Why exactly is Route Middleware invoked twice?

[silk surge]

Hi @clear oar. I'm kapa.ai, your friendly support bot. I will try to answer your question by searching through the knowledge base. :wave:

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Route middleware in Nuxt runs twice because it executes both on the server during the initial page render and again on the client during hydration. This is part of Nuxt's universal rendering approach.

Why Route Middleware Runs Twice

When you access a page in a Nuxt application with universal rendering (the default mode):

1. Server-side execution: The middleware runs first on the server when generating the HTML
2. Client-side execution: After the page loads in the browser, the middleware runs again during hydration when Vue takes control of the document

This behavior is explicitly mentioned in the Nuxt documentation:

If your site is server-rendered or generated, middleware for the initial page will be executed both when the page is rendered and then again on the client.
Nuxt Docs: When Middleware Runs

Common Misconception

You mentioned that you thought middleware "runs only on clientside," but the documentation actually states:

Route middleware run within the Vue part of your Nuxt app. Despite the similar name, they are completely different from server middleware, which are run in the Nitro server part of your app.
Nuxt Docs: Middleware

This means route middleware runs in the Vue application context (which exists both on server and client), as opposed to server middleware which runs in the Nitro server context.

How to Prevent Double Execution

If you want to avoid this behavior, you can skip middleware execution in specific environments:

export default defineNuxtRouteMiddleware(to => {
  // skip middleware on server
  if (import.meta.server) return
  
  // OR skip middleware on client side entirely
  if (import.meta.client) return
  
  // OR only skip middleware on initial client load
  const nuxtApp = useNuxtApp()
  if (import.meta.client && nuxtApp.isHydrating && nuxtApp.payload.serverRendered) return
})

This approach is documented in the Nuxt Docs: When Middleware Runs section.

By using these conditions, you can control when your middleware executes and prevent duplicate API calls to your Flask backend.

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