Skip revalidation for RSC fallback data by default

src/_internal/types.ts

@@ -221,6 +221,11 @@ export interface PublicConfiguration<
    * @see {@link https://swr.vercel.app/docs/revalidation#disable-automatic-revalidations}
    */
   revalidateIfStale: boolean
+  /**
+   * automatically revalidate when data comes from RSC fallback
+   * @defaultValue false
+   */
+  revalidateOnRSCFallback?: boolean
   /**
    * retry when fetcher has an error
    * @defaultValue true

src/index/use-swr.ts

@@ -219,6 +219,14 @@ export const useSWRHandler = <Data = any, Error = any>(
         if (isInitialMount && !isUndefined(revalidateOnMount))
           return revalidateOnMount
         const data = !isUndefined(fallback) ? fallback : snapshot.data
+        // If revalidateOnRSCFallback is explicitly configured, use it for fallback data
+        if (
+          !isUndefined(fallback) &&
+          !isUndefined(data) &&
+          isInitialMount &&
+          !isUndefined(getConfig().revalidateOnRSCFallback)
+        )
+          return getConfig().revalidateOnRSCFallback
         if (suspense) return isUndefined(data) || revalidateIfStale
         return isUndefined(data) || revalidateIfStale
       })()
@@ -367,6 +375,16 @@ export const useSWRHandler = <Data = any, Error = any>(
     // If `revalidateOnMount` is set, we take the value directly.
     if (isInitialMount && !isUndefined(revalidateOnMount))
       return revalidateOnMount
+
+    // If revalidateOnRSCFallback is explicitly configured, use it for fallback data
+    if (
+      !isUndefined(fallback) &&
+      !isUndefined(data) &&
+      !IS_SERVER &&
+      isInitialMount &&
+      !isUndefined(getConfig().revalidateOnRSCFallback)
+    )
+      return getConfig().revalidateOnRSCFallback
     // Under suspense mode, it will always fetch on render if there is no
     // stale data so no need to revalidate immediately mount it again.
     // If data exists, only revalidate if `revalidateIfStale` is true.

test/rsc-fallback.test.tsx

@@ -0,0 +1,99 @@
+import { act, screen } from '@testing-library/react'
+import React from 'react'
+import useSWR from 'swr'
+import { createKey, renderWithConfig, sleep } from './utils'
+
+describe('RSC Fallback', () => {
+  it('should not revalidate on mount with RSC fallback data when revalidateOnRSCFallback is false', async () => {
+    const key = createKey()
+    const fetchFn = jest.fn(() => 'updated data')
+
+    function Page() {
+      const { data, isValidating, isLoading } = useSWR(key, fetchFn, {
+        fallbackData: 'RSC data',
+        revalidateOnRSCFallback: false
+      })
+      return (
+        <div>
+          Data: {data}, isValidating: {String(isValidating)}, isLoading:{' '}
+          {String(isLoading)}
+        </div>
+      )
+    }
+
+    renderWithConfig(<Page />)
+
+    screen.getByText(content => content.includes('Data: RSC data'))
+    screen.getByText(content => content.includes('isValidating: false'))
+    screen.getByText(content => content.includes('isLoading: false'))
+    expect(fetchFn).not.toHaveBeenCalled()
+
+    await act(() => sleep(50))
+
+    expect(fetchFn).not.toHaveBeenCalled()
+  })
+
+  it('should revalidate on mount with RSC fallback data when revalidateOnRSCFallback is true', async () => {
+    const key = createKey()
+    const fetchFn = jest.fn(() => 'updated data')
+
+    function Page() {
+      const { data } = useSWR(key, fetchFn, {
+        fallbackData: 'RSC data',
+        revalidateOnRSCFallback: true
+      })
+      return <div>Data: {data}</div>
+    }
+
+    renderWithConfig(<Page />)
+
+    expect(screen.getByText('Data: RSC data')).toBeInTheDocument()
+
+    await act(() => sleep(50))
+
+    expect(fetchFn).toHaveBeenCalledTimes(1)
+    expect(screen.getByText('Data: updated data')).toBeInTheDocument()
+  })
+
+  it('should not affect existing fallback behavior when revalidateOnRSCFallback is not set', async () => {
+    const key = createKey()
+    const fetchFn = jest.fn(() => 'updated data')
+
+    function Page() {
+      const { data } = useSWR(key, fetchFn, {
+        fallbackData: 'fallback data'
+      })
+      return <div>Data: {data}</div>
+    }
+
+    renderWithConfig(<Page />)
+
+    expect(screen.getByText('Data: fallback data')).toBeInTheDocument()
+
+    // Default behavior: revalidateIfStale is true, so it should revalidate
+    await act(() => sleep(50))
+
+    expect(fetchFn).toHaveBeenCalledTimes(1)
+    expect(screen.getByText('Data: updated data')).toBeInTheDocument()
+  })
+
+  it('should work with SWRConfig provider fallback when revalidateOnRSCFallback is false', async () => {
+    const key = createKey()
+    const fetchFn = jest.fn(() => 'updated data')
+
+    function Page() {
+      const { data } = useSWR(key, fetchFn, {
+        revalidateOnRSCFallback: false
+      })
+      return <div>Data: {data}</div>
+    }
+
+    renderWithConfig(<Page />, { fallback: { [key]: 'provider fallback' } })
+
+    expect(screen.getByText('Data: provider fallback')).toBeInTheDocument()
+
+    await act(() => sleep(50))
+
+    expect(fetchFn).not.toHaveBeenCalled()
+  })
+})