diff --git a/.changeset/fix-build-watch-dynamic-scripts.md b/.changeset/fix-build-watch-dynamic-scripts.md
new file mode 100644
index 000000000..de4971d08
--- /dev/null
+++ b/.changeset/fix-build-watch-dynamic-scripts.md
@@ -0,0 +1,5 @@
+---
+"@crxjs/vite-plugin": patch
+---
+
+fix: dynamic content scripts failing in build --watch mode
diff --git a/.gitignore b/.gitignore
index ca6d238c7..2f4b43744 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,6 +6,8 @@
 .vscode
 **/chromium-data-dir-*
 **/tests/e2e/mv3-*/src
+**/tests/e2e/mv3-*-hmr/src
+**/tests/e2e/mv3-*-build-watch/src
 **/__diff_output__
 TODO
 pnpm-global
diff --git a/packages/vite-plugin/src/node/plugin-contentScripts.ts b/packages/vite-plugin/src/node/plugin-contentScripts.ts
index 81a9c337c..9c60cdd57 100644
--- a/packages/vite-plugin/src/node/plugin-contentScripts.ts
+++ b/packages/vite-plugin/src/node/plugin-contentScripts.ts
@@ -180,11 +180,11 @@ export const pluginContentScripts: CrxPluginFn = () => {
         // emit content script loaders
         for (const [key, script] of contentScripts)
           if (key === script.refId) {
+            const fileName = this.getFileName(script.refId)
+
             if (script.type === 'module') {
-              const fileName = this.getFileName(script.refId)
               script.fileName = fileName
             } else if (script.type === 'loader') {
-              const fileName = this.getFileName(script.refId)
               script.fileName = fileName
 
               const bundleFileInfo = bundle[fileName]
diff --git a/packages/vite-plugin/src/node/plugin-contentScripts_dynamic.ts b/packages/vite-plugin/src/node/plugin-contentScripts_dynamic.ts
index 629ef3984..265851cf2 100644
--- a/packages/vite-plugin/src/node/plugin-contentScripts_dynamic.ts
+++ b/packages/vite-plugin/src/node/plugin-contentScripts_dynamic.ts
@@ -43,6 +43,47 @@ export const pluginDynamicContentScripts: CrxPluginFn = () => {
       configResolved(_config) {
         config = _config
       },
+      buildStart() {
+        // In watch mode, Rollup caches resolveId/load results between rebuilds,
+        // so the contentScripts map may not be repopulated. We must re-emit
+        // dynamic content script files here to ensure their refIds are valid in
+        // the new build context. Without this, getFileName() throws errors.
+        if (config.command === 'build') {
+          // Snapshot unique dynamic scripts (the map has multiple keys per script)
+          const dynamicScripts: ContentScript[] = []
+          for (const [key, script] of contentScripts) {
+            if (script.isDynamicScript && key === script.scriptId) {
+              dynamicScripts.push(script)
+            }
+          }
+
+          // Clear stale entries (old refIds, fileNames are invalid in new build)
+          contentScripts.clear()
+
+          // Re-emit each dynamic script with a fresh refId for this build context
+          for (const script of dynamicScripts) {
+            const absoluteId = script.id.startsWith('/')
+              ? `${config.root}${script.id}`
+              : `${config.root}/${script.id}`
+            const refId = this.emitFile({
+              type: 'chunk',
+              id: absoluteId,
+              name: basename(script.id),
+            })
+            contentScripts.set(
+              script.id,
+              formatFileData({
+                type: script.type,
+                id: script.id,
+                isDynamicScript: true,
+                refId,
+                scriptId: script.scriptId,
+                matches: script.matches,
+              }),
+            )
+          }
+        }
+      },
       configureServer(server) {
         return () => {
           server.middlewares.use(async (req, res, next) => {
@@ -140,9 +181,37 @@ export const pluginDynamicContentScripts: CrxPluginFn = () => {
         const index = id.indexOf('?scriptId=')
         if (index > -1) {
           const scriptId = id.slice(index + '?scriptId='.length)
-          const script = contentScripts.get(scriptId)!
+          let script = contentScripts.get(scriptId)
+
+          // In watch mode, Rollup may cache resolveId() but not load(),
+          // so the map may be empty when load runs. Recreate the entry.
+          if (!script && config.command === 'build') {
+            const fileId = id.slice(0, index)
+            const refId = this.emitFile({
+              type: 'chunk',
+              id: fileId,
+              name: basename(fileId),
+            })
+            script = formatFileData({
+              type: 'loader',
+              id: relative(config.root, fileId),
+              isDynamicScript: true,
+              refId,
+              scriptId,
+              matches: [],
+            })
+            contentScripts.set(script.id, script)
+          }
+
+          if (!script) {
+            throw new Error(`Content script not found for scriptId: "${scriptId}"`)
+          }
+
           if (config.command === 'build') {
-            return `export default import.meta.CRX_DYNAMIC_SCRIPT_${script.refId};`
+            // Use scriptId (deterministic hash) instead of refId for the placeholder.
+            // refIds change between watch mode rebuilds, but scriptId is stable.
+            // This ensures load() cache is valid across rebuilds.
+            return `export default import.meta.CRX_DYNAMIC_SCRIPT_${script.scriptId};`
           } else if (typeof script.fileName === 'string') {
             return `export default ${JSON.stringify(script.fileName)};`
           } else {
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/manifest.json b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/manifest.json
new file mode 100644
index 000000000..3ea20cfd6
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/manifest.json
@@ -0,0 +1,9 @@
+{
+  "description": "test extension",
+  "manifest_version": 3,
+  "name": "test extension",
+  "background": { "service_worker": "src/background.ts" },
+  "version": "1.0.0",
+  "host_permissions": ["<all_urls>"],
+  "permissions": ["scripting"]
+}
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/background.ts b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/background.ts
new file mode 100644
index 000000000..e320ddeed
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/background.ts
@@ -0,0 +1,25 @@
+import contentScript from './content?script'
+
+// This fixes `self`'s type.
+declare const self: ServiceWorkerGlobalScope
+export { }
+
+chrome.runtime.onInstalled.addListener(async ({ reason }) => {
+  if (reason === 'install') {
+    await self.skipWaiting()
+    await new Promise((r) => setTimeout(r, 100))
+
+    chrome.scripting
+      .registerContentScripts([
+        { id: 'contentScript', js: [contentScript], matches: ['<all_urls>'] },
+      ])
+      .catch(console.error)
+      .then(() => {
+        console.log('Content script registered successfully.')
+        
+        chrome.tabs.create({
+          url: 'https://example.com'
+        }).catch(console.error);
+      })
+  }
+})
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/content.ts b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/content.ts
new file mode 100644
index 000000000..f0bfe4386
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/content.ts
@@ -0,0 +1,11 @@
+import { header } from './header'
+
+const app = document.createElement('div')
+app.id = 'app'
+app.innerHTML = `
+  <h1>${header}</h1>
+  <a href="https://vitejs.dev/guide/features.html" target="_blank">Documentation</a>
+`
+document.body.append(app)
+
+chrome.runtime.sendMessage({ type: 'content-script-load' })
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/header.ts b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/header.ts
new file mode 100644
index 000000000..defec8962
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src1/header.ts
@@ -0,0 +1 @@
+export const header = 'Hello Vite!'
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src2/header.ts b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src2/header.ts
new file mode 100644
index 000000000..6f1d50fe0
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/src2/header.ts
@@ -0,0 +1 @@
+export const header = 'Hello Vite + CRX!'
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/vite-build-watch.test.ts b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/vite-build-watch.test.ts
new file mode 100644
index 000000000..17bf2644a
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/vite-build-watch.test.ts
@@ -0,0 +1,57 @@
+import fs from 'fs-extra'
+import path from 'pathe'
+import { expect, test } from 'vitest'
+import { createUpdate, getPage } from '../helpers'
+import { header } from './src2/header'
+import { build } from '../runners'
+import { RollupWatcher } from 'rollup'
+
+test(
+  'crx page update on build --watch',
+  async () => {
+    const src = path.join(__dirname, 'src')
+    const src1 = path.join(__dirname, 'src1')
+    const src2 = path.join(__dirname, 'src2')
+
+    await fs.remove(src)
+    await fs.copy(src1, src, { recursive: true })
+
+    const { browser, output, outDir } = await build(__dirname)
+
+    const update = createUpdate({ target: src, src: src2 })
+    const page = await getPage(browser, 'example.com')
+
+    const app = page.locator('#app')
+    await app.waitFor({ timeout: 15_000 })
+
+    // update header.ts file -> trigger rebuild
+    await update('header.ts')
+
+    // wait for the watch rebuild to complete and produce updated output.
+    // Note: in build --watch mode there is no extension auto-reload, so we
+    // assert on the produced files rather than the running page.
+    const assetsDir = path.join(outDir, 'assets')
+    const deadline = Date.now() + 15_000
+    let updated = false
+    while (Date.now() < deadline) {
+      const assets = await fs.readdir(assetsDir).catch(() => [] as string[])
+      for (const f of assets) {
+        if (!f.startsWith('content.ts.')) continue
+        const source = await fs.readFile(path.join(assetsDir, f), 'utf8')
+        if (source.includes(header)) {
+          updated = true
+          break
+        }
+      }
+      if (updated) break
+      await new Promise((r) => setTimeout(r, 250))
+    }
+    expect(updated).toBe(true)
+
+    // Clean up the watcher
+    if ('close' in output) {
+      ;(output as RollupWatcher).close()
+    }
+  },
+  { retry: 2 },
+)
diff --git a/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/vite.config.ts b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/vite.config.ts
new file mode 100644
index 000000000..299573d08
--- /dev/null
+++ b/packages/vite-plugin/tests/e2e/mv3-vite-dynamic-content-script-build-watch/vite.config.ts
@@ -0,0 +1,10 @@
+import { crx } from '../../plugin-testOptionsProvider'
+import { defineConfig } from 'vite'
+import manifest from './manifest.json'
+
+export default defineConfig({
+  build: { minify: false, watch: {} },
+  clearScreen: false,
+  logLevel: 'error',
+  plugins: [crx({ manifest })],
+})
diff --git a/packages/vite-plugin/tests/e2e/runners.ts b/packages/vite-plugin/tests/e2e/runners.ts
index 350002d63..bf71f2bf3 100644
--- a/packages/vite-plugin/tests/e2e/runners.ts
+++ b/packages/vite-plugin/tests/e2e/runners.ts
@@ -25,7 +25,7 @@ afterEach(async () => {
 })
 
 export async function build(dirname: string) {
-  const { outDir, config } = await _build(dirname)
+  const { outDir, config, output } = await _build(dirname)
 
   const dataDir = path.join(config.cacheDir!, '.chromium')
   await fs.remove(dataDir);
@@ -41,7 +41,7 @@ export async function build(dirname: string) {
     })
   })
 
-  return { browser, outDir, dataDir }
+  return { browser, outDir, dataDir, output }
 }
 
 export async function serve(dirname: string) {
diff --git a/packages/vite-plugin/tests/runners.ts b/packages/vite-plugin/tests/runners.ts
index fe6ad05c0..2ae9a9f27 100644
--- a/packages/vite-plugin/tests/runners.ts
+++ b/packages/vite-plugin/tests/runners.ts
@@ -1,7 +1,7 @@
 import { watch } from 'chokidar'
 import fs from 'fs-extra'
 import { join } from 'pathe'
-import { RollupOutput } from 'rollup'
+import { RollupOutput, RollupWatcher } from 'rollup'
 import {
   delay,
   firstValueFrom,
@@ -27,7 +27,7 @@ import { afterEach, expect } from 'vitest'
 export interface BuildTestResult {
   command: 'build'
   config: ResolvedConfig
-  output: RollupOutput
+  output: RollupOutput | RollupWatcher
   outDir: string
   rootDir: string
 }
@@ -107,7 +107,8 @@ export async function build(
 
   if (Array.isArray(output))
     throw new TypeError('received outputarray from vite build')
-  if ('close' in output) throw new TypeError('received watcher from vite build')
+  // need watcher
+  // if ('close' in output) throw new TypeError('received watcher from vite build')
 
   return { command: 'build', outDir, output, config: config!, rootDir: dirname }
 }