When does PluginContainer.transform get called in Vite 6?

[grahse]

I'm trying to upgrade to Vite 6 and have a custom plugin where, within a configureServer hook, I'm wrapping the PluginContainer.transform hook. For example:

{
   name: "custom-plugin",
   apply: "serve",
   enforce: "post",
   configureServer(server) {
      const wrappedTransform = server.pluginContainer.transform;
      server.pluginContainer.transform = function () {
         // This never gets called.
      }
   }
}

The issue is that the transform hook never seems to get called. This works with Vite 5, so I'm wondering if something changed in Vite 6 with how/when it gets called?

[agammann]

Yes, Vite 6 changed the environment pipeline. I reproduced your wrapper with Vite 5.4.19 and 6.3.5: server.pluginContainer.transform was reached in 5 but not 6 for the same server.transformRequest('/main.js'); a plugin's public transform(code, id) hook ran in both.

Move the logic into your plugin's transform hook instead of replacing the container method in configureServer:

{
  name: 'custom-plugin',
  apply: 'serve',
  transform(code, id) {
    // Inspect or transform this module here.
    return null;
  },
}

If it needs different behavior for client and SSR processing in Vite 6, the hook can inspect this.environment. See the Vite 6 plugin hook docs and environment hooks.

[Icaro0310]

It's lazy. pluginContainer.transform only runs when the dev server actually serves a module, so a browser request for /src/main.ts triggers it through transformRequest, roughly once per module version. It never fires during configureServer or at startup, so if your wrap logs nothing until a page load hits, that's the expected behavior.

One Vite 6 gotcha: transforms now run per environment. server.pluginContainer aliases the client environment's container, so your wrapper catches browser requests but not SSR transforms. If you need those too, wrap server.environments.ssr.pluginContainer.transform the same way.

If it still never fires on requests, double check nothing is short circuiting earlier, like a cached transform result or optimizeDeps serving the file before the container sees it.

—
Related work of mine: github.com/Icaro0310 · Website: icaro0310.github.io