Raw Response from server function not working on Bun runtime

[pricci1]

Which project does this relate to?

Start

Describe the bug

When running with Bun as a runtime (e.g bun --bun run dev), returning a Response object from a server function throws an error.

const expectedValue = 'Hello from a server function!'
export const rawResponseFn = createServerFn().handler(() => {
  return new Response(expectedValue)
})

Uncaught (in promise) Error: Invariant failed: expected content-type header to be set
    at invariant (tiny-invariant.js?v=ff4058f6:12:9)
    at getResponse (serverFnFetcher.js?v=ff4058f6:104:3)
    at async serverFnFetcher (serverFnFetcher.js?v=ff4058f6:43:12)
    at async client (createServerFn.js?v=ff4058f6:205:21)
    at async onClick (demo.start.raw-response.tsx:32:28)

See for example this e2e component. It works with node, but not Bun

https://github.com/TanStack/router/blob/0babe5d805ae18a7624f54eb6948d59ba10f5715/e2e/react-start/server-functions/src/routes/raw-response.tsx

Your Example Website or App

https://github.com/TanStack/router/blob/0babe5d805ae18a7624f54eb6948d59ba10f5715/e2e/react-start/server-functions/src/routes/raw-response.tsx

Steps to Reproduce the Bug or Issue

1. Create new Start app
2. Create a component with a server function that returns a Response
3. Run dev server with Bun runtime: bun --bun run dev
4. Visit site and trigger the call to the function

Expected behavior

Response

Screenshots or Videos

No response

Platform

• Router / Start Version: 1.132.7
• OS: Linux
• Browser: Chrome
• Browser Version: 140
• Bundler: vite
• Bundler Version: 6.3.6
• Runtime: Bun
• Runtime Version: 1.2.22

Additional context

No response

[jph-sw]

Works for me on 1.132.9

[pricci1]

Just tried with Bun 1.2.23 and @tanstack/react-start@1.132.19. The same bug.

The problem is that the response from the server function comes with missing headers (e.g. content-type) when running with Bun runtime (bun --bun run dev), whereas when running with Node (bun run dev) it does.

Bun response headers

HTTP/1.1 200 OK
Vary: Origin
Date: Sun, 28 Sep 2025 15:53:01 GMT
Transfer-Encoding: chunked

Node response headers

HTTP/1.1 200
Vary: Origin
content-type: text/plain;charset=UTF-8
Date: Sun, 28 Sep 2025 15:57:13 GMT
Connection: keep-alive
Keep-Alive: timeout=5
Transfer-Encoding: chunked

[pricci1]

@jph-sw Could you please confirm that when running with bun --bun run dev the "raw response" example response comes with the content-type header set?

[jph-sw]

@pricci1 It was an error on my part.