Don’t index into strings with the theme(…) function (#19111)

Indexing into a string is only ever going to produce a single character
and is almost guaranteed to be a mistake. The legacy keypath notation is
meant to traverse objects and arrays — not strings.

Fixes #19104

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix resolving theme keys when starting with the name of another theme key in JS configs and plugins ([#19097](https://github.com/tailwindlabs/tailwindcss/pull/19097))
 - Allow named groups in combination with `not-*`, `has-*`, and `in-*` ([#19100](https://github.com/tailwindlabs/tailwindcss/pull/19100))
 - Prevent important utilities from affecting other utilities ([#19110](https://github.com/tailwindlabs/tailwindcss/pull/19110))
+- Don’t index into strings with the `theme(…)` function ([#19111](https://github.com/tailwindlabs/tailwindcss/pull/19111))
 - Upgrade: Canonicalize utilities containing `0` values ([#19095](https://github.com/tailwindlabs/tailwindcss/pull/19095))
 
 ## [4.1.14] - 2025-10-01

packages/tailwindcss/src/compat/config.test.ts

@@ -1687,3 +1687,35 @@ test('handles setting theme keys to null', async () => {
     "
   `)
 })
+
+test('The theme() function does not try indexing into strings', async () => {
+  let compiler = await compile(css`
+    @theme default {
+      --color-what-50: #f00;
+      --color-what-950: #f00;
+    }
+
+    @theme {
+      /*
+       * The value of this theme variable is > 50 chars because colors.what.50 was previously
+       * indexing the string: "light-dark(theme(colors.what.950), theme(colors.what.50))"
+       * because the resolved config contained an object with a "what" key that was that string
+       */
+      --color-what: light-dark(theme(colors.what.950), theme(colors.what.50));
+    }
+
+    @source inline("text-what");
+
+    @tailwind utilities;
+  `)
+
+  expect(compiler.build([])).toMatchInlineSnapshot(`
+    ":root, :host {
+      --color-what: light-dark(#f00, #f00);
+    }
+    .text-what {
+      color: var(--color-what);
+    }
+    "
+  `)
+})

packages/tailwindcss/src/compat/plugin-functions.ts

@@ -233,6 +233,11 @@ function get(obj: any, path: string[]) {
       continue
     }
 
+    // We never want to index into strings
+    if (typeof obj === 'string') {
+      return undefined
+    }
+
     obj = obj[key]
   }