I believe all these tests should pass, but one fails:

$ pnpm test

> vitest-define-bug@1.0.0 test /home/user/projects/vitest-define-bug
> vitest --run


 RUN  v4.0.5 /home/user/projects/vitest-define-bug

 ✓  node  src/test.ts (1 test) 3ms
 ✓  browser2 (chromium)  src/test2.ts (1 test) 2ms
 ❯  browser (chromium)  src/test.ts (1 test | 1 failed) 3ms
   × process.env.FOO 2ms

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Failed Tests 1 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

 FAIL   browser (chromium)  src/test.ts > process.env.FOO
ReferenceError: process is not defined
 ❯ src/test.ts:4:9
      2| 
      3| test("process.env.FOO", () => {
      4|   expect(process.env.FOO).toBe("BAR");
       |         ^
      5| });
      6| 

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[1/1]⎯


 Test Files  1 failed | 2 passed (3)
      Tests  1 failed | 2 passed (3)
   Start at  23:20:46
   Duration  1.14s (transform 24ms, setup 0ms, collect 62ms, tests 7ms, environment 0ms, prepare 237ms)

 ELIFECYCLE  Test failed. See above for more details.

node and browser are running the exact same test, but for some reason this configuration is working for node but not for browser:

            define: {
                "process.env.FOO": JSON.stringify("BAR"),
            },

It seems to have something to do with the specific name of the constant, because browser2 is identical to browser except it uses process.FOO.FOO instead of process.env.FOO and that works.

Also if I isolate the browser project into its own file, then it works:

$ pnpm test-browser

> vitest-define-bug@1.0.0 test-browser /home/user/projects/vitest-define-bug
> vitest --run --config vitest.config-browser.ts


 RUN  v4.0.5 /home/user/projects/vitest-define-bug

 ✓  browser (chromium)  src/test.ts (1 test) 2ms
   ✓ process.env.FOO 1ms

 Test Files  1 passed (1)
      Tests  1 passed (1)
   Start at  23:18:40
   Duration  809ms (transform 0ms, setup 0ms, collect 13ms, tests 2ms, environment 0ms, prepare 104ms)

So the requirements for the bug are:

• define called for a constant named something like process.env.FOO
• Run in browser mode
• Configured as a project, not at the root of the vitest config