fix(wrangler): emit an error event for watch-mode rebuild failures by martijnwalraven · Pull Request #14561 · cloudflare/workers-sdk

martijnwalraven · 2026-07-05T08:43:05Z

When unstable_startWorker runs in watch mode and a rebuild fails, the logBuildOutput esbuild plugin logs the failure and swallows it — no error event is dispatched, so programmatic consumers cannot observe that dev is now serving the previous bundle. Initial build failures, by contrast, route through the error path and surface as buildFailed on the DevEnv.

This threads an optional onRebuildError(errors, warnings) callback through logBuildOutput/runBuild; BundlerController provides it and routes rebuild failures through the same error path as initial-build failures, attaching the esbuild messages as the error cause. Terminal output is unchanged (the error path still logs via logBuildFailure), buildFailed now fires symmetrically, and recovery is unaffected (the next successful rebuild emits reloadComplete as before).

Tests
- Tests included/updated (BundleController: rebuild failure emits the event; next good build recovers)
Public documentation
- Documentation not necessary because: unstable_startWorker is an experimental API; a changeset is included.

🤖 Generated with Claude Code

Initial build failures route through the BundlerController's error event, so DevEnv logs them and emits `buildFailed` — but watch-mode rebuild failures were only logged from the log-build-output esbuild plugin and never became events, leaving startWorker consumers unable to observe a broken rebuild (only initial builds and config parse errors fired `buildFailed`). The plugin gains an optional `onRebuildError` callback that takes ownership of the failure (including its logging) when provided; runBuild threads it, and the BundlerController routes rebuild failures through emitErrorEvent with the esbuild messages as a BuildFailure- shaped cause — DevEnv's existing handling then logs via logBuildFailure exactly as before (no double-logging) and emits `buildFailed` symmetrically with initial builds. Existing logBuildOutput callers are unaffected. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

changeset-bot · 2026-07-05T08:43:09Z

🦋 Changeset detected

Latest commit: 69e1bee

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

This PR includes changesets to release 3 packages

Name	Type
wrangler	Patch
@cloudflare/vite-plugin	Patch
@cloudflare/vitest-pool-workers	Patch

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workers-devprod · 2026-07-05T08:43:28Z

Codeowners approval required for this PR:

✅ @cloudflare/wrangler

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

pkg-pr-new · 2026-07-06T12:05:14Z

@cloudflare/autoconfig

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create-cloudflare

npm i https://pkg.pr.new/create-cloudflare@14561

@cloudflare/deploy-helpers

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@cloudflare/kv-asset-handler

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miniflare

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@cloudflare/pages-shared

npm i https://pkg.pr.new/@cloudflare/pages-shared@14561

@cloudflare/unenv-preset

npm i https://pkg.pr.new/@cloudflare/unenv-preset@14561

@cloudflare/vite-plugin

npm i https://pkg.pr.new/@cloudflare/vite-plugin@14561

@cloudflare/vitest-pool-workers

npm i https://pkg.pr.new/@cloudflare/vitest-pool-workers@14561

@cloudflare/workers-auth

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@cloudflare/workers-editor-shared

npm i https://pkg.pr.new/@cloudflare/workers-editor-shared@14561

@cloudflare/workers-utils

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wrangler

npm i https://pkg.pr.new/wrangler@14561

commit: 69e1bee

edmundhung · 2026-07-06T12:09:42Z

The change makes sense to me. Rebuild failures should be observable like initial build failures.

What are you using unstable_startWorker for today?

For context, we are likely going to steer users toward createTestHarness (a new integration harness to be announced soon) for testing use cases, or Vite's createServer when they need a dev server. I don’t think it should block this fix, but it would be helpful to understand the use case this is unblocking.

martijnwalraven · 2026-07-06T14:01:45Z

What are you using unstable_startWorker for today?

I've been building my own daemon that coordinates typecheck, lint, tests, and dev servers for a worktree, and turns output into compact, diffed, actionable evidence (e.g. answering "What diagnostics changed as a result of the latest edits?", "What errors occurred since the last dev server reload?"). It's mainly meant for agents, because it avoids having them run different tools and repeatedly bring raw output into the context to interpret.

For context, we are likely going to steer users toward createTestHarness (a new integration harness to be announced soon) for testing use cases, or Vite's createServer when they need a dev server. I don’t think it should block this fix, but it would be helpful to understand the use case this is unblocking.

For testing, we're already on @cloudflare/vitest-pool-workers, so createTestHarness is likely not a good match.

The Vite plugin may be able to replace unstable_startWorker for us. We've honestly not evaluated it closely enough. It seems most of what we need is there already: dev registry on by default, persistState, inspectorPort, auxiliaryWorkers, and eager re-evaluation on save via the module runner's full-reload path. What's concretely missing for us compared to how we use unstable_startWorker:

A typed, subscribable error surface. Build/config failures and uncaught runtime exceptions reach a consumer only as logger text (customLogger, with workerd structured logs flattened into the logger) — no equivalent of DevEnv's events with structured causes (file/line).
A recovery signal. Nothing announces a successful reload after a failure (DevEnv's reloadComplete), which a consumer needs to clear error state.
(Minor) API-shaped control of the dev-registry path for ephemeral/isolated environments instead of through env variables like MINIFLARE_REGISTRY_PATH.

If those sound like additions that would be helpful to more people, I'm happy to take a stab at it and open a PR.

edmundhung · 2026-07-06T17:43:20Z

I will need to discuss this with my team further. But the list looks reasonable to me.

I am gonna merge the PR first. Thanks for the fix!

martijnwalraven · 2026-07-06T17:54:23Z

I will need to discuss this with my team further. But the list looks reasonable to me.

Thanks! Note that #14562 is related to this, but it's not as straightforward a fix, so it may need more scrutiny.

workers-devprod

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martijnwalraven requested a review from workers-devprod as a code owner July 5, 2026 08:43

workers-devprod requested review from a team and edmundhung and removed request for a team July 5, 2026 08:43

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workers-devprod added this to workers-sdk Jul 6, 2026

github-project-automation Bot moved this to Untriaged in workers-sdk Jul 6, 2026

fix(wrangler): use explicit BuildFailure for rebuild errors

69e1bee

edmundhung approved these changes Jul 6, 2026

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github-project-automation Bot moved this from Untriaged to Approved in workers-sdk Jul 6, 2026

edmundhung merged commit b973ed3 into cloudflare:main Jul 6, 2026
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github-project-automation Bot moved this from Approved to Done in workers-sdk Jul 6, 2026

workers-devprod mentioned this pull request Jul 6, 2026

Version Packages #14536

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martijnwalraven mentioned this pull request Jul 8, 2026

feat: surface uncaught Worker exceptions as a typed runtimeError event in local dev #14562

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This was referenced Jul 30, 2026

[wrangler] Serialize custom build watcher events to avoid concurrent runs #14936

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[wrangler] serialize custom build watch events to avoid concurrent runs #12704

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fix(wrangler): emit an error event for watch-mode rebuild failures - #14561

fix(wrangler): emit an error event for watch-mode rebuild failures#14561
edmundhung merged 2 commits into
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