diff --git a/frontend/src/AGENTS.md b/frontend/src/AGENTS.md
index 55ccf9a61d..610a71302f 100644
--- a/frontend/src/AGENTS.md
+++ b/frontend/src/AGENTS.md
@@ -174,6 +174,8 @@ Array previews coalesce consecutive generated markers only at the end into one o
 
 ### Key Patterns
 
+REST `core/api/fetcher.ts`: preserve Request; CSRF uses effective method/headers.
+
 Conversation exports read every persisted message page through
 `fetchThreadExportMessages` (200 rows/page). Run `mergeMessages` after history
 deduplication to restore earliest trusted positions for identities repeated across runs.
diff --git a/frontend/src/core/api/fetcher.ts b/frontend/src/core/api/fetcher.ts
index 9f3eb779e6..d4016180ee 100644
--- a/frontend/src/core/api/fetcher.ts
+++ b/frontend/src/core/api/fetcher.ts
@@ -66,23 +66,26 @@ export async function fetch(
   input: RequestInfo | string,
   init?: RequestInit,
 ): Promise<Response> {
+  const request = typeof input === "string" ? undefined : input;
   const url = typeof input === "string" ? input : input.url;
+  const method = init?.method ?? request?.method ?? "GET";
+  let headers = init?.headers ?? request?.headers;
 
   // Static demos have no Gateway. Resolve REST calls before credentials,
   // CSRF, or redirects; demo assets and explicit mock routes still use HTTP.
   if (isStaticWebsiteOnly()) {
     const response = await staticApiResponse(url, {
       ...init,
-      method:
-        init?.method ?? (typeof input === "string" ? "GET" : input.method),
+      method,
+      headers,
+      signal: init?.signal === undefined ? request?.signal : init.signal,
     });
     if (response) return response;
   }
 
   // Inject CSRF for state-changing methods. GET/HEAD/OPTIONS/TRACE skip
   // it to mirror the gateway's ``should_check_csrf`` logic exactly.
-  let headers = init?.headers;
-  if (isStateChangingMethod(init?.method ?? "GET")) {
+  if (isStateChangingMethod(method)) {
     const token = readCsrfCookie();
     if (token) {
       // Fresh Headers instance so we don't mutate caller-supplied objects.
@@ -94,7 +97,7 @@ export async function fetch(
     }
   }
 
-  const res = await globalThis.fetch(url, {
+  const res = await globalThis.fetch(input, {
     ...init,
     headers,
     credentials: "include",
diff --git a/frontend/tests/unit/core/api/fetcher.test.ts b/frontend/tests/unit/core/api/fetcher.test.ts
new file mode 100644
index 0000000000..75cf12d1d8
--- /dev/null
+++ b/frontend/tests/unit/core/api/fetcher.test.ts
@@ -0,0 +1,144 @@
+import { afterEach, beforeEach, describe, expect, it, rs } from "@rstest/core";
+
+import { fetch as apiFetch } from "@/core/api/fetcher";
+
+describe("api fetcher Request inputs", () => {
+  let sent: Request[];
+
+  beforeEach(() => {
+    sent = [];
+    rs.stubGlobal("document", { cookie: "csrf_token=cookie-token" });
+    rs.stubGlobal(
+      "fetch",
+      rs.fn(async (input: RequestInfo | URL, init?: RequestInit) => {
+        sent.push(new Request(input, init));
+        return new Response("ok");
+      }),
+    );
+  });
+
+  afterEach(() => {
+    rs.unstubAllGlobals();
+  });
+
+  it("preserves a Request's method, body, headers and cancellation", async () => {
+    const controller = new AbortController();
+    const request = new Request("http://localhost/api/threads", {
+      method: "POST",
+      headers: { "Content-Type": "application/json", "X-Request-ID": "run-1" },
+      body: JSON.stringify({ title: "Research" }),
+      signal: controller.signal,
+    });
+
+    await apiFetch(request);
+
+    expect(sent[0]?.method).toBe("POST");
+    expect(await sent[0]?.text()).toBe(JSON.stringify({ title: "Research" }));
+    expect(sent[0]?.headers.get("Content-Type")).toBe("application/json");
+    expect(sent[0]?.headers.get("X-Request-ID")).toBe("run-1");
+    expect(sent[0]?.headers.get("X-CSRF-Token")).toBe("cookie-token");
+    expect(sent[0]?.credentials).toBe("include");
+    expect(request.headers.has("X-CSRF-Token")).toBe(false);
+    controller.abort();
+    expect(sent[0]?.signal.aborted).toBe(true);
+  });
+
+  it.each(["POST", "PUT", "PATCH", "DELETE"])(
+    "injects CSRF for a Request with method %s",
+    async (method) => {
+      await apiFetch(new Request("http://localhost/api/threads", { method }));
+      expect(sent[0]?.method).toBe(method);
+      expect(sent[0]?.headers.get("X-CSRF-Token")).toBe("cookie-token");
+    },
+  );
+
+  it.each(["GET", "HEAD", "OPTIONS"])(
+    "does not inject CSRF for a Request with method %s",
+    async (method) => {
+      await apiFetch(new Request("http://localhost/api/threads", { method }));
+      expect(sent[0]?.method).toBe(method);
+      expect(sent[0]?.headers.has("X-CSRF-Token")).toBe(false);
+    },
+  );
+
+  it("does not inject CSRF for TRACE in init", async () => {
+    // Native Request rejects TRACE, so inspect options before that boundary.
+    const nativeFetch = rs.fn().mockResolvedValue(new Response("ok"));
+    rs.stubGlobal("fetch", nativeFetch);
+
+    await apiFetch("http://localhost/api/threads", { method: "TRACE" });
+
+    expect(nativeFetch).toHaveBeenCalledWith("http://localhost/api/threads", {
+      method: "TRACE",
+      headers: undefined,
+      credentials: "include",
+    });
+  });
+
+  it("preserves an explicit CSRF header inherited from the Request", async () => {
+    await apiFetch(
+      new Request("http://localhost/api/threads", {
+        method: "POST",
+        headers: { "X-CSRF-Token": "explicit-token" },
+      }),
+    );
+    expect(sent[0]?.headers.get("X-CSRF-Token")).toBe("explicit-token");
+  });
+
+  it("lets init replace Request headers and body without mutating either header set", async () => {
+    const request = new Request("http://localhost/api/threads", {
+      method: "POST",
+      headers: { "X-Original": "original", "X-CSRF-Token": "request-token" },
+      body: "original body",
+    });
+    const headers = new Headers({ "X-Override": "override" });
+
+    await apiFetch(request, {
+      method: "PUT",
+      headers,
+      body: "replacement body",
+    });
+
+    expect(sent[0]?.method).toBe("PUT");
+    expect(await sent[0]?.text()).toBe("replacement body");
+    expect(sent[0]?.headers.get("X-Override")).toBe("override");
+    expect(sent[0]?.headers.has("X-Original")).toBe(false);
+    expect(sent[0]?.headers.get("X-CSRF-Token")).toBe("cookie-token");
+    expect(headers.has("X-CSRF-Token")).toBe(false);
+    expect(request.headers.get("X-CSRF-Token")).toBe("request-token");
+  });
+
+  it("uses an init method override when deciding whether CSRF is required", async () => {
+    await apiFetch(
+      new Request("http://localhost/api/threads", { method: "DELETE" }),
+      { method: "GET" },
+    );
+    expect(sent[0]?.method).toBe("GET");
+    expect(sent[0]?.headers.has("X-CSRF-Token")).toBe(false);
+  });
+
+  it("preserves Request headers and body when there is no CSRF cookie", async () => {
+    document.cookie = "";
+    await apiFetch(
+      new Request("http://localhost/api/threads", {
+        method: "POST",
+        headers: { "X-Request-ID": "run-2" },
+        body: "payload",
+      }),
+    );
+    expect(sent[0]?.headers.get("X-Request-ID")).toBe("run-2");
+    expect(sent[0]?.headers.has("X-CSRF-Token")).toBe(false);
+    expect(await sent[0]?.text()).toBe("payload");
+  });
+
+  it("retains string-input behavior and an explicit init CSRF header", async () => {
+    await apiFetch("http://localhost/api/threads", {
+      method: "POST",
+      headers: { "X-CSRF-Token": "init-token" },
+      body: "payload",
+    });
+    expect(sent[0]?.method).toBe("POST");
+    expect(sent[0]?.headers.get("X-CSRF-Token")).toBe("init-token");
+    expect(await sent[0]?.text()).toBe("payload");
+  });
+});
diff --git a/frontend/tests/unit/core/api/static-mode.test.ts b/frontend/tests/unit/core/api/static-mode.test.ts
index d72974773f..1d904abff2 100644
--- a/frontend/tests/unit/core/api/static-mode.test.ts
+++ b/frontend/tests/unit/core/api/static-mode.test.ts
@@ -168,6 +168,31 @@ describe("static website API requests", () => {
     expect(network).toHaveBeenCalledTimes(2);
   });
 
+  it("respects cancellation inherited from a Request before resolving a static response", async () => {
+    const controller = new AbortController();
+    const request = new Request("http://127.0.0.1:3000/api/features", {
+      signal: controller.signal,
+    });
+    controller.abort();
+
+    await expect(apiFetch(request)).rejects.toThrow();
+    expect(network).not.toHaveBeenCalled();
+  });
+
+  it("lets an init signal override an aborted Request in static mode", async () => {
+    const controller = new AbortController();
+    const override = new AbortController();
+    const request = new Request("http://127.0.0.1:3000/api/features", {
+      signal: controller.signal,
+    });
+    controller.abort();
+
+    expect((await apiFetch(request, { signal: override.signal })).status).toBe(
+      200,
+    );
+    expect(network).not.toHaveBeenCalled();
+  });
+
   it("uses the current browser origin for fixtures and respects cancellation", async () => {
     rs.stubEnv("HOSTNAME", "demo.internal");
     rs.stubEnv("PORT", "4000");