diff --git a/src/start.ts b/src/start.ts
index ace8d04..d51d7ae 100644
--- a/src/start.ts
+++ b/src/start.ts
@@ -2,11 +2,19 @@ import {
   sentryGlobalFunctionMiddleware,
   sentryGlobalRequestMiddleware,
 } from '@sentry/tanstackstart-react';
-import { createStart } from '@tanstack/react-start';
+import { createCsrfMiddleware, createStart } from '@tanstack/react-start';
+
+// Server functions are same-origin RPC endpoints; reject cross-site requests to
+// them (same-origin check via Sec-Fetch-Site / Origin, with a Referer fallback).
+const csrfMiddleware = createCsrfMiddleware({
+  filter: ctx => ctx.handlerType === 'serverFn',
+});
 
 export const startInstance = createStart(() => {
   return {
-    requestMiddleware: [sentryGlobalRequestMiddleware],
+    // Sentry first (outermost) so it observes the whole request, including any
+    // CSRF rejection raised by the middleware after it.
+    requestMiddleware: [sentryGlobalRequestMiddleware, csrfMiddleware],
     functionMiddleware: [sentryGlobalFunctionMiddleware],
   };
 });