diff --git a/src/browser/client-fetch.bridge-auth-registry.test.ts b/src/browser/client-fetch.bridge-auth-registry.test.ts
new file mode 100644
index 000000000..dc4db32ee
--- /dev/null
+++ b/src/browser/client-fetch.bridge-auth-registry.test.ts
@@ -0,0 +1,53 @@
+import type { AddressInfo } from "node:net";
+import { createServer } from "node:http";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { deleteBridgeAuthForPort, setBridgeAuthForPort } from "./bridge-auth-registry.js";
+
+describe("fetchBrowserJson loopback auth (bridge auth registry)", () => {
+  afterEach(() => {
+    vi.resetModules();
+    vi.clearAllMocks();
+  });
+
+  it("falls back to per-port bridge auth when config auth is not available", async () => {
+    vi.doMock("../config/config.js", async (importOriginal) => {
+      const original = await importOriginal<typeof import("../config/config.js")>();
+      return {
+        ...original,
+        loadConfig: () => ({}),
+      };
+    });
+
+    const server = createServer((req, res) => {
+      const auth = String(req.headers.authorization ?? "").trim();
+      if (auth !== "Bearer registry-token") {
+        res.statusCode = 401;
+        res.setHeader("Content-Type", "text/plain; charset=utf-8");
+        res.end("Unauthorized");
+        return;
+      }
+      res.statusCode = 200;
+      res.setHeader("Content-Type", "application/json; charset=utf-8");
+      res.end(JSON.stringify({ ok: true }));
+    });
+
+    await new Promise<void>((resolve, reject) => {
+      server.once("error", reject);
+      server.listen(0, "127.0.0.1", () => resolve());
+    });
+
+    const port = (server.address() as AddressInfo).port;
+    setBridgeAuthForPort(port, { token: "registry-token" });
+
+    try {
+      const { fetchBrowserJson } = await import("./client-fetch.js");
+      const result = await fetchBrowserJson<{ ok: boolean }>(`http://127.0.0.1:${port}/`, {
+        timeoutMs: 2000,
+      });
+      expect(result.ok).toBe(true);
+    } finally {
+      deleteBridgeAuthForPort(port);
+      await new Promise<void>((resolve) => server.close(() => resolve()));
+    }
+  });
+});