From 41f6bc368cc24c06277a611adfc9983fd0a71d99 Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Mon, 10 Nov 2025 15:38:24 +1100
Subject: [PATCH 1/9] Add wasm support for test and SSR

---
 packages/vite/src/node/plugins/wasm.ts        | 41 +++++----
 playground/ssr-wasm/__tests__/serve.ts        | 35 ++++++++
 .../ssr-wasm/__tests__/ssr-wasm.spec.ts       | 15 ++++
 playground/ssr-wasm/package.json              | 15 ++++
 playground/ssr-wasm/server.js                 | 56 +++++++++++++
 playground/ssr-wasm/src/app.js                |  8 ++
 playground/ssr-wasm/src/heavy.wasm            |  1 +
 playground/ssr-wasm/src/light.wasm            |  1 +
 playground/ssr-wasm/src/static-heavy.js       | 12 +++
 playground/ssr-wasm/src/static-light.js       | 12 +++
 playground/ssr-wasm/vite.config.ts            |  7 ++
 playground/test-utils.ts                      |  2 +
 playground/wasm/__tests__/wasm.spec.ts        | 84 +++++++++++++------
 pnpm-lock.yaml                                |  6 ++
 14 files changed, 255 insertions(+), 40 deletions(-)
 create mode 100644 playground/ssr-wasm/__tests__/serve.ts
 create mode 100644 playground/ssr-wasm/__tests__/ssr-wasm.spec.ts
 create mode 100644 playground/ssr-wasm/package.json
 create mode 100644 playground/ssr-wasm/server.js
 create mode 100644 playground/ssr-wasm/src/app.js
 create mode 120000 playground/ssr-wasm/src/heavy.wasm
 create mode 120000 playground/ssr-wasm/src/light.wasm
 create mode 100644 playground/ssr-wasm/src/static-heavy.js
 create mode 100644 playground/ssr-wasm/src/static-light.js
 create mode 100644 playground/ssr-wasm/vite.config.ts

diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index 98cd50309aed59..bce374b6c287b1 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -26,21 +26,32 @@ const wasmHelper = async (opts = {}, url: string) => {
     }
     result = await WebAssembly.instantiate(bytes, opts)
   } else {
-    // https://github.com/mdn/webassembly-examples/issues/5
-    // WebAssembly.instantiateStreaming requires the server to provide the
-    // correct MIME type for .wasm files, which unfortunately doesn't work for
-    // a lot of static file servers, so we just work around it by getting the
-    // raw buffer.
-    const response = await fetch(url)
-    const contentType = response.headers.get('Content-Type') || ''
-    if (
-      'instantiateStreaming' in WebAssembly &&
-      contentType.startsWith('application/wasm')
-    ) {
-      result = await WebAssembly.instantiateStreaming(response, opts)
-    } else {
-      const buffer = await response.arrayBuffer()
+    if (typeof process !== 'undefined' && process.versions?.node) {
+      const fs = await import('node:fs/promises')
+      if (url.startsWith('/@fs/')) {
+        url = url.slice(4)
+      } else if (url.startsWith('/')) {
+        url = url.slice(1)
+      }
+      const buffer = await fs.readFile(url)
       result = await WebAssembly.instantiate(buffer, opts)
+    } else {
+      // https://github.com/mdn/webassembly-examples/issues/5
+      // WebAssembly.instantiateStreaming requires the server to provide the
+      // correct MIME type for .wasm files, which unfortunately doesn't work for
+      // a lot of static file servers, so we just work around it by getting the
+      // raw buffer.
+      const response = await fetch(url)
+      const contentType = response.headers.get('Content-Type') || ''
+      if (
+        'instantiateStreaming' in WebAssembly &&
+        contentType.startsWith('application/wasm')
+      ) {
+        result = await WebAssembly.instantiateStreaming(response, opts)
+      } else {
+        const buffer = await response.arrayBuffer()
+        result = await WebAssembly.instantiate(buffer, opts)
+      }
     }
   }
   return result.instance
@@ -66,7 +77,7 @@ export const wasmHelperPlugin = (): Plugin => {
           return `export default ${wasmHelperCode}`
         }
 
-        const url = await fileToUrl(this, id)
+        const url = (await fileToUrl(this, id)).split('?')[0]
 
         return `
   import initWasm from "${wasmHelperId}"
diff --git a/playground/ssr-wasm/__tests__/serve.ts b/playground/ssr-wasm/__tests__/serve.ts
new file mode 100644
index 00000000000000..b4425c609e8f6f
--- /dev/null
+++ b/playground/ssr-wasm/__tests__/serve.ts
@@ -0,0 +1,35 @@
+// this is automatically detected by playground/vitestSetup.ts and will replace
+// the default e2e test serve behavior
+
+import path from 'node:path'
+import kill from 'kill-port'
+import { hmrPorts, ports, rootDir } from '~utils'
+
+export const port = ports['ssr-wasm']
+
+export async function serve(): Promise<{ close(): Promise<void> }> {
+  await kill(port)
+
+  const { createServer } = await import(path.resolve(rootDir, 'server.js'))
+  const { app, vite } = await createServer(rootDir, hmrPorts['ssr-wasm'])
+
+  return new Promise((resolve, reject) => {
+    try {
+      const server = app.listen(port, () => {
+        resolve({
+          // for test teardown
+          async close() {
+            await new Promise((resolve) => {
+              server.close(resolve)
+            })
+            if (vite) {
+              await vite.close()
+            }
+          },
+        })
+      })
+    } catch (e) {
+      reject(e)
+    }
+  })
+}
diff --git a/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts b/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts
new file mode 100644
index 00000000000000..601f627c3fc62d
--- /dev/null
+++ b/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts
@@ -0,0 +1,15 @@
+import { expect, test } from 'vitest'
+import { port } from './serve'
+import { page } from '~utils'
+
+const url = `http://localhost:${port}`
+
+test('should work when inlined', async () => {
+  await page.goto(`${url}/static-light`)
+  expect(await page.textContent('.static-light')).toMatch('42')
+})
+
+test('should work when output', async () => {
+  await page.goto(`${url}/static-heavy`)
+  expect(await page.textContent('.static-heavy')).toMatch('24')
+})
diff --git a/playground/ssr-wasm/package.json b/playground/ssr-wasm/package.json
new file mode 100644
index 00000000000000..c2a599d4d55cd8
--- /dev/null
+++ b/playground/ssr-wasm/package.json
@@ -0,0 +1,15 @@
+{
+  "name": "@vitejs/test-ssr-wasm",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "node server",
+    "serve": "NODE_ENV=production node server",
+    "debug": "node --inspect-brk server"
+  },
+  "dependencies": {},
+  "devDependencies": {
+    "express": "^5.1.0"
+  }
+}
diff --git a/playground/ssr-wasm/server.js b/playground/ssr-wasm/server.js
new file mode 100644
index 00000000000000..4508300693e825
--- /dev/null
+++ b/playground/ssr-wasm/server.js
@@ -0,0 +1,56 @@
+import express from 'express'
+
+const isTest = process.env.VITEST
+
+export async function createServer(root = process.cwd(), hmrPort) {
+  const app = express()
+
+  /**
+   * @type {import('vite').ViteDevServer}
+   */
+  const vite = await (
+    await import('vite')
+  ).createServer({
+    root,
+    logLevel: isTest ? 'error' : 'info',
+    server: {
+      middlewareMode: true,
+      watch: {
+        // During tests we edit the files too fast and sometimes chokidar
+        // misses change events, so enforce polling for consistency
+        usePolling: true,
+        interval: 100,
+      },
+      hmr: {
+        port: hmrPort,
+      },
+    },
+    appType: 'custom',
+  })
+  // use vite's connect instance as middleware
+  app.use(vite.middlewares)
+
+  app.use('*all', async (req, res, next) => {
+    try {
+      const url = req.originalUrl
+      const render = (await vite.ssrLoadModule('/src/app.js')).render
+      const html = await render(url)
+      res.status(200).set({ 'Content-Type': 'text/html' }).end(html)
+    } catch (e) {
+      vite && vite.ssrFixStacktrace(e)
+      if (isTest) throw e
+      console.log(e.stack)
+      res.status(500).end(e.stack)
+    }
+  })
+
+  return { app, vite }
+}
+
+if (!isTest) {
+  createServer().then(({ app }) =>
+    app.listen(5173, () => {
+      console.log('http://localhost:5173')
+    }),
+  )
+}
diff --git a/playground/ssr-wasm/src/app.js b/playground/ssr-wasm/src/app.js
new file mode 100644
index 00000000000000..a1c9b3336bc8cc
--- /dev/null
+++ b/playground/ssr-wasm/src/app.js
@@ -0,0 +1,8 @@
+export async function render(url) {
+  switch (url) {
+    case '/static-light':
+      return (await import('./static-light')).render()
+    case '/static-heavy':
+      return (await import('./static-heavy')).render()
+  }
+}
diff --git a/playground/ssr-wasm/src/heavy.wasm b/playground/ssr-wasm/src/heavy.wasm
new file mode 120000
index 00000000000000..e606d782a550fd
--- /dev/null
+++ b/playground/ssr-wasm/src/heavy.wasm
@@ -0,0 +1 @@
+../../wasm/heavy.wasm
\ No newline at end of file
diff --git a/playground/ssr-wasm/src/light.wasm b/playground/ssr-wasm/src/light.wasm
new file mode 120000
index 00000000000000..24ab4b13e7fd1c
--- /dev/null
+++ b/playground/ssr-wasm/src/light.wasm
@@ -0,0 +1 @@
+../../wasm/light.wasm
\ No newline at end of file
diff --git a/playground/ssr-wasm/src/static-heavy.js b/playground/ssr-wasm/src/static-heavy.js
new file mode 100644
index 00000000000000..49d1772dd30473
--- /dev/null
+++ b/playground/ssr-wasm/src/static-heavy.js
@@ -0,0 +1,12 @@
+import heavy from './heavy.wasm?init'
+
+export async function render() {
+  let result
+  const { exported_func } = await heavy({
+    imports: {
+      imported_func: (res) => (result = res),
+    },
+  }).then((i) => i.exports)
+  exported_func()
+  return `<div class="static-heavy">${result}</div>`
+}
diff --git a/playground/ssr-wasm/src/static-light.js b/playground/ssr-wasm/src/static-light.js
new file mode 100644
index 00000000000000..18a1ee291b0899
--- /dev/null
+++ b/playground/ssr-wasm/src/static-light.js
@@ -0,0 +1,12 @@
+import light from './light.wasm?init'
+
+export async function render() {
+  let result
+  const { exported_func } = await light({
+    imports: {
+      imported_func: (res) => (result = res),
+    },
+  }).then((i) => i.exports)
+  exported_func()
+  return `<div class="static-light">${result}</div>`
+}
diff --git a/playground/ssr-wasm/vite.config.ts b/playground/ssr-wasm/vite.config.ts
new file mode 100644
index 00000000000000..f70301f75c43bb
--- /dev/null
+++ b/playground/ssr-wasm/vite.config.ts
@@ -0,0 +1,7 @@
+import { defineConfig } from 'vite'
+export default defineConfig({
+  build: {
+    // make cannot emit light.wasm
+    assetsInlineLimit: 80,
+  },
+})
diff --git a/playground/test-utils.ts b/playground/test-utils.ts
index 97e4c57455f7b2..5e084e636d61d6 100644
--- a/playground/test-utils.ts
+++ b/playground/test-utils.ts
@@ -34,6 +34,7 @@ export const ports = {
   'ssr-html': 9602,
   'ssr-noexternal': 9603,
   'ssr-pug': 9604,
+  'ssr-wasm': 9608,
   'ssr-webworker': 9605,
   'proxy-bypass': 9606, // not imported but used in `proxy-hmr/vite.config.js`
   'proxy-bypass/non-existent-app': 9607, // not imported but used in `proxy-hmr/other-app/vite.config.js`
@@ -57,6 +58,7 @@ export const hmrPorts = {
   'ssr-html': 24683,
   'ssr-noexternal': 24684,
   'ssr-pug': 24685,
+  'ssr-wasm': 24691,
   'css/lightningcss-proxy': 24686,
   json: 24687,
   'ssr-conditions': 24688,
diff --git a/playground/wasm/__tests__/wasm.spec.ts b/playground/wasm/__tests__/wasm.spec.ts
index 0f23851fe97ffc..653c3927cb33bd 100644
--- a/playground/wasm/__tests__/wasm.spec.ts
+++ b/playground/wasm/__tests__/wasm.spec.ts
@@ -1,33 +1,67 @@
-import { expect, test } from 'vitest'
+import { describe, expect, test } from 'vitest'
 import { isBuild, page } from '~utils'
 
-test('should work when inlined', async () => {
-  await page.click('.inline-wasm .run')
-  await expect
-    .poll(() => page.textContent('.inline-wasm .result'))
-    .toMatch('42')
-})
+describe('WASM in browser', () => {
+  test('should work when inlined', async () => {
+    await page.click('.inline-wasm .run')
+    await expect
+      .poll(() => page.textContent('.inline-wasm .result'))
+      .toMatch('42')
+  })
 
-test('should work when output', async () => {
-  await page.click('.output-wasm .run')
-  await expect
-    .poll(() => page.textContent('.output-wasm .result'))
-    .toMatch('24')
-})
+  test('should work when output', async () => {
+    await page.click('.output-wasm .run')
+    await expect
+      .poll(() => page.textContent('.output-wasm .result'))
+      .toMatch('24')
+  })
 
-test('init function returns WebAssembly.Instance', async () => {
-  await page.click('.init-returns-instance .run')
-  await expect
-    .poll(() => page.textContent('.init-returns-instance .result'))
-    .toMatch('true')
-})
+  test('init function returns WebAssembly.Instance', async () => {
+    await page.click('.init-returns-instance .run')
+    await expect
+      .poll(() => page.textContent('.init-returns-instance .result'))
+      .toMatch('true')
+  })
 
-test('?url', async () => {
-  expect(await page.textContent('.url')).toMatch(
-    isBuild ? 'data:application/wasm' : '/light.wasm',
-  )
+  test('?url', async () => {
+    expect(await page.textContent('.url')).toMatch(
+      isBuild ? 'data:application/wasm' : '/light.wasm',
+    )
+  })
+
+  test('should work when wasm in worker', async () => {
+    await expect
+      .poll(() => page.textContent('.worker-wasm .result'))
+      .toMatch('3')
+  })
 })
 
-test('should work when wasm in worker', async () => {
-  await expect.poll(() => page.textContent('.worker-wasm .result')).toMatch('3')
+describe('WASM in vitest', () => {
+  test('should work when inlined', async () => {
+    const { default: light } = await import('../light.wasm?init')
+    let result
+    const instance = await light({
+      imports: {
+        imported_func: (res) => (result = res),
+      },
+    })
+    expect(instance instanceof WebAssembly.Instance).toBe(true)
+    // @ts-ignore
+    instance.exports.exported_func()
+    expect(result).toBe(42)
+  })
+
+  test('should work when output', async () => {
+    const { default: heavy } = await import('../heavy.wasm?init')
+    let result
+    const instance = await heavy({
+      imports: {
+        imported_func: (res) => (result = res),
+      },
+    })
+    expect(instance instanceof WebAssembly.Instance).toBe(true)
+    // @ts-ignore
+    instance.exports.exported_func()
+    expect(result).toBe(24)
+  })
 })
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index cd03a2a6817b50..9001a2bb51d26a 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -1675,6 +1675,12 @@ importers:
 
   playground/ssr-resolve/pkg-module-sync: {}
 
+  playground/ssr-wasm:
+    devDependencies:
+      express:
+        specifier: ^5.1.0
+        version: 5.1.0
+
   playground/ssr-webworker:
     dependencies:
       '@vitejs/test-browser-exports':

From 791ca1bf5df097b14ae2246dee093e0b7c690a89 Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Mon, 10 Nov 2025 16:27:00 +1100
Subject: [PATCH 2/9] fixes

---
 packages/vite/src/node/plugins/wasm.ts | 2 +-
 playground/ssr-wasm/server.js          | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index bce374b6c287b1..8c7e8e7d37e472 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -29,7 +29,7 @@ const wasmHelper = async (opts = {}, url: string) => {
     if (typeof process !== 'undefined' && process.versions?.node) {
       const fs = await import('node:fs/promises')
       if (url.startsWith('/@fs/')) {
-        url = url.slice(4)
+        url = url.slice(5)
       } else if (url.startsWith('/')) {
         url = url.slice(1)
       }
diff --git a/playground/ssr-wasm/server.js b/playground/ssr-wasm/server.js
index 4508300693e825..4d103ed67cc5d9 100644
--- a/playground/ssr-wasm/server.js
+++ b/playground/ssr-wasm/server.js
@@ -30,7 +30,7 @@ export async function createServer(root = process.cwd(), hmrPort) {
   // use vite's connect instance as middleware
   app.use(vite.middlewares)
 
-  app.use('*all', async (req, res, next) => {
+  app.use('*', async (req, res, next) => {
     try {
       const url = req.originalUrl
       const render = (await vite.ssrLoadModule('/src/app.js')).render

From 2a9b355881825563734cbed8cf5ac9996e2c9249 Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Mon, 10 Nov 2025 16:32:00 +1100
Subject: [PATCH 3/9] fix

---
 playground/ssr-wasm/server.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/playground/ssr-wasm/server.js b/playground/ssr-wasm/server.js
index 4d103ed67cc5d9..4508300693e825 100644
--- a/playground/ssr-wasm/server.js
+++ b/playground/ssr-wasm/server.js
@@ -30,7 +30,7 @@ export async function createServer(root = process.cwd(), hmrPort) {
   // use vite's connect instance as middleware
   app.use(vite.middlewares)
 
-  app.use('*', async (req, res, next) => {
+  app.use('*all', async (req, res, next) => {
     try {
       const url = req.originalUrl
       const render = (await vite.ssrLoadModule('/src/app.js')).render

From b971467e8e6f04e7eb6c694de37647f32e3ef24e Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Mon, 10 Nov 2025 16:42:05 +1100
Subject: [PATCH 4/9] handle windows and unix path

---
 packages/vite/src/node/plugins/wasm.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index 8c7e8e7d37e472..0e631254fbbbc5 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -30,6 +30,9 @@ const wasmHelper = async (opts = {}, url: string) => {
       const fs = await import('node:fs/promises')
       if (url.startsWith('/@fs/')) {
         url = url.slice(5)
+        if (!/^[A-Z]:/i.test(url)) {
+          url = `/${url}`
+        }
       } else if (url.startsWith('/')) {
         url = url.slice(1)
       }

From a68efaffcb2d4df334d636dfc4e24cce6e961d11 Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Mon, 10 Nov 2025 16:50:55 +1100
Subject: [PATCH 5/9] trying

---
 packages/vite/src/node/plugins/wasm.ts | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index 0e631254fbbbc5..aa0f33146934fe 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -26,7 +26,11 @@ const wasmHelper = async (opts = {}, url: string) => {
     }
     result = await WebAssembly.instantiate(bytes, opts)
   } else {
-    if (typeof process !== 'undefined' && process.versions?.node) {
+    if (
+      typeof process !== 'undefined' &&
+      process.versions &&
+      process.versions.node
+    ) {
       const fs = await import('node:fs/promises')
       if (url.startsWith('/@fs/')) {
         url = url.slice(5)

From 642cfce3fe4ee1ebe59595d5184a3d67898770d6 Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Mon, 10 Nov 2025 17:06:37 +1100
Subject: [PATCH 6/9] fix

---
 packages/vite/src/node/plugins/wasm.ts | 81 ++++++++++++++------------
 1 file changed, 45 insertions(+), 36 deletions(-)

diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index aa0f33146934fe..25691217703ad1 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -1,5 +1,6 @@
 import { exactRegex } from '@rolldown/pluginutils'
 import type { Plugin } from '../plugin'
+import { fsPathFromId } from '../utils'
 import { fileToUrl } from './asset'
 
 const wasmHelperId = '\0vite/wasm-helper.js'
@@ -26,46 +27,45 @@ const wasmHelper = async (opts = {}, url: string) => {
     }
     result = await WebAssembly.instantiate(bytes, opts)
   } else {
-    if (
-      typeof process !== 'undefined' &&
-      process.versions &&
-      process.versions.node
-    ) {
-      const fs = await import('node:fs/promises')
-      if (url.startsWith('/@fs/')) {
-        url = url.slice(5)
-        if (!/^[A-Z]:/i.test(url)) {
-          url = `/${url}`
-        }
-      } else if (url.startsWith('/')) {
-        url = url.slice(1)
-      }
-      const buffer = await fs.readFile(url)
-      result = await WebAssembly.instantiate(buffer, opts)
-    } else {
-      // https://github.com/mdn/webassembly-examples/issues/5
-      // WebAssembly.instantiateStreaming requires the server to provide the
-      // correct MIME type for .wasm files, which unfortunately doesn't work for
-      // a lot of static file servers, so we just work around it by getting the
-      // raw buffer.
-      const response = await fetch(url)
-      const contentType = response.headers.get('Content-Type') || ''
-      if (
-        'instantiateStreaming' in WebAssembly &&
-        contentType.startsWith('application/wasm')
-      ) {
-        result = await WebAssembly.instantiateStreaming(response, opts)
-      } else {
-        const buffer = await response.arrayBuffer()
-        result = await WebAssembly.instantiate(buffer, opts)
-      }
-    }
+    result = await instantiateFromUrl(url, opts)
   }
   return result.instance
 }
 
 const wasmHelperCode = wasmHelper.toString()
 
+const instantiateFromUrl = async (url: string, opts?: WebAssembly.Imports) => {
+  // https://github.com/mdn/webassembly-examples/issues/5
+  // WebAssembly.instantiateStreaming requires the server to provide the
+  // correct MIME type for .wasm files, which unfortunately doesn't work for
+  // a lot of static file servers, so we just work around it by getting the
+  // raw buffer.
+  const response = await fetch(url)
+  const contentType = response.headers.get('Content-Type') || ''
+  if (
+    'instantiateStreaming' in WebAssembly &&
+    contentType.startsWith('application/wasm')
+  ) {
+    return WebAssembly.instantiateStreaming(response, opts)
+  } else {
+    const buffer = await response.arrayBuffer()
+    return WebAssembly.instantiate(buffer, opts)
+  }
+}
+
+const instantiateFromUrlCode = instantiateFromUrl.toString()
+
+const instantiateFromFile = async (
+  fsPath: string,
+  opts?: WebAssembly.Imports,
+) => {
+  const fs = await import('node:fs/promises')
+  const buffer = await fs.readFile(fsPath)
+  return WebAssembly.instantiate(buffer, opts)
+}
+
+const instantiateFromFileCode = instantiateFromFile.toString()
+
 export const wasmHelperPlugin = (): Plugin => {
   return {
     name: 'vite:wasm-helper',
@@ -80,11 +80,20 @@ export const wasmHelperPlugin = (): Plugin => {
     load: {
       filter: { id: [exactRegex(wasmHelperId), wasmInitRE] },
       async handler(id) {
+        const isServer = this.environment.config.consumer === 'server'
+
         if (id === wasmHelperId) {
-          return `export default ${wasmHelperCode}`
+          const instantiateFromUrl = isServer
+            ? instantiateFromFileCode
+            : instantiateFromUrlCode
+          return `
+const instantiateFromUrl = ${instantiateFromUrl}
+export default ${wasmHelperCode}
+`
         }
 
-        const url = (await fileToUrl(this, id)).split('?')[0]
+        id = id.split('?')[0]
+        const url = isServer ? fsPathFromId(id) : await fileToUrl(this, id)
 
         return `
   import initWasm from "${wasmHelperId}"

From 109d41cb5da4b2a36f2132bb3ccf5147f97e787e Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Wed, 12 Nov 2025 12:09:23 +1100
Subject: [PATCH 7/9] correctly handle ssr build

---
 packages/vite/src/node/build.ts        |  2 +-
 packages/vite/src/node/plugins/wasm.ts | 39 ++++++++++++-------
 playground/ssr-wasm/__tests__/serve.ts |  9 ++++-
 playground/ssr-wasm/package.json       |  4 +-
 playground/ssr-wasm/server.js          | 54 ++++++++++++++------------
 playground/ssr-wasm/vite.config.ts     |  2 +
 6 files changed, 68 insertions(+), 42 deletions(-)

diff --git a/packages/vite/src/node/build.ts b/packages/vite/src/node/build.ts
index 844a45c14656a2..301a69de1d3d47 100644
--- a/packages/vite/src/node/build.ts
+++ b/packages/vite/src/node/build.ts
@@ -1404,7 +1404,7 @@ export function toOutputFilePathInJS(
       return result
     }
   }
-  if (relative && !ssr) {
+  if (relative || ssr) {
     return toRelative(filename, hostId)
   }
   return joinUrlSegments(decodedBase, filename)
diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index 25691217703ad1..73d05dcd0966af 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -1,6 +1,9 @@
+import { fileURLToPath, pathToFileURL } from 'node:url'
+import { readFile } from 'node:fs/promises'
 import { exactRegex } from '@rolldown/pluginutils'
 import type { Plugin } from '../plugin'
 import { fsPathFromId } from '../utils'
+import { FS_PREFIX } from '../constants'
 import { fileToUrl } from './asset'
 
 const wasmHelperId = '\0vite/wasm-helper.js'
@@ -55,12 +58,14 @@ const instantiateFromUrl = async (url: string, opts?: WebAssembly.Imports) => {
 
 const instantiateFromUrlCode = instantiateFromUrl.toString()
 
-const instantiateFromFile = async (
-  fsPath: string,
-  opts?: WebAssembly.Imports,
-) => {
-  const fs = await import('node:fs/promises')
-  const buffer = await fs.readFile(fsPath)
+const instantiateFromFile = async (url: string, opts?: WebAssembly.Imports) => {
+  let fsPath = url
+  if (url.startsWith('file:')) {
+    fsPath = fileURLToPath(url)
+  } else if (url.startsWith('/')) {
+    fsPath = url.slice(1)
+  }
+  const buffer = await readFile(fsPath)
   return WebAssembly.instantiate(buffer, opts)
 }
 
@@ -83,18 +88,26 @@ export const wasmHelperPlugin = (): Plugin => {
         const isServer = this.environment.config.consumer === 'server'
 
         if (id === wasmHelperId) {
-          const instantiateFromUrl = isServer
-            ? instantiateFromFileCode
-            : instantiateFromUrlCode
-          return `
-const instantiateFromUrl = ${instantiateFromUrl}
+          if (isServer) {
+            return `
+import { readFile } from 'node:fs/promises'
+import { fileURLToPath } from 'node:url'
+const instantiateFromUrl = ${instantiateFromFileCode}
+export default ${wasmHelperCode}
+`
+          } else {
+            return `
+const instantiateFromUrl = ${instantiateFromUrlCode}
 export default ${wasmHelperCode}
 `
+          }
         }
 
         id = id.split('?')[0]
-        const url = isServer ? fsPathFromId(id) : await fileToUrl(this, id)
-
+        let url = await fileToUrl(this, id)
+        if (isServer && url.startsWith(FS_PREFIX)) {
+          url = pathToFileURL(fsPathFromId(id)).href
+        }
         return `
   import initWasm from "${wasmHelperId}"
   export default opts => initWasm(opts, ${JSON.stringify(url)})
diff --git a/playground/ssr-wasm/__tests__/serve.ts b/playground/ssr-wasm/__tests__/serve.ts
index b4425c609e8f6f..95a3e0552cb17a 100644
--- a/playground/ssr-wasm/__tests__/serve.ts
+++ b/playground/ssr-wasm/__tests__/serve.ts
@@ -3,10 +3,17 @@
 
 import path from 'node:path'
 import kill from 'kill-port'
-import { hmrPorts, ports, rootDir } from '~utils'
+import { build } from 'vite'
+import { hmrPorts, isBuild, ports, rootDir } from '~utils'
 
 export const port = ports['ssr-wasm']
 
+export async function preServe() {
+  if (isBuild) {
+    await build({ root: rootDir })
+  }
+}
+
 export async function serve(): Promise<{ close(): Promise<void> }> {
   await kill(port)
 
diff --git a/playground/ssr-wasm/package.json b/playground/ssr-wasm/package.json
index c2a599d4d55cd8..4530b1bf29de5a 100644
--- a/playground/ssr-wasm/package.json
+++ b/playground/ssr-wasm/package.json
@@ -5,8 +5,8 @@
   "type": "module",
   "scripts": {
     "dev": "node server",
-    "serve": "NODE_ENV=production node server",
-    "debug": "node --inspect-brk server"
+    "build": "vite build",
+    "preview": "NODE_ENV=production node server"
   },
   "dependencies": {},
   "devDependencies": {
diff --git a/playground/ssr-wasm/server.js b/playground/ssr-wasm/server.js
index 4508300693e825..14f594694411bc 100644
--- a/playground/ssr-wasm/server.js
+++ b/playground/ssr-wasm/server.js
@@ -1,43 +1,47 @@
 import express from 'express'
 
 const isTest = process.env.VITEST
+const isProduction = process.env.NODE_ENV === 'production'
 
 export async function createServer(root = process.cwd(), hmrPort) {
   const app = express()
 
-  /**
-   * @type {import('vite').ViteDevServer}
-   */
-  const vite = await (
-    await import('vite')
-  ).createServer({
-    root,
-    logLevel: isTest ? 'error' : 'info',
-    server: {
-      middlewareMode: true,
-      watch: {
-        // During tests we edit the files too fast and sometimes chokidar
-        // misses change events, so enforce polling for consistency
-        usePolling: true,
-        interval: 100,
+  /** @type {import('vite').ViteDevServer} */
+  let vite
+  if (!isProduction) {
+    vite = await (
+      await import('vite')
+    ).createServer({
+      root,
+      logLevel: isTest ? 'error' : 'info',
+      server: {
+        middlewareMode: true,
+        watch: {
+          // During tests we edit the files too fast and sometimes chokidar
+          // misses change events, so enforce polling for consistency
+          usePolling: true,
+          interval: 100,
+        },
+        hmr: {
+          port: hmrPort,
+        },
       },
-      hmr: {
-        port: hmrPort,
-      },
-    },
-    appType: 'custom',
-  })
-  // use vite's connect instance as middleware
-  app.use(vite.middlewares)
+      appType: 'custom',
+    })
+    // use vite's connect instance as middleware
+    app.use(vite.middlewares)
+  }
 
   app.use('*all', async (req, res, next) => {
     try {
       const url = req.originalUrl
-      const render = (await vite.ssrLoadModule('/src/app.js')).render
+      const render = isProduction
+        ? (await import('./dist/app.js')).render
+        : (await vite.ssrLoadModule('/src/app.js')).render
       const html = await render(url)
       res.status(200).set({ 'Content-Type': 'text/html' }).end(html)
     } catch (e) {
-      vite && vite.ssrFixStacktrace(e)
+      vite?.ssrFixStacktrace(e)
       if (isTest) throw e
       console.log(e.stack)
       res.status(500).end(e.stack)
diff --git a/playground/ssr-wasm/vite.config.ts b/playground/ssr-wasm/vite.config.ts
index f70301f75c43bb..e02cb3a969afa5 100644
--- a/playground/ssr-wasm/vite.config.ts
+++ b/playground/ssr-wasm/vite.config.ts
@@ -3,5 +3,7 @@ export default defineConfig({
   build: {
     // make cannot emit light.wasm
     assetsInlineLimit: 80,
+    ssr: './src/app.js',
+    ssrEmitAssets: true,
   },
 })

From 11375b18596a0ac5b288b722626df1bd137f98e0 Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Wed, 12 Nov 2025 13:34:16 +1100
Subject: [PATCH 8/9] add build tests to verify assets

---
 .../ssr-wasm/__tests__/ssr-wasm.spec.ts       | 24 ++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts b/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts
index 601f627c3fc62d..5aba47e37dae80 100644
--- a/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts
+++ b/playground/ssr-wasm/__tests__/ssr-wasm.spec.ts
@@ -1,6 +1,6 @@
 import { expect, test } from 'vitest'
 import { port } from './serve'
-import { page } from '~utils'
+import { findAssetFile, isBuild, listAssets, page } from '~utils'
 
 const url = `http://localhost:${port}`
 
@@ -13,3 +13,25 @@ test('should work when output', async () => {
   await page.goto(`${url}/static-heavy`)
   expect(await page.textContent('.static-heavy')).toMatch('24')
 })
+
+test.runIf(isBuild)('should not contain wasm file when inlined', async () => {
+  const assets = await listAssets()
+  const lightWasm = assets.find((f) => /light-.+\.wasm$/.test(f))
+  expect(lightWasm).toBeUndefined()
+
+  const staticLight = await findAssetFile(/^static-light-.+\.js$/)
+  expect(staticLight).toContain('data:application/wasm;base64,')
+})
+
+test.runIf(isBuild)(
+  'should contain and reference wasm file when output',
+  async () => {
+    const assets = await listAssets()
+    const heavyWasm = assets.find((f) => /heavy-.+\.wasm$/.test(f))
+    expect(heavyWasm).toBeDefined()
+
+    const staticHeavy = await findAssetFile(/^static-heavy-.+\.js$/)
+    expect(staticHeavy).toContain(heavyWasm)
+    expect(staticHeavy).not.toContain('data:application/wasm;base64,')
+  },
+)

From 9f6cdad5aaeacc39b8ee9141a4ae5f0219c1b49e Mon Sep 17 00:00:00 2001
From: Xidorn Quan <me@upsuper.org>
Date: Wed, 26 Nov 2025 09:53:30 +1100
Subject: [PATCH 9/9] handle wasm init specially

---
 packages/vite/src/node/build.ts        |  2 +-
 packages/vite/src/node/plugins/wasm.ts | 50 ++++++++++++++++++++++++--
 pnpm-lock.yaml                         |  2 +-
 3 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/packages/vite/src/node/build.ts b/packages/vite/src/node/build.ts
index 301a69de1d3d47..844a45c14656a2 100644
--- a/packages/vite/src/node/build.ts
+++ b/packages/vite/src/node/build.ts
@@ -1404,7 +1404,7 @@ export function toOutputFilePathInJS(
       return result
     }
   }
-  if (relative || ssr) {
+  if (relative && !ssr) {
     return toRelative(filename, hostId)
   }
   return joinUrlSegments(decodedBase, filename)
diff --git a/packages/vite/src/node/plugins/wasm.ts b/packages/vite/src/node/plugins/wasm.ts
index 73d05dcd0966af..320dd7632c782a 100644
--- a/packages/vite/src/node/plugins/wasm.ts
+++ b/packages/vite/src/node/plugins/wasm.ts
@@ -1,15 +1,20 @@
 import { fileURLToPath, pathToFileURL } from 'node:url'
 import { readFile } from 'node:fs/promises'
+import MagicString from 'magic-string'
 import { exactRegex } from '@rolldown/pluginutils'
+import { createToImportMetaURLBasedRelativeRuntime } from '../build'
 import type { Plugin } from '../plugin'
 import { fsPathFromId } from '../utils'
 import { FS_PREFIX } from '../constants'
-import { fileToUrl } from './asset'
+import { cleanUrl } from '../../shared/utils'
+import { assetUrlRE, fileToUrl } from './asset'
 
 const wasmHelperId = '\0vite/wasm-helper.js'
 
 const wasmInitRE = /(?<![?#].*)\.wasm\?init/
 
+const wasmInitUrlRE: RegExp = /__VITE_WASM_INIT__([\w$]+)__/g
+
 const wasmHelper = async (opts = {}, url: string) => {
   let result
   if (url.startsWith('data:')) {
@@ -105,8 +110,12 @@ export default ${wasmHelperCode}
 
         id = id.split('?')[0]
         let url = await fileToUrl(this, id)
-        if (isServer && url.startsWith(FS_PREFIX)) {
-          url = pathToFileURL(fsPathFromId(id)).href
+        if (isServer) {
+          if (url.startsWith(FS_PREFIX)) {
+            url = pathToFileURL(fsPathFromId(id)).href
+          } else if (assetUrlRE.test(url)) {
+            url = url.replace('__VITE_ASSET__', '__VITE_WASM_INIT__')
+          }
         }
         return `
   import initWasm from "${wasmHelperId}"
@@ -114,6 +123,41 @@ export default ${wasmHelperCode}
   `
       },
     },
+
+    renderChunk(code, chunk, opts) {
+      if (this.environment.config.consumer !== 'server') {
+        return null
+      }
+
+      const toRelativeRuntime = createToImportMetaURLBasedRelativeRuntime(
+        opts.format,
+        this.environment.config.isWorker,
+      )
+
+      let match: RegExpExecArray | null
+      let s: MagicString | undefined
+
+      wasmInitUrlRE.lastIndex = 0
+      while ((match = wasmInitUrlRE.exec(code))) {
+        const [full, referenceId] = match
+        const file = this.getFileName(referenceId)
+        chunk.viteMetadata!.importedAssets.add(cleanUrl(file))
+        const { runtime } = toRelativeRuntime(file, chunk.fileName)
+        s ||= new MagicString(code)
+        s.update(match.index, match.index + full.length, `"+${runtime}+"`)
+      }
+
+      if (s) {
+        return {
+          code: s.toString(),
+          map: this.environment.config.build.sourcemap
+            ? s.generateMap({ hires: 'boundary' })
+            : null,
+        }
+      } else {
+        return null
+      }
+    },
   }
 }
 
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index a753132cba2aac..7d35087c24b367 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -1677,7 +1677,7 @@ importers:
     devDependencies:
       express:
         specifier: ^5.1.0
-        version: 5.1.0
+        version: 5.1.0(ms@2.1.3)
 
   playground/ssr-webworker:
     dependencies: