bug: tRPC breaks when using revalidatePath/revalidateTag in server action

[Allypost]

Provide environment information

  System:
    OS: Linux 6.5 Arch Linux
    CPU: (8) x64 Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
    Memory: 11.91 GB / 31.29 GB
    Container: Yes
    Shell: 3.6.1 - /bin/fish
  Binaries:
    Node: 21.1.0 - ~/.local/share/rtx/installs/node/21.1.0/bin/node
    Yarn: 1.22.19 - /usr/bin/yarn
    npm: 10.2.0 - ~/.local/share/rtx/installs/node/21.1.0/bin/npm
    pnpm: 8.6.7 - ~/.local/share/pnpm/pnpm
    bun: 1.0.7 - ~/.local/share/rtx/installs/bun/1.0.7/bin/bun
  C3A:
    Init version: 7.22.0

Describe the bug

The issue occurs when using both server actions and tRPC inside the same component.

If a revalidatePath is used inside the server action, the component breaks after submitting the form with the error

Unhandled Runtime Error
Error: fetch failed

Call Stack
TRPCClientError.from
node_modules/@trpc/client/dist/TRPCClientError-0de4d231.mjs (37:15)
from
node_modules/@trpc/client/dist/httpBatchLink-204206a5.mjs (200:55)

This is because the tRPC proxy as is implemented in the current way forwards all headers. The issue is that next generates additional content and thus breaks the content-length header.

The (temporary) fix is to omit the content-length header in the requests.
Adding heads.delete("content-length") in the tRPC providers should solve that.

Reproduction repo

https://github.com/Allypost/create-t3-app-trpc-breaks-server-actions

To reproduce

Run the provided repo and simply click the "Broken" button.

Alternatively, add a server action with a revalidatePath to a page that has a server-component tRPC request

Additional information

Error screenshot

[juliusmarminge]

we should just forward the cookies i think. there are more headers that seem to break stuff and it's only really the cookie we want to be able to auth users. any extra headers can be added by individual needs (#1644)?