[svelte-triage-bot]

Fixes #17681

Concurrent syncs share node_modules/$app even when their outDirs differ. Their generated tsconfigs can legitimately contain different bytes. Since writeFileSync truncates before writing, overlapping opens can let a shorter write land after a longer one and leave trailing content. Readers can also observe the truncation window.

Publish through unique sibling temporary files, then rename only complete contents into place. Temporary basenames have a fixed length independent of the destination, so valid long filenames—including prerendered output paths—remain supported. Failed writes/publication clean up the temporary file and propagate the error rather than falling back to another non-atomic write.

The disk comparison introduced in #17310 remains intact: it still prevents needless generated-file rewrites and HMR reloads. Likewise, preserve the shared parent-config placement from #16458 and #16589, and the generated-type rootDirs mapping protected by #17242. Making that mapping independent of outDir would break custom-output type resolution while merely hiding one source of disagreement.

Fixing the shared writer covers both parent tsconfigs, generated declarations/manifests/environment modules, and its existing builder/prerender consumers. This preserves last-complete-writer semantics; it does not introduce separate simultaneous TypeScript configurations for each process.

• Minimal two-server reproduction

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from fcc745c:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/fcc745c67710619f91db351e9ad52c5b1b66e97f

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17682

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: fcc745c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[evdama]

Is there anything this still needs to come out of draft? Happy to help test it.

This is the half of #17310 that 8fec3b0 took out before the merge, since no failure had been captured behind it then. #17681 now captures one, with a reproduction, and this version also answers two of the three points in that commit message: the cleanup is a finally with force: true and there is no fallback write, and the temporary name is a UUID in place of a per-pid suffix.

For context, we have carried an equivalent patch to write since August on a project where syncs overlap all the time - svelte-kit sync runs from our type check and from every Vitest run, and in one two-hour window 12 of 20 runs overlapped another, up to three at once. We would drop the patch once this ships. route_meta_data.json (#17095) is the same window in write_types, which does not go through write, so that one stays open after this.