diff --git a/packages/bun-plugin/__regression__/css-emit.bun.ts b/packages/bun-plugin/__regression__/css-emit.bun.ts
new file mode 100644
index 00000000..836570bd
--- /dev/null
+++ b/packages/bun-plugin/__regression__/css-emit.bun.ts
@@ -0,0 +1,70 @@
+import { existsSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join, resolve } from 'node:path'
+
+import { expect, it } from 'bun:test'
+
+const pluginEntry = resolve(import.meta.dir, '..', 'dist', 'index.mjs')
+
+// Separate processes keep the real WASM sheet and plugin hooks isolated from
+// the root suite's mocks. Every process starts without a df directory.
+it.each(['empty theme', 'configured theme', 'extracted styles'])(
+  'emits an importable stylesheet from a clean checkout: %s',
+  (scenario) => {
+    const cwd = mkdtempSync(join(tmpdir(), 'devup-css-emit-'))
+    try {
+      writeFileSync(join(cwd, 'bunfig.toml'), '')
+      if (scenario === 'configured theme') {
+        writeFileSync(
+          join(cwd, 'devup.json'),
+          JSON.stringify({
+            theme: { colors: { default: { primary: '#123456' } } },
+          }),
+        )
+      }
+      const widths = Array.from({ length: 8 }, (_, i) => 101 + i)
+      for (const width of widths) {
+        writeFileSync(
+          join(cwd, `fixture-${width}.ts`),
+          `import { css } from '@devup-ui/react'
+export const cls = css({ width: '${width}px' })
+`,
+        )
+      }
+      writeFileSync(
+        join(cwd, 'check.ts'),
+        `import { existsSync, readFileSync } from 'node:fs'
+import { expect } from 'bun:test'
+
+expect(existsSync('df')).toBe(false)
+await import(${JSON.stringify(pluginEntry.replaceAll('\\', '/'))})
+const cssPath = './df/devup-ui/devup-ui.css'
+${
+  scenario === 'extracted styles'
+    ? `const widths = ${JSON.stringify(widths)}
+const modules = await Promise.all(widths.map(width => import('./fixture-' + width + '.ts')))
+for (const mod of modules) expect(mod.cls).toBeTruthy()
+const css = readFileSync(cssPath, 'utf-8')
+for (const width of widths) expect(css).toContain('width:' + width + 'px')`
+    : `expect(existsSync(cssPath)).toBe(true)
+${scenario === 'configured theme' ? "expect(readFileSync(cssPath, 'utf-8')).toContain('--primary:#123456')" : ''}`
+}
+await import(cssPath)
+`,
+      )
+      expect(existsSync(join(cwd, 'df'))).toBe(false)
+      const result = Bun.spawnSync([process.execPath, 'run', 'check.ts'], {
+        cwd,
+        stdout: 'pipe',
+        stderr: 'pipe',
+        env: { ...process.env, BUN_RUNTIME_TRANSPILER_CACHE_PATH: '0' },
+      })
+      expect(
+        result.exitCode,
+        result.stdout.toString() + result.stderr.toString(),
+      ).toBe(0)
+    } finally {
+      rmSync(cwd, { recursive: true, force: true })
+    }
+  },
+)
diff --git a/packages/bun-plugin/__regression__/css-emit.md b/packages/bun-plugin/__regression__/css-emit.md
new file mode 100644
index 00000000..18630b10
--- /dev/null
+++ b/packages/bun-plugin/__regression__/css-emit.md
@@ -0,0 +1,62 @@
+# Bun CSS emission regression
+
+The source at `6f9079ec` already resolves injected CSS imports to a path-free
+virtual module (`src/css-id.ts`) and loads it as an empty JavaScript module.
+That fixes Bun runtime loading and cross-worktree transpiler-cache isolation,
+but `writeDataFiles()` only creates the CSS directory and `loadSourceFile()`
+discards extracted CSS. No stylesheet is written.
+
+The other plugins have different bundler lifecycles:
+
+- Next initializes `devup-ui.css` with `getCss(null, false)` and refreshes CSS
+  through its loaders/coordinator.
+- Webpack writes initial CSS in watch mode, writes extracted CSS in its source
+  loader, serves CSS through its CSS loader, and writes the final base sheet
+  after a successful build.
+- Vite creates the directory before its initial base stylesheet, materializes
+  CSS imports during transformation, and serves/finalizes CSS from `getCss`.
+- Rsbuild materializes stylesheet imports during transformation and serves
+  their contents through its CSS transform.
+
+Bun uses `singleCss = true`, so its artifact is `df/devup-ui/devup-ui.css`.
+Initialize it after directory creation with actual theme CSS, then refresh it
+from `getCss(null, false)` after extraction. A placeholder alone would discard
+the styles, since Bun's virtual runtime module intentionally contains no CSS.
+Keep virtual resolution for cache isolation. No shared utility is needed for
+this small change: the other plugins' emission timing and loaders differ.
+
+Run `bun run build`, then
+`bun run --filter @devup-ui/bun-plugin test:regression`.
+The CSS regression runs isolated Bun processes in temporary directories without
+`df`, using the built plugin and real WASM. It checks initial emission with and
+without a theme, parallel extraction of eight distinct styles, and CSS imports.
+It disables the transpiler cache for these cold-start cases; the existing
+worktree-isolation regression continues to exercise the shared cache.
+
+## Verification (Windows, Bun 1.4.1)
+
+Before implementation:
+
+- Initial `bun test`: 0 pass, 66 fail/errors because workspace build artifacts
+  were missing. Initial `bun run lint`: Rust checks passed; ESLint could not
+  load the unbuilt local ESLint plugin.
+- Initial `bun run test`: Rust tests passed, coverage 98.21% (8572/8728 lines);
+  the Bun phase failed because build artifacts were still missing.
+- `bun run build`: passed.
+- Prepared `bun test`: 5180 pass, 0 fail; 87 snapshots; 100% function/line
+  coverage under the existing repository configuration.
+- Prepared `bun run lint`: passed (format, Clippy, ESLint).
+- Regression RED, before production edits: 2 existing tests passed; all 3 new
+  cases failed because the stylesheet did not exist (`false` / `ENOENT`).
+
+After implementation:
+
+- `bun run --filter @devup-ui/bun-plugin build`: passed.
+- `bun run --filter @devup-ui/bun-plugin test:regression`: 5 pass, 0 fail,
+  including the unchanged preload and worktree-cache regressions.
+- `bun run test`: passed. Rust coverage remains 98.21% (8572/8728 lines,
+  +0.00%); Bun remains 5180 pass, 0 fail, 87 snapshots and 100% function/line
+  coverage. No coverage configuration or existing assertions were changed.
+- Running Tarpaulin and Clippy concurrently caused transient missing `target`
+  fingerprint files on Windows. Validation commands were rerun sequentially.
+- Sequential `bun run lint`: passed, matching the prepared baseline.
diff --git a/packages/bun-plugin/package.json b/packages/bun-plugin/package.json
index 0862abfc..48c14ec7 100644
--- a/packages/bun-plugin/package.json
+++ b/packages/bun-plugin/package.json
@@ -22,7 +22,7 @@
   "scripts": {
     "lint": "eslint",
     "build": "bun ../../node_modules/@typescript/native/bin/tsc && bun build --target node src/index.cjs.ts --production --env=disable --outfile dist/index.cjs --format cjs --packages external && bun build --target node src/index.ts --production --env=disable --outfile dist/index.mjs --format esm --packages external && bun build --target node src/register.ts --production --env=disable --outfile dist/register.cjs --format cjs --packages external && bun build --target node src/register.ts --production --env=disable --outfile dist/register.mjs --format esm --packages external",
-    "test:regression": "cd __regression__ && bun test ./preload-race.bun.ts ./worktree-isolation.bun.ts"
+    "test:regression": "cd __regression__ && bun test ./preload-race.bun.ts ./worktree-isolation.bun.ts ./css-emit.bun.ts"
   },
   "publishConfig": {
     "access": "public"
diff --git a/packages/bun-plugin/src/plugin.ts b/packages/bun-plugin/src/plugin.ts
index 3738d560..9b2230fc 100644
--- a/packages/bun-plugin/src/plugin.ts
+++ b/packages/bun-plugin/src/plugin.ts
@@ -1,4 +1,4 @@
-import { existsSync } from 'node:fs'
+import { existsSync, writeFileSync } from 'node:fs'
 import { mkdir, writeFile } from 'node:fs/promises'
 import { dirname, join, relative, resolve } from 'node:path'
 
@@ -10,6 +10,7 @@ import {
 } from '@devup-ui/plugin-utils'
 import {
   codeExtract,
+  getCss,
   getThemeInterface,
   hasDevupUI,
   registerShorthands,
@@ -51,6 +52,7 @@ async function writeDataFiles() {
   if (!existsSync(cssDir)) {
     await mkdir(cssDir, { recursive: true })
   }
+  await writeFile(join(cssDir, 'devup-ui.css'), getCss(null, false), 'utf-8')
 }
 
 async function initialize({ shorthands }: DevupUIBunPluginOptions = {}) {
@@ -89,6 +91,10 @@ async function loadSourceFile(filePath: string) {
       false,
       importAliases,
     )
+    // singleCss stores every extracted style in the base sheet. Finish the
+    // write before returning the injected import; synchronous writes also keep
+    // concurrent source loads from overwriting a newer sheet with an older one.
+    writeFileSync(join(cssDir, 'devup-ui.css'), getCss(null, false), 'utf-8')
     return { contents: code.code, loader }
   }
   return { contents, loader }