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SSR & Hydration

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This guide covers server-side rendering (SSR) with Better i18n and TanStack Start, ensuring consistent rendering between server and client.

How SSR Works #

Code
1. Request arrives at server
2. Middleware detects locale (URL, cookie, header)
3. Loader fetches messages from CDN
4. React renders HTML with translations
5. HTML sent to client
6. Client hydrates with same messages
7. No flash of untranslated content!

Loading Messages on Server #

Use getMessages in your route loaders:

app/routes/__root.tsx
import { getMessages } from "@better-i18n/use-intl/server" // [!code ++]

export const Route = createRootRouteWithContext<{ locale: string }>()({
  staleTime: 0, // re-run loader on locale switch // [!code highlight]
  loader: async ({ context }) => { // [!code highlight]
    const messages = await getMessages({ // [!code ++]
      projectId: "org/project", // [!code ++]
      locale: context.locale || "en", // [!code ++]
    }) // [!code ++]

    return { messages, locale: context.locale } // [!code ++]
  },

  component: RootComponent,
})

Providing Messages to Client #

Pass messages to the provider to avoid client-side fetching:

TSX
function RootComponent() {
  const { messages, locale } = Route.useLoaderData()

  return (
    <BetterI18nProvider // [!code ++]
      projectId="org/project" // [!code ++]
      locale={locale} // [!code ++]
      messages={messages}  // Pre-loaded from server // [!code ++]
      timeZone="UTC" // [!code ++]
    > // [!code ++]
      <Outlet />
    </BetterI18nProvider> // [!code ++]
  )
}

Timezone Configuration #

use-intl v4+ requires a timezone during SSR to prevent hydration mismatches:

TSX
<BetterI18nProvider
  projectId="org/project"
  locale={locale}
  messages={messages}
  timeZone="UTC"  // Required for SSR // [!code highlight]
>
  {children}
</BetterI18nProvider>

Why Timezone Matters #

Date formatting can differ between server and client:

TSX
// Server (UTC): "January 18, 2026"
// Client (PST): "January 17, 2026"  ← Hydration mismatch!

By setting a consistent timeZone, both render identically.

User Timezone #

To use the user's timezone, detect it on the client and update:

TSX
function RootComponent() {
  const { messages, locale } = Route.useLoaderData()
  const [timeZone, setTimeZone] = useState("UTC")

  useEffect(() => {
    setTimeZone(Intl.DateTimeFormat().resolvedOptions().timeZone)
  }, [])

  return (
    <BetterI18nProvider
      projectId="org/project"
      locale={locale}
      messages={messages}
      timeZone={timeZone}
    >
      <Outlet />
    </BetterI18nProvider>
  )
}

Route-Level Messages #

For route-specific namespaces, you can load additional messages:

app/routes/dashboard.tsx
export const Route = createFileRoute("/dashboard")({
  loader: async ({ context }) => {
    // Dashboard-specific translations
    const dashboardMessages = await getMessages({
      projectId: "org/project",
      locale: context.locale,
      namespace: "dashboard",  // Load only dashboard namespace
    })

    return { dashboardMessages }
  },
})

Server-Side Translation #

Use translations in loaders for metadata:

app/routes/about.tsx
import { getMessages, createServerTranslator } from "@better-i18n/use-intl/server"

export const Route = createFileRoute("/about")({
  loader: async ({ context }) => {
    const messages = await getMessages({
      projectId: "org/project",
      locale: context.locale,
    })

    const t = createServerTranslator({
      locale: context.locale,
      messages,
      namespace: "about",
    })

    return {
      messages,
      meta: {
        title: t("meta.title"),
        description: t("meta.description"),
      },
    }
  },

  head: ({ loaderData }) => ({
    meta: [
      { title: loaderData.meta.title },
      { name: "description", content: loaderData.meta.description },
    ],
  }),

  component: AboutPage,
})

Caching Considerations #

Messages are cached at multiple levels:

LevelDurationInvalidation
CDN1 hourOn publish
ServerRequestPer request
ClientSessionOn navigation

For production, consider adding server-side caching:

TSX
const messageCache = new Map<string, Messages>()

async function getCachedMessages(locale: string) {
  if (!messageCache.has(locale)) {
    messageCache.set(locale, await getMessages({
      projectId: "org/project",
      locale,
    }))
  }
  return messageCache.get(locale)!
}

Debug Mode #

Enable debug logging to troubleshoot SSR issues:

TSX
const messages = await getMessages({
  projectId: "org/project",
  locale: context.locale,
  debug: true,  // Logs CDN requests
})

Common Issues #

Hydration Mismatch #

Symptom: Console warning about hydration mismatch.

Solution: Ensure timeZone is set consistently:

TSX
<BetterI18nProvider timeZone="UTC" ... />

Flash of English #

Symptom: Page briefly shows English before correct locale.

Solution: Pre-load messages in the loader:

TSX
loader: async ({ context }) => {
  const messages = await getMessages({ ... })
  return { messages }  // Pass to provider
}

Locale Switch Updates Content but Not UI Translations #

Symptom: Switching locale updates CMS content (cards, articles) but useTranslations() strings stay in the old language.

Solution: Set staleTime: 0 on the root route so the loader re-runs on every SPA navigation:

TSX
export const Route = createRootRouteWithContext<RouterContext>()({
  staleTime: 0, // [!code highlight]
  loader: async ({ context }) => {
    const messages = await getMessages({ projectId: "org/project", locale: context.locale })
    return { messages, locale: context.locale }
  },
})

With staleTime: Infinity (or any high value), TanStack Router considers the SSR loader result "forever fresh" and skips re-running it on client-side navigations — even when the locale changes. staleTime: 0 fixes this. Performance is unaffected because getMessages() uses an in-memory TtlCache (60s) — same-locale navigations hit cache instantly.

Wrong Locale on First Load #

Symptom: First page load shows wrong locale.

Solution: Ensure middleware sets locale in context:

TSX
// middleware/i18n.ts
export const i18nMiddleware = createBetterI18nMiddleware({
  projectId: "org/project",
  defaultLocale: "en",
})