diff --git a/eslint.config.js b/eslint.config.js
index 69130fd..5f717bb 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -18,5 +18,16 @@ export default tseslint.config(
     },
   },
   ...layerRules,
+  // domain is bundled into the browser with web: Node-only globals break there at runtime, not in Vitest (Node).
+  {
+    files: ['src/domain/**/*.ts', 'src/web/**/*.{ts,tsx}'],
+    ignores: ['**/*.test.{ts,tsx}'],
+    rules: {
+      'no-restricted-globals': ['error',
+        { name: 'Buffer', message: 'Not in the browser: use TextEncoder/TextDecoder.' },
+        { name: 'process', message: 'Not in the browser.' },
+      ],
+    },
+  },
   { files: ['*.config.js'], ...tseslint.configs.disableTypeChecked },
 );
diff --git a/src/domain/workflow-function.ts b/src/domain/workflow-function.ts
index 9a0040a..670cda6 100644
--- a/src/domain/workflow-function.ts
+++ b/src/domain/workflow-function.ts
@@ -43,7 +43,7 @@ export type FunctionTestCase = z.infer<typeof FunctionTestCase>;
 export const WorkflowFunctionDocumentV1 = z.object({
   schemaVersion: z.literal(1),
   language: z.enum(['typescript', 'posix']),
-  source: z.string().min(1).refine((value) => !value.includes('\0'), 'source cannot contain NUL').refine((value) => Buffer.byteLength(value, 'utf8') <= FUNCTION_MAX_SOURCE_BYTES, 'source is too large'),
+  source: z.string().min(1).refine((value) => !value.includes('\0'), 'source cannot contain NUL').refine((value) => new TextEncoder().encode(value).length <= FUNCTION_MAX_SOURCE_BYTES, 'source is too large'),
   capabilities: FunctionCapabilitySetV1,
   cases: z.array(FunctionTestCase).min(1).max(FUNCTION_MAX_CASES),
 }).strict().superRefine((document, context) => {