vitest >=5.0.0 changed the default of `extends`. Projects defined with an inline configuration inherit options from the root-level configuration. 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/inlineproject";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
function write(file, text) {
  const p = path.join(dir, file);
  fs.mkdirSync(path.dirname(p), { recursive: true });
  fs.writeFileSync(p, text);
}
function vitest(args, cwd) {
  const r = cp.spawnSync(process.execPath, ["/w/node_modules/vitest/vitest.mjs", ...args], {
    cwd: cwd || dir,
    encoding: "utf8",
    env: { ...process.env, NO_COLOR: "1", CI: "1" },
    timeout: 40000,
  });
  return { status: r.status, out: (r.stdout || "") + (r.stderr || "") + (r.error ? String(r.error) : "") };
}

// Root config: setupFiles at the root, one inline project without extends.
write("vitest.config.mjs", [
  "export default {",
  "  test: {",
  "    setupFiles: ['./setup.mjs'],",
  "    projects: [{ test: { name: 'inline', include: ['probe.test.mjs'] } }],",
  "  },",
  "}",
].join("\n") + "\n");
write("setup.mjs", "globalThis.__ROOT_SETUP_RAN__ = true\n");
write("probe.test.mjs", [
  "import { test } from 'vitest'",
  "import { writeFileSync } from 'node:fs'",
  "test('probe', () => { writeFileSync(new URL('./result.txt', import.meta.url), String(globalThis.__ROOT_SETUP_RAN__ === true)) })",
].join("\n") + "\n");
const r = vitest(["run"]);
console.log(r.out);
const resultFile = path.join(dir, "result.txt");
if (!fs.existsSync(resultFile)) {
  console.error("ERROR: the inline project's test did not run (exit " + r.status + ")");
  process.exit(2);
}
const inherited = fs.readFileSync(resultFile, "utf8");
if (inherited !== "true") {
  console.error("FAIL: the inline project did not inherit the root setupFiles (extends is not on by default)");
  process.exit(1);
}
if (r.status !== 0) {
  console.error("ERROR: vitest exited " + r.status);
  process.exit(2);
}
console.log("PASS: the inline project inherited the root setupFiles without extends: true");

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/inlineproject";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
function write(file, text) {
  const p = path.join(dir, file);
  fs.mkdirSync(path.dirname(p), { recursive: true });
  fs.writeFileSync(p, text);
}
function vitest(args, cwd) {
  const r = cp.spawnSync(process.execPath, ["/w/node_modules/vitest/vitest.mjs", ...args], {
    cwd: cwd || dir,
    encoding: "utf8",
    env: { ...process.env, NO_COLOR: "1", CI: "1" },
    timeout: 40000,
  });
  return { status: r.status, out: (r.stdout || "") + (r.stderr || "") + (r.error ? String(r.error) : "") };
}

// Root config: setupFiles at the root, one inline project without extends.
write("vitest.config.mjs", [
  "export default {",
  "  test: {",
  "    setupFiles: ['./setup.mjs'],",
  "    projects: [{ test: { name: 'inline', include: ['probe.test.mjs'] } }],",
  "  },",
  "}",
].join("\n") + "\n");
write("setup.mjs", "globalThis.__ROOT_SETUP_RAN__ = true\n");
write("probe.test.mjs", [
  "import { test } from 'vitest'",
  "import { writeFileSync } from 'node:fs'",
  "test('probe', () => { writeFileSync(new URL('./result.txt', import.meta.url), String(globalThis.__ROOT_SETUP_RAN__ === true)) })",
].join("\n") + "\n");
const r = vitest(["run"]);
console.log(r.out);
const resultFile = path.join(dir, "result.txt");
if (!fs.existsSync(resultFile)) {
  console.error("ERROR: the inline project's test did not run (exit " + r.status + ")");
  process.exit(2);
}
const inherited = fs.readFileSync(resultFile, "utf8");
if (inherited !== "true") {
  console.error("FAIL: the inline project did not inherit the root setupFiles (extends is not on by default)");
  process.exit(1);
}
if (r.status !== 0) {
  console.error("ERROR: vitest exited " + r.status);
  process.exit(2);
}
console.log("PASS: the inline project inherited the root setupFiles without extends: true");