feat: allow error boundaries to catch errors on the server by dummdidumm · Pull Request #15308 · sveltejs/kit

dummdidumm · 2026-02-12T22:44:49Z

Take advantage of sveltejs/svelte#17672 to add the handleError hook as transformError so that error boundaries run on the server. Behind an experimental flag.

Closes #14808
Closes #14410
Closes #14398
Closes #14932

New tests fail right now, install the other PR via pkg.new locally to see it in effect.

Todos:

what about page.status, should it be passed as a prop to +error.svelte, too? What would it be in case of a rendering error though? -> answer: not pass, but update it on error
what about the navigation event on the client? handleError receives one, but for render errors we don't have this context. Do a best effort of a stub, like "this was the URL this happened on" etc? Or just pass null? Solved: We can use the last navigation event we know of
a few others (see code)

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Take advantage of sveltejs/svelte#17672 to add the `handleError` hook as `transformError` so that error boundaries run on the server. Behind an experimental flag. Closes #14808 Closes #14410 Closes #14398

changeset-bot · 2026-02-12T22:44:55Z

🦋 Changeset detected

Latest commit: bb79369

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svelte-docs-bot · 2026-02-12T22:45:27Z

Preview: https://svelte-dev-git-preview-kit-15308-svelte.vercel.app/

packages/kit/src/core/config/index.spec.js

packages/kit/src/core/sync/write_root.js

packages/kit/src/runtime/client/client.js

Rich-Harris · 2026-02-13T00:39:14Z

packages/kit/src/runtime/client/client.js


+	if (errors && __SVELTEKIT_EXPERIMENTAL_USE_TRANSFORM_ERROR__) {
+		let last_idx = -1;
+		result.props.errors = (


in an ideal world, we wouldn't need to do all this — the error boundary self-activate. that would involve turning load errors into render errors. how achievable does that sound? (if the answer is 'lol not at all' then that's fine)

...maybe by making the data prop a getter that throws? 🤔

I don't know what you mean by this. This is about loading the +error.svelte components. We have to do this here upfront because we can't rely on using await in the template - it's still experimental and will be for a while.

Rich-Harris · 2026-02-13T00:40:20Z

packages/kit/src/runtime/server/page/index.js


+		/** @type {Array<import('types').SSRComponent | undefined> | undefined} */
+		let error_components;
+		if (options.server_error_boundaries && ssr) {


see https://github.com/sveltejs/kit/pull/15308/changes#r2801722105

documentation/docs/30-advanced/25-errors.md

This will get the tests in sveltejs/kit#15308 green, right now they fail because page state cannot be found because context not available

packages/kit/src/runtime/client/client.js

documentation/docs/30-advanced/25-errors.md

packages/kit/src/runtime/server/page/render.js

… double colons instead of single colon. This commit fixes the issue reported at documentation/docs/30-advanced/25-errors.md:149 **Bug Explanation:** In the SvelteKit documentation file `documentation/docs/30-advanced/25-errors.md`, there is a typo in a code example demonstrating the use of `<svelte:boundary>` for error handling. The closing tag was written as `</svelte::boundary>` (with two colons) instead of `</svelte:boundary>` (with one colon). This is inconsistent with the opening tag `<svelte:boundary>` which correctly uses a single colon. This typo would confuse developers reading the documentation, as it shows invalid Svelte syntax. Users copying this code example would get a syntax error when trying to use it. **Fix Explanation:** Changed the closing tag from `</svelte::boundary>` to `</svelte:boundary>` to match the opening tag and use the correct Svelte special element syntax. Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com> Co-authored-by: dummdidumm <sholthausen@web.de>

…after `error` has been reassigned to the transformed `App.Error`, causing the original HTTP status to always be replaced with 500. This commit fixes the issue reported at packages/kit/src/runtime/server/page/render.js:194 ## Bug Explanation The bug is located in `packages/kit/src/runtime/server/page/render.js` in the `transformError` callback (line 194). ### Why it happens: 1. The `transformError` callback receives `e` as a parameter - this is the original error which could be an `HttpError` (e.g., from `error(403, 'Forbidden')`) or a `SvelteKitError`, both of which have a `.status` property. 2. The callback calls `handle_error_and_jsonify(event, event_state, options, e)` which transforms the error into an `App.Error` object. This `App.Error` is NOT an `HttpError` or `SvelteKitError`. 3. The code then reassigns: `props.page.error = props.error = error = transformed;` 4. The bug: `props.page.status = status = get_status(error);` - This calls `get_status()` on the transformed error (now an `App.Error`), NOT the original error. 5. The `get_status()` function (in `packages/kit/src/utils/error.js`) checks: `error instanceof HttpError || error instanceof SvelteKitError ? error.status : 500` 6. Since the transformed `App.Error` is neither `HttpError` nor `SvelteKitError`, `get_status()` always returns 500. ### Impact: When a component throws `error(403, 'Forbidden')` or any other HTTP error during rendering (when `handleRenderingErrors` is enabled), the 403 status would be lost and replaced with 500. This affects the HTTP response status code and the page's status property. ## Fix Explanation Changed `get_status(error)` to `get_status(e)` on line 194. This ensures we extract the status from the original error `e` (which may be an `HttpError` or `SvelteKitError` with the correct status) rather than the transformed error which is always `App.Error`. The fix is minimal and targeted - it only changes the argument passed to `get_status()` to use the correct variable that still contains the original error with its status information. Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com> Co-authored-by: dummdidumm <sholthausen@web.de>

…with 500 when errors are transformed in the client-side `transformError` callback because `get_status()` is called on the transformed `App.Error` instead of the original error. This commit fixes the issue reported at packages/kit/src/runtime/client/client.js:622 ## Bug Explanation The bug exists in the client-side `transformError` callback when the experimental error handling feature is enabled. ### How it happens: 1. When an error is thrown (e.g., `error(403, 'Forbidden')`), it's passed to `transformError` as parameter `e` 2. The error is transformed via `handle_error(e, ...)` which returns an `App.Error` object (a user-friendly error representation) 3. `get_status(error)` is then called on the **transformed** error to set the HTTP status 4. However, `get_status()` (in `packages/kit/src/utils/error.js`) only extracts the correct status from `HttpError` or `SvelteKitError` instances - for any other type, it returns 500 5. Since the transformed error is an `App.Error` (plain object), `get_status()` always returns 500, losing the original status ### Impact: If a user throws `error(403, 'Forbidden')` in a component, the page would incorrectly show status 500 instead of 403. This breaks the expected behavior where `$page.status` should reflect the original error status. ## Fix Explanation The fix changes `get_status(error)` to `get_status(e)` in the client-side code (`packages/kit/src/runtime/client/client.js` line 621): * Before: `rendering_error = { error, status: get_status(error) };` * After: `rendering_error = { error, status: get_status(e) };` By calling `get_status(e)` on the **original** error (parameter `e`) before transformation, we correctly extract the HTTP status from the `HttpError` or `SvelteKitError` instance, preserving the intended status code (403, 404, etc.). Note: The server-side fix in `render.js` was already applied in a separate commit. Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com> Co-authored-by: dummdidumm <sholthausen@web.de>

documentation/docs/30-advanced/25-errors.md

Rich-Harris · 2026-03-10T13:46:38Z

very excited for this, and even more excited to merge it into version-3 then delete a whole bunch of it

Co-authored-by: Rich Harris <richard.a.harris@gmail.com>

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 main, this PR will be updated. # Releases ## @sveltejs/kit@2.54.0 ### Minor Changes - feat: allow error boundaries to catch errors on the server ([#15308](#15308)) ### Patch Changes - chore: upgrade `devalue` ([#15535](#15535)) - fix: don't wait for remote functions that are not awaited in the template ([#15280](#15280)) - feat: allow `resolve()` to accept pathnames with a search string and/or hash ([#15458](#15458)) - chore: remove deprecation warnings for `config.kit.files.*` options when validating the Svelte config file ([#15482](#15482)) - fix: handles form target attribute in remote form redirects ([#15457](#15457)) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

feat: allow error boundaries to catch errors on the server

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Take advantage of sveltejs/svelte#17672 to add the `handleError` hook as `transformError` so that error boundaries run on the server. Behind an experimental flag. Closes #14808 Closes #14410 Closes #14398

Rich-Harris reviewed Feb 13, 2026

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dummdidumm and others added 15 commits February 13, 2026 11:04

better code output; pass error prop

4dc9665

ensure NavigationEvent is set

59867d4

test it

ed183f7

fixes/tweaks

4747589

root.svelte snippet simplifcation

081cc30

fix data prop passing

54c76be

fix test app, client/write_root/render fixes

4201313

root layout wraps root error component, not the other way around

893d207

Merge branch 'main' into error-boundaries-server

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bump packages

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lint

46901d8

docs

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54d51d5

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missed a spot

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dummdidumm and others added 2 commits February 22, 2026 22:27

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update page.error/status

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dummdidumm added a commit to sveltejs/svelte that referenced this pull request Feb 24, 2026

fix: set server context after async transformError

d798dfe

This will get the tests in sveltejs/kit#15308 green, right now they fail because page state cannot be found because context not available

dummdidumm mentioned this pull request Feb 24, 2026

fix: set server context after async transformError sveltejs/svelte#17799

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Merge branch 'main' into error-boundaries-server

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dummdidumm marked this pull request as ready for review February 26, 2026 18:38

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dummdidumm mentioned this pull request Mar 10, 2026

Invalid svelte:elements are not caught by svelte:boundary during SSR sveltejs/svelte#17890

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Merge branch 'main' into error-boundaries-server

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Update documentation/docs/30-advanced/25-errors.md

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Co-authored-by: Rich Harris <richard.a.harris@gmail.com>

dummdidumm merged commit a85396c into main Mar 10, 2026
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dummdidumm deleted the error-boundaries-server branch March 10, 2026 14:16

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feat: allow error boundaries to catch errors on the server#15308
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