diff --git a/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js b/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js
index df3a58c4..42faee53 100644
--- a/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js
+++ b/packages/tailwindcss-language-server/tests/diagnostics/diagnostics.test.js
@@ -433,7 +433,7 @@ defineTest({
     'package.json': json`
       {
         "dependencies": {
-          "tailwindcss": "0.0.0-insiders.efe084b"
+          "tailwindcss": "0.0.0-insiders.249bed0"
         }
       }
     `,
@@ -447,6 +447,7 @@ defineTest({
       settings: {
         tailwindCSS: {
           lint: { suggestCanonicalClasses: 'warning' },
+          rootFontSize: 16,
         },
       },
     }),
@@ -454,7 +455,7 @@ defineTest({
   handle: async ({ client }) => {
     let doc = await client.open({
       lang: 'html',
-      text: '<div class="[@media_print]:flex [color:red]/50">',
+      text: '<div class="[@media_print]:flex [color:red]/50 mt-[16px]">',
     })
 
     let diagnostics = await doc.diagnostics()
@@ -480,6 +481,16 @@ defineTest({
         severity: 2,
         suggestions: ['text-[red]/50'],
       },
+      {
+        code: 'suggestCanonicalClasses',
+        message: 'The class `mt-[16px]` can be written as `mt-4`',
+        range: {
+          start: { line: 0, character: 47 },
+          end: { line: 0, character: 56 },
+        },
+        severity: 2,
+        suggestions: ['mt-4'],
+      },
     ])
   },
 })
diff --git a/packages/tailwindcss-language-service/src/diagnostics/canonical-classes.ts b/packages/tailwindcss-language-service/src/diagnostics/canonical-classes.ts
index c47c40ce..3f30f337 100644
--- a/packages/tailwindcss-language-service/src/diagnostics/canonical-classes.ts
+++ b/packages/tailwindcss-language-service/src/diagnostics/canonical-classes.ts
@@ -37,7 +37,9 @@ export async function getSuggestCanonicalClassesDiagnostics(
     // diagnostics in a given class list.
 
     for (let className of classNames) {
-      let canonicalized = state.designSystem.canonicalizeCandidates([className.className])[0]
+      let canonicalized = state.designSystem.canonicalizeCandidates([className.className], {
+        rem: settings.tailwindCSS.rootFontSize,
+      })[0]
       let isCanonical = canonicalized === className.className
 
       if (isCanonical) continue
diff --git a/packages/tailwindcss-language-service/src/util/v4/design-system.ts b/packages/tailwindcss-language-service/src/util/v4/design-system.ts
index 3b3e3153..2ddcc2b3 100644
--- a/packages/tailwindcss-language-service/src/util/v4/design-system.ts
+++ b/packages/tailwindcss-language-service/src/util/v4/design-system.ts
@@ -29,6 +29,10 @@ export interface ThemeEntry {
   name: string
 }
 
+export interface CanonicalizeOptions {
+  rem?: number
+}
+
 export interface DesignSystem {
   theme: Theme
   variants: Map<string, VariantFn>
@@ -45,7 +49,7 @@ export interface DesignSystem {
   invalidCandidates?: Set<string>
 
   // Added in v4.1.15
-  canonicalizeCandidates?(classes: string[]): string[]
+  canonicalizeCandidates?(classes: string[], options?: CanonicalizeOptions): string[]
 }
 
 export interface DesignSystem {