diff --git a/packages/tailwindcss-language-server/package.json b/packages/tailwindcss-language-server/package.json
index 813e139be..d877569a0 100644
--- a/packages/tailwindcss-language-server/package.json
+++ b/packages/tailwindcss-language-server/package.json
@@ -36,7 +36,7 @@
     "@tailwindcss/forms": "0.5.3",
     "@tailwindcss/language-service": "workspace:*",
     "@tailwindcss/line-clamp": "0.4.2",
-    "@tailwindcss/oxide": "^4.0.0-alpha.16",
+    "@tailwindcss/oxide": "^4.0.0-alpha.19",
     "@tailwindcss/typography": "0.5.7",
     "@types/color-name": "^1.1.3",
     "@types/culori": "^2.1.0",
diff --git a/packages/tailwindcss-language-server/src/css/extract-source-directives.ts b/packages/tailwindcss-language-server/src/css/extract-source-directives.ts
new file mode 100644
index 000000000..9de33a9b3
--- /dev/null
+++ b/packages/tailwindcss-language-server/src/css/extract-source-directives.ts
@@ -0,0 +1,13 @@
+import type { Plugin } from 'postcss'
+
+export function extractSourceDirectives(sources: string[]): Plugin {
+  return {
+    postcssPlugin: 'extract-at-rules',
+    AtRule: {
+      source: ({ params }) => {
+        if (params[0] !== '"' && params[0] !== "'") return
+        sources.push(params.slice(1, -1))
+      },
+    },
+  }
+}
diff --git a/packages/tailwindcss-language-server/src/css/fix-relative-paths.ts b/packages/tailwindcss-language-server/src/css/fix-relative-paths.ts
new file mode 100644
index 000000000..46d5d1693
--- /dev/null
+++ b/packages/tailwindcss-language-server/src/css/fix-relative-paths.ts
@@ -0,0 +1,69 @@
+import path from 'node:path'
+import type { AtRule, Plugin } from 'postcss'
+import { normalizePath } from '../utils'
+
+const SINGLE_QUOTE = "'"
+const DOUBLE_QUOTE = '"'
+
+export function fixRelativePaths(): Plugin {
+  // Retain a list of touched at-rules to avoid infinite loops
+  let touched: WeakSet<AtRule> = new WeakSet()
+
+  function fixRelativePath(atRule: AtRule) {
+    if (touched.has(atRule)) return
+
+    let rootPath = atRule.root().source?.input.file
+    if (!rootPath) return
+
+    let inputFilePath = atRule.source?.input.file
+    if (!inputFilePath) return
+
+    let value = atRule.params[0]
+
+    let quote =
+      value[0] === DOUBLE_QUOTE && value[value.length - 1] === DOUBLE_QUOTE
+        ? DOUBLE_QUOTE
+        : value[0] === SINGLE_QUOTE && value[value.length - 1] === SINGLE_QUOTE
+          ? SINGLE_QUOTE
+          : null
+
+    if (!quote) return
+
+    let glob = atRule.params.slice(1, -1)
+
+    // Handle eventual negative rules. We only support one level of negation.
+    let negativePrefix = ''
+    if (glob.startsWith('!')) {
+      glob = glob.slice(1)
+      negativePrefix = '!'
+    }
+
+    // We only want to rewrite relative paths.
+    if (!glob.startsWith('./') && !glob.startsWith('../')) {
+      return
+    }
+
+    let absoluteGlob = path.posix.join(normalizePath(path.dirname(inputFilePath)), glob)
+    let absoluteRootPosixPath = path.posix.dirname(normalizePath(rootPath))
+
+    let relative = path.posix.relative(absoluteRootPosixPath, absoluteGlob)
+
+    // If the path points to a file in the same directory, `path.relative` will
+    // remove the leading `./` and we need to add it back in order to still
+    // consider the path relative
+    if (!relative.startsWith('.')) {
+      relative = './' + relative
+    }
+
+    atRule.params = quote + negativePrefix + relative + quote
+    touched.add(atRule)
+  }
+
+  return {
+    postcssPlugin: 'tailwindcss-postcss-fix-relative-paths',
+    AtRule: {
+      source: fixRelativePath,
+      plugin: fixRelativePath,
+    },
+  }
+}
diff --git a/packages/tailwindcss-language-server/src/css/index.ts b/packages/tailwindcss-language-server/src/css/index.ts
new file mode 100644
index 000000000..9028d9de1
--- /dev/null
+++ b/packages/tailwindcss-language-server/src/css/index.ts
@@ -0,0 +1,2 @@
+export * from './resolve-css-imports'
+export * from './extract-source-directives'
diff --git a/packages/tailwindcss-language-server/src/resolve-css-imports.ts b/packages/tailwindcss-language-server/src/css/resolve-css-imports.ts
similarity index 77%
rename from packages/tailwindcss-language-server/src/resolve-css-imports.ts
rename to packages/tailwindcss-language-server/src/css/resolve-css-imports.ts
index 2824c35aa..cf0e130a9 100644
--- a/packages/tailwindcss-language-server/src/resolve-css-imports.ts
+++ b/packages/tailwindcss-language-server/src/css/resolve-css-imports.ts
@@ -1,6 +1,7 @@
 import postcss from 'postcss'
 import postcssImport from 'postcss-import'
-import { createResolver } from './util/resolve'
+import { createResolver } from '../util/resolve'
+import { fixRelativePaths } from './fix-relative-paths'
 
 const resolver = createResolver({
   extensions: ['.css'],
@@ -15,6 +16,7 @@ const resolveImports = postcss([
       return paths ? paths : id
     },
   }),
+  fixRelativePaths(),
 ])
 
 export function resolveCssImports() {
diff --git a/packages/tailwindcss-language-server/src/language/cssServer.ts b/packages/tailwindcss-language-server/src/language/cssServer.ts
index a9b854983..6c47a727b 100644
--- a/packages/tailwindcss-language-server/src/language/cssServer.ts
+++ b/packages/tailwindcss-language-server/src/language/cssServer.ts
@@ -381,7 +381,7 @@ async function validateTextDocument(textDocument: TextDocument): Promise<void> {
     .filter((diagnostic) => {
       if (
         diagnostic.code === 'unknownAtRules' &&
-        /Unknown at rule @(tailwind|apply|config|theme)/.test(diagnostic.message)
+        /Unknown at rule @(tailwind|apply|config|theme|plugin|source)/.test(diagnostic.message)
       ) {
         return false
       }
diff --git a/packages/tailwindcss-language-server/src/oxide.ts b/packages/tailwindcss-language-server/src/oxide.ts
new file mode 100644
index 000000000..ba9f2b5d7
--- /dev/null
+++ b/packages/tailwindcss-language-server/src/oxide.ts
@@ -0,0 +1,143 @@
+import { lte } from 'tailwindcss-language-service/src/util/semver'
+
+// This covers the Oxide API from v4.0.0-alpha.1 to v4.0.0-alpha.18
+declare namespace OxideV1 {
+  interface GlobEntry {
+    base: string
+    glob: string
+  }
+
+  interface ScanOptions {
+    base: string
+    globs?: boolean
+  }
+
+  interface ScanResult {
+    files: Array<string>
+    globs: Array<GlobEntry>
+  }
+}
+
+// This covers the Oxide API from v4.0.0-alpha.19
+declare namespace OxideV2 {
+  interface GlobEntry {
+    base: string
+    pattern: string
+  }
+
+  interface ScanOptions {
+    base: string
+    sources: Array<GlobEntry>
+  }
+
+  interface ScanResult {
+    files: Array<string>
+    globs: Array<GlobEntry>
+  }
+}
+
+// This covers the Oxide API from v4.0.0-alpha.20+
+declare namespace OxideV3 {
+  interface GlobEntry {
+    base: string
+    pattern: string
+  }
+
+  interface ScannerOptions {
+    detectSources?: { base: string }
+    sources: Array<GlobEntry>
+  }
+
+  interface ScannerConstructor {
+    new (options: ScannerOptions): Scanner
+  }
+
+  interface Scanner {
+    files: Array<string>
+    globs: Array<GlobEntry>
+  }
+}
+
+interface Oxide {
+  scanDir?(options: OxideV1.ScanOptions): OxideV1.ScanResult
+  scanDir?(options: OxideV2.ScanOptions): OxideV2.ScanResult
+  Scanner?: OxideV3.ScannerConstructor
+}
+
+async function loadOxideAtPath(id: string): Promise<Oxide | null> {
+  let oxide = await import(id)
+
+  // This is a much older, unsupported version of Oxide before v4.0.0-alpha.1
+  if (!oxide.scanDir) return null
+
+  return oxide
+}
+
+interface GlobEntry {
+  base: string
+  pattern: string
+}
+
+interface ScanOptions {
+  oxidePath: string
+  oxideVersion: string
+  basePath: string
+  sources: Array<GlobEntry>
+}
+
+interface ScanResult {
+  files: Array<string>
+  globs: Array<GlobEntry>
+}
+
+/**
+ * This is a helper function that leverages the Oxide API to scan a directory
+ * and a set of sources and turn them into files and globs.
+ *
+ * Because the Oxide API has changed over time this function presents a unified
+ * interface that works with all versions of the Oxide API but the results may
+ * be different depending on the version of Oxide that is being used.
+ *
+ * For example, the `sources` option is ignored before v4.0.0-alpha.19.
+ */
+export async function scan(options: ScanOptions): Promise<ScanResult | null> {
+  const oxide = await loadOxideAtPath(options.oxidePath)
+  if (!oxide) return null
+
+  // V1
+  if (lte(options.oxideVersion, '4.0.0-alpha.18')) {
+    let result = oxide.scanDir?.({
+      base: options.basePath,
+      globs: true,
+    })
+
+    return {
+      files: result.files,
+      globs: result.globs.map((g) => ({ base: g.base, pattern: g.glob })),
+    }
+  }
+
+  // V2
+  if (lte(options.oxideVersion, '4.0.0-alpha.19')) {
+    let result = oxide.scanDir({
+      base: options.basePath,
+      sources: options.sources,
+    })
+
+    return {
+      files: result.files,
+      globs: result.globs,
+    }
+  }
+
+  // V3
+  let scanner = new oxide.Scanner({
+    detectSources: { base: options.basePath },
+    sources: options.sources,
+  })
+
+  return {
+    files: scanner.files,
+    globs: scanner.globs,
+  }
+}
diff --git a/packages/tailwindcss-language-server/src/project-locator.test.ts b/packages/tailwindcss-language-server/src/project-locator.test.ts
index 34bb73ec5..7db5f1b19 100644
--- a/packages/tailwindcss-language-server/src/project-locator.test.ts
+++ b/packages/tailwindcss-language-server/src/project-locator.test.ts
@@ -141,3 +141,29 @@ testFixture('v4/auto-content', [
     ],
   },
 ])
+
+testFixture('v4/custom-source', [
+  //
+  {
+    config: 'admin/app.css',
+    content: [
+      '{URL}/admin/**/*.{py,tpl,js,vue,php,mjs,cts,jsx,tsx,rhtml,slim,handlebars,twig,rs,njk,svelte,liquid,pug,md,ts,heex,mts,astro,nunjucks,rb,eex,haml,cjs,html,hbs,jade,aspx,razor,erb,mustache,mdx}',
+      '{URL}/admin/**/*.bin',
+      '{URL}/admin/foo.bin',
+      '{URL}/package.json',
+      '{URL}/shared.html',
+      '{URL}/web/**/*.{py,tpl,js,vue,php,mjs,cts,jsx,tsx,rhtml,slim,handlebars,twig,rs,njk,svelte,liquid,pug,md,ts,heex,mts,astro,nunjucks,rb,eex,haml,cjs,html,hbs,jade,aspx,razor,erb,mustache,mdx}',
+    ],
+  },
+  {
+    config: 'web/app.css',
+    content: [
+      '{URL}/admin/**/*.{py,tpl,js,vue,php,mjs,cts,jsx,tsx,rhtml,slim,handlebars,twig,rs,njk,svelte,liquid,pug,md,ts,heex,mts,astro,nunjucks,rb,eex,haml,cjs,html,hbs,jade,aspx,razor,erb,mustache,mdx}',
+      '{URL}/web/*.bin',
+      '{URL}/web/bar.bin',
+      '{URL}/package.json',
+      '{URL}/shared.html',
+      '{URL}/web/**/*.{py,tpl,js,vue,php,mjs,cts,jsx,tsx,rhtml,slim,handlebars,twig,rs,njk,svelte,liquid,pug,md,ts,heex,mts,astro,nunjucks,rb,eex,haml,cjs,html,hbs,jade,aspx,razor,erb,mustache,mdx}',
+    ],
+  },
+])
diff --git a/packages/tailwindcss-language-server/src/project-locator.ts b/packages/tailwindcss-language-server/src/project-locator.ts
index 6a96a9671..50c3d426f 100644
--- a/packages/tailwindcss-language-server/src/project-locator.ts
+++ b/packages/tailwindcss-language-server/src/project-locator.ts
@@ -7,14 +7,16 @@ import type { Settings } from '@tailwindcss/language-service/src/util/state'
 import { CONFIG_GLOB, CSS_GLOB } from './lib/constants'
 import { readCssFile } from './util/css'
 import { Graph } from './graph'
-import type { Message } from 'postcss'
+import type { AtRule, Message } from 'postcss'
 import { type DocumentSelector, DocumentSelectorPriority } from './projects'
 import { CacheMap } from './cache-map'
 import { getPackageRoot } from './util/get-package-root'
 import resolveFrom from './util/resolveFrom'
 import { type Feature, supportedFeatures } from '@tailwindcss/language-service/src/features'
-import { resolveCssImports } from './resolve-css-imports'
+import { extractSourceDirectives, resolveCssImports } from './css'
 import { normalizeDriveLetter, normalizePath, pathToFileURL } from './utils'
+import postcss from 'postcss'
+import * as oxide from './oxide'
 
 export interface ProjectConfig {
   /** The folder that contains the project */
@@ -341,6 +343,9 @@ export class ProjectLocator {
     // Resolve real paths for all the files in the CSS import graph
     await Promise.all(imports.map((file) => file.resolveRealpaths()))
 
+    // Resolve all @source directives
+    await Promise.all(imports.map((file) => file.resolveSourceDirectives()))
+
     // Create a graph of all the CSS files that might (indirectly) use Tailwind
     let graph = new Graph<FileEntry>()
 
@@ -500,7 +505,13 @@ async function* contentSelectorsFromCssConfig(entry: ConfigEntry): AsyncIterable
       }
     } else if (item.kind === 'auto' && !auto) {
       auto = true
-      for await (let pattern of detectContentFiles(entry.packageRoot)) {
+
+      // Note: the file representing `entry` is not guaranteed to be the first
+      // file so we use `flatMap`` here to simplify the logic but none of the
+      // other entries should have sources.
+      let sources = entry.entries.flatMap((entry) => entry.sources)
+
+      for await (let pattern of detectContentFiles(entry.packageRoot, entry.path, sources)) {
         yield {
           pattern,
           priority: DocumentSelectorPriority.CONTENT_FILE,
@@ -510,25 +521,37 @@ async function* contentSelectorsFromCssConfig(entry: ConfigEntry): AsyncIterable
   }
 }
 
-async function* detectContentFiles(base: string): AsyncIterable<string> {
+async function* detectContentFiles(
+  base: string,
+  inputFile: string,
+  sources: string[],
+): AsyncIterable<string> {
   try {
     let oxidePath = resolveFrom(path.dirname(base), '@tailwindcss/oxide')
     oxidePath = pathToFileURL(oxidePath).href
-
-    const oxide: typeof import('@tailwindcss/oxide') = await import(oxidePath)
+    let oxidePackageJsonPath = resolveFrom(path.dirname(base), '@tailwindcss/oxide/package.json')
+    let oxidePackageJson = JSON.parse(await fs.readFile(oxidePackageJsonPath, 'utf8'))
+
+    let result = await oxide.scan({
+      oxidePath,
+      oxideVersion: oxidePackageJson.version,
+      basePath: base,
+      sources: sources.map((pattern) => ({
+        base: path.dirname(inputFile),
+        pattern,
+      })),
+    })
 
     // This isn't a v4 project
-    if (!oxide.scanDir) return
-
-    let { files, globs } = oxide.scanDir({ base, globs: true })
+    if (!result) return
 
-    for (let file of files) {
+    for (let file of result.files) {
       yield normalizePath(file)
     }
 
-    for (let { base, glob } of globs) {
+    for (let { base, pattern } of result.globs) {
       // Do not normalize the glob itself as it may contain escape sequences
-      yield normalizePath(base) + '/' + glob
+      yield normalizePath(base) + '/' + pattern
     }
   } catch {
     //
@@ -553,6 +576,7 @@ class FileEntry {
   content: string | null
   deps: FileEntry[] = []
   realpath: string | null
+  sources: string[] = []
 
   constructor(
     public type: 'js' | 'css',
@@ -579,7 +603,8 @@ class FileEntry {
       // Replace the file content with the processed CSS
       this.content = result.css
     } catch {
-      //
+      // TODO: Errors here should be surfaced in tests and possibly the user in
+      // `trace` logs or something like that
     }
   }
 
@@ -589,6 +614,20 @@ class FileEntry {
     await Promise.all(this.deps.map((entry) => entry.resolveRealpaths()))
   }
 
+  async resolveSourceDirectives() {
+    if (this.sources.length > 0) {
+      return
+    }
+
+    // Note: This should eventually use the DesignSystem to extract the same
+    // sources also discovered by tailwind. Since we don't have everything yet
+    // to initialize the design system though, we set up a simple postcss at
+    // rule exporter instead for now.
+    await postcss([extractSourceDirectives(this.sources)]).process(this.content, {
+      from: this.realpath,
+    })
+  }
+
   /**
    * Look for `@config` directives in a CSS file and return the path to the config
    * file that it points to. This path is (possibly) relative to the CSS file so
diff --git a/packages/tailwindcss-language-server/src/util/v4/design-system.ts b/packages/tailwindcss-language-server/src/util/v4/design-system.ts
index 818902da0..07cc3766d 100644
--- a/packages/tailwindcss-language-server/src/util/v4/design-system.ts
+++ b/packages/tailwindcss-language-server/src/util/v4/design-system.ts
@@ -1,7 +1,7 @@
 import type { DesignSystem } from '@tailwindcss/language-service/src/util/v4'
 
 import postcss from 'postcss'
-import { resolveCssImports } from '../../resolve-css-imports'
+import { resolveCssImports } from '../../css'
 
 const HAS_V4_IMPORT = /@import\s*(?:'tailwindcss'|"tailwindcss")/
 const HAS_V4_THEME = /@theme\s*\{/
diff --git a/packages/tailwindcss-language-server/tests/completions/completions.test.js b/packages/tailwindcss-language-server/tests/completions/completions.test.js
index 32b283cd8..6f44f8b15 100644
--- a/packages/tailwindcss-language-server/tests/completions/completions.test.js
+++ b/packages/tailwindcss-language-server/tests/completions/completions.test.js
@@ -310,8 +310,8 @@ withFixture('v4/basic', (c) => {
     let result = await completion({ lang, text, position, settings })
     let textEdit = expect.objectContaining({ range: { start: position, end: position } })
 
-    expect(result.items.length).toBe(12376)
-    expect(result.items.filter((item) => item.label.endsWith(':')).length).toBe(220)
+    expect(result.items.length).toBe(12400)
+    expect(result.items.filter((item) => item.label.endsWith(':')).length).toBe(224)
     expect(result).toEqual({
       isIncomplete: false,
       items: expect.arrayContaining([
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package-lock.json b/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package-lock.json
index 803850fa7..318dad5e1 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package-lock.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package-lock.json
@@ -5,37 +5,39 @@
   "packages": {
     "": {
       "dependencies": {
-        "@tailwindcss/oxide": "^4.0.0-alpha.16",
-        "tailwindcss": "^4.0.0-alpha.16"
+        "@tailwindcss/oxide": "^4.0.0-alpha.19",
+        "tailwindcss": "^4.0.0-alpha.19"
       }
     },
     "node_modules/@tailwindcss/oxide": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide/-/oxide-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-sm/Y8dcTyM7WtNqGhpBkzcjWwhMqt46CN2VQ0KxAFH+FAz4BElnl/8eWaSd1ZKWosxDQCXB8d2Yy38h7Yqbw8g==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide/-/oxide-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-DdkrVz/MKPoe9v7W3c0+SEFKRDIPMSsxgN7gPC+xeTnTL4BGoT5b1EiVGFuXWEyLbDmWztuN6z75Yuze2BwvMQ==",
+      "license": "MIT",
       "engines": {
         "node": ">= 10"
       },
       "optionalDependencies": {
-        "@tailwindcss/oxide-android-arm64": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-darwin-arm64": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-darwin-x64": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-freebsd-x64": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-linux-arm-gnueabihf": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-linux-arm64-gnu": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-linux-arm64-musl": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-linux-x64-gnu": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-linux-x64-musl": "4.0.0-alpha.16",
-        "@tailwindcss/oxide-win32-x64-msvc": "4.0.0-alpha.16"
+        "@tailwindcss/oxide-android-arm64": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-darwin-arm64": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-darwin-x64": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-freebsd-x64": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-linux-arm-gnueabihf": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-linux-arm64-gnu": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-linux-arm64-musl": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-linux-x64-gnu": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-linux-x64-musl": "4.0.0-alpha.19",
+        "@tailwindcss/oxide-win32-x64-msvc": "4.0.0-alpha.19"
       }
     },
     "node_modules/@tailwindcss/oxide-android-arm64": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-android-arm64/-/oxide-android-arm64-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-duaTHvkAeUJQoqfA5XnYIp6F0PtqdcjXILuUF43wV0hC3NH2CECaxAgG2Ca5OVFAGqCI3fo29iqPMVcEEDlyjA==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-android-arm64/-/oxide-android-arm64-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-NhpXem1j7g0uSGyLucmMj0VVQMeUrWc6kR/Ymnri3tpw2eaykgFYwLfdnI7jdJRxUxa/nNJip9yBJ3diZXl60w==",
       "cpu": [
         "arm64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "android"
@@ -45,12 +47,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-darwin-arm64": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-darwin-arm64/-/oxide-darwin-arm64-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-4T5+35t5Qb0hZLLmclhVzsV5tmnjMwCqEySMnG8YLMB7YlATvmZG9TL8JqJLQjxqwjMDsl5tCddkui4FAxgLbA==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-darwin-arm64/-/oxide-darwin-arm64-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-KCdalT+huX2cW9snNmPr+B66V91cSzIobBCXVgYCPCh0NZF4ueKu+X+kQN2gFxurDUm/D+aKW/0rQUIsUmFpdQ==",
       "cpu": [
         "arm64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "darwin"
@@ -60,12 +63,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-darwin-x64": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-darwin-x64/-/oxide-darwin-x64-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-dchop1QRdOcnh8hwI/w1HrUgE3ZAvvz8iCEv5akEA0zOglBsHd3hGA2u8zAt5PrDz/wBmdOpr+R5H2bYLw1MPw==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-darwin-x64/-/oxide-darwin-x64-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-ETOWA08loUmOVTEa3zhRhY8HyqdGtR9DNhXdrRZBi67ZwCAmA+jg5B+mZaYeQJ6CjETx07BnhcGmmxGz3/6c8w==",
       "cpu": [
         "x64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "darwin"
@@ -75,12 +79,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-freebsd-x64": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-freebsd-x64/-/oxide-freebsd-x64-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-K5otxfNigxsY2fkgHI63Jjm+hvSI4gCFa2xGtsvTVUEHPUTOEo4n+aj9yIkNFgGpeIDii2nt3DtKYjhKyfUirw==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-freebsd-x64/-/oxide-freebsd-x64-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-uB0rYLpqPnmyqtYSKHu1AtnHeerNcVH+de0sIufGCBDFGYNxmW8feCKNZwo6r7U/Fzg+AF9BOjwvdvd4yLfQ8g==",
       "cpu": [
         "x64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "freebsd"
@@ -90,12 +95,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-linux-arm-gnueabihf": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-arm-gnueabihf/-/oxide-linux-arm-gnueabihf-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-AT8tYba/32q5FVLnJThcvYS8zmOBwLU5JzScaTY0Lc34WbGQ0+y6dtPlZoyyW+e+OBI8mDsTiD2BR3h0rdqb7g==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-arm-gnueabihf/-/oxide-linux-arm-gnueabihf-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-hIfm6DNh18rkz2PFRsQANINH0tpso6/vaU8p0Qw7rgYqqrxJTRpyLVsnvx3ahMOplJrDT6Z+Nfak8udnZN2C/Q==",
       "cpu": [
         "arm"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "linux"
@@ -105,12 +111,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-linux-arm64-gnu": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-arm64-gnu/-/oxide-linux-arm64-gnu-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-RcKr+fXs0kOT679UM2SEBRqGkXTP+jzk9+G96gwqa4OLgp6fiW1TSRB22V8j+Q10oWqfMHxsBSe9awM7F2ebuw==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-arm64-gnu/-/oxide-linux-arm64-gnu-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-n1Hr+8Hup2GLmeonQy9ydZxMBCs0FR1rcv4K7AHip+6PbD0se8k9LBIZac3OguFNj2hTehiadaiRb18rsVUw0g==",
       "cpu": [
         "arm64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "linux"
@@ -120,12 +127,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-linux-arm64-musl": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-arm64-musl/-/oxide-linux-arm64-musl-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-lsUf21WkPufMVSOmj3EwoSGdb0KbCq0czMChkeIyLlt5WC/ZvH0ZMd9U5sfHQ7c1Q9usWfhz+Is9SbX7n2WvuA==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-arm64-musl/-/oxide-linux-arm64-musl-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-KhwthLZh9Js3t5URkuRURw45iU3rSh9vhuHRaV4KQT10ZFiXQMUlFfMKJyxRMcgC2fcL5vsiqwjOaMwp7Y8vsQ==",
       "cpu": [
         "arm64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "linux"
@@ -135,12 +143,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-linux-x64-gnu": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-x64-gnu/-/oxide-linux-x64-gnu-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-D+uu2PCFb1fOuVWWS+xhtKVfbNbPmGYdMy4xwplOdHn8gacokUvDGsKdW/nogFoHtSws4+U6O4+mFjSQH3heSg==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-x64-gnu/-/oxide-linux-x64-gnu-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-qqEuULSiczyZkdWVzwkiiFyOYqx5RR2De75iwYREzXUuHRHval1ep2qO7tvZdgt37t2vgjoQwaPA6zO+JGUa+Q==",
       "cpu": [
         "x64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "linux"
@@ -150,12 +159,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-linux-x64-musl": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-x64-musl/-/oxide-linux-x64-musl-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-zFoaEQvx9DhXO7LUNRlmUFm8N92LXs9n1YD/60MOYJqpVzPdqLBplk+Ltpw1NPE/Y2BZ7XvXyrBl11XH6Wj0/A==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-linux-x64-musl/-/oxide-linux-x64-musl-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-JKTYCiNz83sYl2FgKJk3dL11FS4dAj7Rgmuz3TVANmeMYTBtwmFPthoH0qAmF+hjPJgT5Ne7lSwplfuHJAD3MQ==",
       "cpu": [
         "x64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "linux"
@@ -165,12 +175,13 @@
       }
     },
     "node_modules/@tailwindcss/oxide-win32-x64-msvc": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-win32-x64-msvc/-/oxide-win32-x64-msvc-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-ClCzUFuD6xptvcksYtoLJekUdSN9TVoSrr66eNVAErtA+vKKTThOyliEz/pZfe7lHsI93sDR22HMtu/zP0prJA==",
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/oxide-win32-x64-msvc/-/oxide-win32-x64-msvc-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-D43tji14+i/GhJn5YZX8c2FXFmAqlI6DDGX8caUM35dga/uT+sJUfLJ84YigJyvAdSF1gBVdm7MvhYRcVYHOwg==",
       "cpu": [
         "x64"
       ],
+      "license": "MIT",
       "optional": true,
       "os": [
         "win32"
@@ -180,9 +191,10 @@
       }
     },
     "node_modules/tailwindcss": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-h6UIkQEpOJZy0N8tXeWgIhsEYPfUyqST9Oidr46+1W78p8S9hjJDfnW08/bKW17NA9/ro8sZvFHT98LtwwxtSQ=="
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-4k8i6ml9ZQaj4xmdTHA2ZVp4YGsA+9+8VsIj/VkwyD38J5jX3L3NSi7TnPvlYT+WXcqAJF7EX8x76yCTfi2CyA==",
+      "license": "MIT"
     }
   }
 }
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package.json b/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package.json
index 874817d09..50f951e76 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/auto-content/package.json
@@ -1,6 +1,6 @@
 {
   "dependencies": {
-    "tailwindcss": "^4.0.0-alpha.16",
-    "@tailwindcss/oxide": "^4.0.0-alpha.16"
+    "tailwindcss": "^4.0.0-alpha.19",
+    "@tailwindcss/oxide": "^4.0.0-alpha.19"
   }
 }
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package-lock.json b/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package-lock.json
index 4e476083b..cf1b17a6d 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package-lock.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package-lock.json
@@ -5,13 +5,14 @@
   "packages": {
     "": {
       "dependencies": {
-        "tailwindcss": "^4.0.0-alpha.16"
+        "tailwindcss": "^4.0.0-alpha.19"
       }
     },
     "node_modules/tailwindcss": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-h6UIkQEpOJZy0N8tXeWgIhsEYPfUyqST9Oidr46+1W78p8S9hjJDfnW08/bKW17NA9/ro8sZvFHT98LtwwxtSQ=="
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-4k8i6ml9ZQaj4xmdTHA2ZVp4YGsA+9+8VsIj/VkwyD38J5jX3L3NSi7TnPvlYT+WXcqAJF7EX8x76yCTfi2CyA==",
+      "license": "MIT"
     }
   }
 }
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package.json b/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package.json
index 996efe041..efa461d1b 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/basic/package.json
@@ -1,5 +1,5 @@
 {
   "dependencies": {
-    "tailwindcss": "^4.0.0-alpha.16"
+    "tailwindcss": "^4.0.0-alpha.19"
   }
 }
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/app.css b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/app.css
new file mode 100644
index 000000000..41dcb5f80
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/app.css
@@ -0,0 +1,2 @@
+@import './tw.css';
+@import './ui.css';
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/foo.bin b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/foo.bin
new file mode 100644
index 000000000..ec56c4341
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/foo.bin
@@ -0,0 +1 @@
+<p class="underline">Admin</p>
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/tw.css b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/tw.css
new file mode 100644
index 000000000..9c0a79199
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/tw.css
@@ -0,0 +1,2 @@
+@import 'tailwindcss';
+@source './**/*.bin';
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/ui.css b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/ui.css
new file mode 100644
index 000000000..24ea64d28
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/admin/ui.css
@@ -0,0 +1,3 @@
+@theme {
+  --color-potato: #907a70;
+}
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/package-lock.json b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/package-lock.json
new file mode 100644
index 000000000..2bc339890
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/package-lock.json
@@ -0,0 +1,16 @@
+{
+  "name": "custom-source",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "dependencies": {
+        "tailwindcss": "^4.0.0-alpha.19"
+      }
+    },
+    "node_modules/tailwindcss": {
+      "version": "4.0.0-alpha.19",
+      "license": "MIT"
+    }
+  }
+}
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/package.json b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/package.json
new file mode 100644
index 000000000..efa461d1b
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/package.json
@@ -0,0 +1,5 @@
+{
+  "dependencies": {
+    "tailwindcss": "^4.0.0-alpha.19"
+  }
+}
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/shared.html b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/shared.html
new file mode 100644
index 000000000..49b293bf4
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/shared.html
@@ -0,0 +1 @@
+<p>I belong to no one!</p>
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/web/app.css b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/web/app.css
new file mode 100644
index 000000000..9357eb024
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/web/app.css
@@ -0,0 +1,2 @@
+@import 'tailwindcss';
+@source './*.bin';
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/web/bar.bin b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/web/bar.bin
new file mode 100644
index 000000000..56ee61a2a
--- /dev/null
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/custom-source/web/bar.bin
@@ -0,0 +1 @@
+<p class="underline">Web</p>
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package-lock.json b/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package-lock.json
index 697a00965..b64395e49 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package-lock.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package-lock.json
@@ -5,13 +5,14 @@
   "packages": {
     "": {
       "dependencies": {
-        "tailwindcss": "^4.0.0-alpha.16"
+        "tailwindcss": "^4.0.0-alpha.19"
       }
     },
     "node_modules/tailwindcss": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-h6UIkQEpOJZy0N8tXeWgIhsEYPfUyqST9Oidr46+1W78p8S9hjJDfnW08/bKW17NA9/ro8sZvFHT98LtwwxtSQ=="
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-4k8i6ml9ZQaj4xmdTHA2ZVp4YGsA+9+8VsIj/VkwyD38J5jX3L3NSi7TnPvlYT+WXcqAJF7EX8x76yCTfi2CyA==",
+      "license": "MIT"
     }
   }
 }
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package.json b/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package.json
index 996efe041..efa461d1b 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/multi-config/package.json
@@ -1,5 +1,5 @@
 {
   "dependencies": {
-    "tailwindcss": "^4.0.0-alpha.16"
+    "tailwindcss": "^4.0.0-alpha.19"
   }
 }
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package-lock.json b/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package-lock.json
index 1dd6e822d..df3317ef2 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package-lock.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package-lock.json
@@ -8,7 +8,7 @@
         "packages/*"
       ],
       "dependencies": {
-        "tailwindcss": "^4.0.0-alpha.16"
+        "tailwindcss": "^4.0.0-alpha.19"
       }
     },
     "node_modules/@private/admin": {
@@ -32,9 +32,10 @@
       "link": true
     },
     "node_modules/tailwindcss": {
-      "version": "4.0.0-alpha.16",
-      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.16.tgz",
-      "integrity": "sha512-h6UIkQEpOJZy0N8tXeWgIhsEYPfUyqST9Oidr46+1W78p8S9hjJDfnW08/bKW17NA9/ro8sZvFHT98LtwwxtSQ=="
+      "version": "4.0.0-alpha.19",
+      "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.0.0-alpha.19.tgz",
+      "integrity": "sha512-4k8i6ml9ZQaj4xmdTHA2ZVp4YGsA+9+8VsIj/VkwyD38J5jX3L3NSi7TnPvlYT+WXcqAJF7EX8x76yCTfi2CyA==",
+      "license": "MIT"
     },
     "packages/admin": {
       "name": "@private/admin"
diff --git a/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package.json b/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package.json
index 3f0204c30..700667f29 100644
--- a/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package.json
+++ b/packages/tailwindcss-language-server/tests/fixtures/v4/workspaces/package.json
@@ -3,6 +3,6 @@
     "packages/*"
   ],
   "dependencies": {
-    "tailwindcss": "^4.0.0-alpha.16"
+    "tailwindcss": "^4.0.0-alpha.19"
   }
 }
diff --git a/packages/tailwindcss-language-service/src/util/doc.ts b/packages/tailwindcss-language-service/src/util/doc.ts
index 4975ec463..770612d70 100644
--- a/packages/tailwindcss-language-service/src/util/doc.ts
+++ b/packages/tailwindcss-language-service/src/util/doc.ts
@@ -1,6 +1,7 @@
 import type { Range } from 'vscode-languageserver'
 import type { TextDocument } from 'vscode-languageserver-textdocument'
 import moo from 'moo'
+import { spliceChangesIntoString, StringChange } from './splice-changes-into-string'
 
 export function getTextWithoutComments(
   doc: TextDocument,
@@ -8,6 +9,7 @@ export function getTextWithoutComments(
   range?: Range,
 ): string
 export function getTextWithoutComments(text: string, type: 'html' | 'js' | 'jsx' | 'css'): string
+
 export function getTextWithoutComments(
   docOrText: TextDocument | string,
   type: 'html' | 'js' | 'jsx' | 'css',
@@ -20,12 +22,78 @@ export function getTextWithoutComments(
   }
 
   if (type === 'css') {
-    return text.replace(/\/\*.*?\*\//gs, replace)
+    return getCssWithoutComments(text)
   }
 
   return text.replace(/<!--.*?-->/gs, replace)
 }
 
+function getCssWithoutComments(input: string) {
+  const DOUBLE_QUOTE = 0x22 // "
+  const SINGLE_QUOTE = 0x27 // '
+  const BACKSLASH = 0x5c // \
+  const SLASH = 0x2f // /
+  const ASTERISK = 0x2a // *
+  const LINE_BREAK = 0x0a // \n
+
+  let changes: StringChange[] = []
+
+  // Collect ranges for every comment in the input.
+  for (let i = 0; i < input.length; ++i) {
+    let currentChar = input.charCodeAt(i)
+
+    if (currentChar === BACKSLASH) {
+      i += 1
+    }
+
+    // Skip over strings — they are to be left untouched
+    else if (currentChar === SINGLE_QUOTE || currentChar === DOUBLE_QUOTE) {
+      for (let j = i + 1; j < input.length; ++j) {
+        let peekChar = input.charCodeAt(j)
+
+        // Current character is a `\` therefore the next character is escaped.
+        if (peekChar === BACKSLASH) {
+          j += 1
+        }
+
+        // End of the string.
+        else if (peekChar === currentChar) {
+          i = j
+          break
+        } else if (peekChar === LINE_BREAK) {
+          i = j
+          break
+        }
+      }
+    } else if (currentChar === SLASH && input.charCodeAt(i + 1) === ASTERISK) {
+      let start = i
+
+      for (let j = i + 2; j < input.length; j++) {
+        let peekChar = input.charCodeAt(j)
+
+        // Current character is a `\` therefore the next character is escaped.
+        if (peekChar === BACKSLASH) {
+          j += 1
+        }
+
+        // End of the comment
+        else if (peekChar === ASTERISK && input.charCodeAt(j + 1) === SLASH) {
+          i = j + 1
+          break
+        }
+      }
+
+      changes.push({
+        start,
+        end: i + 1,
+        replacement: replace(input.slice(start, i + 1)),
+      })
+    }
+  }
+
+  return spliceChangesIntoString(input, changes)
+}
+
 function replace(match: string): string {
   return match.replace(/./gs, (char) => (char === '\n' ? '\n' : ' '))
 }
diff --git a/packages/tailwindcss-language-service/src/util/splice-changes-into-string.ts b/packages/tailwindcss-language-service/src/util/splice-changes-into-string.ts
new file mode 100644
index 000000000..e1e6f1685
--- /dev/null
+++ b/packages/tailwindcss-language-service/src/util/splice-changes-into-string.ts
@@ -0,0 +1,42 @@
+export interface StringChange {
+  start: number
+  end: number
+  replacement: string
+}
+
+/**
+ * Apply the changes to the string such that a change in the length
+ * of the string does not break the indexes of the subsequent changes.
+ */
+export function spliceChangesIntoString(str: string, changes: StringChange[]) {
+  // If there are no changes, return the original string
+  if (!changes[0]) return str
+
+  // Sort all changes in order to make it easier to apply them
+  changes.sort((a, b) => {
+    return a.end - b.end || a.start - b.start
+  })
+
+  // Append original string between each chunk, and then the chunk itself
+  // This is sort of a String Builder pattern, thus creating less memory pressure
+  let result = ''
+
+  let previous = changes[0]
+
+  result += str.slice(0, previous.start)
+  result += previous.replacement
+
+  for (let i = 1; i < changes.length; ++i) {
+    let change = changes[i]
+
+    result += str.slice(previous.end, change.start)
+    result += change.replacement
+
+    previous = change
+  }
+
+  // Add leftover string from last chunk to end
+  result += str.slice(previous.end)
+
+  return result
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index d746a236c..851a1c973 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -51,8 +51,8 @@ importers:
         specifier: 0.4.2
         version: 0.4.2(tailwindcss@3.4.4)
       '@tailwindcss/oxide':
-        specifier: ^4.0.0-alpha.16
-        version: 4.0.0-alpha.18
+        specifier: ^4.0.0-alpha.19
+        version: 4.0.0-alpha.19
       '@tailwindcss/typography':
         specifier: 0.5.7
         version: 0.5.7(tailwindcss@3.4.4)
@@ -840,68 +840,68 @@ packages:
     peerDependencies:
       tailwindcss: '>=2.0.0 || >=3.0.0 || >=3.0.0-alpha.1'
 
-  '@tailwindcss/oxide-android-arm64@4.0.0-alpha.18':
-    resolution: {integrity: sha512-fAdyr3GC5CDEdiVs/wg6aZFr0BD/RZxlEAjzGUL9mFyqyX5HsGbu0rVwyJvDaFjKMw5nB6hjfyaZMpuwHfQr8w==}
+  '@tailwindcss/oxide-android-arm64@4.0.0-alpha.19':
+    resolution: {integrity: sha512-NhpXem1j7g0uSGyLucmMj0VVQMeUrWc6kR/Ymnri3tpw2eaykgFYwLfdnI7jdJRxUxa/nNJip9yBJ3diZXl60w==}
     engines: {node: '>= 10'}
     cpu: [arm64]
     os: [android]
 
-  '@tailwindcss/oxide-darwin-arm64@4.0.0-alpha.18':
-    resolution: {integrity: sha512-K3YoYtAQNyaLXJ8q6f2ib4K6hm1+MT2ajh59C8rLOjfBp0jL4Whd8k9DuL8ctjlVoRvRT9ggQO90fKqx5g+6HA==}
+  '@tailwindcss/oxide-darwin-arm64@4.0.0-alpha.19':
+    resolution: {integrity: sha512-KCdalT+huX2cW9snNmPr+B66V91cSzIobBCXVgYCPCh0NZF4ueKu+X+kQN2gFxurDUm/D+aKW/0rQUIsUmFpdQ==}
     engines: {node: '>= 10'}
     cpu: [arm64]
     os: [darwin]
 
-  '@tailwindcss/oxide-darwin-x64@4.0.0-alpha.18':
-    resolution: {integrity: sha512-WYIK59UaKWhh1LVV6E3VdfPjrZZl/d1q1B3pBVXSoxOWU2fX1OP1f6pqs78/cADAK3FDJ24MByn2m/rDyfFXvg==}
+  '@tailwindcss/oxide-darwin-x64@4.0.0-alpha.19':
+    resolution: {integrity: sha512-ETOWA08loUmOVTEa3zhRhY8HyqdGtR9DNhXdrRZBi67ZwCAmA+jg5B+mZaYeQJ6CjETx07BnhcGmmxGz3/6c8w==}
     engines: {node: '>= 10'}
     cpu: [x64]
     os: [darwin]
 
-  '@tailwindcss/oxide-freebsd-x64@4.0.0-alpha.18':
-    resolution: {integrity: sha512-awG9qCHfVEa7yyEnFziDf0jkbJ/zB7n8CYv6jnDt43nfrdiytTmXOD7+QUt1PEY+mKw5mOJR4yWH1rMVnWIV5w==}
+  '@tailwindcss/oxide-freebsd-x64@4.0.0-alpha.19':
+    resolution: {integrity: sha512-uB0rYLpqPnmyqtYSKHu1AtnHeerNcVH+de0sIufGCBDFGYNxmW8feCKNZwo6r7U/Fzg+AF9BOjwvdvd4yLfQ8g==}
     engines: {node: '>= 10'}
     cpu: [x64]
     os: [freebsd]
 
-  '@tailwindcss/oxide-linux-arm-gnueabihf@4.0.0-alpha.18':
-    resolution: {integrity: sha512-4aY4B9Ips0KuvCF9vDLKE99APi7GxyueQPzKWkHE1T9CJVFk1JpsPF1hnSzaZewxloM18yLXBFCl7SImoaGh0A==}
+  '@tailwindcss/oxide-linux-arm-gnueabihf@4.0.0-alpha.19':
+    resolution: {integrity: sha512-hIfm6DNh18rkz2PFRsQANINH0tpso6/vaU8p0Qw7rgYqqrxJTRpyLVsnvx3ahMOplJrDT6Z+Nfak8udnZN2C/Q==}
     engines: {node: '>= 10'}
     cpu: [arm]
     os: [linux]
 
-  '@tailwindcss/oxide-linux-arm64-gnu@4.0.0-alpha.18':
-    resolution: {integrity: sha512-Ky9gcVhJZQBaRLL5w35dE+fHoHXVn9PJy7iNAqgauh+kkPeU7L/EoWu70XMBVaxu7jw8J7KLAUgLR9E6Atwldg==}
+  '@tailwindcss/oxide-linux-arm64-gnu@4.0.0-alpha.19':
+    resolution: {integrity: sha512-n1Hr+8Hup2GLmeonQy9ydZxMBCs0FR1rcv4K7AHip+6PbD0se8k9LBIZac3OguFNj2hTehiadaiRb18rsVUw0g==}
     engines: {node: '>= 10'}
     cpu: [arm64]
     os: [linux]
 
-  '@tailwindcss/oxide-linux-arm64-musl@4.0.0-alpha.18':
-    resolution: {integrity: sha512-dcq/3wleXfqc9+QAe1eZO00yJEGJRhwA6U80p3C1cgVSg3sZOeKwLPi/69MbkXN1I+VoKqLbYgtv/GzzLY9g2A==}
+  '@tailwindcss/oxide-linux-arm64-musl@4.0.0-alpha.19':
+    resolution: {integrity: sha512-KhwthLZh9Js3t5URkuRURw45iU3rSh9vhuHRaV4KQT10ZFiXQMUlFfMKJyxRMcgC2fcL5vsiqwjOaMwp7Y8vsQ==}
     engines: {node: '>= 10'}
     cpu: [arm64]
     os: [linux]
 
-  '@tailwindcss/oxide-linux-x64-gnu@4.0.0-alpha.18':
-    resolution: {integrity: sha512-R0NeALYR7vhGNvY5PJpmwPdAfp5X2CyYo8FqR2fMgqVyKHpGhJsj+BR2Hg6AcSFg7t4DA1T+fOOcGUupK9cnkQ==}
+  '@tailwindcss/oxide-linux-x64-gnu@4.0.0-alpha.19':
+    resolution: {integrity: sha512-qqEuULSiczyZkdWVzwkiiFyOYqx5RR2De75iwYREzXUuHRHval1ep2qO7tvZdgt37t2vgjoQwaPA6zO+JGUa+Q==}
     engines: {node: '>= 10'}
     cpu: [x64]
     os: [linux]
 
-  '@tailwindcss/oxide-linux-x64-musl@4.0.0-alpha.18':
-    resolution: {integrity: sha512-dY/j372Jc+MwGoyHnFW8eGBW6Gab1lU1cMIFm+4TcA63HuYAJfnzt+KU1Jai/5huYj+cy6qrHnWAdO9V3yWpdQ==}
+  '@tailwindcss/oxide-linux-x64-musl@4.0.0-alpha.19':
+    resolution: {integrity: sha512-JKTYCiNz83sYl2FgKJk3dL11FS4dAj7Rgmuz3TVANmeMYTBtwmFPthoH0qAmF+hjPJgT5Ne7lSwplfuHJAD3MQ==}
     engines: {node: '>= 10'}
     cpu: [x64]
     os: [linux]
 
-  '@tailwindcss/oxide-win32-x64-msvc@4.0.0-alpha.18':
-    resolution: {integrity: sha512-B7eSqgxLfJYZCOX+PZZRWJB5pKTRAVWDRzlMy86cdOIT12TRL4OPT4KQuWnrvfNKVKgcQv6eZhG6CZS6pLi20Q==}
+  '@tailwindcss/oxide-win32-x64-msvc@4.0.0-alpha.19':
+    resolution: {integrity: sha512-D43tji14+i/GhJn5YZX8c2FXFmAqlI6DDGX8caUM35dga/uT+sJUfLJ84YigJyvAdSF1gBVdm7MvhYRcVYHOwg==}
     engines: {node: '>= 10'}
     cpu: [x64]
     os: [win32]
 
-  '@tailwindcss/oxide@4.0.0-alpha.18':
-    resolution: {integrity: sha512-Pc4c8khaklIqLWWSVqbVX8P3imjmM3SChJaSPkajbzlQRx17iwqZWVQJIhwP/rZh1aGeJDOVb0kduVlw/J5bIA==}
+  '@tailwindcss/oxide@4.0.0-alpha.19':
+    resolution: {integrity: sha512-DdkrVz/MKPoe9v7W3c0+SEFKRDIPMSsxgN7gPC+xeTnTL4BGoT5b1EiVGFuXWEyLbDmWztuN6z75Yuze2BwvMQ==}
     engines: {node: '>= 10'}
 
   '@tailwindcss/typography@0.5.7':
@@ -3056,48 +3056,48 @@ snapshots:
     dependencies:
       tailwindcss: 3.4.4
 
-  '@tailwindcss/oxide-android-arm64@4.0.0-alpha.18':
+  '@tailwindcss/oxide-android-arm64@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-darwin-arm64@4.0.0-alpha.18':
+  '@tailwindcss/oxide-darwin-arm64@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-darwin-x64@4.0.0-alpha.18':
+  '@tailwindcss/oxide-darwin-x64@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-freebsd-x64@4.0.0-alpha.18':
+  '@tailwindcss/oxide-freebsd-x64@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-linux-arm-gnueabihf@4.0.0-alpha.18':
+  '@tailwindcss/oxide-linux-arm-gnueabihf@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-linux-arm64-gnu@4.0.0-alpha.18':
+  '@tailwindcss/oxide-linux-arm64-gnu@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-linux-arm64-musl@4.0.0-alpha.18':
+  '@tailwindcss/oxide-linux-arm64-musl@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-linux-x64-gnu@4.0.0-alpha.18':
+  '@tailwindcss/oxide-linux-x64-gnu@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-linux-x64-musl@4.0.0-alpha.18':
+  '@tailwindcss/oxide-linux-x64-musl@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide-win32-x64-msvc@4.0.0-alpha.18':
+  '@tailwindcss/oxide-win32-x64-msvc@4.0.0-alpha.19':
     optional: true
 
-  '@tailwindcss/oxide@4.0.0-alpha.18':
+  '@tailwindcss/oxide@4.0.0-alpha.19':
     optionalDependencies:
-      '@tailwindcss/oxide-android-arm64': 4.0.0-alpha.18
-      '@tailwindcss/oxide-darwin-arm64': 4.0.0-alpha.18
-      '@tailwindcss/oxide-darwin-x64': 4.0.0-alpha.18
-      '@tailwindcss/oxide-freebsd-x64': 4.0.0-alpha.18
-      '@tailwindcss/oxide-linux-arm-gnueabihf': 4.0.0-alpha.18
-      '@tailwindcss/oxide-linux-arm64-gnu': 4.0.0-alpha.18
-      '@tailwindcss/oxide-linux-arm64-musl': 4.0.0-alpha.18
-      '@tailwindcss/oxide-linux-x64-gnu': 4.0.0-alpha.18
-      '@tailwindcss/oxide-linux-x64-musl': 4.0.0-alpha.18
-      '@tailwindcss/oxide-win32-x64-msvc': 4.0.0-alpha.18
+      '@tailwindcss/oxide-android-arm64': 4.0.0-alpha.19
+      '@tailwindcss/oxide-darwin-arm64': 4.0.0-alpha.19
+      '@tailwindcss/oxide-darwin-x64': 4.0.0-alpha.19
+      '@tailwindcss/oxide-freebsd-x64': 4.0.0-alpha.19
+      '@tailwindcss/oxide-linux-arm-gnueabihf': 4.0.0-alpha.19
+      '@tailwindcss/oxide-linux-arm64-gnu': 4.0.0-alpha.19
+      '@tailwindcss/oxide-linux-arm64-musl': 4.0.0-alpha.19
+      '@tailwindcss/oxide-linux-x64-gnu': 4.0.0-alpha.19
+      '@tailwindcss/oxide-linux-x64-musl': 4.0.0-alpha.19
+      '@tailwindcss/oxide-win32-x64-msvc': 4.0.0-alpha.19
 
   '@tailwindcss/typography@0.5.7(tailwindcss@3.4.4)':
     dependencies: