Describe the bug

I found a potential mistake in 2.36.2 => 2.36.3 (commit hash: 8cf71f05b8600e6c4f050316a24aa8d39fb12179). There was an idea to consolidate the DEV and __SVELTEKIT_DEV__ vars. __SVELTEKIT_DEV__ was simply removed, probably assuming it's the same thing as DEV. But actually it's not 😁

Reproduction

The problem lies in the definition of those vars.
As I understand from the esm-env lib: DEV = NODE_ENV !== 'production'
Whereas __SVELTEKIT_DEV__ is defined in packages/kit/src/exports/vite/index.js:375 and it's eventually __SVELTEKIT_DEV__ = !is_build.

That was the reason why my project was making a broken build when building for staging. Previously the staging build had __SVELTEKIT_DEV__ as false, but the DEV is true now in the same case (as I mentioned before, DEV = NODE_ENV !== 'production')

I didn't dig deeper for the particular line of code that caused a problem for me, but I can see many config options that depend on __SVELTEKIT_DEV__. So the fact that they all work differently now should potentially lead to errors.

Logs

System Info

System:
    OS: macOS 26.2
    CPU: (10) arm64 Apple M1 Pro
    Memory: 269.84 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.16.0 - /usr/local/bin/node
    npm: 10.9.2 - /usr/local/bin/npm
  Browsers:
    Chrome: 146.0.7680.177
    Safari: 26.2
  npmPackages:
    @sveltejs/adapter-static: 3.0.8 => 3.0.8 
    @sveltejs/kit: 2.50.0 => 2.50.0 
    @sveltejs/vite-plugin-svelte: 5.1.1 => 5.1.1 
    svelte: 5.14.4 => 5.14.4 
    vite: 6.3.6 => 6.3.6

Severity

serious, but I can work around it

Additional Information

No response