fix(vite-plugin): avoid duplicate environment CSS chunks

.changepacks/changepack_log_kQbMx_I1A9iUKgf2hMJkR.json

@@ -0,0 +1,7 @@
+{
+  "changes": {
+    "packages/vite-plugin/package.json": "Patch"
+  },
+  "note": "Avoid re-emitting identical Devup CSS chunks across Vite environment builds",
+  "date": "2026-08-30T10:18:56.558349Z"
+}

packages/vite-plugin/src/__tests__/plugin.test.ts

@@ -43,6 +43,7 @@ interface ViteConfig {
 
 interface ViteTestPlugin {
   name: string
+  sharedDuringBuild: true
   enforce: 'pre'
   apply: () => boolean
   config: (
@@ -66,10 +67,38 @@ interface ViteTestPlugin {
     timestamp: number
   }) => Promise<unknown[] | undefined>
   load: (id: string) => string | undefined
-  transform: (code: string, id: string) => Promise<{ code: string } | undefined>
+  transform: (
+    this: {
+      environment?: {
+        name: string
+        config: {
+          consumer: 'client' | 'server'
+          build?: { write?: boolean }
+        }
+      }
+    },
+    code: string,
+    id: string,
+  ) => Promise<{ code: string } | undefined>
   generateBundle: (
+    this: {
+      environment?: {
+        name: string
+        config: {
+          consumer: 'client' | 'server'
+          build?: { write?: boolean }
+        }
+      }
+    },
     options: object,
-    bundle: Record<string, { source: string; name: string }>,
+    bundle: Record<
+      string,
+      {
+        source?: string
+        name: string
+        viteMetadata?: { importedCss?: Set<string> }
+      }
+    >,
   ) => Promise<void>
   resolveId: (source: string, importer?: string) => string | undefined
 }
@@ -174,6 +203,7 @@ describe('devupUIVitePlugin', () => {
     const plugin = createPlugin({})
     expect(plugin).toEqual({
       name: 'devup-ui',
+      sharedDuringBuild: true,
       config: expect.any(Function),
       load: expect.any(Function),
       watchChange: expect.any(Function),
@@ -572,6 +602,117 @@ describe('devupUIVitePlugin', () => {
       expect(bundle['base.css'].source).toEqual('final complete sheet')
     })
 
+    it('does not forward server css that the client already emits', async () => {
+      const plugin = createPlugin({})
+      getCssSpy.mockImplementation((fileNum: number | null) =>
+        fileNum === null ? 'base sheet' : 'file sheet',
+      )
+      const serverBundle = {
+        'base.css': { source: 'stale', name: 'devup-ui.css' },
+        'file.css': { source: 'stale', name: 'devup-ui-3.css' },
+        'entry.js': {
+          name: 'entry',
+          viteMetadata: {
+            importedCss: new Set(['base.css', 'file.css', 'server-only.css']),
+          },
+        },
+      }
+      const clientBundle = {
+        'base.css': { source: 'stale', name: 'devup-ui.css' },
+        'file.css': { source: 'stale', name: 'devup-ui-3.css' },
+      }
+
+      await plugin.generateBundle.call(
+        { environment: { name: 'rsc', config: { consumer: 'server' } } },
+        {},
+        serverBundle,
+      )
+      await plugin.generateBundle.call(
+        { environment: { name: 'client', config: { consumer: 'client' } } },
+        {},
+        clientBundle,
+      )
+
+      expect(serverBundle['base.css'].source).toEqual('base sheet')
+      expect(serverBundle['file.css'].source).toEqual('file sheet')
+      expect(clientBundle['base.css'].source).toEqual('base sheet')
+      expect(clientBundle['file.css'].source).toEqual('file sheet')
+      expect(serverBundle['entry.js'].viteMetadata.importedCss).toEqual(
+        new Set(['server-only.css']),
+      )
+    })
+
+    it('ignores no-write analysis bundles when tracking server css', async () => {
+      const plugin = createPlugin({})
+      const serverBundle = {
+        'file.css': { source: 'stale', name: 'devup-ui-3.css' },
+        'entry.js': {
+          name: 'entry',
+          viteMetadata: { importedCss: new Set(['file.css']) },
+        },
+      }
+      await plugin.generateBundle.call(
+        {
+          environment: {
+            name: 'rsc',
+            config: { consumer: 'server', build: { write: false } },
+          },
+        },
+        {},
+        serverBundle,
+      )
+      const clientBundle = {
+        'file.css': { source: 'stale', name: 'devup-ui-3.css' },
+      }
+
+      await plugin.generateBundle.call(
+        { environment: { name: 'client', config: { consumer: 'client' } } },
+        {},
+        clientBundle,
+      )
+
+      expect(serverBundle['entry.js'].viteMetadata.importedCss).toEqual(
+        new Set(['file.css']),
+      )
+    })
+
+    it('keeps server forwarding for a different output file name', async () => {
+      const plugin = createPlugin({})
+      const serverBundle = {
+        'devup-ui-3.server.css': {
+          source: 'stale',
+          name: 'devup-ui-3.css',
+        },
+        'entry.js': {
+          name: 'entry',
+          viteMetadata: {
+            importedCss: new Set(['devup-ui-3.server.css']),
+          },
+        },
+      }
+      await plugin.generateBundle.call(
+        { environment: { name: 'rsc', config: { consumer: 'server' } } },
+        {},
+        serverBundle,
+      )
+      const clientBundle = {
+        'devup-ui-3.client.css': {
+          source: 'stale',
+          name: 'devup-ui-3.css',
+        },
+      }
+
+      await plugin.generateBundle.call(
+        { environment: { name: 'client', config: { consumer: 'client' } } },
+        {},
+        clientBundle,
+      )
+
+      expect(serverBundle['entry.js'].viteMetadata.importedCss).toEqual(
+        new Set(['devup-ui-3.server.css']),
+      )
+    })
+
     it('resolves a stable id during build', async () => {
       const plugin = createPlugin({})
       await plugin.configResolved({ command: 'build' })

packages/vite-plugin/src/plugin.ts

@@ -113,6 +113,12 @@ interface ConfigHookMeta {
   rolldownVersion?: string
 }
 
+interface ViteOutputWithMetadata {
+  viteMetadata?: {
+    importedCss?: Set<string>
+  }
+}
+
 /**
  * Vite merges a plugin's `config()` result over the user's, replacing function
  * values outright, so returning a bare `manualChunks` silently drops one the
@@ -262,9 +268,14 @@ export function DevupUI({
   }
   const importAliases = mergeImportAliases(userImportAliases)
   const cssMap = new Map()
+  let serverBundleToForward: Record<string, ViteOutputWithMetadata> | undefined
   let isServe = false
   return {
     name: 'devup-ui',
+    // The WASM sheet and transform state are intentionally shared. Vite
+    // otherwise recreates this plugin for every environment build, which makes
+    // each environment independently emit the same CSS asset.
+    sharedDuringBuild: true,
     async configResolved(config) {
       isServe = config?.command === 'serve'
       const projectRoot = config?.root ?? process.cwd()
@@ -455,7 +466,7 @@ export function DevupUI({
       if (!rel.startsWith('./')) rel = `./${rel}`
 
       const {
-        code: retCode,
+        code: extractedCode,
         css = '',
         map,
         cssFile,
@@ -494,12 +505,14 @@ export function DevupUI({
       }
       await Promise.all(promises)
       return {
-        code: retCode,
+        code: extractedCode,
         map,
       }
     },
     async generateBundle(_options, bundle) {
       if (!extractCss) return
+      const writesOutput = this.environment?.config.build?.write !== false
+      const cssFiles = new Set<string>()
 
       // `load` can only snapshot the sheet as it stood when the module was
       // pulled in, and module order varies per build, so the emitted asset was
@@ -512,7 +525,29 @@ export function DevupUI({
         const cssName = getDevupCssChunkName(asset.name)
         if (!cssName) continue
         if (!('source' in asset)) continue
-        asset.source = getCss(getFileNumByFilename(cssName), false)
+        const source = getCss(getFileNumByFilename(cssName), false)
+        asset.source = source
+        cssFiles.add(file)
+      }
+
+      const environment = this.environment
+      if (!environment || !writesOutput) return
+      if (environment.config.consumer === 'client' && serverBundleToForward) {
+        // @vitejs/plugin-rsc forwards every CSS file referenced by the RSC
+        // bundle into the client bundle. Files the client already emitted are
+        // registered twice and trigger FILE_NAME_CONFLICT. Keep both bundles'
+        // imports and client metadata intact, but remove overlaps from the RSC
+        // forwarding set before its later generateBundle hook reads it.
+        for (const output of Object.values(serverBundleToForward)) {
+          for (const file of cssFiles) {
+            output.viteMetadata?.importedCss?.delete(file)
+          }
+        }
+      } else if (environment.config.consumer === 'server') {
+        serverBundleToForward = bundle as unknown as Record<
+          string,
+          ViteOutputWithMetadata
+        >
       }
     },
   }