bug: ReadableStream uploading is not supported on safari

Provide environment information

System:
    OS: macOS 15.5
    CPU: (12) arm64 Apple M4 Pro
    Memory: 547.69 MB / 24.00 GB
    Shell: 5.9 - /opt/homebrew/bin/zsh
  Binaries:
    Node: 22.13.1 - ~/.nvm/versions/node/v22.13.1/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.13.1/bin/npm
    pnpm: 10.12.1 - ~/.nvm/versions/node/v22.13.1/bin/pnpm
    bun: 1.2.18 - ~/.bun/bin/bun
    Watchman: 2025.07.28.00 - /opt/homebrew/bin/watchman
  Browsers:
    Chrome: 139.0.7258.127
    Safari: 18.5
  npmPackages:
    @tanstack/react-query: ^4.40.0 => 4.40.0
    @trpc/client: ^10.45.2 => 10.45.2
    @trpc/react-query: ^10.45.2 => 10.45.2
    @trpc/server: ^10.45.2 => 10.45.2
    react: ^18.3.1 => 18.3.1
    typescript: ~5.8.3 => 5.8.3

Describe the bug

TRPCClientError: ReadableStream uploading is not supported

Getting this error in a few .mutate or mutate/mutateAsync calls using useMutations in safari. It works perfectly well in Chrome and Firebox based browsers.

I initially thought it might be due to batching and changed httpBatchLink to httpLink, also tried to use splitLink while checking for Safari but getting the same issue. It also doesn't fail for all mutations but only some.

Link to reproduction

N/A

To reproduce

N/A

Additional information

No response

👨‍👧‍👦 Contributing

• 🙋‍♂️ Yes, I'd be down to file a PR fixing this bug!

Start by tracing the TypeScript tRPC client path that handles ReadableStream request bodies and compare its behavior in Safari 18.5 with Chrome; the issue names no file, test, or reproduction. Done means the affected mutations no longer throw the ReadableStream uploading error in Safari, with regression coverage if the project has a suitable test entry point.