[start] Unhandled 500 (TypeError: immutable) when a server route returns Response.redirect() while a set-cookie is pending on the event

Which project does this relate to?

Start

Describe the bug

When a server route handler returns a Response whose headers are immutable — most commonly Response.redirect(url), but also a passed-through fetch() response — and a cookie is pending on the h3 event (via setCookie(), or indirectly via better-auth's tanstackStartCookies() plugin which calls setCookie after every auth API call), the request fails with an unhandled 500 instead of returning the redirect:

{"status":500,"unhandled":true,"message":"HTTPError"}

Root cause is mergeEventResponseHeaders in @tanstack/start-server-core (src/request-response.ts):

function mergeEventResponseHeaders(response, event) {
  if (response.ok) return;
  const eventSetCookies = getSetCookieValues(event.res.headers);
  if (eventSetCookies.length === 0) return;
  const responseSetCookies = getSetCookieValues(response.headers);
  response.headers.delete("set-cookie"); // ← throws TypeError: immutable
  ...
}

A 3xx redirect is not response.ok, so the merge path runs, and Response.redirect() returns a response with an immutable headers guard per the Fetch spec. The TypeError propagates as an unhandled HTTPError 500.

This is easy to hit in production without realizing: any authenticated server route that ends in Response.redirect(...) breaks as soon as the auth library refreshes a session cookie during the request (better-auth with cookieCache does this on every getSession). The handler completes successfully — side effects and DB writes land — and then the framework 500s while post-processing the response, which makes it look like a routing/dispatch failure. Meanwhile unauthenticated requests to the same route (no pending cookie → early return) work fine, which is very confusing to debug.

Your Example Website or App

Minimal repro route below (any Start app):

// src/routes/api/redirect-repro.ts
import { createFileRoute } from "@tanstack/react-router";
import { setCookie } from "@tanstack/react-start/server";

export const Route = createFileRoute("/api/redirect-repro")({
  server: {
    handlers: {
      GET: () => {
        setCookie("example", "value"); // anything that puts a pending cookie on the event
        return Response.redirect("https://example.com/", 302);
      },
    },
  },
});

Steps to Reproduce the Bug or Issue

1. Add the route above to a Start app.
2. curl -i http://localhost:3000/api/redirect-repro
3. Observe 500 with body {"status":500,"unhandled":true,"message":"HTTPError"} instead of a 302.

Also reproducible directly against the dist internals:

import { requestHandler, setCookie } from "@tanstack/start-server-core/dist/esm/request-response.js";

const handler = requestHandler(async () => {
  setCookie("example", "value");
  return Response.redirect("https://example.com/", 302);
});
const res = await handler(new Request("http://localhost/api/x"));
console.log(res.status); // 500, cause: TypeError: immutable at mergeEventResponseHeaders

Expected behavior

The redirect should be returned (with the pending set-cookie headers merged). When the returned response has immutable headers, mergeEventResponseHeaders could clone it instead of mutating in place, e.g.:

// new Response(response.body, response) yields mutable headers

or guard the mutation with a try/catch fallback to a cloned response.

Platform

• OS: macOS / Cloudflare Workers (workerd) — reproduces on both Node (undici) and workerd, since both enforce the immutable headers guard
• @tanstack/react-start: 1.168.27
• @tanstack/start-server-core: 1.169.16

Additional context

Workaround for anyone hitting this: return new Response(null, { status: 302, headers: { location: url } }) instead of Response.redirect(url) from server route handlers.

Possibly related to the change that introduced event-cookie merging for non-2xx responses (#7189 / #5464) — the merge itself is desirable, it just can't assume the handler's response headers are mutable.

Start in @tanstack/start-server-core/src/request-response.ts at mergeEventResponseHeaders, then reproduce the issue with the provided redirect-repro route or requestHandler example. Done means a response with pending cookies can return a 302 redirect without an unhandled 500, including when its headers are immutable.