[gonzaloriestra]

WHY are these changes introduced?

Fixes https://github.com/shop/issues-develop/issues/24323.

Agents need structured rebuild results and errors while production build watchers keep running. build --json currently rejects --watch.

WHAT is this pull request doing?

Add typed JSONL records for watch startup, build cycles, output paths, recoverable compilation errors, and shutdown. Coordinate client and server watchers with a temporary trigger so client-only edits, including plugin-managed inputs, complete their JSON cycles. Ignore repeated trigger notifications for an already-consumed client build when reporting cycles and invoking server callbacks.

Close both Vite watchers on cancellation and clean up the client watcher and trigger if server setup fails. Keep finite JSON results and text presentation intact, refresh schema help and the manifest, and add a minor changeset.

HOW to test your changes?

In a Hydrogen 2026-04 project, run shopify hydrogen build --watch --json > /tmp/hydrogen-build.jsonl 2> /tmp/hydrogen-build.log. Change a server source file, introduce a syntax error, then fix it. Check that failures are followed by successful rebuild records with existing artifact paths. Edit a client-only entry or a file watched by a client plugin and verify another completed cycle. Press Ctrl-C and verify watch-stopped and exit 130. Compare text watch output and finite build --json; inspect build --json-schema.

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset
• I've added tests to cover my changes
• I've added or updated the documentation

[shopify]

Oxygen deployed a preview of your gonzalo/hydrogen-json-watch-build branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	October 28, 2026 8:41 AM

Learn more about Hydrogen's GitHub integration.

[fredericoo]

thank u so much, this is great! gotta do a few fixes but overall it's near done! lmk when you change it so we merge it asap

[fredericoo]

thanks for turning these around so quickly! the trigger dedupe and the JSON-only comment are sorted, but the cycle: 4 flake still reproduces and the slimmed-down unit test lost the normal-stop cleanup checks - details inline

[fredericoo]

all sorted, thanks! looped the build suite 40 times (plus 20 under CPU load) with no flakes, and the new normal-stop test catches each cleanup line being removed. LGTM

one thing for a follow-up: a doubled fs event still gives JSON consumers an extra build-started/build-failed pair, so skipping client cycles with no changes would be worth doing at some point