diff --git a/src/browser/client-fetch.loopback-auth.test.ts b/src/browser/client-fetch.loopback-auth.test.ts
index f0c49b3163d..209f87d9fd0 100644
--- a/src/browser/client-fetch.loopback-auth.test.ts
+++ b/src/browser/client-fetch.loopback-auth.test.ts
@@ -31,6 +31,18 @@ vi.mock("./routes/dispatcher.js", () => ({
 
 import { fetchBrowserJson } from "./client-fetch.js";
 
+function stubJsonFetchOk() {
+  const fetchMock = vi.fn<(input: RequestInfo | URL, init?: RequestInit) => Promise<Response>>(
+    async () =>
+      new Response(JSON.stringify({ ok: true }), {
+        status: 200,
+        headers: { "Content-Type": "application/json" },
+      }),
+  );
+  vi.stubGlobal("fetch", fetchMock);
+  return fetchMock;
+}
+
 describe("fetchBrowserJson loopback auth", () => {
   beforeEach(() => {
     vi.restoreAllMocks();
@@ -49,14 +61,7 @@ describe("fetchBrowserJson loopback auth", () => {
   });
 
   it("adds bearer auth for loopback absolute HTTP URLs", async () => {
-    const fetchMock = vi.fn<(input: RequestInfo | URL, init?: RequestInit) => Promise<Response>>(
-      async () =>
-        new Response(JSON.stringify({ ok: true }), {
-          status: 200,
-          headers: { "Content-Type": "application/json" },
-        }),
-    );
-    vi.stubGlobal("fetch", fetchMock);
+    const fetchMock = stubJsonFetchOk();
 
     const res = await fetchBrowserJson<{ ok: boolean }>("http://127.0.0.1:18888/");
     expect(res.ok).toBe(true);
@@ -67,14 +72,7 @@ describe("fetchBrowserJson loopback auth", () => {
   });
 
   it("does not inject auth for non-loopback absolute URLs", async () => {
-    const fetchMock = vi.fn<(input: RequestInfo | URL, init?: RequestInit) => Promise<Response>>(
-      async () =>
-        new Response(JSON.stringify({ ok: true }), {
-          status: 200,
-          headers: { "Content-Type": "application/json" },
-        }),
-    );
-    vi.stubGlobal("fetch", fetchMock);
+    const fetchMock = stubJsonFetchOk();
 
     await fetchBrowserJson<{ ok: boolean }>("http://example.com/");
 
@@ -84,14 +82,7 @@ describe("fetchBrowserJson loopback auth", () => {
   });
 
   it("keeps caller-supplied auth header", async () => {
-    const fetchMock = vi.fn<(input: RequestInfo | URL, init?: RequestInit) => Promise<Response>>(
-      async () =>
-        new Response(JSON.stringify({ ok: true }), {
-          status: 200,
-          headers: { "Content-Type": "application/json" },
-        }),
-    );
-    vi.stubGlobal("fetch", fetchMock);
+    const fetchMock = stubJsonFetchOk();
 
     await fetchBrowserJson<{ ok: boolean }>("http://localhost:18888/", {
       headers: {
@@ -103,4 +94,14 @@ describe("fetchBrowserJson loopback auth", () => {
     const headers = new Headers(init?.headers);
     expect(headers.get("authorization")).toBe("Bearer caller-token");
   });
+
+  it("injects auth for IPv6 loopback absolute URLs", async () => {
+    const fetchMock = stubJsonFetchOk();
+
+    await fetchBrowserJson<{ ok: boolean }>("http://[::1]:18888/");
+
+    const init = fetchMock.mock.calls[0]?.[1];
+    const headers = new Headers(init?.headers);
+    expect(headers.get("authorization")).toBe("Bearer loopback-token");
+  });
 });
diff --git a/src/browser/client-fetch.ts b/src/browser/client-fetch.ts
index c8617d0f79c..2fc0bacf396 100644
--- a/src/browser/client-fetch.ts
+++ b/src/browser/client-fetch.ts
@@ -21,7 +21,11 @@ function isAbsoluteHttp(url: string): boolean {
 function isLoopbackHttpUrl(url: string): boolean {
   try {
     const host = new URL(url).hostname.trim().toLowerCase();
-    return host === "127.0.0.1" || host === "localhost" || host === "::1";
+    // URL hostnames may keep IPv6 brackets (for example "[::1]"); normalize before checks.
+    const normalizedHost = host.startsWith("[") && host.endsWith("]") ? host.slice(1, -1) : host;
+    return (
+      normalizedHost === "127.0.0.1" || normalizedHost === "localhost" || normalizedHost === "::1"
+    );
   } catch {
     return false;
   }