Fix webpack loader cache key for resource queries (#19723)

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## Summary

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Provide a summary of the issue and the changes you're making. How does
your change solve the problem?

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`@tailwindcss/webpack` currently uses `this.resourcePath` as the cache
key, which ignores the resource query. When the same CSS file is
imported multiple times with different `resourceQuery` values, all of
those imports share a single `CacheEntry`. That means the utilities
discovered for one entry can leak into the CSS output for another entry.

This PR changes the cache key to use `this.resource` (path + query)
instead, while still using `this.resourcePath` for all filesystem work.

## Test plan

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- `pnpm test:integrations -- webpack/loader.test.ts`  
  - Confirms all existing webpack loader integration tests pass.  
- Confirms the new `@tailwindcss/webpack loader isolates cache by
resource including query` test passes, verifying that two entries
importing the same CSS file with different queries produce isolated
outputs (`dist/a.css` only contains `only-a` / `--color-red-500`, and
`dist/b.css` only contains `only-b` / `--color-blue-500`).

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Improve canonicalizations for `tracking-*` utilities ([#19827](https://github.com/tailwindlabs/tailwindcss/pull/19827))
 - Fix crash due to invalid characters in candidate ([#19829](https://github.com/tailwindlabs/tailwindcss/pull/19829))
+- Ensure query params in imports are considered unique resources when using `@tailwindcss/webpack` ([#19723](https://github.com/tailwindlabs/tailwindcss/pull/19723))
 
 ## [4.2.2] - 2026-03-18
 

integrations/webpack/loader.test.ts

@@ -418,3 +418,78 @@ test(
     `)
   },
 )
+
+test(
+  '@tailwindcss/webpack loader isolates cache by resource including query',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "main": "./src/index.js",
+          "browser": "./src/index.js",
+          "dependencies": {
+            "css-loader": "^6",
+            "webpack": "^5",
+            "webpack-cli": "^5",
+            "mini-css-extract-plugin": "^2",
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/webpack": "workspace:^"
+          }
+        }
+      `,
+      'webpack.config.js': js`
+        let MiniCssExtractPlugin = require('mini-css-extract-plugin')
+        let path = require('node:path')
+
+        module.exports = {
+          mode: 'development',
+          entry: {
+            a: './src/a.js',
+            b: './src/b.js',
+          },
+          output: {
+            clean: true,
+          },
+          plugins: [new MiniCssExtractPlugin()],
+          module: {
+            rules: [
+              {
+                test: /.css$/i,
+                use: [
+                  MiniCssExtractPlugin.loader,
+                  'css-loader',
+                  '@tailwindcss/webpack',
+                  path.resolve(__dirname, 'query-loader.js'),
+                ],
+              },
+            ],
+          },
+        }
+      `,
+      'query-loader.js': js`
+        module.exports = function (source) {
+          if (this.resourceQuery.includes('a')) {
+            return '@import "tailwindcss/utilities"; @utility only-a {color: var(--color-red-500);}'
+          }
+
+          if (this.resourceQuery.includes('b')) {
+            return '@import "tailwindcss/utilities"; @utility only-b {color: var(--color-blue-500);}'
+          }
+
+          return source
+        }
+      `,
+      'src/a.js': js`import './index.css?a'`,
+      'src/b.js': js`import './index.css?b'`,
+      'src/index.css': css``,
+    },
+  },
+  async ({ fs, exec }) => {
+    await exec('pnpm webpack --mode=development')
+
+    await fs.expectFileToContain('dist/a.css', ['only-a', '--color-red-500'])
+    await fs.expectFileToContain('dist/b.css', ['only-b', '--color-blue-500'])
+    await fs.expectFileNotToContain('dist/a.css', ['only-b', '--color-blue-500'])
+    await fs.expectFileNotToContain('dist/b.css', ['only-a', '--color-red-500'])
+  },
+)

packages/@tailwindcss-webpack/src/index.ts

@@ -40,8 +40,12 @@ interface CacheEntry {
 
 const cache = new QuickLRU<string, CacheEntry>({ maxSize: 50 })
 
-function getContextFromCache(inputFile: string, opts: LoaderOptions): CacheEntry {
-  let key = `${inputFile}:${opts.base ?? ''}:${JSON.stringify(opts.optimize)}`
+function getCacheKey(resourceId: string, opts: LoaderOptions): string {
+  return `${resourceId}:${opts.base ?? ''}:${JSON.stringify(opts.optimize)}`
+}
+
+function getContextFromCache(resourceId: string, opts: LoaderOptions): CacheEntry {
+  let key = getCacheKey(resourceId, opts)
   if (cache.has(key)) return cache.get(key)!
   let entry: CacheEntry = {
     mtimes: new Map<string, number>(),
@@ -61,6 +65,7 @@ export default async function tailwindLoader(
   let callback = this.async()
   let options = this.getOptions() ?? {}
   let inputFile = this.resourcePath
+  let resourceId = this.resource
   let base = options.base ?? process.cwd()
   let shouldOptimize = options.optimize ?? process.env.NODE_ENV === 'production'
   let isCSSModuleFile = inputFile.endsWith('.module.css')
@@ -83,7 +88,7 @@ export default async function tailwindLoader(
   }
 
   try {
-    let context = getContextFromCache(inputFile, options)
+    let context = getContextFromCache(resourceId, options)
     let inputBasePath = path.dirname(path.resolve(inputFile))
 
     // Whether this is the first build or not
@@ -273,7 +278,7 @@ export default async function tailwindLoader(
     callback(null, result)
   } catch (error) {
     // Clear the cache entry on error to force a full rebuild next time
-    let key = `${inputFile}:${options.base ?? ''}:${JSON.stringify(options.optimize)}`
+    let key = getCacheKey(resourceId, options)
     cache.delete(key)
 
     DEBUG && I.end(`[@tailwindcss/webpack] ${path.relative(base, inputFile)}`)