ssr: ssr.external is not the top level priority

[zhangyuang]

Describe the bug

On the server side, we hope the third-party module will be externalized correctly.
If I set an alias for the third-party module, Vite will transform the URL to a local file system path instead of an externalized path. Even though I set ssr.external to the module.

It's a simple reproduct code

// server-entry.js
import * as Vue from 'vue'

// vite.config.js
import { resolve } from 'node:path'
import { defineConfig } from 'vite'

// in order to trigger ssr server restart
export default defineConfig({
  resolve: {
    alias: {
      vue: resolve(process.cwd(), 'node_modules/vue'),
    },
  },
  ssr: {
    external: ['vue'],
  },
})

After running dev with this config below, server-entry will be transfrormd to this code

import * as Vue from "/@fs/xxx/node_modules/.pnpm/vue@3.5.18_typescript@5.7.3/node_modules/vue/index.mjs"

It's not expected.

I remember this logic is expected in vite@^2.0.0.

We use alias for Vue or React to keep a single instance on the client-side, and we don't want to split client-config and server-config to seperate config files. I hope Vite can handle the externalized module path correctly even if I set an alias.

If you agree with my opinion, I will be happy to provide a pull request for it.

Reproduction

git@github.com:zhangyuang/vite-ssr-external-bug.git

Steps to reproduce

$ git clone git@github.com:zhangyuang/vite-ssr-external-bug.git
$ cd vite-ssr-external-bug && yarn && npm run dev

System Info

System:
    OS: macOS 15.5
    CPU: (14) arm64 Apple M4 Pro
    Memory: 2.76 GB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.4.1 - ~/Library/pnpm/node
    Yarn: 1.22.22 - ~/Library/pnpm/yarn
    npm: 10.8.2 - ~/Library/pnpm/npm
    pnpm: 10.13.1 - ~/Library/pnpm/pnpm
  Browsers:
    Chrome: 138.0.7204.169
    Firefox Nightly: 140.0a1
    Safari: 18.5

Used Package Manager

yarn

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

The change is likely done by #15602. Though the PR also fixed unintuitive inconsistency, resolve.alias being applied before externalization is a good behavior as it matches with bare rollup's own external behavior if I remember correctly.

[zhangyuang]

I agree with resolve.alias should be applied before externalization, but i also hope third-party modules can be externalized on server-side. Maybe there can add an additional judgement?

[hi-ogawa]

For that, what we need is per-environment resolve.alias feature, but it's not implemented yet (I think there was a discussion before). I think the workaround is to alias from own plugin based on environment such as:

export default defineConfig({
  plugins: [
    {
      name: "client-only-alias",
      enforce: "pre",
      applyToEnvironment: e => e.name === 'client',
      resolveId(source) { 
        if (source === 'vue') return this.resolve(resolve(process.cwd(), 'node_modules/vue'))
      }
    }
  ]
})

[zhangyuang]

Yes, I can resolve this issue at the business code level, but I also hope that a unified solution can be implemented at the Vite tool level. For instance, we can more accurately determine if a module is a third-party one and then mark it as external; alternatively, if it matches ssr.external, it should definitely be set as external which is the top level priority.If you agree with my view, I would like to submit a PR.

[hi-ogawa]

Ah, alright the point is that ssr.external: ["vue"] is redundant since that's the default behavior. If users have put there explicitly, that should override resolve.alias?

[zhangyuang]

Yes, it is redundant. Because Vue should be marked as an external third-party dependency during the server-side rendering process. The reason I set this configuration is to verify whether setting it can solve the problem I encountered. If we can reach a consensus and prioritize this, then it can be resolved in this way. 发自我的iPhone

…

------------------ Original ------------------ From: Hiroshi Ogawa ***@***.***> Date: Tue,Jul 29,2025 6:05 PM To: vitejs/vite ***@***.***> Cc: yuuang ***@***.***>, Author ***@***.***> Subject: Re: [vitejs/vite] ssr: ssr.external is not the top level priority(Issue #20497) hi-ogawa left a comment (vitejs/vite#20497) Ah, alright the point is that ssr.external: ["vue"] is redundant since that's the default behavior. If users have put there explicitly, that should override resolve.alias? — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you authored the thread.Message ID: ***@***.***>