staticFunctionMiddleware does not respect basePath configuration

Discussed in https://github.com/TanStack/router/discussions/6146

^{Originally posted by busy-dog December 19, 2025}

Description of the Issue

staticFunctionMiddleware defaults to mounting __tsr/staticServerFnCache at the root path /__tsr/...
Even if base or basePath is set in SSR or Router configuration, the URLs generated by the middleware still access from the root path, leading to 404 errors when accessing resources.

Reproduction Steps

Configure vite.config.ts, set base: '/docs/', and deploy to Ng with corresponding resource routing configured.
Open the browser and navigate to any route. The chromedevtools network shows /__tsr/staticServerFnCache/xxx.json 404.

Expected behavior
The actual request should be to /docs/__tsr/staticServerFnCache/xxx.json

Possible Solution

Add a parameter to the getStaticCacheUrl method in the staticFunctionMiddleware.ts file of start-static-server-functions, or try reading the basePath to correct the fetch url

Start in the staticFunctionMiddleware.ts file of start-static-server-functions, especially getStaticCacheUrl, and review the base setting in vite.config.ts. Reproduce with base set to /docs/ and inspect the generated staticServerFnCache request. Done means the request uses /docs/__tsr/staticServerFnCache/... instead of the root path and no longer returns a 404.