From d6eb18e0ebabd273746a99199aa55dc334639b3c Mon Sep 17 00:00:00 2001
From: James <james@eli.cx>
Date: Thu, 13 Aug 2026 09:00:06 +0100
Subject: [PATCH] fix(cache): preserve binary fetch response bodies

---
 packages/vinext/src/shims/fetch-cache.ts |  8 +++---
 tests/fetch-cache.test.ts                | 32 ++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/packages/vinext/src/shims/fetch-cache.ts b/packages/vinext/src/shims/fetch-cache.ts
index 72e98b4d0c..22d8c93c93 100644
--- a/packages/vinext/src/shims/fetch-cache.ts
+++ b/packages/vinext/src/shims/fetch-cache.ts
@@ -19,6 +19,8 @@
  *   await runWithFetchCache(async () => { ... render ... });
  */
 
+import { Buffer } from "node:buffer";
+
 import { getDataCacheHandler, type CachedFetchValue, type CacheHandler } from "./cache-handler.js";
 import { encodeCacheTags } from "../utils/encode-cache-tag.js";
 import { getOrCreateAls } from "./internal/als-registry.js";
@@ -43,7 +45,7 @@ import {
 const HEADER_BLOCKLIST = ["traceparent", "tracestate"];
 
 // Cache key version — bump when changing the key format to bust stale entries
-const CACHE_KEY_PREFIX = "v4";
+const CACHE_KEY_PREFIX = "v5";
 const MAX_CACHE_KEY_BODY_BYTES = 1024 * 1024; // 1 MiB
 
 // "Cache indefinitely" duration — mirrors upstream's CACHE_ONE_YEAR_SECONDS.
@@ -656,7 +658,7 @@ async function buildFetchCacheValue(
   if (response.status !== 200) return null;
 
   const responseForCache = options?.cloneForReturn === false ? response : response.clone();
-  const body = await responseForCache.text();
+  const body = Buffer.from(await responseForCache.arrayBuffer()).toString("base64");
   const headers: Record<string, string> = {};
   responseForCache.headers.forEach((v, k) => {
     if (k.toLowerCase() === "set-cookie") return;
@@ -995,7 +997,7 @@ function buildCachedFetchResponse(
   data: CachedFetchValue["data"],
   input: string | URL | Request,
 ): Response {
-  const response = new Response(data.body, {
+  const response = new Response(Buffer.from(data.body, "base64"), {
     status: data.status ?? 200,
     headers: data.headers,
   });
diff --git a/tests/fetch-cache.test.ts b/tests/fetch-cache.test.ts
index 9c62bd11a1..40eb9da822 100644
--- a/tests/fetch-cache.test.ts
+++ b/tests/fetch-cache.test.ts
@@ -122,6 +122,38 @@ describe("fetch cache shim", () => {
     expect(fetchMock).toHaveBeenCalledTimes(1);
   });
 
+  // Next.js stores CachedFetchData.body as base64:
+  // https://github.com/vercel/next.js/blob/canary/packages/next/src/server/lib/patch-fetch.ts
+  it("preserves binary response bodies when replaying the fetch cache", async () => {
+    const url = "https://api.example.com/compressed";
+    const body = new Uint8Array([0x1f, 0x8b, 0x08, 0x00, 0xff, 0x80, 0x00, 0x7f]);
+
+    fetchMock.mockImplementationOnce(async () => {
+      const response = new Response(body, {
+        status: 200,
+        headers: {
+          "content-encoding": "gzip",
+          "content-type": "application/octet-stream",
+        },
+      });
+      Object.defineProperty(response, "url", {
+        value: url,
+        configurable: true,
+        enumerable: true,
+        writable: false,
+      });
+      return response;
+    });
+
+    const cold = await fetch(url, { cache: "force-cache" });
+    expect(new Uint8Array(await cold.arrayBuffer())).toEqual(body);
+
+    const cached = await fetch(url, { cache: "force-cache" });
+    expect(new Uint8Array(await cached.arrayBuffer())).toEqual(body);
+    expect(cached.headers.get("content-encoding")).toBe("gzip");
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+  });
+
   it("preserves Response.url on cached fetch responses", async () => {
     const url = "https://api.example.com/force-url";