breaking: remove `experimental.handleRenderingErrors` flag by Rich-Harris · Pull Request #16265 · sveltejs/kit

Rich-Harris · 2026-07-07T21:13:19Z

Removes the flag, as this is the default behaviour in SvelteKit 3

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pkg-svelte-dev · 2026-07-07T21:13:22Z

Install the latest version of @sveltejs/kit from fa432ff:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/fa432ff20a8e4c72673143933ae4f6baff3ac03c

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16265

changeset-bot · 2026-07-07T21:13:29Z

🦋 Changeset detected

Latest commit: fa432ff

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vercel

Additional Suggestion:

Docs still instruct users to enable the removed experimental.handleRenderingErrors flag, which now throws a hard config-validation error.

svelte-docs-bot · 2026-07-07T21:18:47Z

Preview: https://svelte-dev-git-preview-kit-16265-svelte.vercel.app/

Rich-Harris · 2026-07-07T21:33:26Z

gah — this is tricky. transformError is an async function, because handleError might be async. but if transformError returns a Promise then Svelte throws an error if experimental.async isn't enabled. I think this means we need to

keep this under an experimental flag for now, which would really suck (we badly need this feature)
disallow async handleError client hooks unless experimental.async is enabled

… the error status because the reworked `transformError` no longer reassigns the local `status` variable This commit fixes the issue reported at packages/kit/src/runtime/server/page/render.js:205 ## Bug In `packages/kit/src/runtime/server/page/render.js`, `status` is a mutable destructured parameter of `render_response` (line 59). It is consumed in two places: - Hydration payload (line 512): `if (status !== 200 && !error) hydrate.push(...)` - The final HTTP response (line 670): `text(transformed, { status, headers })` The **base** implementation's `transformError` callback reassigned this closure-scoped variable when an error was caught: ```js props.page.status = status = transformed.status; ``` The **reworked** callback (commit `2718a1d`) only updated `props.page.status` in both the sync and async branches, dropping the `status = ...` assignment: ```js props.page.status = error.status; // local `status` never updated ``` ## Trigger / failure mode A page whose `load` succeeds (so `render_response` is invoked with `status: 200`) but whose Svelte component **throws during rendering** and is caught by an error boundary. `transformError` runs, produces e.g. a 500 error object, sets `props.page.status = 500`, but the local `status` stays `200`. The response is then built with `text(transformed, { status: 200 })`, so the client receives an HTTP `200 OK` containing the error page instead of the correct `500`. This is a behavioral regression from the base implementation. ## Fix Restore the local reassignment in both the sync and async branches: ```js props.page.status = status = error.status; ``` This keeps the HTTP status code (and the hydration `status` field) in sync with the error status, matching the previous behavior. I left `props.error` unchanged, as the boundary renders the error via the `failed` snippet using the `transformError` return value rather than the root `error` prop. Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com> Co-authored-by: Rich-Harris <hello@rich-harris.dev>

Rich-Harris · 2026-07-08T00:21:44Z

I went with 'don't wrap sync handleError hooks in an async function', which means that the await_invalid error doesn't happen if your hook is synchronous. If you do hit the error, it's a little cryptic, but I'm honestly not sure how best to address that other than docs

dummdidumm

One question otherwise LGTM. Update: Nevermind, I should've read the comments before reviewing. But I've gotta say it's somewhat of a big ask - "hey you have an async handleError on the server, you gotta add async: true now whether you want to or not".

…c flag

Rich-Harris · 2026-07-08T17:59:13Z

But I've gotta say it's somewhat of a big ask

Existing async hooks will continue to work for the errors they work for today (i.e. load errors etc). For render errors it'll now fail somewhat gracefully, by returning a generic error and printing a 'please enable the flag' warning

…ch can cause an unhandled promise rejection that crashes the server This commit fixes the issue reported at packages/kit/src/runtime/server/utils.js:122 ## Bug In `handle_error_and_jsonify` (`packages/kit/src/runtime/server/utils.js`), when the user's `handleError` hook returns a `Promise` but the app is **not** running with `compilerOptions.experimental.async` enabled and the error occurs during rendering (`!__SVELTEKIT_SUPPORTS_ASYNC__ && state.is_in_render`), the function warns and returns a generic error object: ```js if (result instanceof Promise) { if (!__SVELTEKIT_SUPPORTS_ASYNC__ && state.is_in_render) { console.warn(...); return { status, message: 'Internal Error' }; // <-- `result` promise discarded } ... } ``` The `result` promise is **discarded without being awaited or given a `.catch()` handler**. `with_request_store` (`packages/kit/src/exports/internal/server/event.js`) only wraps the call in `try/finally` — **not** `try/catch` — so it passes the promise straight through without attaching any rejection handler: ```js export function with_request_store(store, fn) { try { sync_store = store; return als ? als.run(store, fn) : fn(); } finally { if (!IN_WEBCONTAINER) sync_store = null; } } ``` Nothing else along the path handles it. ## Concrete trigger 1. User defines an **async** `handleError` hook that can reject (e.g. an `await`ed call to an external logging/Sentry service that throws). 2. `compilerOptions.experimental.async` is **not** enabled — exactly the misconfiguration this branch warns about. 3. A rendering error occurs → `handle_error_and_jsonify` is reached with `state.is_in_render === true` and the hook returns a promise. 4. The warn branch returns the generic object and drops `result`. 5. When `result` later rejects, there is no handler → **unhandled promise rejection**. Under Node.js v15+ default behavior (`--unhandled-rejections=throw`), this can terminate the server process. ## Fix Attach a no-op `.catch(() => {})` to the discarded promise before returning, preventing the unhandled rejection: ```js // we're discarding the result, but we still need to prevent an unhandled // rejection if the user's async `handleError` hook rejects result.catch(() => {}); return { status, message: 'Internal Error' }; ``` This matches the codebase's existing convention for intentionally-ignored promises (`event.js:14`, `core/postbuild/queue.js:20`, `app/server/remote/requested.js:263`). Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com> Co-authored-by: Rich-Harris <hello@rich-harris.dev>

This PR was opened by the [Changesets release](https://github.com/changesets/action) GitHub action. When you're ready to do a release, you can merge this and the packages will be published to npm automatically. If you're not ready to do a release yet, that's fine, whenever you add more changesets to version-3, this PR will be updated. ⚠️

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⚠️ # Releases ## @sveltejs/kit@3.0.0-next.8 ### Major Changes - breaking: remove `experimental.handleRenderingErrors` flag ([#16265](#16265)) - breaking: make `getRequest` and `setResponse` synchronous ([#16280](#16280)) - breaking: make `page.url` immutable on a type level ([#16256](#16256)) - breaking: add `refreshAll` and deprecate `invalidateAll` ([#16289](#16289)) ### Minor Changes - feat: allow hyphens in param and matcher names ([#16284](#16284)) - feat: add `ErrorProps` to generated types ([#16272](#16272)) ### Patch Changes - fix: detect destructured `load` and `actions` exports during type generation ([#16329](#16329)) - fix: ensure CSS URL references are absolute when `paths.relative` is `false` ([#16315](#16315)) - fix: exclude deleted cookies from `cookies.getAll()` so it stays consistent with `cookies.get()` ([#16297](#16297)) - fix: reset failed `<svelte:boundary>` on client navigation so a stale `+error.svelte` is torn down ([#16296](#16296)) - fix: preserve shared client chunk hashes when the app version changes ([#16324](#16324)) - fix: align MAX_COOKIE_SIZE with RFC 6265bis ([#16322](#16322)) - fix: use mouseover+mousemove for preloading to reduce events ([#16325](#16325)) ## @sveltejs/package@3.0.0-next.2 ### Minor Changes - feat: warn when using a `.server.` file or file inside a `server` directory without importing a server-only module ([#16266](#16266)) ## @sveltejs/adapter-auto@8.0.0-next.1 ### Patch Changes - fix: allow prerelease versions of SvelteKit 3 to satisfy the peer dependency range ([#16286](#16286)) - Updated dependencies [[`737d119`](737d119), [`fa78efb`](fa78efb), [`07c207e`](07c207e), [`a47071b`](a47071b), [`14d7d5a`](14d7d5a), [`5c38e51`](5c38e51), [`0702baa`](0702baa), [`e1938c6`](e1938c6), [`8293144`](8293144), [`f76d7d9`](f76d7d9), [`ab5c253`](ab5c253), [`b557b1b`](b557b1b), [`4a513e2`](4a513e2)]: - @sveltejs/kit@3.0.0-next.8 ## @sveltejs/adapter-cloudflare@8.0.0-next.2 ### Patch Changes - fix: allow prerelease versions of SvelteKit 3 to satisfy the peer dependency range ([#16286](#16286)) - Updated dependencies [[`737d119`](737d119), [`fa78efb`](fa78efb), [`07c207e`](07c207e), [`a47071b`](a47071b), [`14d7d5a`](14d7d5a), [`5c38e51`](5c38e51), [`0702baa`](0702baa), [`e1938c6`](e1938c6), [`8293144`](8293144), [`f76d7d9`](f76d7d9), [`ab5c253`](ab5c253), [`b557b1b`](b557b1b), [`4a513e2`](4a513e2)]: - @sveltejs/kit@3.0.0-next.8 ## @sveltejs/adapter-netlify@7.0.0-next.3 ### Patch Changes - fix: include `utils.js` in package.json `files` ([#16298](#16298)) - Updated dependencies [[`737d119`](737d119), [`fa78efb`](fa78efb), [`07c207e`](07c207e), [`a47071b`](a47071b), [`14d7d5a`](14d7d5a), [`5c38e51`](5c38e51), [`0702baa`](0702baa), [`e1938c6`](e1938c6), [`8293144`](8293144), [`f76d7d9`](f76d7d9), [`ab5c253`](ab5c253), [`b557b1b`](b557b1b), [`4a513e2`](4a513e2)]: - @sveltejs/kit@3.0.0-next.8 ## @sveltejs/adapter-node@6.0.0-next.3 ### Patch Changes - fix: allow prerelease versions of SvelteKit 3 to satisfy the peer dependency range ([#16286](#16286)) - Updated dependencies [[`737d119`](737d119), [`fa78efb`](fa78efb), [`07c207e`](07c207e), [`a47071b`](a47071b), [`14d7d5a`](14d7d5a), [`5c38e51`](5c38e51), [`0702baa`](0702baa), [`e1938c6`](e1938c6), [`8293144`](8293144), [`f76d7d9`](f76d7d9), [`ab5c253`](ab5c253), [`b557b1b`](b557b1b), [`4a513e2`](4a513e2)]: - @sveltejs/kit@3.0.0-next.8 ## @sveltejs/adapter-static@4.0.0-next.1 ### Patch Changes - fix: allow prerelease versions of SvelteKit 3 to satisfy the peer dependency range ([#16286](#16286)) - Updated dependencies [[`737d119`](737d119), [`fa78efb`](fa78efb), [`07c207e`](07c207e), [`a47071b`](a47071b), [`14d7d5a`](14d7d5a), [`5c38e51`](5c38e51), [`0702baa`](0702baa), [`e1938c6`](e1938c6), [`8293144`](8293144), [`f76d7d9`](f76d7d9), [`ab5c253`](ab5c253), [`b557b1b`](b557b1b), [`4a513e2`](4a513e2)]: - @sveltejs/kit@3.0.0-next.8 ## @sveltejs/adapter-vercel@7.0.0-next.2 ### Patch Changes - fix: allow prerelease versions of SvelteKit 3 to satisfy the peer dependency range ([#16286](#16286)) - Updated dependencies [[`737d119`](737d119), [`fa78efb`](fa78efb), [`07c207e`](07c207e), [`a47071b`](a47071b), [`14d7d5a`](14d7d5a), [`5c38e51`](5c38e51), [`0702baa`](0702baa), [`e1938c6`](e1938c6), [`8293144`](8293144), [`f76d7d9`](f76d7d9), [`ab5c253`](ab5c253), [`b557b1b`](b557b1b), [`4a513e2`](4a513e2)]: - @sveltejs/kit@3.0.0-next.8 Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

breaking: remove experimental.handleRenderingErrors flag

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Rich-Harris deleted the remove-rendering-error-flag branch July 8, 2026 20:01

github-actions Bot mentioned this pull request Jul 8, 2026

Version Packages (next) #16282

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Nic-Polumeyv mentioned this pull request Jul 17, 2026

[SvelteKit 3.0.0-next.8]: In dev mode with remote functions, +error.page selects the wrong layout #16345

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Nic-Polumeyv mentioned this pull request Jul 21, 2026

refactor: split remote handler + add unit tests #16064

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