diff --git a/src/shared/transport.ts b/src/shared/transport.ts
index f9b21bed32..242493cc25 100644
--- a/src/shared/transport.ts
+++ b/src/shared/transport.ts
@@ -6,7 +6,7 @@ export type FetchLike = (url: string | URL, init?: RequestInit) => Promise<Respo
  * Normalizes HeadersInit to a plain Record<string, string> for manipulation.
  * Handles Headers objects, arrays of tuples, and plain objects.
  */
-export function normalizeHeaders(headers: HeadersInit | undefined): Record<string, string> {
+export function normalizeHeaders(headers: RequestInit['headers'] | undefined): Record<string, string> {
     if (!headers) return {};
 
     if (headers instanceof Headers) {
diff --git a/test/shared/transport.test.ts b/test/shared/transport.test.ts
new file mode 100644
index 0000000000..e12e6fd672
--- /dev/null
+++ b/test/shared/transport.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, test } from 'vitest';
+import { normalizeHeaders } from '../../src/shared/transport.js';
+
+describe('normalizeHeaders', () => {
+  test('returns empty object for undefined', () => {
+    expect(normalizeHeaders(undefined)).toEqual({});
+  });
+
+  test('copies plain object', () => {
+    const input = { 'Content-Type': 'application/json', 'X-Custom': 'value' };
+    const result = normalizeHeaders(input);
+    expect(result).toEqual(input);
+  });
+
+  test('normalizes array of tuples', () => {
+    const result = normalizeHeaders([['Content-Type', 'text/plain'], ['Accept', '*/*']]);
+    expect(result).toEqual({ 'Content-Type': 'text/plain', 'Accept': '*/*' });
+  });
+
+  test('accepts Headers instance', () => {
+    const h = new Headers();
+    h.set('Authorization', 'Bearer token');
+    const result = normalizeHeaders(h);
+    expect(result).toEqual({ 'authorization': 'Bearer token' });
+  });
+});