vitest >=5.0.0 changed the default of `--merge-reports`. By default, Vitest stores all blob test results in .vitest/blob/, so they can be later merged using --merge-reports. 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/blob";
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) : "") };
}

write("a.test.mjs", "import { test, expect } from 'vitest'\ntest('blob probe', () => { expect(1).toBe(1) })\n");
const run = vitest(["run", "--reporter=blob"]);
console.log(run.out);
if (run.status !== 0) {
  console.error("ERROR: vitest run --reporter=blob exited " + run.status);
  process.exit(2);
}
const blob = path.join(dir, ".vitest", "blob", "blob.json");
if (!fs.existsSync(blob)) {
  // The symptom of the old default: the report went to .vitest-reports/ instead.
  if (fs.existsSync(path.join(dir, ".vitest-reports", "blob.json"))) {
    console.error("FAIL: the blob reporter did not write .vitest/blob/blob.json by default");
    process.exit(1);
  }
  console.error("ERROR: no blob report was found in either folder");
  process.exit(2);
}
// --merge-reports without a path must read the same default folder.
const merge = vitest(["run", "--merge-reports"]);
console.log(merge.out);
if (merge.status !== 0 || !/1 passed/.test(merge.out)) {
  console.error("FAIL: --merge-reports without a path did not merge .vitest/blob (exit " + merge.status + ")");
  process.exit(1);
}
console.log("PASS: blob report written to and merged from .vitest/blob/");

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/blob";
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) : "") };
}

write("a.test.mjs", "import { test, expect } from 'vitest'\ntest('blob probe', () => { expect(1).toBe(1) })\n");
const run = vitest(["run", "--reporter=blob"]);
console.log(run.out);
if (run.status !== 0) {
  console.error("ERROR: vitest run --reporter=blob exited " + run.status);
  process.exit(2);
}
const blob = path.join(dir, ".vitest", "blob", "blob.json");
if (!fs.existsSync(blob)) {
  // The symptom of the old default: the report went to .vitest-reports/ instead.
  if (fs.existsSync(path.join(dir, ".vitest-reports", "blob.json"))) {
    console.error("FAIL: the blob reporter did not write .vitest/blob/blob.json by default");
    process.exit(1);
  }
  console.error("ERROR: no blob report was found in either folder");
  process.exit(2);
}
// --merge-reports without a path must read the same default folder.
const merge = vitest(["run", "--merge-reports"]);
console.log(merge.out);
if (merge.status !== 0 || !/1 passed/.test(merge.out)) {
  console.error("FAIL: --merge-reports without a path did not merge .vitest/blob (exit " + merge.status + ")");
  process.exit(1);
}
console.log("PASS: blob report written to and merged from .vitest/blob/");