Show more specific error for functional-like but invalid utility names (#18568)

Fixes #18566

The behavior is by design but the error message could definitely be
improved.

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ignore consecutive semicolons in the CSS parser ([#18532](https://github.com/tailwindlabs/tailwindcss/pull/18532))
 - Center the dropdown icon added to an input with a paired datalist ([#18511](https://github.com/tailwindlabs/tailwindcss/pull/18511))
 - Extract candidates in Slang templates ([#18565](https://github.com/tailwindlabs/tailwindcss/pull/18565))
+- Improve error messages when encountering invalid functional utility names ([#18568](https://github.com/tailwindlabs/tailwindcss/pull/18568))
 
 ## [4.1.11] - 2025-06-26
 

packages/tailwindcss/src/index.test.ts

@@ -4354,6 +4354,30 @@ describe('@utility', () => {
       `[Error: \`@utility 💨\` defines an invalid utility name. Utilities should be alphanumeric and start with a lowercase letter.]`,
     )
   })
+
+  test('A functional @utility must end in -*', () => {
+    return expect(
+      compileCss(css`
+        @utility foo* {
+          color: red;
+        }
+      `),
+    ).rejects.toThrowErrorMatchingInlineSnapshot(
+      `[Error: \`@utility foo*\` defines an invalid utility name. A functional utility must end in \`-*\`.]`,
+    )
+  })
+
+  test('Only the last part of a functional @utility can be dynamic', () => {
+    return expect(
+      compileCss(css`
+        @utility my-*-utility {
+          color: red;
+        }
+      `),
+    ).rejects.toThrowErrorMatchingInlineSnapshot(
+      `[Error: \`@utility my-*-utility\` defines an invalid utility name. The dynamic portion marked by \`-*\` must appear once at the end.]`,
+    )
+  })
 })
 
 test('addBase', async () => {

packages/tailwindcss/src/index.ts

@@ -229,6 +229,18 @@ async function parseCss(
 
       let utility = createCssUtility(node)
       if (utility === null) {
+        if (!node.params.endsWith('-*')) {
+          if (node.params.endsWith('*')) {
+            throw new Error(
+              `\`@utility ${node.params}\` defines an invalid utility name. A functional utility must end in \`-*\`.`,
+            )
+          } else if (node.params.includes('*')) {
+            throw new Error(
+              `\`@utility ${node.params}\` defines an invalid utility name. The dynamic portion marked by \`-*\` must appear once at the end.`,
+            )
+          }
+        }
+
         throw new Error(
           `\`@utility ${node.params}\` defines an invalid utility name. Utilities should be alphanumeric and start with a lowercase letter.`,
         )