Fix recursion in legacy CSS theme(…) function

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- Fix infinite loop when `theme()` function is called recursively inside `@theme` blocks
 
 ## [4.1.6] - 2025-05-09
 

packages/tailwindcss/src/css-functions.test.ts

@@ -433,6 +433,27 @@ describe('--theme(…)', () => {
       }"
     `)
   })
+
+  test('--theme(…) prevents infinite loops with circular references', async () => {
+    expect(
+      await compileCss(css`
+        @theme {
+          --font-sans: 'Inter', --theme(--font-sans);
+        }
+        .font {
+          font-family: var(--font-sans);
+        }
+      `),
+    ).toMatchInlineSnapshot(`
+      ":root, :host {
+        --font-sans: "Inter", var(--font-sans);
+      }
+
+      .font {
+        font-family: var(--font-sans);
+      }"
+    `)
+  })
 })
 
 describe('theme(…)', () => {
@@ -903,6 +924,19 @@ describe('theme(…)', () => {
           }"
         `)
       })
+
+      test('theme(…) prevents infinite loops with circular references', async () => {
+        await expect(
+          compileCss(css`
+            @theme {
+              --font-sans: 'Inter', theme(fontFamily.sans);
+            }
+            .font {
+              font-family: var(--font-sans);
+            }
+          `),
+        ).rejects.toThrowErrorMatchingInlineSnapshot(`[Error: Could not resolve value for theme function: \`theme(fontFamily.sans)\`. The resolved value \`'Inter', theme(fontFamily.sans)\` contains a recursive reference to itself.]`)
+      })
     })
 
     describe('with CSS variable syntax', () => {

packages/tailwindcss/src/css-functions.ts

@@ -145,6 +145,14 @@ function legacyTheme(
     )
   }
 
+  // Detect eventual recursive theme function calls.
+  let regex = new RegExp('theme\\(\\s*[\'\"]?' + path)
+  if (typeof resolvedValue === 'string' && resolvedValue.match(regex)) {
+    throw new Error(
+      `Could not resolve value for theme function: \`theme(${path})\`. The resolved value \`${resolvedValue}\` contains a recursive reference to itself.`,
+    )
+  }
+
   return resolvedValue
 }