Client-first with opt-in prerendering: Allow ssr: true per route when defaultSsr: false

[fullheart]

I love TanStack Start and its promise of being "client-first" — building an SPA that feels like Vite, with the option to selectively prerender content-heavy pages at build time.

What I want to build

A fully client-first app (defaultSsr: false) where most routes are classic SPA — but a few specific routes (e.g. /blog, /docs) are statically prerendered at build time using staticFunctionMiddleware and ssr: true.

Where it breaks today

defaultSsr: false prevents route-level ssr: true from working — the staticServerFnCache is never generated at build time. This was reported in #7239 and confirmed as by design:

with defaultSsr: false you also disabled SSR on the root route, and thus all routes below it, regardless of whether ssg-page has ssr:true

The Selective SSR docs explain this inheritance model — a parent's ssr cascades down to all children.

The purpose of this discussion is to determine whether the inheritance constraint can be relaxed.

Workaround (hacky)

Doing a two-phase build: first with defaultSsr: "data-only" to generate the cache, then copy assets and rebuild with defaultSsr: false. It works, but it's fragile and shouldn't be necessary: https://github.com/fullheart/tanstack-start-static-server-function-build-workaround

Inspiration from Astro

Astro takes a different approach: the global output mode sets a default, but every page can independently override it with export const prerender = true | false — no inheritance. A page-level declaration always wins over the global default.

That's the model I'd love for TanStack Start: defaultSsr as the fallback, ssr per route as the override.

Questions

1. What's the best path forward? Should defaultSsr: false only disable request-time SSR but still allow build-time SSG via route-level ssr: true? Or should we introduce a separate concept like prerender that's independent of the SSR inheritance chain?

2. Would love to hear your thoughts — especially from the team and anyone else building client-first apps with selective static pages.

[cherniavskii]

I also stumbled upon the same issue: defaultSsr: false effectively makes selective SSR impossible.
As far as I understand, SPA mode isn't the correct direction either (selective SSR won't work).
Would be great to have a guide on how to create a client-side app with selected pages rendered statically (ideally with ISG) if this is even possible with TanStack Start.

[cherniavskii]

Hmm, I think it's possible to have defaultSsr: false and selective ssr: true in the end.
The only missing thing for me was to add ssr: true to the root route (createRootRoute).
In this setup, the root shell is still rendered on the server (this is fine for me), but all pages render only on client.
And then all pages with ssr: true have SSR enabled.
fyi @fullheart