Pre-hydration redirect from server actions does not prepend base path

### Link to the code that reproduces this issue https://github.com/greenlynx/nextjs-bad-redirect-repro ### To Reproduce Our application has a base path set, and a form that submits to a server action that causes a redirect. When the app is tested using Playwright, and the form is submitted before the page is hydrated, the redirect location in the 303 response is wrong (it is missing the base path). # Steps to reproduce 1. Disable Javascript in browser 2. `pnpm dev` 3. Click submit button ### Current vs. Expected behavior # Current behaviour - Redirect to `/success` and see a 404 error (because the base path `/custom` was not prepended to the redirect location) # Expected behaviour - Redirect to `/custom/success`, and see "OK" message ### Provide environment information ```bash Operating System: Platform: darwin Arch: arm64 Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6041 Available memory (MB): 49152 Available CPU cores: 14 Binaries: Node: 22.20.0 npm: 10.9.3 Yarn: 1.22.22 pnpm: 10.18.3 Relevant Packages: next: 16.0.1-canary.5 // Latest available version is detected (16.0.1-canary.5). eslint-config-next: N/A react: 19.2.0 react-dom: 19.2.0 typescript: 5.9.3 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Redirects, Server Actions, Runtime ### Which stage(s) are affected? (Select all that apply) next dev (local), Other (Deployed), Vercel (Deployed) ### Additional context _No response_