vitest >=5.0.0 changed the default of `clearMocks`. clearMocks default is true: Vitest automatically calls vi.clearAllMocks before each test. Silent: code written for older versions still runs, without an error.

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/clearmocks";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
fs.writeFileSync(path.join(dir, "mock.test.mjs"), [
  "import { test, vi } from 'vitest'",
  "import { writeFileSync } from 'node:fs'",
  "const fn = vi.fn()",
  "test('first', () => { fn(); fn() })",
  "test('second', () => { writeFileSync(new URL('./calls.txt', import.meta.url), String(fn.mock.calls.length)) })",
].join("\n") + "\n");
const r = cp.spawnSync(process.execPath, ["/w/node_modules/vitest/vitest.mjs", "run"], {
  cwd: dir,
  encoding: "utf8",
  env: { ...process.env, NO_COLOR: "1", CI: "1" },
  timeout: 60000,
});
console.log((r.stdout || "") + (r.stderr || "") + (r.error ? String(r.error) : ""));
const callsFile = path.join(dir, "calls.txt");
if (r.status !== 0 || !fs.existsSync(callsFile)) {
  console.error("ERROR: vitest run did not complete (exit " + r.status + ")");
  process.exit(2);
}
const calls = fs.readFileSync(callsFile, "utf8").trim();
if (calls === "0") {
  console.log("PASS: mock history was cleared before the second test");
  process.exit(0);
}
if (calls === "2") {
  console.error("FAIL: mock calls from the first test were still recorded in the second test");
  process.exit(1);
}
console.error("ERROR: unexpected mock call count " + calls);
process.exit(2);

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/clearmocks";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
fs.writeFileSync(path.join(dir, "mock.test.mjs"), [
  "import { test, vi } from 'vitest'",
  "import { writeFileSync } from 'node:fs'",
  "const fn = vi.fn()",
  "test('first', () => { fn(); fn() })",
  "test('second', () => { writeFileSync(new URL('./calls.txt', import.meta.url), String(fn.mock.calls.length)) })",
].join("\n") + "\n");
const r = cp.spawnSync(process.execPath, ["/w/node_modules/vitest/vitest.mjs", "run"], {
  cwd: dir,
  encoding: "utf8",
  env: { ...process.env, NO_COLOR: "1", CI: "1" },
  timeout: 60000,
});
console.log((r.stdout || "") + (r.stderr || "") + (r.error ? String(r.error) : ""));
const callsFile = path.join(dir, "calls.txt");
if (r.status !== 0 || !fs.existsSync(callsFile)) {
  console.error("ERROR: vitest run did not complete (exit " + r.status + ")");
  process.exit(2);
}
const calls = fs.readFileSync(callsFile, "utf8").trim();
if (calls === "0") {
  console.log("PASS: mock history was cleared before the second test");
  process.exit(0);
}
if (calls === "2") {
  console.error("FAIL: mock calls from the first test were still recorded in the second test");
  process.exit(1);
}
console.error("ERROR: unexpected mock call count " + calls);
process.exit(2);