Describe the bug

On Vite v8.0.11 the dev server serves /@vite/client (i.e. node_modules/vite/dist/client/client.mjs) with two build-time constants unreplaced, so the browser throws Uncaught ReferenceError: __BUNDLED_DEV__ is not defined before main.ts runs and the entire app fails to boot.

Specifically, in the chain inside clientInjectionsPlugin.createClientConfigValueReplacer:

.replace(`__WS_TOKEN__`, wsTokenReplacement)                          // ✅ replaced
.replace(`__SERVER_FORWARD_CONSOLE__`, serverForwardConsoleReplacement) // ❌ NOT replaced in served output
.replaceAll(`__BUNDLED_DEV__`, bundleDevReplacement)                  // ❌ NOT replaced in served output

__WS_TOKEN__ does get replaced in the served file (verified via curl), but the two new constants added in #20916 and #21235 pass through verbatim.

Reproduction

Hard to share a public minimal repro right now (private monorepo), but the symptom reproduces 100% on a Vite 8.0.11 project with a standard vite.config.ts:

import { defineConfig } from 'vite'
import vue from '@vitejs/plugin-vue'

export default defineConfig({
  plugins: [vue()],
  server: { host: '127.0.0.1', port: 5177 },
})

$ npm run dev
$ curl http://localhost:5177/@vite/client | grep -E 'wsToken|isBundleMode|forwardConsole'
const wsToken = \"7095Bis0DD6P\";                       # ✅ replaced
const isBundleMode = __BUNDLED_DEV__;                  # ❌ raw identifier
const forwardConsole = __SERVER_FORWARD_CONSOLE__;     # ❌ raw identifier

The browser then throws:

Uncaught ReferenceError: BUNDLED_DEV is not defined

…which kills HMR setup before main.ts runs.

System info

• Vite 8.0.11 (latest as of 2026-05-10)
• Node 22
• macOS 25.4 (arm64)
• @vitejs/plugin-vue 5.x
• Plain dev server, no SSR, no bundledDev / no forwardConsole user config

Workaround

Adding the two constants to user define in vite.config.ts papers over it (Vite's __DEFINES__ mechanism kicks in for user defines and replaces them at runtime):

export default defineConfig({
  plugins: [vue()],
  define: {
    __BUNDLED_DEV__: 'false',
    __SERVER_FORWARD_CONSOLE__: 'false',
  },
})

After restart, curl /@vite/client shows:

const isBundleMode = false;
const forwardConsole = {\"enabled\":true,...};

…and the browser boots normally. But this is clearly a workaround — the dedicated __BUNDLED_DEV__ / __SERVER_FORWARD_CONSOLE__ replacement chain in clientInjectionsPlugin should handle this without user intervention.

What's surprising

The other constants on the same .replace() chain (__WS_TOKEN__, __HMR_*, __MODE__, __BASE__) all get replaced correctly in the served output. Only these two are skipped — even though they're in the same function in createClientConfigValueReplacer. Suggests either:

• escapeReplacement(undefined) ends up returning a function that signals "don't replace" (the function returns JSON.stringify(undefined) which is the value undefined not a string — but I expected JS to coerce to \"undefined\")
• or the values of config.server.forwardConsole / config.experimental.bundledDev are accessed before they're initialized (race in plugin lifecycle)

Related PRs

• feat: forward browser console logs and errors to dev server terminal #20916 — feat: forward browser console logs and errors to dev server terminal (introduced __SERVER_FORWARD_CONSOLE__)
• feat: highly experimental full bundle mode #21235 — feat: highly experimental full bundle mode (introduced __BUNDLED_DEV__)

Happy to put together a public minimal repro if it helps.