diff --git a/.changeset/great-peaches-yell.md b/.changeset/great-peaches-yell.md
new file mode 100644
index 00000000000..f95f3e9e066
--- /dev/null
+++ b/.changeset/great-peaches-yell.md
@@ -0,0 +1,5 @@
+---
+"@clerk/tanstack-start": patch
+---
+
+Fix type mismatch between TanStack Start and Clerk event handlers
diff --git a/packages/tanstack-start/package.json b/packages/tanstack-start/package.json
index 9e1176c5fe7..f74ffcad7f3 100644
--- a/packages/tanstack-start/package.json
+++ b/packages/tanstack-start/package.json
@@ -69,7 +69,8 @@
   "devDependencies": {
     "@tanstack/react-router": "^1.97.25",
     "@tanstack/start": "^1.97.25",
-    "esbuild-plugin-file-path-extensions": "^2.1.2"
+    "esbuild-plugin-file-path-extensions": "^2.1.2",
+    "h3": "^1.13.0"
   },
   "peerDependencies": {
     "@tanstack/react-router": ">=1.85.9",
diff --git a/packages/tanstack-start/src/server/middlewareHandler.ts b/packages/tanstack-start/src/server/middlewareHandler.ts
index 3faa31b9eae..ec56e4711a9 100644
--- a/packages/tanstack-start/src/server/middlewareHandler.ts
+++ b/packages/tanstack-start/src/server/middlewareHandler.ts
@@ -1,5 +1,5 @@
 import type { AnyRouter } from '@tanstack/react-router';
-import type { EventHandler } from 'vinxi/http';
+import type { eventHandler } from 'h3';
 
 import { authenticateRequest } from './authenticateRequest';
 import { loadOptions } from './loadOptions';
@@ -11,13 +11,16 @@ export type HandlerCallback<TRouter extends AnyRouter> = (ctx: {
   router: TRouter;
   responseHeaders: Headers;
 }) => Response | Promise<Response>;
-export type CustomizeStartHandler<TRouter extends AnyRouter> = (cb: HandlerCallback<TRouter>) => EventHandler;
+
+export type CustomizeStartHandler<TRouter extends AnyRouter> = (
+  cb: HandlerCallback<TRouter>,
+) => ReturnType<typeof eventHandler>;
 
 export function createClerkHandler<TRouter extends AnyRouter>(
   eventHandler: CustomizeStartHandler<TRouter>,
   clerkOptions: LoaderOptions = {},
 ) {
-  return (cb: HandlerCallback<TRouter>): EventHandler => {
+  return (cb: HandlerCallback<TRouter>) => {
     return eventHandler(async ({ request, router, responseHeaders }) => {
       try {
         const loadedOptions = loadOptions(request, clerkOptions);
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 0907dfefcd3..a51a16dd604 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -963,6 +963,9 @@ importers:
       esbuild-plugin-file-path-extensions:
         specifier: ^2.1.2
         version: 2.1.4
+      h3:
+        specifier: ^1.13.0
+        version: 1.14.0
 
   packages/testing:
     dependencies: