From eb37ff35789203844db78cf1abbe163daf32bcd0 Mon Sep 17 00:00:00 2001
From: Edmund Hung <edmund@cloudflare.com>
Date: Tue, 25 Feb 2025 12:30:17 +0000
Subject: [PATCH 1/3] test if response header is immutable

---
 .../misc/test/fetch-mock.test.ts                    | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fixtures/vitest-pool-workers-examples/misc/test/fetch-mock.test.ts b/fixtures/vitest-pool-workers-examples/misc/test/fetch-mock.test.ts
index 4db01193ecca..7bfaa953c9ab 100644
--- a/fixtures/vitest-pool-workers-examples/misc/test/fetch-mock.test.ts
+++ b/fixtures/vitest-pool-workers-examples/misc/test/fetch-mock.test.ts
@@ -61,6 +61,19 @@ it("intercepts URLs with query parameters with repeated keys", async () => {
 	expect(await response3.text()).toBe("baz");
 });
 
+it("throws if you try to mutate the headers", async () => {
+	fetchMock
+		.get("https://example.com")
+		.intercept({ path: "/" })
+		.reply(200, "body");
+
+	let response = await fetch("https://example.com");
+
+	expect(() => response.headers.set("foo", "bar")).toThrowError();
+	expect(() => response.headers.append("foo", "baz")).toThrowError();
+	expect(() => response.headers.delete("foo")).toThrowError();
+});
+
 describe("AbortSignal", () => {
 	let abortSignalTimeoutMock: MockInstance;
 

From 5f7cd7c0113088bf7ca5e455bc06ec6ec75e34fb Mon Sep 17 00:00:00 2001
From: Edmund Hung <edmund@cloudflare.com>
Date: Tue, 25 Feb 2025 12:40:14 +0000
Subject: [PATCH 2/3] fix: responses from fetchMock should have immutable
 headers

---
 .changeset/vast-trains-joke.md                       |  5 +++++
 .../vitest-pool-workers/src/worker/fetch-mock.ts     | 12 ++++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 .changeset/vast-trains-joke.md

diff --git a/.changeset/vast-trains-joke.md b/.changeset/vast-trains-joke.md
new file mode 100644
index 000000000000..0ee39f6563ba
--- /dev/null
+++ b/.changeset/vast-trains-joke.md
@@ -0,0 +1,5 @@
+---
+"@cloudflare/vitest-pool-workers": patch
+---
+
+fix: responses from `fetchMock` should have immutable headers
diff --git a/packages/vitest-pool-workers/src/worker/fetch-mock.ts b/packages/vitest-pool-workers/src/worker/fetch-mock.ts
index 3c5e64e0cd30..52179b5a4a3b 100644
--- a/packages/vitest-pool-workers/src/worker/fetch-mock.ts
+++ b/packages/vitest-pool-workers/src/worker/fetch-mock.ts
@@ -170,7 +170,19 @@ globalThis.fetch = async (input, init) => {
 					statusText: responseStatusText,
 					headers: responseHeaders,
 				});
+				const throwImmutableHeaderError = () => {
+					throw new TypeError("Can't modify immutable headers");
+				};
 				Object.defineProperty(response, "url", { value: url.href });
+				Object.defineProperty(response.headers, "set", {
+					value: throwImmutableHeaderError,
+				});
+				Object.defineProperty(response.headers, "append", {
+					value: throwImmutableHeaderError,
+				});
+				Object.defineProperty(response.headers, "delete", {
+					value: throwImmutableHeaderError,
+				});
 				responseResolve(response);
 			} else {
 				responseResolve(maybeResponse);

From e54d234c1a71e5d23dd9b985e5a76e39f3d8763f Mon Sep 17 00:00:00 2001
From: Edmund Hung <edmund@cloudflare.com>
Date: Wed, 26 Feb 2025 18:20:31 +0000
Subject: [PATCH 3/3] address feedback

---
 .../vitest-pool-workers/src/worker/fetch-mock.ts   | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/packages/vitest-pool-workers/src/worker/fetch-mock.ts b/packages/vitest-pool-workers/src/worker/fetch-mock.ts
index 52179b5a4a3b..987b0b17c607 100644
--- a/packages/vitest-pool-workers/src/worker/fetch-mock.ts
+++ b/packages/vitest-pool-workers/src/worker/fetch-mock.ts
@@ -170,18 +170,14 @@ globalThis.fetch = async (input, init) => {
 					statusText: responseStatusText,
 					headers: responseHeaders,
 				});
-				const throwImmutableHeaderError = () => {
+				const throwImmutableHeadersError = () => {
 					throw new TypeError("Can't modify immutable headers");
 				};
 				Object.defineProperty(response, "url", { value: url.href });
-				Object.defineProperty(response.headers, "set", {
-					value: throwImmutableHeaderError,
-				});
-				Object.defineProperty(response.headers, "append", {
-					value: throwImmutableHeaderError,
-				});
-				Object.defineProperty(response.headers, "delete", {
-					value: throwImmutableHeaderError,
+				Object.defineProperties(response.headers, {
+					set: { value: throwImmutableHeadersError },
+					append: { value: throwImmutableHeadersError },
+					delete: { value: throwImmutableHeadersError },
 				});
 				responseResolve(response);
 			} else {