`staticFunctionMiddleware` `Unexpected token '<', "<!DOCTYPE "... is not valid JSON `

Which project does this relate to?

Router

Describe the bug

When I add staticFunctionMiddleware to my server function, it throws an error after the build:

Unexpected token '<', "<!DOCTYPE "... is not valid JSON

The returned object is serializable as JSON.

I also tried JSON.parse(JSON.stringify(result)), but I get the same error.

Complete minimal reproducer

https://github.com/aymericzip/staticFunctionMiddleware_repro

Steps to Reproduce the Bug

bun install 
bun run build
cd ./apps/website-tanstack
uncomment middleware in `apps/website-tanstack/src/serverFunctions/docs.ts`
bun run build
bun run start 

navigate to /doc

I also included a demo for https://github.com/TanStack/router/issues/7516
comment code in packages/@intlayer/design-system/src/providers/ReactQueryProvider.tsx

cd ../../
bun run build ## rebuild design system
cd ./apps/website-tanstack
bun run build

Expected behavior

Screenshots or Videos

Demo: https://drive.google.com/open?id=1gBbIZ2okGl099IfONT5lGjToc2TeMtBf&usp=drive_fs

Platform

"@tanstack/react-devtools": "0.10.5",
"@tanstack/react-query": "5.101.0",
"@tanstack/react-router": "1.170.15",
"@tanstack/react-router-ssr-query": "1.167.1",
"@tanstack/react-start": "1.168.25",
"@tanstack/react-virtual": "3.14.2",
"@tanstack/router-plugin": "1.168.18",
"bun": "1.3.14",

mac

Additional context

No response

Start with the linked minimal reproducer and run the listed Bun build/start steps. Then inspect apps/website-tanstack/src/serverFunctions/docs.ts and the referenced ReactQueryProvider.tsx setup while reproducing the failure at /doc. Done means the server function result loads without the unexpected JSON parse error after building.