• Type: boolean
  • Default: false

Enalbe SSR configuration.

Boolean Type

When the value type is boolean, it indicates whether to enable SSR deployment mode. The default is false to disable it.

export default defineConfig({
  server: {
    ssr: true,

Object Type

When the value type is Object, the following properties can be configured:

  • mode: string = 'string', which defaults to using renderToString for rendering. Configure stream to enable streaming rendering.
  • forceCSR: boolean = false, which is off by default for forcing CSR rendering. Configure true to force CSR by adding ?csr=true or adding x-modern-ssr-fallback header when accessing the page.
  • inlineScript: boolean = true, by default, SSR data is injected into HTML as inline scripts and assigned directly to global variables. Configure false to distribute JSON instead of assigning to global variables, this configuration doesn't work in Streaming SSR.
  • disablePrerender: boolean = fasle, To ensure compatibility with the old data request method (useLoader), by default, Modern.js performs pre-rendering of components. However, if developers want to reduce one rendering when there is no use of the useLoader API in your project, you can set the configuration disablePrerender=true.
  • unsafeHeaders: string[] = [], For safety reasons, Modern.js does not add excessive content to SSR_DATA. Developers can use this configuration to specify the headers that need to be injected.
  • scriptLoading: 'defer' | 'blocking' | 'module' | 'async', The configuration is the same as html.scriptLoading, supporting SSR injected script set to async loading. The priority is ssr.scriptLoading > html.scriptLoading.
export default defineConfig({
  server: {
    ssr: {
      forceCSR: true,
      mode: 'stream',
      inlineScript: false,
      unsafeHeaders: ['User-Agent'],
      scriptLoading: 'async',

Active Fallback

In a production environment, there are scenarios where it is necessary to actively fallback an SSR project to CSR. Examples include

  1. When the SSR fails, a fallback to the CSR is required to ensure product availability.

  2. When the SSR is working normally, but there are rendering failures during csr, debugging is required.

  3. When the SSR server is under heavy load, it may be necessary to fallback some traffic directly to the CSR to avoid service downtime.

By configuring server.ssr.forceCSR to true in the project, you can control this behavior through query strings or request headers.

For example, in a custom Web Server middleware, you can actively fallback when traffic exceeds a certain threshold:

export const middleware = (ctx, next) => {
  const { req, res } = ctx;
  if (condition) {
    req.headers['x-modern-ssr-fallback'] = '1';