vitest >=5.0.0 changed what `vi.fn` does. Instances of a class passed to vi.fn keep the prototype chain of the class, so prototype methods stay available and instanceof checks against the class pass. 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/classproto";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
fs.writeFileSync(path.join(dir, "dog.test.mjs"), [
  "import { test, vi } from 'vitest'",
  "import { writeFileSync } from 'node:fs'",
  "class Dog { speak() { return 'bark!' } }",
  "test('dog', () => {",
  "  const MockedDog = vi.fn(Dog)",
  "  const dog = new MockedDog()",
  "  const result = { speak: typeof dog.speak, isDog: dog instanceof Dog }",
  "  writeFileSync(new URL('./result.json', import.meta.url), JSON.stringify(result))",
  "})",
].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 resultFile = path.join(dir, "result.json");
if (!fs.existsSync(resultFile)) {
  console.error("ERROR: the test file wrote no result (exit " + r.status + ")");
  process.exit(2);
}
const res = JSON.parse(fs.readFileSync(resultFile, "utf8"));
console.log(JSON.stringify(res));
if (res.speak === "function" && res.isDog === true) {
  console.log("PASS: new (vi.fn(Dog))() has the prototype method speak and is instanceof Dog");
  process.exit(0);
}
if (res.speak === "undefined" && res.isDog === false) {
  console.error("FAIL: new (vi.fn(Dog))() lacks the prototype method speak and is not instanceof Dog");
  process.exit(1);
}
console.error("ERROR: unexpected result " + JSON.stringify(res));
process.exit(2);

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/classproto";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
fs.writeFileSync(path.join(dir, "dog.test.mjs"), [
  "import { test, vi } from 'vitest'",
  "import { writeFileSync } from 'node:fs'",
  "class Dog { speak() { return 'bark!' } }",
  "test('dog', () => {",
  "  const MockedDog = vi.fn(Dog)",
  "  const dog = new MockedDog()",
  "  const result = { speak: typeof dog.speak, isDog: dog instanceof Dog }",
  "  writeFileSync(new URL('./result.json', import.meta.url), JSON.stringify(result))",
  "})",
].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 resultFile = path.join(dir, "result.json");
if (!fs.existsSync(resultFile)) {
  console.error("ERROR: the test file wrote no result (exit " + r.status + ")");
  process.exit(2);
}
const res = JSON.parse(fs.readFileSync(resultFile, "utf8"));
console.log(JSON.stringify(res));
if (res.speak === "function" && res.isDog === true) {
  console.log("PASS: new (vi.fn(Dog))() has the prototype method speak and is instanceof Dog");
  process.exit(0);
}
if (res.speak === "undefined" && res.isDog === false) {
  console.error("FAIL: new (vi.fn(Dog))() lacks the prototype method speak and is not instanceof Dog");
  process.exit(1);
}
console.error("ERROR: unexpected result " + JSON.stringify(res));
process.exit(2);