feat: allow error boundaries to work on the server by dummdidumm · Pull Request #17672 · sveltejs/svelte

dummdidumm · 2026-02-10T22:24:33Z

This makes error boundaries run on the server if a new onerror handler is passed to render. onerror can either synchronously or asynchronously return a value. It should be a sanitized JSON.stringify-able value so that it can be passed to the client for hydration via a comment. mount/hydrate also get the onerror property.

If no onerror is passed to render it will just throw just like before, hence this is backwards compatible.

This work is important for SvelteKit to allow +error.svelte to make use of them and in general to make boundaries properly work during SSR (also see sveltejs/kit#14398).

closes #15370

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This makes error boundaries run on the server if a new `onerror` handler is passed to `render`. `onerror` can either synchronously or asynchronously return a value. It should be a sanitized JSON.stringify-able value so that it can be passed to the client for hydration via a comment. `mount/hydrate` also get the `onerror` property. If no `onerror` is passed to `render` it will just throw just like before, hence this is backwards compatible. This work is important for SvelteKit to allow `+error.svelte` to make use of them and in general to make boundaries properly work during SSR (also see sveltejs/kit#14398). closes #15370

changeset-bot · 2026-02-10T22:24:43Z

🦋 Changeset detected

Latest commit: 584d271

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svelte-docs-bot · 2026-02-10T22:25:24Z

Preview: https://svelte-dev-git-preview-svelte-17672-svelte.vercel.app/

github-actions · 2026-02-10T22:26:03Z

Playground

pnpm add https://pkg.pr.new/svelte@17672

Rich-Harris · 2026-02-10T23:21:11Z

I wonder if we should call the app-level onerror handleError instead, to a) differentiate it from the boundary-level handler and b) align it with SvelteKit's handleError?

dummdidumm · 2026-02-10T23:23:20Z

Marking as draft, want to see if this really does what it should in combination with SvelteKit

packages/svelte/src/internal/server/renderer.js

packages/svelte/src/compiler/phases/2-analyze/index.js

Rich-Harris · 2026-02-11T00:08:02Z

If a failed snippet is SSR'd, should the boundary's onerror be called with the serialized error upon hydration? My gut says yes

Rich-Harris · 2026-02-11T00:18:07Z

this was too cumbersome to do with the suggestion UI: #17673

packages/svelte/src/internal/client/render.js

#17672

Rich-Harris

with the caveat that we should try this out in SvelteKit to make sure we haven't missed anything obvious, this looks excellent

Rich-Harris · 2026-02-12T16:45:15Z

Recapping offline discussion: it would be much better if transformError could be synchronous. This would have the knock-on effect of forcing handleError in SvelteKit to be synchronous, which would be a breaking change. Personally I think it's a change that makes sense; if we require handleError to be sync during the flagged-in period we can at least flush out any valid use cases for returning a promise

Rich-Harris · 2026-02-12T16:46:08Z

Counterpoint: sveltejs/kit#7756

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

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 ## svelte@5.53.0 ### Minor Changes - feat: allow comments in tags ([#17671](#17671)) - feat: allow error boundaries to work on the server ([#17672](#17672)) ### Patch Changes - fix: use TrustedHTML to test for customizable <select> support, where necessary ([#17743](#17743)) - fix: ensure head effects are kept in the effect tree ([#17746](#17746)) - chore: deactivate current_batch by default in unset_context ([#17738](#17738)) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

dummdidumm temporarily deployed to Publish pkg.pr.new (maintainers) February 10, 2026 22:24 — with GitHub Actions Inactive

fix

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dummdidumm marked this pull request as draft February 10, 2026 23:25

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chore: DRY out boundary compiler code #17673

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feat: Support for <svelte:boundary> on SSR / static generation (POC) #15462

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6 tasks

chore: DRY out boundary compiler code (#17673)

3b7f434

#17672

dummdidumm temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 09:42 — with GitHub Actions Inactive

dummdidumm added 3 commits February 11, 2026 10:45

revert this was committed by accident

020bdfb

fix / don't swallow error (user might want to throw a different one)

f288618

unnecessary

9d298fb

dummdidumm temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 10:27 — with GitHub Actions Inactive

onerror -> handleError

2edee31

Rich-Harris temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 18:27 — with GitHub Actions Inactive

flesh out docs

03940d7

Rich-Harris temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 19:10 — with GitHub Actions Inactive

on second thoughts, handleError -> transformError

1acec3b

regenerate

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Rich-Harris temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 19:35 — with GitHub Actions Inactive

more detail

1550be6

Rich-Harris temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 19:41 — with GitHub Actions Inactive

get the preview to deploy, hopefully

9472ff6

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small tweak

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reset context on async error

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dummdidumm temporarily deployed to Publish pkg.pr.new (maintainers) February 11, 2026 21:20 — with GitHub Actions Inactive

add to legacy entry points

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dummdidumm mentioned this pull request Feb 12, 2026

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

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5 tasks

This was referenced Feb 14, 2026

+error.svelte not triggered correctly when navigating, leaving the app broken #17700

Closed

fix: run effects in pending snippets #17719

Merged

Rich-Harris and others added 6 commits February 17, 2026 08:15

merge main

d304f42

handle snippet attributes

270a9ae

Merge branch 'main' into ssr-error-boundary

e254b47

prettier

de24364

catch this one, too

8cc86a0

tweak

584d271

Rich-Harris marked this pull request as ready for review February 18, 2026 21:50

Rich-Harris merged commit 2661513 into main Feb 18, 2026
20 checks passed

Rich-Harris deleted the ssr-error-boundary branch February 18, 2026 21:50

github-actions bot mentioned this pull request Feb 18, 2026

Version Packages #17747

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dummdidumm commented Feb 10, 2026

Before submitting the PR, please make sure you do the following

Tests and linting

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