fix: apply redirect Set-Cookie headers during single flight mutations - #2243
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brenelz
approved these changes
Jul 27, 2026
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When a server action returns or throws a response carrying cookies, for example
redirect("/", { headers: { "Set-Cookie": ... } }), those cookies reached the browser but not the single flight re-render of the redirect target. Queries reading that cookie during the same flight still saw the old request cookies, so the result differed from a real browser round trip.createSingleFlightHeadersnow also merges theSet-Cookievalues from the action result, applied after the ones already on the event response so they win on conflict. Only cookies are merged, not all response headers: copyingLocation,Content-Typeand friends onto a synthetic request would be wrong, and cookies are what a round trip would actually have carried back.Covered by unit tests in
handler.spec.tsand an e2e repro atapps/tests/src/routes/single-flight-cookie.tsx.