diff --git a/.changeset/tidy-donkeys-cheat.md b/.changeset/tidy-donkeys-cheat.md
new file mode 100644
index 000000000..1572fd0c1
--- /dev/null
+++ b/.changeset/tidy-donkeys-cheat.md
@@ -0,0 +1,5 @@
+---
+"@solidjs/start": patch
+---
+
+Apply cookies set on a returned or thrown response during single flight mutations. `redirect(to, { headers: { "Set-Cookie": ... } })` previously only reached the browser: the single flight re-render of the redirect target still ran with the old request cookies, so queries reading that cookie saw stale values. Those cookies are now merged into the request the re-render sees, matching what a browser round trip would have sent.
diff --git a/apps/tests/src/e2e/single-flight-cookie.test.ts b/apps/tests/src/e2e/single-flight-cookie.test.ts
new file mode 100644
index 000000000..f93cef4f5
--- /dev/null
+++ b/apps/tests/src/e2e/single-flight-cookie.test.ts
@@ -0,0 +1,12 @@
+import { expect, test } from "@playwright/test";
+
+test.describe("single flight mutation", () => {
+  test("should apply a cookie set by a thrown redirect to the same flight", async ({ page }) => {
+    await page.goto("/single-flight-cookie");
+    await expect(page.locator("#cookie-value")).toHaveText("none");
+
+    await page.getByRole("button", { name: "set cookie" }).click();
+
+    await expect(page.locator("#cookie-value")).toHaveText("1234");
+  });
+});
diff --git a/apps/tests/src/routes/single-flight-cookie.tsx b/apps/tests/src/routes/single-flight-cookie.tsx
new file mode 100644
index 000000000..3750e4b44
--- /dev/null
+++ b/apps/tests/src/routes/single-flight-cookie.tsx
@@ -0,0 +1,38 @@
+import { action, createAsync, query, redirect } from "@solidjs/router";
+import { getRequestEvent } from "solid-js/web";
+
+const readCookie = query(async () => {
+  "use server";
+  const cookies = getRequestEvent()!.request.headers.get("cookie") ?? "";
+  const match = /(?:^|;\s*)single_flight_cookie=([^;]*)/.exec(cookies);
+  return match ? match[1] : "none";
+}, "single-flight-cookie");
+
+const setCookie = action(async () => {
+  "use server";
+  throw redirect("/single-flight-cookie", {
+    headers: {
+      "Set-Cookie": "single_flight_cookie=1234; Path=/; HttpOnly; SameSite=Lax; Max-Age=3600",
+    },
+  });
+}, "single-flight-cookie-set");
+
+// the preload is what the single flight re-render runs to collect data for the
+// redirect target, so the mutation response carries the fresh cookie value
+export const route = {
+  preload: () => readCookie(),
+};
+
+export default function SingleFlightCookie() {
+  const value = createAsync(() => readCookie(), { deferStream: true });
+
+  return (
+    <main>
+      <h1>Single Flight Cookie</h1>
+      <p id="cookie-value">{value()}</p>
+      <form action={setCookie} method="post">
+        <button type="submit">set cookie</button>
+      </form>
+    </main>
+  );
+}
diff --git a/packages/start/src/fns/handler.spec.ts b/packages/start/src/fns/handler.spec.ts
index 29dad2f25..16d2726cc 100644
--- a/packages/start/src/fns/handler.spec.ts
+++ b/packages/start/src/fns/handler.spec.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it, vi, beforeEach } from "vitest";
+import { parseCookies } from "h3";
 import type { FetchEvent } from "../server/types.ts";
 
 vi.mock("h3", () => ({
@@ -24,12 +25,15 @@ vi.mock("../server/fetchEvent.ts", () => ({
   mergeResponseHeaders: vi.fn(),
 }));
 
-function createMockFetchEvent(headers: Record<string, string> = {}): FetchEvent {
+function createMockFetchEvent(
+  headers: Record<string, string> = {},
+  setCookies: string[] = [],
+): FetchEvent {
   return {
     request: new Request("http://localhost/test", { headers }),
     response: {
       headers: {
-        getSetCookie: () => [],
+        getSetCookie: () => [...setCookies],
       },
     },
     nativeEvent: {},
@@ -38,10 +42,11 @@ function createMockFetchEvent(headers: Record<string, string> = {}): FetchEvent
 }
 
 describe("createSingleFlightHeaders", () => {
-  let createSingleFlightHeaders: (sourceEvent: FetchEvent) => Headers;
+  let createSingleFlightHeaders: (sourceEvent: FetchEvent, result?: unknown) => Headers;
 
   beforeEach(async () => {
     vi.clearAllMocks();
+    vi.mocked(parseCookies).mockReturnValue({});
     const module = await import("./handler.ts");
     createSingleFlightHeaders = module.createSingleFlightHeaders;
   });
@@ -82,4 +87,74 @@ describe("createSingleFlightHeaders", () => {
     expect(sourceEvent.request.headers.get("cookie")).toBe(originalCookieHeader);
     expect(sourceEvent.request.headers.get("cf-ray")).toBe(originalCfRay);
   });
+
+  it("should apply cookies set on the event response", () => {
+    const sourceEvent = createMockFetchEvent({}, [
+      "val=1234; Path=/; HttpOnly; SameSite=Lax; Max-Age=3600",
+    ]);
+
+    const result = createSingleFlightHeaders(sourceEvent);
+
+    expect(result.get("cookie")).toBe("val=1234");
+  });
+
+  it("should apply cookies set on a thrown redirect response", () => {
+    const sourceEvent = createMockFetchEvent();
+    const redirect = new Response(null, {
+      status: 302,
+      headers: {
+        Location: "/",
+        "Set-Cookie": "val=1234; Path=/; HttpOnly; SameSite=Lax; Max-Age=3600",
+      },
+    });
+
+    const result = createSingleFlightHeaders(sourceEvent, redirect);
+
+    expect(result.get("cookie")).toBe("val=1234");
+  });
+
+  it("should let response cookies win over ones already on the request", () => {
+    vi.mocked(parseCookies).mockReturnValue({ session: "old" });
+    const sourceEvent = createMockFetchEvent({ cookie: "session=old" });
+    const redirect = new Response(null, {
+      status: 302,
+      headers: { "Set-Cookie": "session=new; Path=/" },
+    });
+
+    const result = createSingleFlightHeaders(sourceEvent, redirect);
+
+    expect(result.get("cookie")).toBe("session=new");
+  });
+
+  it("should remove cookies cleared by the response", () => {
+    vi.mocked(parseCookies).mockReturnValue({ session: "abc123" });
+    const sourceEvent = createMockFetchEvent({ cookie: "session=abc123" });
+    const redirect = new Response(null, {
+      status: 302,
+      headers: { "Set-Cookie": "session=; Path=/; Max-Age=0" },
+    });
+
+    const result = createSingleFlightHeaders(sourceEvent, redirect);
+
+    expect(result.get("cookie")).toBe(null);
+  });
+
+  it("should not copy non-cookie response headers onto the request", () => {
+    const sourceEvent = createMockFetchEvent();
+    const redirect = new Response(null, {
+      status: 302,
+      headers: { Location: "/", "X-Revalidate": "user" },
+    });
+
+    const result = createSingleFlightHeaders(sourceEvent, redirect);
+
+    expect(result.get("location")).toBe(null);
+    expect(result.get("x-revalidate")).toBe(null);
+  });
+
+  it("should ignore non-Response results", () => {
+    const sourceEvent = createMockFetchEvent();
+
+    expect(() => createSingleFlightHeaders(sourceEvent, { some: "value" })).not.toThrow();
+  });
 });
diff --git a/packages/start/src/fns/handler.ts b/packages/start/src/fns/handler.ts
index 470992e76..2683087a9 100644
--- a/packages/start/src/fns/handler.ts
+++ b/packages/start/src/fns/handler.ts
@@ -241,7 +241,7 @@ async function handleNoJS(result: any, request: Request, parsed: any[], thrown?:
 }
 
 let App: any;
-export function createSingleFlightHeaders(sourceEvent: FetchEvent) {
+export function createSingleFlightHeaders(sourceEvent: FetchEvent, result?: unknown) {
   // cookie handling logic is pretty simplistic so this might be imperfect
   // unclear if h3 internals are available on all platforms but we need a way to
   // update request headers on the underlying H3 event.
@@ -249,6 +249,11 @@ export function createSingleFlightHeaders(sourceEvent: FetchEvent) {
   const headers = new Headers(sourceEvent.request.headers);
   const cookies = parseCookies(sourceEvent.nativeEvent);
   const SetCookies = sourceEvent.response.headers.getSetCookie();
+  // cookies attached to the returned/thrown response (eg. `redirect(to, { headers })`)
+  // haven't been merged onto the event response yet, but a browser round trip would
+  // have sent them back with the next request, so apply them here too. They come after
+  // the ones on the event response, so they win on conflict.
+  if (result instanceof Response) SetCookies.push(...result.headers.getSetCookie());
   headers.delete("cookie");
   // let useH3Internals = false;
   // if (sourceEvent.nativeEvent.node?.req) {
@@ -294,7 +299,7 @@ async function handleSingleFlight(sourceEvent: FetchEvent, result: any): Promise
   }
   const event = { ...sourceEvent } as PageEvent;
   event.request = new Request(url, {
-    headers: createSingleFlightHeaders(sourceEvent),
+    headers: createSingleFlightHeaders(sourceEvent, result),
   });
   return await provideRequestEvent(event, async () => {
     await createPageEvent(event);