git clone https://github.com/mkizka/nextjs-serveractions-jest
cd nextjs-serveractions-jest
yarn
yarn test

$ cat next.config.js
/** @type {import("next").NextConfig} */
module.exports = {
  reactStrictMode: true,
  experimental: {
    serverActions: true,
  },
};

$ yarn test
yarn run v1.22.19
$ jest
- warn You have enabled experimental feature (serverActions) in next.config.js.
- warn Experimental features are not covered by semver, and may cause unexpected or broken application behavior. Use at your own risk.

 FAIL  app/action.test.ts
  ● Test suite failed to run


      × To use Server Actions, please enable the feature flag in your Next.js config. Read more: https://nextjs.org/docs/app/building-your-application/data-fetching/server-actions#convention
       ╭─[/home/ubuntu/nextjs-serveractions-jest/app/action.ts:1:1]
     1 │ "use server";
       · ────────────
     2 │
     3 │ export const action = () => {
     4 │   console.log("action called");
       ╰────

      3 | jest.spyOn(console, "log");
      4 |
    > 5 | test("action", () => {
        |                 ^
      6 |   action();
      7 |   expect(console.log).toHaveBeenCalledWith("action called");
      8 | });

      at Object.transformSync (node_modules/next/src/build/swc/index.ts:1000:25)
      at Object.<anonymous> (app/action.test.ts:5:17)

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        0.15 s
Ran all test suites.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.