From ad3c30329b3633826be597ca63b69f9d51a8e1ea Mon Sep 17 00:00:00 2001
From: mktbsh <t93.hasebe@gmail.com>
Date: Sat, 14 Mar 2026 11:12:40 +0900
Subject: [PATCH 1/2] Fix fetch RequestInit override handling

---
 src/fetch.test.ts | 59 +++++++++++++++++++++++++++++++++++++++++++++++
 src/fetch.ts      |  2 +-
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/src/fetch.test.ts b/src/fetch.test.ts
index 919f788..933a5d6 100644
--- a/src/fetch.test.ts
+++ b/src/fetch.test.ts
@@ -21,6 +21,31 @@ describe("createFetchRequest", () => {
 		expect(await clonedRequest.text()).toBe("hello");
 	});
 
+	it("applies init overrides to Request inputs without consuming the original body", async () => {
+		const originalRequest = new Request("https://example.com/base", {
+			body: "original-body",
+			headers: {
+				"x-original": "1",
+			},
+			method: "POST",
+		});
+
+		const request = createFetchRequest(originalRequest, {
+			body: "override-body",
+			headers: {
+				"x-override": "1",
+			},
+			method: "PUT",
+		});
+
+		expect(request.method).toBe("PUT");
+		expect(request.headers.get("x-original")).toBeNull();
+		expect(request.headers.get("x-override")).toBe("1");
+		expect(await request.text()).toBe("override-body");
+		expect(originalRequest.bodyUsed).toBe(false);
+		expect(await originalRequest.text()).toBe("original-body");
+	});
+
 	it("creates a Request from fetch arguments", async () => {
 		const request = createFetchRequest("https://example.com/from-args", {
 			body: JSON.stringify({ hello: "world" }),
@@ -35,6 +60,40 @@ describe("createFetchRequest", () => {
 });
 
 describe("createFetchInterceptorHandler", () => {
+	it("matches against the effective Request when fetch receives Request and init", async () => {
+		const onIntercept = vi.fn();
+		const originalFetch = vi.fn(async () => new Response("ok"));
+		const interceptedFetch = createFetchInterceptorHandler(originalFetch, {
+			matcher: (request) => request.method === "POST",
+			onIntercept,
+		});
+		const baseRequest = new Request("https://example.com/base", {
+			headers: {
+				"x-original": "1",
+			},
+			method: "PUT",
+		});
+
+		await interceptedFetch(baseRequest, {
+			body: "override-body",
+			headers: {
+				"x-override": "1",
+			},
+			method: "POST",
+		});
+
+		expect(originalFetch).toHaveBeenCalledOnce();
+		expect(onIntercept).toHaveBeenCalledOnce();
+
+		const [request] = onIntercept.mock.calls[0];
+
+		expect(request.method).toBe("POST");
+		expect(request.headers.get("x-original")).toBeNull();
+		expect(request.headers.get("x-override")).toBe("1");
+		expect(await request.text()).toBe("override-body");
+		expect(baseRequest.bodyUsed).toBe(false);
+	});
+
 	it("intercepts matching requests with a cloned response", async () => {
 		const onIntercept = vi.fn();
 		const originalFetch = vi.fn(async () => {
diff --git a/src/fetch.ts b/src/fetch.ts
index fa1e6e5..5739a8a 100644
--- a/src/fetch.ts
+++ b/src/fetch.ts
@@ -11,7 +11,7 @@ export function createFetchRequest(
 	const [input, init] = args;
 
 	if (input instanceof Request) {
-		return input.clone();
+		return new Request(input.clone(), init);
 	}
 
 	return new Request(input, init);

From aded3479bb66bd6237655cbb4f6609454e509981 Mon Sep 17 00:00:00 2001
From: mktbsh <t93.hasebe@gmail.com>
Date: Sat, 14 Mar 2026 11:12:49 +0900
Subject: [PATCH 2/2] Add changeset for fetch request override fix

---
 .changeset/effective-fetch-request.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/effective-fetch-request.md

diff --git a/.changeset/effective-fetch-request.md b/.changeset/effective-fetch-request.md
new file mode 100644
index 0000000..20be3a8
--- /dev/null
+++ b/.changeset/effective-fetch-request.md
@@ -0,0 +1,5 @@
+---
+"@hsblabs/fetch-interceptor": patch
+---
+
+Preserve `RequestInit` overrides when intercepting `fetch(request, init)` calls.