Hydrated match loaderData overwritten during initial client load

[roduyemi]

Which project does this relate to?

Start

Describe the bug

Description:

When using TanStack Start with React 19.1 (stable), the Transitioner component fires
commitLocation → router.load() during the initial hydration render. This re-runs route
loaders, overwriting the dehydrated loaderData.

The root cause:

https://github.com/TanStack/router/blob/main/packages/router-core/src/ssr/ssr-client.ts clears match._nonReactive.dehydrated = undefined for fully SSR'd matches before the Transitioner effect fires. When load() runs, shouldSkipLoader in https://github.com/TanStack/router/blob/main/packages/router-core/src/load-matches.ts should skip dehydrated matches — but _nonReactive.dehydrated has already been cleared by hydrate(), so loaders re-run.

This doesn't happen with React canary (used in our other project) because the effect timing is different — Transitioner's commitLocation doesn't fire during hydration.

Workaround:

Re-set _nonReactive.dehydrated = true on matches before the first router.load() call.

Your Example Website or App

N/A

Steps to Reproduce the Bug or Issue

1. Create a TanStack Start project with defaultSsr: true
2. Add a route loader that returns a defer()'d value
3. Use useLoaderData() in the route component to read the deferred value
4. Build and serve with streaming SSR
5. Hard refresh the page
6. The deferred value from useLoaderData() is undefined on the client — the loader re-ran
   and cleared the dehydrated data

Expected behavior

Expected behavior:

Dehydrated matches should not have their loaders re-run during the initial hydration cycle. Either _nonReactive.dehydrated should be preserved until after the first load, or load() should recognise it's running during hydration and skip matches that already have loaderData.

Screenshots or Videos

No response

Platform

• Router / Start Version: [e.g. 1.121.0]
  • @tanstack/react-router: 1.163.3
  • @tanstack/react-start: 1.166.1
  • @tanstack/router-plugin: 1.164.0
  • react: 19.1.4
  • react-dom: 19.1.4
• OS:
  • macOS
• Browser: Chrome
• Browser Version: 146.0.7680.80
• Bundler: vite@8.0.0-beta.8
• Bundler Version: 8.0.0-beta.8

Additional context

No response