diff --git a/.gitignore b/.gitignore
index aa17b3e4f04..4b6e857425f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -41,6 +41,7 @@ build.log
 tcr-dry.log
 .agent-setup/
 .agent-recordings/
+.vitest/
 .vitest-profile/
 .pi-subagents/
 sbom-source.cdx.json
diff --git a/scripts/gitignore.test.mjs b/scripts/gitignore.test.mjs
new file mode 100644
index 00000000000..84aa06231f3
--- /dev/null
+++ b/scripts/gitignore.test.mjs
@@ -0,0 +1,35 @@
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { test } from 'node:test';
+import { fileURLToPath } from 'node:url';
+
+const repoRoot = fileURLToPath(new URL('..', import.meta.url));
+
+function ignoreRule(path) {
+	const result = spawnSync('git', ['check-ignore', '-v', '--no-index', '--', path], {
+		cwd: repoRoot,
+		encoding: 'utf8',
+	});
+	assert.ifError(result.error);
+	assert.equal(result.status, 0, `${path} must be ignored by Git`);
+	const [rule, ignoredPath] = result.stdout.trim().split('\t');
+	assert.equal(ignoredPath, path);
+	return rule;
+}
+
+test('ignores existing Vitest profile output', () => {
+	assert.match(
+		ignoreRule('packages/cli/.vitest-profile/json/output.json'),
+		/^\.gitignore:\d+:\.vitest-profile\/$/,
+	);
+});
+
+// DEVP-1240: Vitest 5 writes JSON reports into a package-local .vitest directory.
+test('ignores Vitest JSON reports in workspace packages', () => {
+	for (const path of [
+		'packages/cli/.vitest/json/output.json',
+		'packages/frontend/editor-ui/.vitest/json/output.json',
+	]) {
+		assert.match(ignoreRule(path), /^\.gitignore:\d+:\.vitest\/$/);
+	}
+});