diff --git a/src/lib/__tests__/githubViewer.test.ts b/src/lib/__tests__/githubViewer.test.ts
new file mode 100644
index 0000000..435bc77
--- /dev/null
+++ b/src/lib/__tests__/githubViewer.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { fetchViewerLogin } from "../githubViewer";
+import { GitHubApiError } from "../types";
+
+const mockFetch = vi.fn();
+
+describe("fetchViewerLogin", () => {
+  beforeEach(() => {
+    vi.stubGlobal("fetch", mockFetch);
+    mockFetch.mockReset();
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("should return login on successful fetch", async () => {
+    mockFetch.mockResolvedValue({
+      ok: true,
+      json: async () => ({ login: "testuser" }),
+    });
+
+    const login = await fetchViewerLogin("test-token");
+    expect(login).toBe("testuser");
+
+    expect(mockFetch).toHaveBeenCalledTimes(1);
+    expect(mockFetch).toHaveBeenCalledWith("https://api.github.com/user", {
+      headers: {
+        Accept: "application/vnd.github+json",
+        Authorization: "Bearer test-token",
+        "User-Agent": "github-user-summary",
+      },
+      cache: "no-store",
+    });
+  });
+
+  it("should throw GitHubApiError when fetch is not ok", async () => {
+    mockFetch.mockResolvedValue({
+      ok: false,
+      status: 401,
+      json: async () => ({ message: "Bad credentials" }),
+    });
+
+    await expect(fetchViewerLogin("invalid-token")).rejects.toThrow(GitHubApiError);
+
+    // We can also check specific properties on the error
+    try {
+      await fetchViewerLogin("invalid-token");
+    } catch (e) {
+      expect(e).toBeInstanceOf(GitHubApiError);
+      expect((e as GitHubApiError).status).toBe(401);
+      expect((e as GitHubApiError).message).toBe("Failed to resolve current GitHub user");
+    }
+
+    expect(mockFetch).toHaveBeenCalledTimes(2);
+  });
+});