diff --git a/packages/core/test/shared/transport.test.ts b/packages/core/test/shared/transport.test.ts
new file mode 100644
index 000000000..bdef03a5e
--- /dev/null
+++ b/packages/core/test/shared/transport.test.ts
@@ -0,0 +1,182 @@
+import { createFetchWithInit, type FetchLike, normalizeHeaders } from '../../src/shared/transport.js';
+
+describe('normalizeHeaders', () => {
+    test('returns empty object for undefined', () => {
+        expect(normalizeHeaders(undefined)).toEqual({});
+    });
+
+    test('handles Headers instance', () => {
+        const headers = new Headers({
+            'x-foo': 'bar',
+            'content-type': 'application/json'
+        });
+        expect(normalizeHeaders(headers)).toEqual({
+            'x-foo': 'bar',
+            'content-type': 'application/json'
+        });
+    });
+
+    test('handles array of tuples', () => {
+        const headers: [string, string][] = [
+            ['x-foo', 'bar'],
+            ['x-baz', 'qux']
+        ];
+        expect(normalizeHeaders(headers)).toEqual({
+            'x-foo': 'bar',
+            'x-baz': 'qux'
+        });
+    });
+
+    test('handles plain object', () => {
+        const headers = { 'x-foo': 'bar', 'x-baz': 'qux' };
+        expect(normalizeHeaders(headers)).toEqual({
+            'x-foo': 'bar',
+            'x-baz': 'qux'
+        });
+    });
+
+    test('returns a shallow copy for plain objects', () => {
+        const headers = { 'x-foo': 'bar' };
+        const result = normalizeHeaders(headers);
+        expect(result).not.toBe(headers);
+        expect(result).toEqual(headers);
+    });
+});
+
+describe('createFetchWithInit', () => {
+    test('returns baseFetch unchanged when no baseInit provided', () => {
+        const mockFetch: FetchLike = vi.fn();
+        const result = createFetchWithInit(mockFetch);
+        expect(result).toBe(mockFetch);
+    });
+
+    test('passes baseInit to fetch when no call init provided', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseInit: RequestInit = {
+            method: 'POST',
+            credentials: 'include'
+        };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        await wrappedFetch('https://example.com');
+
+        expect(mockFetch).toHaveBeenCalledWith(
+            'https://example.com',
+            expect.objectContaining({
+                method: 'POST',
+                credentials: 'include'
+            })
+        );
+    });
+
+    test('merges baseInit with call init, call init wins for non-header fields', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseInit: RequestInit = {
+            method: 'POST',
+            credentials: 'include'
+        };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        await wrappedFetch('https://example.com', { method: 'PUT' });
+
+        expect(mockFetch).toHaveBeenCalledWith(
+            'https://example.com',
+            expect.objectContaining({
+                method: 'PUT',
+                credentials: 'include'
+            })
+        );
+    });
+
+    test('merges headers from both base and call init', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseInit: RequestInit = {
+            headers: { 'x-base': 'base-value', 'x-shared': 'base' }
+        };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        await wrappedFetch('https://example.com', {
+            headers: { 'x-call': 'call-value', 'x-shared': 'call' }
+        });
+
+        expect(mockFetch).toHaveBeenCalledWith(
+            'https://example.com',
+            expect.objectContaining({
+                headers: {
+                    'x-base': 'base-value',
+                    'x-call': 'call-value',
+                    'x-shared': 'call'
+                }
+            })
+        );
+    });
+
+    test('uses baseInit headers when call init has no headers', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseInit: RequestInit = {
+            headers: { 'x-base': 'base-value' }
+        };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        await wrappedFetch('https://example.com', { method: 'POST' });
+
+        expect(mockFetch).toHaveBeenCalledWith(
+            'https://example.com',
+            expect.objectContaining({
+                method: 'POST',
+                headers: { 'x-base': 'base-value' }
+            })
+        );
+    });
+
+    test('handles URL object as first argument', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseInit: RequestInit = { method: 'GET' };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        const url = new URL('https://example.com/path');
+        await wrappedFetch(url);
+
+        expect(mockFetch).toHaveBeenCalledWith(url, expect.objectContaining({ method: 'GET' }));
+    });
+
+    test('passes all baseInit properties when call init is empty object', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseInit: RequestInit = {
+            method: 'POST',
+            credentials: 'include',
+            headers: { 'x-base': 'value' }
+        };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        await wrappedFetch('https://example.com', {});
+
+        expect(mockFetch).toHaveBeenCalledWith(
+            'https://example.com',
+            expect.objectContaining({
+                method: 'POST',
+                credentials: 'include',
+                headers: { 'x-base': 'value' }
+            })
+        );
+    });
+
+    test('passes Headers instance through when call init has no headers', async () => {
+        const mockFetch: FetchLike = vi.fn();
+        const baseHeaders = new Headers({ 'x-base': 'value' });
+        const baseInit: RequestInit = {
+            headers: baseHeaders
+        };
+
+        const wrappedFetch = createFetchWithInit(mockFetch, baseInit);
+        await wrappedFetch('https://example.com', { method: 'POST' });
+
+        expect(mockFetch).toHaveBeenCalledWith(
+            'https://example.com',
+            expect.objectContaining({
+                method: 'POST',
+                headers: baseHeaders
+            })
+        );
+    });
+});