts mocks @actions/core with `spy: true`, so the real implementation still runs and
+// its output functions write `::debug::`, `::warning::`, `::error::` and `::add-mask::`
+// workflow commands straight to process.stdout (bypassing vitest's console capture).
+// Under GitHub Actions the runner parses those lines: a green build-test run carried ten
+// `failure` and ten `warning` annotations that were test fixtures, `::add-mask::` redacted
+// fixture strings from the rest of the log, and `setFailed` set process.exitCode in the
+// worker. Keep the spies — tests assert on the calls — but drop the writes. The
+// implementation is reinstalled per test because `mockReset: true` restores the originals.
+const outputFunctions = [
+  'debug',
+  'info',
+  'warning',
+  'error',
+  'notice',
+  'setFailed',
+  'setSecret',
+  'setOutput',
+  'exportVariable',
+  'addPath',
+  'saveState',
+  'startGroup',
+  'endGroup',
+  'setCommandEcho',
+] as const
+
+beforeEach(() => {
+  for (const name of outputFunctions)
+    vi.mocked(core[name]).mockImplementation(() => {})
+})
diff --git a/vitest.config.mjs b/vitest.config.mjs
index 998045b5..52c1bf50 100644
--- a/vitest.config.mjs
+++ b/vitest.config.mjs
@@ -5,7 +5,7 @@ export default defineConfig({
     environment: 'node',
     include: ['__tests__/**/*.test.ts'],
     globals: false,
-    setupFiles: ['__tests__/setup.ts'],
+    setupFiles: ['__tests__/setup.ts', '__tests__/setupQuietActionsCore.ts'],
     globalSetup: ['__tests__/worker/globalSetup.ts'],
     // Mirrors Jest's `clearMocks: true` + `resetMocks: true` + `restoreMocks: true`.
     // Note: Vitest's mockReset restores a spy's original implementation (Jest 29